(PDF) Adaptations for survival - Wiley · survival of animals and plants in a variety of environments. An adaptation is a genetically controlled structural, ... and with no water flowing - DOKUMEN.TIPS (2023)

key kNOWledGe

This chapter is designed to enable students to: recognise howstructural, physiological and behavioural features can contributeto the survival of organisms

recognise that a feature is adaptive in a specifi c set ofenvironmental conditions gain knowledge of particular adaptationsthat equip organisms for survival in various environmentalconditions in Australia

understand that each organism has a tolerance range for everyenvironmental factor and that beyond that range survival is atrisk.

5 Adaptations for survivalchaPTeR

fiGuRe 5.1 The spinifex hopping mouse (Notomys alexis), alsoknown as the tarrkawarra, is a marsupial mammal that lives in sandydesert regions in parts of Australia. They display many adaptationsthat equip them to survive in a hot and arid environment. In thischapter we will explore some of the many structural, physiologicaland behavioural adaptations that enable plants and animals tosurvive in deserts and in other extreme environments.

c05AdaptationsForSurvival 195 3 August 2015 7:10 AM

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the Rule of ThreesThe Rule of Threes provides a reasonableguideline for priority-setting for anyone lost in a harshenvironment. The rule states: You can live for about 3 minuteswithout air. (Exceed that and a person is at

risk of death from asphyxiation.) You can live for about 3 hourswithout shelter (in a harsh environment).

(Exceed that and a person is at risk of death from exposure.)You can live for about 3 days without water (if you have shelter).(Exceed

that and a person is at risk of death from dehydration.) You canlive for about 3 weeks without food (if you have shelter andwater).

(Exceed that and a person is at risk of death from starvation.)Leaving aside the immediate requirement for air (in fact, theoxygen in air),

these times are guidelines only and will vary according to theseverity of the conditions, as well as a persons body weight,genetic factors and whether or not the person is alreadydehydrated.

The Rule of Threes is a useful rule-of-thumb for human survivalin the harsh cold conditions of the high latitudes of the northernhemisphere. Teachers of basic skills for survival in a freezingwilderness may refer to the Rule of Threes because it stresses thepriorities for survival. First priority direct your focus and yourenergy into securing shelter; second priority water; third prioritythen, only after shelter and water are secured, worry about food.In the freezing cold of a northern hemisphere snowstorm, onechallenge is to find shelter in order to avoid a drop in bodytemperature (hypothermia). In contrast, in the desert heat of theAustralian outback in summer, the challenge is to find protectionagainst the heat and avoid heat stroke (hyperthermia). Regardlessof whether people are challenged by the freezing tundra of Canadaor in the desert heat of outback Australia, another essential fortheir short-term survival is water: prolonged dehydration cankill.

Food is a requirement for survival of heterotrophs (animals andfungi) but, in contrast to water, food is not necessary forshort-term survival. In 1981, a number of political prisoners inNorthern Ireland staged hunger strikes in protest against thepresence of British military personnel in the region. Tenprotestors, who did take water, died after periods without food ofbetween 46 and 73 days. The period of survival without food isinfluenced by a persons health and stored energy reserves.

Bushfires are a serious threat to survival in fire-prone areasof Australia, especially the south-east sector of the country. Thebox on page xx relates to a tragic event during a bushfire inVictoria in 1998. Sadly, some firefighters died when a fiercebushfire suddenly changed direction.

In this chapter, we will look at a range of adaptations thatcontribute to the survival of animals and plants in a variety ofenvironments. An adaptation is a genetically controlled structural,behavioural or physiological feature that enhances the survival ofan organism in particular environmental conditions. It is importantto note that the value of a feature as an adaptation exists inrelation to a specific way of life and in a particular set ofenviron-mental conditions. In another set of environmentalconditions and a different way of life, the same feature may bemaladaptive. For example, freshwater fish extract dissolved oxygenfrom the water in which they live using their gills, the organ forgas exchange (see figure 5.2). Gills provide an efficient structurefor extracting oxygen from water. However, if removed from thewater, the fish gills are maladaptive. Without the buoyancy ofwater to support them, the feathery gill filaments collapse, andwith no water flowing over them the fish cannot obtain oxygen andsuffocates. Likewise, mammalian lungs are adapted for gas exchangewith the air, but they are useless for extracting dissolved oxygenfrom water.

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fiGuRe 5.2 Magnifi ed image of the rows of gill fi laments of afreshwater fi sh. Note that each fi lament has many smallprojections (lamellae) on it. These greatly increase the surfacearea available for the life-supporting process of gas exchange.Normally, water enters the mouth of the fi sh, passes to thepharynx and is forced over the gill surfaces and exits. Would youpredict that a rich blood supply would be present in the gilllamellae?

Adaptive features of an organism are innate, that is, built intoits genetic makeup. Some adaptations refl ect the Rule of Th rees.For animals, survival depends

on their structural, physiological and behavioural features thatenable them to exploit the available resources of shelter, waterand food in a particular environment. For plants, survival alsodepends on their structural, physio-logical and behaviouralfeatures that contribute to their success in accessing water andsunlight in their environments.

tolerance rangeTh e particular environmental conditions in whicha particular species can suc-cessfully live and reproduce defi neits tolerance range. Every organism has a tolerance range forenvironmental factors such as temperature, desiccation, oxygenconcentration, light intensity and ultraviolet exposure. Atolerance range identifi es the variation within which organismscan survive. Figure 5.3 shows the tolerance range for a fi shspecies in terms of water temperature. Th e extremes of this rangeare the tolerance limits for that environmental factor.

low highTemperature

Optimum range

Zone of physiological stress

Zone of intolerance

Tolerance range

fiGuRe 5.3 Tolerance range in terms of temperature for a fi shspecies. Notice that, as water temperature moves closer to thetolerance limits, fewer fi sh are found. This is the so-called zoneof physiological stress. What happens beyond the tolerancelimits?

Odd facT

Some bacteria have a very high temperature tolerance. Thebacterial species known as Sulfolobus acidocaldarius survivestemperatures in boiling hot sulfur springs. This species ofbacterium dies from cold at temperatures below 55 C.

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If an environmental factor has a value above or below the rangeof tolerance of an organism, that organism will not survive unlessit can escape from, or somehow compensate for, the change. In somespecies migration is one such escape behav-iour, while othersretreat underground. Tolerance ranges diff er between species andare infl uenced by structural, physiological and behaviouralfeatures of organ-isms. For example, the cold tolerance of variousmammals is infl uenced by struc-tural features such as fur density,shape of the body (see fi gure 5.4) and extent of insulating fatdeposits, and by their behaviours, such as hibernating(cop-ing-with-it strategy). In fi gure 5.4, can you identify whichfox species is the Arctic fox (Vulpes lagopus) and which is theSimien Fox (Canis simensis)?

fiGuRe 5.4 Apart from the difference in their fur thickness,these two fox species differ in their ear sizes. Smaller ears havea lower surface-area-to-volume ratio than larger ears. Which foxwould be better able to conserve its body heat and tolerate lowertemperatures? Why? (Surface-area-to-volume ratio is discussed inchapter 1, pp. xxxx.)

Any condition that approaches or exceeds the limits of tolerancefor an organism is said to be a limiting factor for that organism.Terrestrial and aquatic environments can diff er in their limitingfactors. Table 5.1 shows environmental factors that infl uencewhich kinds of organism can survive in various habitats.

Table 5.1 Examples of limiting factors in various habitats. Onlyone example of a limiting factor is given for each environment. Canyou identify another limiting factor for one of these habitats?Those species that can survive under certain environmentalconditions have tolerance ranges that accommodate thoseconditions.

Habitat limiting factor Comment

fl oor of tropical rainforest light intensity Low lightintensity limits the kinds of plants that can survive.

desert water availability Limited water supply means that onlyplants able to tolerate desiccation can survive.

littoral zone desiccation Exposure to air and sun limits thetypes of organism that survive.

polar region temperature Low temperatures limit the types oforganism that are found.

stagnant pond dissolved oxygen levels Low dissolved oxygenlevels limit the types of organism that can live there.

Th e structure and the physiology of plants and animalsdetermine their tol-erance range. For each organism, the limits ofits tolerance ranges for various environmental factors are fi xed,except for the occurrence of an enabling mutation.

Th e human species is the only organism that makes extensive useof tech-nology to extend the limits of its natural tolerance range.As air-breathing mammals, we are not prevented from entering thewatery world of the fi sh; technology such as scuba tanks enable usto do this. Technology enables people to survive in hostileenvironments on and beyond Earth, where con-ditions are outside thetolerance range of an unaided person. Equipment and hi-techclothing enables a mountaineer to survive an ascent to the peak ofone of the highest mountains on Earth (see fi gure 5.5a). Anextremely sophisticated spacesuit enables an astronaut to leave theInternational Space Station, which is orbiting about 250 km aboveEarth, and install cables (see fi gure 5.5b).

Odd facT

Reef-building coral polyps live in warm, clear shallow seas, butif the temperature of the water falls below 18 C the polyps die.What is the lower end of the temperature tolerance range for coralpolyps?

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(a) (b)

fiGuRe 5.5 (a) Andrew Lock is Australias most accomplishedhigh-altitude mountaineer and the only member of the BritishCommonwealth to have climbed all 14 of the worlds 8000 metre plusmountains. Here he is ascending to the peak of Mt Annapurna, 8091 mabove sea level, one of the worlds most dangerous climbs. You canfi nd more information about these Andrews ascent of the 14 peaksat http://andrew-lock.com/the-fourteen-8000ers. (b) Commander BarryWilmore, a US astronaut, on a space walk from the InternationalSpace Station on 1 March 2015.

key ideas

Adaptations are structural, physiological or behaviouralfeatures that enable an organism to survive and reproduce inparticular environmental conditions.

The tolerance range of an organism defi nes the range ofenvironmental conditions in which a particular species cansuccessfully live and reproduce.

The human species uses technology to extend the limits of itsnatural tolerance range.

Quick check

1 What is meant when a structural feature of an organism is saidto be maladaptive?

2 Give an example of a physical feature that could be labelledas maladaptive and the conditions in which that label could begiven.

3 Identify whether each of the following statements is true orfalse.a Excluding technological support, survival is not possiblebeyond the

extremes of an organisms tolerance range.b Adaptations arefeatures that equip organisms to survive in all conditions.c Theextreme ends of a temperature tolerance range mark thetolerance

limits of an organism for temperature.d A behaviour that islearned is an example of an adaptation.

the desert environmentAustralia is the driest inhabitedcontinent on Earth and has been arid for millions of years. As vastareas of the continent dried out, ancestral species evolved overmany generations. As a result of mutations, individuals of somespecies devel-oped features that enhanced their survival in aridconditions and it was their off spring that had a greater chance ofsurvival. Th eir descendants are the native plants and animals thatlive successfully in the vast desert regions and semi-arid areas ofoutback Australia. Today, much of inland Australia is covered bydeserts, mainly sandy deserts with some stony deserts (see fi gure5.6). Seventy per cent of Australias land surface is either arid(average annual rainfall of 250 mm or less) or semi-arid (averageannual rainfall of 250 to 350 mm).

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fiGuRe 5.6 (a) Much of inland Australia is hot and arid andcovered by deserts. This fi gure shows our 10 major deserts,ranging from the largest, the Great Victoria Desert, more than 400000 km2, to the smallest, the Pedirka, just over 1000 km2. (b) Mostof these deserts are sandy deserts, such as the Simpson Desert, alarge area of sand plains and dunes in central Australia. (c) Someareas are stony deserts, such as Sturt Stony desert, a rock-covereddesolate area.

(b) (c)

GreatAustralian Bight

Timor Sea

Arafura Sea

Coral Sea

I N D I A N O C E A N

I N D I A N

O C E A N

Darwin

Gibson

Desert

Little

Sandy

Desert

WesternAustral ia

South

Austral ia New South Wales

Victoria

Tasmania

ACT

Queensland

NorthernTerritory

JBT

Perth

Adelaide

Hobart

Canberra

Melbourne

Sydney

Brisbane

Simpson

Desert

1 3

1 Pedirka Desert

2 Tirari Desert

3 Sturt Stony Desert

Alice

SpringsUluru

ACT = Australian Capital Territory

JBT = Jervis Bay Territory0 200 1000 km

Great Victoria Desert

Great Sandy Desert

(a)

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Th e environmental factors that dominate desert environments aretempera-ture and aridity (absence of surface water).

Th e temperatures in Australian deserts in summer are very high.For example, the average daily maximum temperature for the month ofFeb-ruary 2015 across much of inland Australia was in the range of36 to 42 C. For coastal Victoria, the fi gure was 24 to 27 C (seefi gure 5.7a). Th e rainfall in cen-tral Australia in the samemonth was low, with most inland regions receiving rainfall of 0 to25 mm. In contrast, in the same month a few coastal regions ofeastern Australia received 600 to 800 mm of rain (see fi gure5.7b). Because of the high temperatures of the desert areas, anysmall amounts of summer rain quickly evaporate and do not exist asfree-standing water. In spite of the high temperatures and aridityof deserts, some plants and animals live successfully in theseareas because they have adaptations that equip them for life inthis environment.

fiGuRe 5.7 Much of inland Australia consists of large areas ofarid, hot deserts. (a) Map showing the mean maximum temperatureacross Australia in February 2015. (The mean maximum temperature isthe average daily maximum air temperature for the month.) (b) Mapshowing the rainfall in millimetres for the month of February2015.

Over 30

20 to 30

10 to 20

C

Source: World Climate5000 1000 km

(a)

Over 1600

1200 to 1600

800 to 1200

400 to 800

200 to 400

Under 200

mm

Source: World Climate

(b)

Water is essential for lifeOne of the key threats to survival inthe arid Australian outback is dehydration. Water loss by an animalis normally compensated for by water gain so that, overall, over aperiod of time, water balance exists. Th at is:

water-in = water-out

OR

water gain water loss = 0

In desert conditions, the water content of the body can becomeunbalanced if water loss exceeds water gain over an extendedperiod, producing a state of dehydration. Th e more severe thedehydration and the faster it occurs, the more deadly the potentialconsequences.

Water is a remarkable substance with properties that arecritical for living organisms (see the following feature).

Odd facT

Sturt Stony Desert was reached by an expedition led by CharlesSturt in August 1844. A member of his expedition exclaimed: Goodheavens! Did ever man see such country.

In chapter 6 we will examine water balance and itsregulation.

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Water: a special molecule

Water has several special properties that enable it to playimportant roles in biological settings.

Water molecules tend to stick togetherThe tendency of watermolecules (H2O) to stick together can be expressed more scientifically as mol-ecules having high cohesion. Th e cohesiveness is dueto the fact that the oxygen end of a water molecule has a slightnegative charge, while the hydrogen ends have a slight positivecharge (see fi gure 5.8).

H

O

H

e

+

e

e

ee

e

e

e

fiGuRe 5.8 A water molecule showing its arrangement of electrons(e-), two in the inner shell and six electrons in the outer shell.Each of the two hydrogen atoms shares its single electron with theoxygen atom. Oxygen is a larger atom than hydrogen and because ofthis the electrons are pulled toward oxygen and away from thehydrogen, resulting in a net negative charge for the oxygen end ofa water molecule and a net positive charge for the hydrogenend.

= oxygen

= hydrogen

(a) (b)

+ +

fiGuRe 5.9 (a) Each water molecule consists of one oxygen atom(O) joined to two hydrogen atoms (H) by a strong covalent bond. (b)Water molecules are attracted to each other because of the slightnegative charge on the oxygen atom and the slight positive chargeon each hydrogen atom. The attractive forces that hold watermolecules together are called hydrogen bonds, shown here as shortcurved lines.

Because of these slight positive and negative charges, watermolecules are attracted to each other and stick together (in groupsof up to 4 water molecules). Th is attractive force is known ashydrogen bonding (see fi gure 5.9). Th e cohesive nature of wateris what makes it a versatile solvent for polar molecules (see nextpara-graph). Th e cohesive nature of water is one key factor thatenables water columns in the xylem tissue of vas-cular plants tomove at the top of trees.

Water is a versatile solventWater is the predominant solvent inliving organ-isms. Th e chemical reactions that occur in cellsinvolve the synthesis of complex molecules from simple ones and thebreakdown of complex mol-ecules. Th ese chemical reactions occur insolution so that water, as a solvent, is necessary. Likewise,nutrients can only be absorbed from the alimentary canal into thebloodstream if they are in solution. How does water act as asolvent for hydrophilic or polar compounds? Lets look at how waterdissolves a hydrophilic solid, such as a crystal of commonsalt.

Th e top of fi gure 5.10 shows a crystal of common salt (NaCl)when it is fi rst placed in water. At this stage, the salt crystalhas not dissolved. However, after the salt comes into contact withthe water, the salt dissociates (separates) into sodium ions (Na+)and chloride ions (Cl) (see bottom of fi gure 5.10). Note that, insolution, the sodium ion (Na+) is sur-rounded by water moleculesarranged with their oxygen atoms closest to the sodium ion. Checkout the arrangement of water molecules that surround the chlorideion (Cl). Which end of the water mol-ecule is closer to thision?

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(continued)

Each sodium ion and each chloride ion becomes surrounded by ashell of water molecules; this is possible because of the cohesivenature of water molecules. For the chloride ions, it is thepositively charged side of the water molecules that surround them.In contrast, for the sodium ions, it is the nega-tively chargedportion of the water molecules that surrounds them. Th ese shellsof water molecules separate the sodium ions from the chloride ionsand, by removing the attraction between these ions, keep the saltin solution.

Water resists temperature changesResistance to temperaturechanges means that, relative to other compounds, more heat energymust be added to or removed from water to pro-duce a given changein its temperature. Th is rela-tively greater resistance is becauseof the many hydrogen bonds that exist between water molecules

in solution. (Remember, water mol-ecules are cohesive!)

Water has a relatively high speci c heat capacitySpecifi c heatcapacity refers to the heat energy required to raise thetem-perature of a given mass of a sub-stance by one degree. Forexample, the specifi c heat of liquid water is 4186 joules per kgper degree Celsius, while that of air is 1046 J. So, given the sameinput of heat energy, the tem-perature of a body of water changesfar less than the temperature of the surrounding atmosphere.

Biologically, this means that large bodies of water providerelatively more stable thermal living conditions for the organismsthat live in those environ-ments in comparison with terrestrialorganisms.

Water has a high heat of vaporisationTh e heat of vaporisationrefers to the input of heat energy required to con-vert a liquid tovapour (gas). For water, the heat of vaporisation of liquid wateris 2260 kJ/kg; for comparison, that of petrol is about 600. Th ehigh heat of vaporisation of water is an important factor incooling mammals exposed to heat stress.

Th e major mechanism for cooling in humans and other primates isthe evaporation of sweat that is produced when the body starts tooverheat. Not all mammalian species produce sweat in order to loseheat. Some species, such as dogs, use panting to cool themselves.Yet other species, such as kanga-roos and wallabies, lick the furand skin of their fore-arms and paws (see fi gure 5.11). Cooling inall these cases sweating, panting and saliva spreading is due tothe evaporation of water from the sweat or the saliva on the skinor fur surface. Th e heat energy needed to evaporate the water istaken from warm blood in vessels close to the skin surface, coolingthe blood. Th is blood then returns to the body core, coolingit.

Both the high heat of vaporisation and the high specifi c heatcapacity of water were factors in pre-venting the deaths of the firefi ghters caught in a bushfi re discussed in the box on pagexxx.

Cl

Cl

Na+

Na+

Cl

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fiGuRe 5.10 A salt crystal (top) in water will dissociate intoits constituent sodium ions (Na+) and chloride ions (Cl) (bottom).These ions are attracted to different parts of the water moleculesand become surrounded by them. Which part of a water molecule has aslight positive charge?

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fiGuRe 5.11 Evaporation of saliva from the kangaroos forearmsand paws requires heat energy. The heat energy needed to change thestate of water from liquid to vapour comes from the kangaroos body,cooling it.

Water has a high heat of fusionTh is refers to the heat thatmust be removed from liquid water to convert it to a solid (ice).For example, the heat of fusion of liquid water is 333 kJ/kg, whilethat of ethanol, another liquid, is 104 kJ/kg.

Solid water is less dense than liquid waterDensity is a measureof the mass of a substance per unit volume (mass/volume). Mostsubstances are more dense in their solid state than their liquidstate. Th is is not the case for water; liquid water is more densethan solid water (ice) and, as a result, ice fl oats on water. Animportant consequence of this is that

when small bodies of water such as ponds and lakes freeze invery cold climates ice forms at the sur-face. Th is ice acts as aninsulating layer that assists in keeping the underlying waterliquid. If ice were more dense than liquid water bodies of waterwould freeze in very cold weather from the bottom up, with deadlyconsequences for aquatic life (see fi gure 5.12).

fiGuRe 5.12 Water in two of its physical states, as liquid waterand as ice. Ice fl oats on water because it has a lowerdensity.

Some density values are: liquid water = 1.00 g/cm3 seawater =1.02 g/cm3 solid water (ice) = 0.93 g/cm3 coconut oil = 0.93g/cm3

Refer to fi gure 5.10 and see if you can explain why seawater ismore dense than pure water. (Answer: Adding Na+ ions and Cl ions tothe water increases the volume of the water. However, because themass of the ions (Na+ = 23, Cl = 35) is greater than the mass ofthe water molecules (18), adding these ions to the water increasesits mass by a greater factor, thus increasing its density).

To appreciate both the importance of water in living organismsand to understand how some animals can thrive in desert conditions,let us fi rst look briefl y at the water content of healthy adultpeople and their sources of water gain and water loss.

Water in the human bodyLike all living organisms, the human bodyconsists mainly of water. An average adult male consists of about60 to 65 per cent water by weight, equivalent to about 45 litres.For an average adult female, the fi gure is about 50 to 55 percent, corresponding to about 30 litres (see fi gure 5.13a). Th evalues can vary according to a persons age, state of health andweight. People with obesity, for example, have a lower percentageof water than those who are lean. Th e water content of the different tissues and organs of the human body varies from more than 80per cent in the blood to about 10 per cent in adipose tissue (seefi gure 5.13b).

unit 1 humans: Water balance

aOs 2

Topic 1

concept 9

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fiGuRe 5.13 (a) Average percentage of body water in adulthumans. Note the sex difference in the average percentage of totalbody weight that is composed of water. The average fi gure is lowerfor females because they have a higher percentage of fat. (b)Percentage water content of various human tissues and organs. Whichtissues have the lowest water content? Do these data explain thedifference between the average water content of adult males andfemales shown in part (a)?

40%

60%

50%

50%

(a)

Brain 75%

Skin 72%

Heart 79%

Lungs 79%

Intestine 75%

Skeleton(bone) 22%

Blood 83%

Liver 68%

Kideny 83%

Adiposetissue 10%

Muscle 76%

(b)

Of the total water content of the body, most is contained withinintracellular fl uid (about two-thirds). Th is is the fl uid insidethe body cells, mainly the aqueous fl uid of the cytosol. Th eremaining one-third of water is present as extracellular fl uid;this is composed of interstitial fl uid, which fi lls the spacesbetween cells and bathes their plasma membranes (about 26%), andplasma, the liquid portion of the blood (about 7%) (see fi gure5.14a). Th e water content of the body is of course not just water:it contains dissolved solutes including proteins, sugars andminerals ions, such as sodium, potassium and chloride.

fiGuRe 5.14 (a) Average distribution of the total water contentof the body across the three compartments in an adult human. In anaverage adult male weighing 70 kg, the total water of about 42 L isdistributed as about 28 L of intracellular fl uid within cells,about 11 L of interstitial fl uid surrounding cells and about 3 Lcirculating in the blood plasma. (b) The body water compartmentsare separated but water can move between them. The cellular wall ofthe capillaries separates the plasma from the interstitial fl uid.Plasma membranes of cells separate the interstitial fl uid from theintracellular fl uid.

67%

26%

7%

Plasma

Interstitial uid

Intracell

(a)

Plasmamembrane

Plasma Interstitialuid

Intracellularuid

(b)

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Figure 5.15 shows the routes by which water is lost from andgained by the human body.Water lossIn people, as in other mammals,water is lost through several avenues. A healthy adult human loseswater from: the kidneys in the urine the lungs in exhaled breath.When we

breathe out, we lose water vapour from the lungs and itspassages. Typically we do not notice this loss, so this loss issaid to be insensible water loss. We can, however, see this waterloss on a cold

day when the warm water vapour in exhaled breath condenses intotiny water droplets on contact with the cold air outside (see figure 5.16)

the gut in egested faeces the skin, in water lost via pores inthe skin and as sweat secreted by sweat

glands. Th e loss of water via pores in the skin is an exampleof insensible (not noticed) water loss, and is the loss of purewater. In contrast, sweat con-tains dissolved solutes. Note thatsweating does not occur until the body is subjected to heat stress,but the insensible loss of water from the pores of the skin occursall the time.Th e total daily loss of water in a person in atemperate climate and in a

non-exercising state is on average 2.5 L. Table 5.2 shows theproportion of this loss through the diff erent avenues of waterloss.

Table 5.2 Average minimum daily water loss by a non-exercisingadult in a temperate climate

organ Volume lost (l/day)

kidneys 1.5

lungs 0.4

skin (via pores) 0.4

skin (from sweat glands) 0.0*

gut (via faeces) 0.2

*Sweating can be a major source of water loss, but sweatingoccurs only under conditions of heat stress.

Under changed conditions, water loss by a person can increasemarkedly, for example: Water loss from the skin is greatlyincreased when sweating occurs in res-

ponse to an increase in core body temperature, such as duringperiods of vigorous exercise or during periods of exposure to highenvironmental tem-peratures. Sweating is initiated by the region ofthe hypothalamus of the brain in response to an increase in corebody temperature. One study found that water loss from sweating canexceed 1.5 L per hour in persons working in very hot environmentalconditions.

Water loss from the gut is greatly increased when a person hassevere diarrhoea or bouts of vomiting. In the case of diarrhoeacaused by a cholera infection water loss can be fatal if medicaltreatment is not received (refer to chapter 1, p. xxx). Th ishighlights the dangers of severe dehydration.

Water inuid and food

Metabolic water

Skin

Lungs

Kidney

Gut

fiGuRe 5.15 Water is both gained by and lost from the humanbody. Water losses (shown in purple) occur from the skin, lungs,gut and kidneys. Water gain (shown in green) occurs by absorptionof water from fl uid and food taken into the gut. Water gain alsocomes from metabolic water produced by aerobic cellularrespiration.

fiGuRe 5.16 Small amounts of water vapour are lost from thelungs in every exhaled breath. Normally, we are unaware of thisloss. On a cold day, however, when the warm water vapour hits thecold air, it condenses into tiny droplets of water that arevisible.

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Water gainWater is gained from two sources: one external and oneinternal (see below).

Th e external sources of water gained by the human body are theaqueous fl uids we drink and the water content of foods that weingest (see table 5.3).

Table 5.3 Water content of some fresh fruits and vegetables

food item Percentage water

apple 84

banana 74

carrot 87

lettuce 96

orange 87

peas 79

watermelon 92

zucchini 95

Source: Adapted from www2.ca.uky.edu/enri/pubs/enri129.pdf

Water enters the gut from where it is rapidly absorbed into theblood circu-latory system within 5 minutes of drinking the fl uid.Absorption of water occurs mainly in the small intestine and, to alesser extent, in the colon of the large intestine. Water is thendistributed via the blood stream throughout the body to theinterstitial fl uid and then to cells. (Passage of water into cellsacross the plasma membrane may occur either by osmosis or byfacilitated diff usion through channel proteins known as aquaporins(refer to chapter 1, p. xx.)

Th e internal source of water is produced during aerobiccellular respiration. Recall the summary equation for aerobicrespiration (refer to chapter 3, p. xxx):

glucose + oxygen carbon dioxide + water

Note that water is a product of aerobic respiration, and thiswater is called metabolic water. Th is water is produced in one ofthe last steps in aerobic respiration:

O2 + 4H+ + 4e 2H2O

Th e volume of metabolic water produced each day is about 0.4 L.Th is amount is not suffi cient for human survival, and must besupplemented by an intake of external water. One study identifi edthat, on average, humans gain the water they need from fl uids theydrink (about 60%), from food they ingest (about 30%) and frommetabolic water (about 10%).

key ideas

Most of inland Australia is arid or semi-arid and is covered bysandy deserts.

Water is essential for life and is a major component of thehuman body. In the human body, water is present in threecompartments: plasma, interstitial fl uid and intracellular fluid.

For survival, water loss must be balanced by water gain. Waterloss from the human body occurs through several channels, the mainone being via the kidneys.

Water gain by people is either external, from food and drink, orinternal, from metabolic water.

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Quick check

4 Which human tissue has:a the highest water content b thelowest water content?

5 Where would you fi nd:a intracellular fl uid b plasma cinterstitial fl uid?

6 In a healthy human adult, where is most water located?7 Listthe routes for water loss by a person.8 What is the origin ofmetabolic water?9 True or false? A person continually loses waterin sweat.

adaptations for survival: desert animalsIn this section, we willlook at some examples of adaptations that enable ani-mals tosurvive and reproduce in a desert environment. Th e keyenvironmental challenges of desert life are avoiding excessivewater loss that can result in dehydration and avoiding overheatingthat can result in hyperthermia, both conditions being potentiallydeadly.

In the previous section we saw that people, like most mammals,replace most of the water that they lose by drinking liquid water.Th is is easy when access to clean piped water is available. In thedesert, however, free-standing water is neither predictably norreliably available. After an occasional heavy rain or storm,temporary creeks, transient lakes and small pools of water exist inthe desert. For most of the time, however, creek beds are dry,lakes are dry salt-pans, and pools and puddles of water do notexist. Th is lack of free-standing water in the desert can persistfor years, even decades. How is survival possible for animals andplants under these conditions?

Survival without drinking: the mulgaraSome mammals have featuresthat equip them to survive in hot desert environ-ments, includingthe ability to survive without drinking liquid water. Among them isthe crest-tailed mulgara (Dasycercus cristicauda), a small nativemar-

supial mammal that lives in sandy desert regions of centralAustralia (see fi gure 5.17). Mul-garas are carnivorous marsu-pialsand their prey includes insects, scorpions and spiders. Like allmammals, mulgaras need water to egest their faeces, to excretetheir wastes in urine and to cover evaporative loss of water vapourfrom their air-ways. How can they survive without drinking water?Th e mulgara achieves this by min-imising its water loss to such anextent that it can meet all its water needs from the water contentof its food and from metabolic water alone.

fiGuRe 5.17 (a) The crest-tailed mulgara survives in the hot andarid desert environments of central Australia. Here the mulgara iseating a locust that provides it with both nutrients and water. Themulgara can survive without an intake of liquid water. In additionto its food, what is the other important source of water gain forthe mulgara? (b) Distribution of the crest-tailed mulgara.

(a) (b)

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Water-conserving features present in mulgaras includestructural, physio-logical and behavioural adaptations. Onestructural and physiological adap-tation is that mulgaras minimisewater loss by producing very concentrated urine (nearly 4000mOsm/L) compared to humans (1200 mOsm/L). Th is means that amulgara can rid its body of nitrogenous wastes, in the form ofurea, using far less water than a person. Th is is achieved by twomeasures relating to the function and structure of the nephrons ofthe kidney (refer to fi gure 4.45, p. xxx). Th ese two measuresare:1. a reduction in glomerular fi ltration, meaning that less fluid leaves the blood

and enters the kidney tubules2. an increase in tubularreabsorption, meaning that more fl uid is reabsorbed

from the tubules and returned to the blood, particularly in theloop of Henle.Studies have shown that a strong correlation existsbetween the structure of

a kidney and its ability to concentrate urine. In particular,the effi ciency of the kidney is associated with a thicker medullasuch that kidneys with a thicker

medulla can produce urine that is more concen-trated thankidneys with a thinner medulla. A thicker medulla allows for longerkidney tubules, in particular for longer loops of Henle. Th eseU-shaped loops are where the greatest concentration of the kidneyfi ltrate occurs. Figure 5.18 shows a longitudinal section of akidney with a relatively thick medulla.

Th e relative medullary thickness of a mamma-lian kidney (i.e.the thickness of medulla divided by kidney size) is positivelycorrelated with the capacity of the kidney to concentrate the urineand so reduce water loss. Th e higher the relative medullarythick-ness, the higher the maximum concentration of the urine. Inaddition, the relative medullary thickness is greater indesert-dwelling mammals than in non-de-sert dwellers. Who do youthink has kidneys with a thicker medulla: mulgaras or people?

Other structural and physiological adaptations include: Mulgarasproduce very dry faeces and this means

that they lose less water via their gut than animals thatproduce moist faeces.

Mulgaras reduce insensible water loss from their airways byexhaling breath that is a few degrees cooler than the air theyinhale. Warmer air holds more water than cooler air. Th e nasalpassages allow outgoing breath to lose heat through the blood inthe vessels in their nasal tissues, so that the air is cooledbefore it is breathed out. Th is conserves some water that wouldotherwise be lost as vapour if the exhaled breath were warmer.

Mulgaras have few sweat glands so that the loss of water bysweating is minimised. In summary, mulgaras minimise water lossthrough

various structural and physiological adaptations. Because ofthis, mulgaras can obtain suffi cient water for their needs fromthe water content of their food and from the metabolic waterproduced in aerobic respiration. If necessary, mulgaras can survivein the desert environment without drinking liquid water. Overall,mulgaras succeed in balancing their water loss and water gain, sothat water-in equals water-out.

fiGuRe 5.18 (a) Longitudinal section of the kidney of the numbat(Myrmecobius fasciatus), a marsupial mammal. (image suppliedcourtesy of CE Cooper and PC Withers). Termites form the exclusivediet of the numbat. Note the medulla (inner and outer) where theloops of Henle of its kidney tubules are situated. (b) The numbat,an Australian marsupial mammal.

(a)

(b)

Th e units of concentration milliosmoles per litre (mOsm/L)refers to the osmotic strength of a solution.

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In addition, mulgaras exhibit adaptive behaviours that assistthem to avoid overheating, for example: Mulgaras avoid the desertheat, particularly in summer, by sheltering during

the day in their burrows and being active at night whenconditions are cooler. Th is behaviour also assists in waterconservation because sheltering in a humid burrow reduces the lossof water vapour from breathing and by diff usion from the skin.(For a person, the average daily loss from these channels is about800 mL.)

Mulgaras have their fat stores concentrated in their tail.Desert animals tend to store their body fat in a single locationrather than having fat deposits spread under their skin and acrossthe entire body surface. (Th e camels fat store is concentrated inits hump). A possible explanation is that body fat acts as aninsulator and slows heat loss from the body.

Survival without drinking: the tarrkawarraLets meet anotherwater saver. Th e tarrkawarra, or spinifex hopping mouse (Notomysalexis), is a placental mammal that lives in sandy deserts inAustralia (see fi gure 5.1 and 5.19). Because it can survivewithout drinking liquid water, the tarrkawarra can endure longperiods of drought. Its kidney tubules reabsorb almost all thewater from the kidney fi ltrate so that it produces highlyconcen-

trated and almost solid urine. In fact, tarrkawarras produce themost concentrated urine of any mammal. Th eir kidneys can produceurine with a con-centration of 9370 mOsm/L.

Table 5.4 shows a com-parison of the maximum con-centratingabilities of the kidneys of various mammals. Th is table also showsthe urine-to-plasma (U/P) ratio, that is, the concentration ofelectrolytes, such as sodium, potassium and chloride ions, in theplasma relative to that in the urine.

Table 5.4 Comparison of maximum concentration of urine inseveral mammals (max. mOsm/L) and maximum ratio of electrolyteconcentrations in the urine versus in the blood plasma (max. U/P).What does a ratio of 4 mean?

Species Max mosm/l Max u/P

human 1200 4

dog 2500 7

camel 2800 8

rat 2900 9

sheep 3500 11

tarrkawarra 9000 25

Th e sources of water gain and water loss for the tarrkawarraare outlined in the following section, as well as the adaptationsthat enable the it to minimise its water loss so that water-inequals water-out. Achieving this water balance is essential forsurvival.

fiGuRe 5.19 (a) The tarrkawarra, or spinifex hopping mouse, hasthe distinction of producing the most concentrated urine of anymammal. (b) The tarrkawarra lives in arid and semi-arid regions ofAustralia.

(a) (b)

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Water balance in the tarrkawarraWater sources for thetarrkawarra are food, metabolic water and free-standing water.Water loss by the tarrkawarra can occur via the skin, faeces,exhaled air, urine and, in females only, milk. Water gained throughfoodTh e main food for the tarrkawarra is dry seeds. Th e amount ofwater these con-tain depends on the humidity of the air in whichthe seeds are found. Th e rela-tive humidity at night is greaterthan that during the day. Th e nocturnal habits of the tarrkawarraresult in the animal collecting seeds at a time when the watercontent is likely to be at its highest. In addition, seed is storedin the burrows in which the tarrkawarra lives. Th e burrows aremore than a metre deep, well insulated and have a relatively highhumidity because animals huddle together there during the day.Seeds stored in burrows also have a greater water content thanseeds collected from a plant. Th e tarrkawarra also eats greenleafy shoots and insects when they are available but can gainweight on a diet containing dry seed only. Metabolic waterWhencarbohydrate and fatty foods are oxidised in an animals body, themain end products are carbon dioxide and water. Th is oxidationwater, or metabolic water, is used by the tarrkawarra. Th etarrkawarra does drink free-standing water if it is available butcan survive without it. Free-standing water sources may include dewthat can appears after a cold night and rainwater (although rare).A summary of the sources of water for the tarrkawarra is shown infi gure 5.20.

Water in food depends onhow much water is in seeds andwhetherinsects and green plants

are available.Metabolic water

in mouseavailable for use

Free water(dew or rain)

intake may belittle or none.

Some evaporation from skin,but minimised by animals

huddling together in burrow,which causes humidity in

burrow to rise.

Loss in urinemay be as little

as a drop per day.

Very little lossin faeces

Loss in exhaled airreduced by nasalheat exchange

WATEROUT

WATERIN

fiGuRe 5.20 An outline of how the tarrkawarra achieves waterbalance. For survival, water-in must balance water-out.

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Water loss from the skinAlthough a tarrkawarra has no sweatglands, some water is lost by diffusion through the skin.Evaporation from the skin occurs but this is minimised. During hotdays, animals stay in their burrows huddled together. Airsur-rounding the group increases in humidity and has the effect ofreducing water loss from the skin.

Water loss in faeces Tarrkawarra faeces are very dry and littlewater is lost in this way.

Water loss in exhaled air Air that moves from the lungs to thesurrounding atmosphere is saturated with water vapour. This couldresult in significant water loss. In the tarrka-warra, a specialheat exchange system in the nasal passages reduces that loss. Thetemperature of air entering the body is lower than bodytem-perature and so nasal passages are cooled as air enters. Warmair exhaled from the lungs passes over these cooled areas and isalso cooled. Exhaled air is at a lower temperature than bodytemperature. As the air is cooled, some of the water vapour fromthe lungs recondenses on the walls of the nasal passages. Hence,not all the water vapour that leaves the lungs leaves the body.

Water loss in urineMammals must produce urine to be able toexcrete their nitrogenous waste: urea (see chapter 3, p. xxx).Oxidation of proteins results in urea that must be excreted.Tarrkawarras produce the most concentrated urine recorded for anymammal. Although some water loss occurs through the kidneys, it isclear that the kidneys are a significant site of water conservationin tarrkawarras.

Water loss in milk for the youngFemale tarrkawarras, like allmammals, feed their young with milk. The loss of water throughhaving to feed young is balanced to some extent by a motherdrinking the urine her young produce. The water in urine isrecycled. It has been estimated that a female who is feeding heryoung requires only one milli-litre of water per day. This waterfor lactation is obtained from fresh green food, rainwater or dew.Although tarrkawarras live in very dry areas with littlefree-standing water, their structural, behavioural andphysiological characteristics enable them to survive in harshdesert environments.

Surviving by dormancyfrogs in the outback? Surely not! Some frogspecies live in arid inland Australia. Frogs typically live inmoist sur-roundings and need a body of water in which to reproduce.How do they sur-vive long periods of drought in the inland? Somefrog species that live near and breed in ephemeral waterholesrespond in an amazing manner when the waterholes begin to dry out.The frogs burrow deeply into the soft mud at the bottom of theirwaterholes. Once underground at depths of up to 30 cm, theburrowing frogs, such as the trilling frog (Neobatrachus centralis)(see figure 5.21), make a chamber that they seal with a mucoussecretion. The frogs then go into an inactive state known asdormancy in which breathing rates and heart rates are minimal andenergy needs are greatly reduced. Their low energy requirements aremet from their fat reserves. Read the account written by twoexplorers about burrowing frogs:

One day during the dry season we came to a small clay-panbordered with withered shrubs . . . It looked about the mostunlikely spot imaginable to search for frogs, as there was not adrop of surface water or anything moist within many miles . . .

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Th e ground was as hard as a rock and we had to cut it away witha hatchet, but, sure enough, about a foot [30 cm] below thesurface, we came upon a little spherical chamber, about threeinches [76 mm] in diameter, in which lay a dirty yellow frog. Itsbody was shaped like an orange . . . with its head and legs drawnup so as to occupy as little room as possible. Th e walls of itsburrow were moist and slimy . . . Since then we have found plentyof these frogs, all safely buried in hard ground.

(Source: WB Spencer and FJ Gillen, Across Australia, Macmillanand Co., London, 1912)

fiGuRe 5.21 (a) The trilling frog (Neobatrachus centralis) is socalled because of its high-pitched trill. (b) Distribution map ofthe trilling frog in Australia.

(a)

Neobatrachus centralis

(b)

Th e frogs remain buried and are protected from desiccationuntil the next rains come this may be a wait of 1 or 2 years. Th efrogs come out of their dormant state only when soaking rains falland soil moisture rises. Once acti-vated, the frogs return to thesurface to feed and breed in temporary pools. Th e completion ofthe life cycle is very fast. Within days of being laid, eggsundergo embryonic development, hatch and the resulting tadpolesmetamorphose to produce small frogs. Th ese new populations offrogs feed on larvae of crus-taceans and insects that have alsohatched from dormant eggs.

Other animal species survive extended periods of drought bysealing them-selves off from the drying conditions. For example,the univalve (one-shell) freshwater mollusc (Coxiella striata)seals itself inside its shell by closing the shell opening with ahard lid (operculum). Th ese inland molluscs must stay sealedtightly in their shells for months or years.

Surviving by moving aroundSome species cope with drought bymoving from aff ected areas to areas where conditions are morefavourable. For example, banded stilts (Cladorhy-nchusleucocephalus) live near salt lakes in inland Australia and rely onthese lakes for brine shrimps, which are their main food source(see fi gure 5.22). When one salt lake dries up, these birds simplyfl y to another salt lake.

fiGuRe 5.22 A group of banded stilts. What strategy do they usewhen their salt lake habitat dries up?

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Another species that moves widely throughout desert areas is thebudgerigar (Melopsittacus undulatus). Flocks of these birds move tomore favourable areas in search of food and water (see fi gure5.23). In order to avoid the desert heat, they travel in the coolerperiods of the day.

Th e strategy of moving quickly over large dis-tances to seekout transient free-standing water in the arid outback of Australiais largely restricted to birds that are capable of fl ight. Manyanimal species and all plant species, however, cannot use aget-up-and- go strategy in periods of drought.

Surviving through offspringSurvival can be viewed in terms ofthe successful survival of an individual organism that lives toreproduce on many occasions. Survival can also be considered fromthe point of view of survival of a species. Members of some speciesfound in waterholes in the arid outback cannot survive long periodsof drought. When the waterhole dries up, all the organisms die.Yet, these species are suc-cessful residents of the arid outback.How is this achieved?

Some species are unable to survive long dry periods and allmembers of the species die. In this case, the species survivesthrough its off spring. Th is occurs in the case of crustaceanspecies, such as fairy shrimps and shield shrimps. How?

When water is present in abundance, female shrimps produce eggsthat are not drought resistant. As waterholes begin to dry out,fairy shrimps and shield shrimps (see fi gure 5.24) producedrought-resistant fertilised eggs. Th ese eggs are in fact cystsand each contains a fully developed embryo encased in a hardprotective shell. By the time the water has gone, all the adultshrimps are dead but the cysts they have left behind can withstanddesiccation for long periods. Th ese cysts are in a state ofdormancy and can lie in the dust of dry waterholes for more than 20years.

fiGuRe 5.24 (a) A fairy shrimp (Branchinella sp.) about 3 to 4cm in length. Fairy shrimps swim with their legs uppermost. (b) Ashield shrimp (Triops australiensis) about 1.5 cm in length. Howdoes this species survive drought?

(a) (b)

fiGuRe 5.23 The Australian budgerigar lives in large fl ocks inthe arid inland of Australia. By taking fl ight, they can move awayfrom areas when conditions deteriorate and seek more favourableconditions elsewhere.

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When the drought breaks and the waterholes temporarily refi ll,the cysts hatch. Within just a few days, the newly hatched shrimpsmature and reproduce. Th is is necessary because the waterholes andpuddles in which they live will soon dry out. Male shrimps dieafter mating. Females carries large numbers of tinydrought-resistant cysts in a brood sac on their underbodies. Beforethe pools and waterholes turn to mud and then to dust, the femaleshrimps release their cysts and then die. Th ese dried-out dormantcysts will lie in the dust or be blown by the desert wind, and thenext generation of shrimp will only emerge when the rains come,perhaps years later, and short-lived waterholes and poolsreappear.

What about the camel?Camels, both the dromedary (Camelusdromedarius) and the Bactrian camel (C. bactrianus) are known asthe ships of the desert. Camels are large pla-

cental mammals. Th ey are not native to Australia but largeferal herds of camels, mainly dromedaries, live in arid areas ofAustralia, being descendants of camels imported in the 1800s. Whatadaptations do camels possess that enable them to survive in adesert environment?

Structural features that enable camels to survive in desertconditions include: a double row of long eyelashes and slit-shapednos-

trils that can be closed both features protecting the camel fromwind-borne sand particles (see fi gure 5.25)

bony structures in their nasal passages that enable the watervapour in their outgoing breath to be absorbed; it is then exhaledas dry air

oval shaped red blood cells; the oval shape enables them tocontinue circulating even when the vis-cosity (thickness) of theblood increases due to the camel becoming dehydrated and losingbody water

the inbuilt fat store in the hump, which can be metabolised forenergy production if food is not available. Th e oxidation reactioninvolved is also a source of metabolic water for the camel.

Physiological adaptations that minimise water loss in camelsinclude: the ability to produce concentrated urine because of efficient kidneys; the

urine they do produce is released and runs down their legs andits evap-oration cools them

the ability to produce very dry faeces because of a long colonin their gut the ability to allow their body temperature to varyover a wide range, from

34 to 42C, depending on the external temperature (unlike othermammals that maintain their internal body temperature within anarrow range). When it is hot during the day, the camels bodytemperature rises. Sweating only comes into play when the camelsbody temperature reaches 42C. (Th is is an important waterconservation measure because sweating involves signifi -cant waterloss.) At night, when it is cooler, this body heat is lost and thecamels body temperature falls. Camels can lose up to 40 per cent oftheir body water, whereas an adult

person can lose only 15 per cent. When water does becomeavailable, camels are able to drink large volumes of water morethan 100 litres in a day. Th is large intake of water, however,does not cause osmotic complications; for example, a camels redblood cells can swell to more than double their volume before theyburst, while the red blood cells of other large mammals would burstbefore this.

fiGuRe 5.25 The long eyelashes and slit-shaped nostril of thecamel enables it to survive in the arid inland of Australia.

unit 1 Physiological and behavioural adaptationsaOs 2

Topic 1

concept 2

unit 1 structural adaptations

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key ideas

Native mammals of the Australian deserts show many structural,physiological and behavioural adaptations that equip them forliving in arid Australia.

A range of adaptations may be seen in desert-dwelling marsupialsthat enable them to minimise water loss and, if necessary, survivewithout drinking water.

Other survival strategies seen in various animal species includebecoming dormant, moving around and producing drought-resistantoffspring.

Quick check

10 List three water-conserving adaptations that may be seen inthe mulgara.11 Which organism can produce the more concentratedurine: a person or a

mulgara?12 Identify two features relating to kidney functionthat enables desert-

dwelling marsupials to produce very concentrated urine.13 Givean example of an animal that survives the long periods of droughtin

the desert:a by becoming dormantb by moving around c byproducing drought-resistant offspring.

Vegetation types of arid australiaFigure 5.26 shows thedistribution of the major vegetation types in Australia.

Herbaceous stony desert

Tussock grassland

Shrubland

0 400 800 km

N

Rainforest (closed forest)

Tall eucalypt forestWet and dry low woodlandor mulga in drierareas

Arid and semi-arid spinifexor hummock grassland

fiGuRe 5.26 Major vegetation types in Australia

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In terms of area, the dominant vegetation type in Australia ishummock grassland, which covers almost a quarter of the Australianland surface, including the sandy plains and dunes of the majordeserts. Hum-mock grasslands are dominated by species of spinifexgrasses (Triodia spp.) (see fi gure 5.27). Do not think about thesegrasslands in terms of the green grass of a front lawn or asuburban park. Spinifex grasses, such as buck spinifex (Triodiabasedowii) are stiff , drought-resistant grasses.

fiGuRe 5.27 Hummock grasslands are dominated by spinifex(Triodia spp.). These hummock grasslands cover much of theAustralian sandy deserts. Note the typical hummock shape of thespinifex plant that gives these grasslands their name.

In terms of area covered, the next largest vegetation type isshrubland. Dif-ferent kinds of shrubland exist. Th e kind ofshrubland that is present in an area depends on rainfall and soiltype, with each kind having a diff erent dominant plant species.Acacia shrublands occur across the arid and semi-arid areas ofAustralia and are dominated by mulga (Acacia aneura) (see fi gure5.28a). Chenopod shrublands occur in arid regions with salty soilsand are dominated by saltbushes (Atriplex spp.) and bluebushes(Maireana spp.) (see fi gure 5.28b).

fiGuRe 5.28 Australian shrublands cover much of the continent.Included among them are (a) acacia shrublands, dominated by mulgaand (b) chenopod shrublands, dominated by saltbushes andbluebushes.

(a) (b)

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Table 5.5 shows the pattern of distribution of the majorvegetation types in Australia as determined by environmentalphysical factors of rainfall, tempera-ture, evaporation rate, andmineral-nutrient levels (rich or poor) and salt levels of the soil(salinity).

Table 5.5 Patterns in the distribution of various types ofvegetation in Australia

Vegetation type Climate

hummock grasslands arid: lowest and erratic rainfall, highevaporation rates, high temperature

acacia shrublands

chenopod shrublands arid and semi-arid: low rainfall, hightemperature, salty or alkaline soils

tussock grasslands dominated by Astrebla spp.

semi-arid: annual rainfall between 200 and 500 mm, claysoils

tropical grasslands dominated by Sorghum spp.

tropical: summer monsoons and winter drought

mallee woodlands temperate: intermediate rainfall, poor soil

eucalypt forests temperate: high rainfall, poor soil

rainforests tropical or temperate: high and reliable rainfall,rich soil

adaptations in desert plantsLets look at how plants of the aridinland of Australia survive through struc-tural and physiologicaladaptations that equip them to: maximise water uptake minimisewater loss produce drought-resistant seeds.

Maximising water uptakeThe part of a plant that takes up wateris the root system. In arid areas of Aus-tralia, some trees growingalong dry creek beds produce long, unbranched roots that penetrateto moist soil at or near the watertable. Once moisture is reached,the major root branches and forms lateral roots. Plants thatpro-duce these deep roots are called water tappers and their majorroot can grow to depths of 30 m. The part of the root that islocated in the upper dry soil is covered by a corky waterprooflayer of cells that prevents water loss.

Other plants growing in arid regions develop extensive rootsystems that spread out horizontally, far beyond the tree canopybut just below the soil sur-face. In this case, the plant takes upwater from an extensive area around it.

Minimising water lossTranspiration is the loss of water vapourby evaporation from moist surfaces inside the plan (see figure5.29). This loss of water vapour occurs through pores, known asstomata (singular: stoma), which are typically present on the lowersurface of plant leaves (see figure 3.30). The higher the windspeed and the higher the temperature of the leaf, the greater therate of water loss.

For a plant to reduce its water loss, the principal strategy isto reduce the loss of water vapour by transpiration through thestomata on its leaves. Transpi-ration cannot be stopped permanentlybecause it is essential for the process of moving columns of waterthrough xylem tissue, from where water is sup-plied to all cells ofa plant (refer to chapter 4, p. xxx). Stomata are also the poresthrough which the carbon dioxide required for photosynthesis entersleaves and they must open to allow carbon dioxide to diffuse intothe leaves.

Odd facT

The difference between woodlands and forests is the area of skyblocked by the upper canopy of leaves, as seen by a person lookingup to the sky from below. For forests, the foliage coverage is from30 to 100 per cent of the sky, while for woodlands, the coverage isless than 30 per cent.

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Palisadeparenchymacell

Spongyparenchymacell

Mesophyllcells

Cuticle

Upper epidermis

Chloroplast

Phloem

Xylem

Air space

Water loss throughstomatal pore

Water loss through cuticle

Vascularbundle

fiGuRe 5.29 In a leaf, water moves from xylem into surroundingmesophyll cells. As water vapour moves out of the leaf throughstomata, water evaporates from the moist surfaces of the mesophyllcells. Most water is lost from a leaf through stomata, but a littleevaporates from the cuticle.

Lets explore how various adaptations of leaves, mainlystruc-tural, can reduce the water loss in plants.

Presence of a thick cuticleMost water loss from plants occursvia their stomata, but some water is also lost directly across thesurfaces of cells exposed to the external environment. Th is latterwater loss is minimised by the presence of a waxy cuticle on theexposed upper surface of leaves (see fi gure 5.31). Th e cuticle iscomposed of a waterproof material called cutin. Plants that live inarid environments typi-cally have leaves with thicker cuticles thanthose in non-arid regions.

fiGuRe 5.31 Transverse sections through leaves from twodifferent species of plant: (a) Eucalyptus globulus and (b)waterlily (Nymphaea sp.). The two leaves are at the same magnification. Note the thick cuticle (arrow) of the Eucalyptus leaf andapparent lack of cuticle on the waterlily leaf. The thicker thecuticle, the less transpiration occurs.

(a) (b)

fiGuRe 5.30 Transverse section ( 200) through a monocotyledonleaf. Note the stomata (arrowed) on each surface, and the largevascular bundle comprising large thick-walled cells of the xylemtissue and thin-walled cells of phloem. This plant has stomata onboth its upper and lower leaf surfaces. More commonly, stomata arerestricted to the lower surface only.UN

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Reduced number of stomata per unit of surface areaFewer stomataper unit area of leaves means that water loss by transpiration isreduced.

Presence of sunken stomataTh e rate of water loss of a leaf fromits stomata is aff ected by several factors including the humidityof the air surrounding the stomata. Water vapour is lost fromleaves via their stomata more rapidly when they are immediatelysurrounded by dry air than when they are surrounded by humid air.Why? Th e concentration gradient of water vapour from inside theleaf to outside is steeper in drier conditions relative to morehumid conditions.

Another related factor that aff ects water loss by transpirationis wind speed. On a windless day, the loss of water bytranspiration is low because the stomata are surrounded by aboundary of still air with a level of humidity similar to thatinside the leaves. In contrast, on a windy day, the water vapourthat transpires from leaves is immediately blown away so that morewater vapour diff uses from the leaves. Th e windier the day, thehigher the rate of transpiration.

Unlike typical stomata that are situated at the leaf surface,sunken stomata are located in pits below the leaf surface. Th isposition below the leaf sur-faces creates a region of relativelyhigher humidity in the air space immedi-ately surrounding thesunken stomata as compared with stomata at the leaf surface. Th epresence of sunken stomata reduces water loss by transpiration (seefi gure 5.32). Likewise, the presence of hairs on the uppersurfaces of leaves and around the stomata would be expected toreduce the speed of airfl ow over leaves and contribute to areduction in water loss.

(a)

(b)

fiGuRe 5.32 (a) Transverse section of a typical leaf withstomatal openings fl ush with the lower leaf surface. Guard cellssurrounding the stoma are shaded red. (b) Transverse section of aleaf from a plant with its stomata sunk below the leaf surface.Note also the hairs on the lower leaf surface and the thick cuticleon the upper surface.

Leaf colour, size and marginsTemperature is another factor thataff ects the rate of water loss by transpiration; lowertemperatures mean less transpiration. Leaves of some shapes gatherless heat from exposure to the sun than other shapes and so reducewater loss. Leaves with a small surface area reduce the area fromwhich transpiration occurs. Leaf colour. Silver or glossy leavesrefl ect relatively more sunlight producing

lower leaf temperatures. Leaf shape. Small, narrow orcylindrical leaves have a small surface area.

When exposed to the sun, these leaves gather less heat thanlarger fl at leaves and so stay cooler, minimising water loss (seefi gure 5.33).

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(a) (b)

fiGuRe 5.33 Some leaf shapes expose a smaller surface area tothe sun and so stay cooler, such as (a) sm all leaves and (b) cylindrical leaves.

Plants of the Australian genus Hakea show a variety ofadaptations that enable the plants to minimise water loss. As wellas cylindrical leaves, Hakea plants also have a thick cuticle ontheir leaf epidermal cells and have sunken stomata. Look at fi gure5.34a. Note the cylindrical cross-section shape of the Hakea leafand the position of some stomata (arrowed). Figure 5.34b shows thedetail of a sunken stoma in a Hakea leaf.

(b)

Epidermis

Scleroticcell

Pit with stoma at its base(a)

fiGuRe 5.34 (a) A transverse section through a Hakea leaf withindications (arrows) of positions of some of its stomata. (b)Details of a sunken stoma of Hakea. Note the position of the stomabelow the surface of the leaf and the pit above the stomalopening.

Leaf margin. Leaves are thinnest where their upper and lowersurfaces meet, that is, at their margins. Plants lose more heatfrom thinner regions than from thicker regions. Th e larger theratio of edge length to surface area of a leaf, the faster a leafwill be cooled. Cooler leaves have lower transpiration rates.Leaves with incised margins have a largeredge-length-to-surface-area ratio than leaves with entire margins.Th ese leaves are thus cooler and so have a lower transpirationrate (see fi gure 5.35a).

Leaf orientation. Th e orientation of leaves can infl uence leaftemperature (see fi gure 5.35b). Leaves with a vertical orientationhave less exposure to sunshine and so gain less heat and arecooler. Cooler means less water loss.

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A B

(a)

(b)

fiGuRe 5.35 (a) At left is a leaf with an indented margin and atright is a leaf with an entire margin. Which leaf will cool morequickly? (b) Diagram showing how the orientation of leaves canaffect the amount of heat gained when exposed to the sun. Whichleaf orientation would be expected to gain more heat: vertical orhorizontal? (c) Vertically oriented leaves are common in Eucalyptusspecies.

(c)

Rolled-up leavesFigure 5.36 shows a rolled-up leaf of marramgrass (Ammophila arenaria). Although this is not an Australianspecies, some related species in Australia have the samecharacteristics. Th ese leaves have a number of features torestrict water loss, including: hinge cells that lose turgor ifwater is lost and cause the leaf to curl inwards,

creating a humid chamber for the stomata stomata on only oneside of the leaf so that when the leaf curls, no stomata

are directly exposed to the environment stomata located in foldsof the leaf so that they are shielded from air cur-

rents even when the leaf is unrolled a thickened cuticle on thesurface that is exposed when the leaf curls.

Note the hairs on the upper epidermis of the leaf in fi gure5.36b.

fiGuRe 5.36 Transverse section of marram grass (Ammophilaarenaria) at three different, increasing, magnifi cations. (a)Water loss in hinge cells (marked by ) causes the leaf to curl.Note the thickness of the cuticles on the upper surface (thin) andlower surface (thick). (b) Note the vascular tissue, photosynthetictissue, thickened sclerenchyma cells and hair cells. (c) Can youidentify stomata, photosynthetic tissue and hair cells in the upperepidermal layer?

(a) (b)

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No visible leaves Various plants that survive in droughtconditions have no visible leaves, for example, cacti, succu-lentplants that are member of the Cacatacea family and native to theAmericas (see fi gure 5.37). Th eir leaves are reduced to spinesand their stems are swollen with cells that contain water stores intheir vacuoles. Th ese plants have exten-sive shallow roots, thickcuticles on their surfaces and few stomata.

Leaves that arent leavesMembers of the genus Acacia, commonlycalled wattles, are widespread in Australian environ-ments,including arid and semi-arid regions. Mulgas (Acacia aneura) arethe dominant species in the acacia scrublands. As many

Acacia species mature, their feathery leaves are replaced bytheir fl attened leaf stalks. Th ese fl attened leaf stalks areknown as phyllodes. Figure 5.38a shows a transitional state inwhich the feathery true leaves of an Acacia plant are starting tobe replaced by leaf stalks that are gradually thickening. Phyllodesenable plants to survive in arid conditions because they pro-vide astore of water in large parenchyma cells at their centre. Inaddition, phyllodes have fewer stomata than true leaves and so loseless water by transpiration.

(a)(b)

A. denticulosa

A. verticillata

A. rigens

A. podalyriifolia

A. oxycedrus

A. triptera

fiGuRe 5.38 (a) Formation of a phyllode from the leaf stalk ofthe original feathery leaf. (b) Examples of phyllodes from variousAcacia species.

Plants of the genus Casuarina and Allocasuarina show a different adap-tation. Th ese plants have what appear to be fi neneedle-like leaves, but in fact they are modifi ed branches thatfunction as leaves and are known as cladodes. Th e leavesarereduced to tiny scales that encircling each joint of the cladode(see fi gure 5.39). Cladodes have fewer stomata than true leavesand so lose less water by transpiration.

fiGuRe 5.37 Cacti have succulent stems, few or no leaves andextensive shallow roots. Note the succulent stems of prickly pearcactus, Opunta stricta, which can store large quantities ofnutrients and water. Also note the berries. These contain seedsthat can be distributed over large areas by birds and otheranimals.

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fiGuRe 5.39 (a) Cl adodes from a plant belonging to theAustralian genus, Casuarina. Note the rings of tiny scales that areall that remain of the true leaves. (b) Asparagus is anotherexample of a cladode.

(a) (b)

Shedding leavesTo minimise water loss, when plants becomestressed in drought conditions, they may conserve water by droppingtheir leaves. Th is is a strategy that is seen in the bladdersaltbush (Atriplex vesicularia). In conditions of severe droughtthis saltbush closes its stomata, drops its leaves and sheds its fine roots.

Producing drought-resistant seedsPopulations of some herbaceousfl owering plants can survive in arid regions of Australia. Th eseplants germinate from seeds, then fl ower and pro-duce new seeds ina very short period. Plants that complete their life cycles in justtwo to three weeks are said to be ephemeral. Because they producedrought-resistant seeds, populations of ephemeral plants cansurvive in arid regions. Th e outer coats of the seeds of theseplants contain a water-soluble chemical that inhibits seedgermination. So, dry conditions = no germination. When heavy rainsfall, this chemical is dissolved and the seeds germinate to produceseedlings. Shortly after, the new plants produce fl owers in asynchro-nised display (see fi gure 5.40). Th e plants soon die, butnot before they have produced seeds that will lie dormant until thenext heavy rains.

As was seen in fi gure 5.40, local rainfall over a region ofdesert can transform areas of the red centre of Aus-tralia to agreen centre.

Some years, very heavy rains that fall hundreds of kilometresfrom the desert can reach the desert months later. Th is can occurwith the monsoon rains that fall in Queensland and fl ow throughrivers of the Channel Country, with the water spilling out overextensive fl oodplains (see fi gure 5.41). When this happens vastregions of the desert are transformed into watery habi-tats wherefi sh and waterbirds breed.

unit 1 structural adaptations

aOs 2

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concept 1

unit 1 Physiological and behavioural adaptationsaOs 2

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concept 2

fiGuRe 5.40 A magnifi cent display of fl owers of ephemeralplants in the Australian desert. What event caused this blooming ofthe desert? Populations of these plant species exist most of thetime as seeds, not plants!

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fiGuRe 5.41 A fl ooded river plain in the Simpson Desert. Thiswater originated from torrential rainfall in western Queenslandthat eventually fl owed into dry riverbeds in the desert,transforming the sandy plains to shallow lakes. (Image courtesy ofStan Sheldon)

On rare occasions, the water may reach as far south as Lake Eyrein South Australia. Normally, Lake Eyre is a dry saltpan but itsometimes becomes a giant lake and breeding ground for birds,including pelicans (see fi gure 5.42).

(b)

fiGuRe 5.42 (a) Torrential rains in Queensland early in 2011resulted in water fl owing into Lake Eyre, transforming much of itinto a shallow lake by August. This view of Lake Eyre (taken on 5December 2011 from the International Space Station) shows waterstill present in some sections, including Belt Bay and MadiganGulf. The green and pink colours are due to high densities ofarchaeas that thrive in these highly salty aquatic conditions. Thebright white surface of Lake Eyre South (at lower right handcorner) shows that the water has already evaporated from there,returning it to its dry saltpan state. (b) Pelicans in largenumbers breed on the shores and on mounds in Lake Eyre when it fills with water.

LakeEyre

(a)

key ideas

The arid and semi-arid areas of inland Australia are dominatedby hummock grasslands, acacia shrublands and chenopodsshrublands.

Desert plants show a variety of adaptations to maximise wateruptake and to reduce water loss.

The major loss of water in plants occurs as water vapour lost bytranspiration from the leaf stomata.

Many adaptations exist in desert plants for reducing water lossfrom their leaves. Ephemeral plant species of desert regionsproduce drought-resistant seeds that germinate only after rain orfl ooding.

Weblink rain brings red Centres desert landscape to life

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Quick check

14 What is the major avenue of water loss by plants?15 Identifyone example of an adaptation that enables a plant to conserve

water by:a reducing water loss from cells at the leaf surface breducing water loss from their leaf stomatac reducing theabsorption of heat, thus staying cooler and reducing the

rate of transpiration. 16 What is a phyllode?17 How do phyllodesenable a plant to conserve water?18 What feature preventsdrought-resistant seeds from germinating before

rain falls?

the dominant plantsLet us now have a closer look at three plantsthat are dominant species in the arid and semi-arid areas ofAustralia: the mulgas of the acacia shrublands, the saltbushes ofthe chenopod shrublands and the spinifexes of the hummockgrasslands found in the major deserts.

Mulgas: tree of the arid inlandAcacia shrublands of arid inlandAustralia are dominated by mulga (Acacia aneura), which can existeither as trees or small shrubs (refer to fi gure 5.28a, p. xx).Mulga trees have many features or adaptations that equip them forsurvival in arid conditions (see fi gure 5.43).

Grey-green phyllodes reflect sunlight

Phyllodes in placeof leaves minimise water loss

Phyllodes with uprightorientation minimise exposure to sunlightchannel rainwater down plant to ground

Phyllodes shed during drought minimise water loss providerecycled nutrients when rain comes

Nodules with nitrogen-xing bacteria provide nitrates allowinggrowth in nutrient-poor soil

Deep root system maximises water water uptake

fiGuRe 5.43 Some of the adaptations of the mulga tree. Thevertical orientation of sparse foliage of mulga ensures that thelittle rain that falls is directed to the roots of the plant.

Th e root system of a mulga tree is concentrated around the baseof the tree. When rain falls, it is caught by the upward-pointingleaves of this tree and fun-nelled down the branches to the centreof the tree. From there, the water falls to the ground around thetrunk where the root system is most concentrated.

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Mulga trees grow in regions where the rainfall is low andunreliable because they are drought resistant and can survive ayear or more without water. In dry years, a mulga tree does notproduce any fl owers. If, however, heavy rains fall in the summer,a mulga produces fl owers and, if rains occur in the followingwinter, seeds are formed. Th e seeds germinate to produce seedlingsthe following summer and require rain to survive. Notice that apattern of rainfall over three seasons (summer-winter-summer) isrequired for a new generation of mulgas to be produced. Th ispattern of rainfall occurs during a La Nia event.

Saltbushes (chenopods) Soils of some areas of the hot, aridinland contain high concentrations of salt. Many species ofsalt-tolerant plants live in this environment, such as species ofsaltbush (Atriplex spp.) (see fi gure 5.44) and bluebush (Maireanaspp.). Th ese plants are also drought resistant.

fiGuRe 5.44 The saltbush (Atriplex cinerea) is salt tolerant anddrought resistant.

Saltbushes grow in soils that are too salty for many other plantspecies. Th ey can survive because they excrete the dissolved saltthat is taken up by their roots from cells in their leaves. As aresult, the leaves of saltbushes are covered in fi ne saltcrystals. As well as being an excretory product, these saltcrystals refl ect the suns heat and contribute to keeping theplants from overheating.

Saltbushes have structural adaptations to conserve water. Th eirleaves: have sunken stomata are covered in hairs are oriented sothat they expose a minimal surface to the suns rays.

Saltbushes produce seeds that have high concentrations of saltin their outer coats and this salt prevents germination. Saltbushseeds germinate only after the salt has been washed out after heavyrainfall. As soon as the salt inhibition is removed, the seedsgerminate and new seedlings quickly become estab-lished. Th e saltinhibition of germination means the next generation of salt-bushplants appears in times of good rainfall when their chance ofsurvival is maximised.

Spinifexes of the hummock grasslandsTh e leaves of spinifexplants contain high levels of silica grains, which makes them rigidand sharp-pointed (see fi gure 5.45). Spinifex hummocks provideshelter for desert animals: mammals such as the tarrkawarra andreptiles such as many species of lizard and snake. Th e onlyanimals that use spinifex as a food source are termites that feedon the litter fr