Townsend C.R., Begon M., Harper J.L. Essentials of Ecology

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their photosynthetic tissues during periods of drought. Some species then replace

them with new leaf forms that are less extravagant of water or spend the driest

season with no leaves at all – just green stems.

Other plants, tolerators, have evolved a different compromise, producing

long-lived leaves that transpire slowly (for example, by having few and sunken

stomata). They tolerate drought, but of course their photosynthesis is slower.

These plants have sacriﬁced their ability to achieve rapid photosynthesis when

water is abundant but gained the insurance of being able to photosynthesize

throughout the seasons. This is not only a property of plants from arid areas but

also of the pines and spruces that survive where water may be abundant but is

usually frozen and therefore inaccessible.

The viability of alternative strategies to solve the problem of photosynthesiz-

ing in a dry environment is nicely illustrated by the trees of seasonally dry tropical

forests and woodlands. These communities are found naturally in Africa, the

Americas, Australia and India; but whereas, for example, the savannas of Africa

and India are dominated by deciduous species (losing all leaves for at least 1 and

usually 2–4 months each year), and the Llanos of South America are dominated

by evergreens (a full canopy all year), in the savannas of Australia there are

roughly equal numbers of deciduous and evergreen species (Figure 3.17). The

deciduous species avoid drought in the dry season (April–November in Australia)

as a result of their vastly reduced rates of transpiration, having shed their leaves

(Figure 3.17a, b). The evergreens tolerate the threat of drought in the dry

season (Figure 3.17b), but maintain a positive carbon balance throughout the

year (Figure 3.17c), whereas the deciduous species make no net photosynthate

at all for around 3 months.

The evaporation of water lowers the temperature of the body with which it

is in contact. For this reason, if plants are prevented from transpiring they may

overheat. This, rather than water loss itself, may be lethal. The desert honey-

sweet (Tidestromia oblongifolia) grows vigorously in Death Valley, California

despite the fact that its leaves are killed if they reach 50°C, a temperature that

Chapter 3 Physical conditions and the availability of resources

Percentage canopy fullness

100

Month

(a)

Predawn water potential (MPa)

0.0

–0.5

–1.0

–1.5

–2.0

(b)

(c)

Assimilation rate (µmol m

–2

–1

)

JFMAMJJASONDJ

Month

JFMAMJJASONDJ

Month

JFMAMJJASONDJ

Evergreen

Deciduous

Figure 3.17

(a) Percentage canopy fullness for deciduous and evergreen trees in Australian savannas throughout the year. (Note that the southern hemisphere

dry season runs from around April to November.) (b) Susceptibility to drought as measured by increasingly negative values of ‘predawn water

potential’ for deciduous and evergreen trees. (c) Net photosynthesis as measured by the carbon assimilation rate for deciduous and evergreen trees.

AFTER EAMUS, 1999

coexisting alternative strategies in

Australian savannas

water and overheating

9781405156585_4_003.qxd 11/5/07 14:44 Page 89

is commonly reached in the surrounding air. Transpiration cools the surface of

the leaf to a tolerable 40–45°C. Most desert plants bear hairs, spines and waxes

on the leaf surface. These reﬂect a high proportion of incident radiation and help

to prevent overheating. Other more general modiﬁcations in desert plants include

the characteristic ‘chunky’ shape of succulents with few branches, giving a low

surface area to volume ratio over which radiant heat is absorbed.

Specialized biochemical processes may increase the amount of photosynthesis

that can be achieved per unit of water lost. The majority of plants on Earth photo-

synthesize using what is termed the C3 pathway. Although these plants are highly

productive photosynthesizers, they are relatively wasteful of water, reach their

maximum rates of photosynthesis at relatively low intensities of radiation, and are

less successful in arid areas. Alternative pathways of photosynthesis – termed the

C4 pathway and CAM (crassulacean acid metabolism) – are more economical in

their water use. C4 plants have a particularly high afﬁnity for carbon dioxide, and

so absorb more per unit of water lost. CAM plants open their stomata at night and

absorb carbon dioxide and ﬁx it as malic acid. They close their stomata during the

day and release the carbon dioxide internally for photosynthesis. C4 and CAM

plants are most common in arid and, in particular, hot arid areas. They are

restricted in range because the associated costs of their systems apparently make

them less competitive under less arid conditions. For example, the photosynthesis

of C4 plants is inefﬁcient at low radiation intensities (see Figure 3.14) and so

they are poor shade plants; while CAM plants must store their accumulated malic

acid every night: most of them are succulents with extensive swollen water-storage

tissues that cope with this problem.

Almost all falling water (rain, snow, etc.) bypasses the plants and passes to the

soil. Some drains through the soil, but much is held against gravity by capillary

forces and as colloids. Plants obtain virtually all their water from this stored

reserve. Sandy soils have wide pores: these do not hold much water but what is

there is held with weak forces and plants can withdraw it easily. Clay soils have

very ﬁne pores. They retain more water against the force of gravity, but surface

tension in the ﬁne pores makes it more difﬁcult for the plants to withdraw it.

The primary water-absorbing zone on roots is covered with root hairs that make

intimate contact with soil particles (Figure 3.18). As water is withdrawn from the

soil, the ﬁrst to be released is from the wider pores, where capillary forces retain

it only weakly. Subsequent water is withdrawn from narrower paths in which the

water is more tightly held. Consequently, the more the soil around the roots is

depleted of water, the more resistance there is to water ﬂow. As a result of water

withdrawal, roots create water depletion zones (or, more generally, resource

depletion zones or RDZs) around themselves. The faster the roots draw water

from the soil, the more sharply deﬁned the RDZs, and the more slowly water

will move into that zone. In a soil that contains abundant water, rapidly tran-

spiring plants may still wilt because water does not ﬂow fast enough to replenish

the RDZs around their root systems (or because the roots cannot explore new soil

volumes fast enough).

The shapes of root systems are much less tightly programmed than those

of shoots. The root architecture that a plant establishes early in its life can deter-

mine its responsiveness to later events. Plants that develop under waterlogged

conditions usually set down only a superﬁcial root system. If drought develops

later in the season, these same plants can suffer because their root system did not

Part II Conditions and Resources

increasing the efﬁciency of

water use: C4 and CAM

obtaining water from the soil

9781405156585_4_003.qxd 11/5/07 14:44 Page 90

tap the deeper soil layers. A deep tap root, however, will be of little use to a plant

in which most water is received from occasional showers on a dry substrate.

Figure 3.19 illustrates some characteristic differences between the root systems

of plants from damp temperate and dry desert habitats.

3.3.3 Mineral nutrients

Roots extract water from the soil, but they also extract key minerals. Plants require

mineral resources of nitrogen (N), phosphorus (P), sulfur (S), potassium (K),

calcium (Ca), magnesium (Mg) and iron (Fe), together with traces of manganese

(Mn), zinc (Zn), copper (Cu) and boron (B). All of these must be obtained from

the soil (or directly from the water in the case of free-living aquatic plants). Soils

are patchy and heterogeneous, and as roots grow through them, they may meet

regions that vary in nutrient and water content. They tend to branch profusely in

the richer patches (Figure 3.20).

Root architecture is particularly important in this process, because differ-

ent nutrients behave differently and are held in the soil by different forces.

Nitrate ions diffuse rapidly in soil water, and rapidly transpiring plants may

bring nitrates to the root surface faster than they are accumulated in the body of

the plant. However, other key nutrients such as phosphate are tightly bound

in the soil (have low diffusion coefﬁcients). The phosphate RDZs of two roots

0.2 mm apart scarcely overlap, and the parts of a ﬁnely branched root system

scarcely compete with each other. Consequently, if phosphate is in short supply,

a highly branched surface root will greatly improve phosphate absorption. A

more widely spaced extensive root system, in contrast, will tend to maximize

access to nitrate.

Chapter 3 Physical conditions and the availability of resources

Figure 3.18

Highly diagrammatic picture of a root hair withdrawing water from pores in

a very wet soil. Even the widest pores shown are full of water. As water is

withdrawn, the wider pores become emptied and water ﬂows only along the

twisted pathways through narrower pores.

the architecture of roots

determines their foraging

efﬁciency

9781405156585_4_003.qxd 11/5/07 14:44 Page 91

3.3.4 Carbon dioxide

Plants take in carbon dioxide through the stomatal pores on leaf surfaces and,

as we have seen, using the energy of sunlight, capture the carbon atoms during

photosynthesis and release oxygen. Carbon dioxide varies in its concentration

at a variety of scales. In 1750, atmospheric carbon dioxide concentrations were

approximately 280 µll

−1

. Currently, the ﬁgure is over 370 µll

−1

and is increasing

by 0.4–0.5% per year, mainly as a result of the burning of fossil fuels (Box 3.2).

Part II Conditions and Resources

100

120

140

160

180

200

000

Depth (cm)

(a)

Lolium multiflorum

(b)

Mercurialis

annua

(c)

Aphanes

arvensis

(d)

Sagina

procumbens

Depth (cm)

(e)

Hymenoclea

salsola

(f)

Echinocereus

engelmanii

(g)

Gutierrezia

microcephala

(h)

Salazaria

mexicana

(i)

Salvia

dorrii

(a–d) FROM FITTER, 1991; (e–i) REDRAWN FROM A VARIETY OF SOURCES

Figure 3.19

Proﬁles of root systems of plants from contrasting environments. (a–d) Northern temperate species of open ground: (a) Lolium multiﬂorum, an

annual grass; (b) Mercurialis annua, an annual weed; and (c) Aphanes arvensis and (d) Sagina procumbens, both ephemeral weeds. (e–i) Desert

shrub and semishrub species, Mid Hills, eastern Mojave Desert, California.

9781405156585_4_003.qxd 11/5/07 14:44 Page 92

Chapter 3 Physical conditions and the availability of resources

Sand

Clay

Sand

Soil

surface

Figure 3.20

The root system developed by a young plant of wheat growing

through a sandy soil with a layer of clay. Clays offer more nutrient

resources and hold more water than sand and the roots respond

by branching more intensively in the clay.

COURTESY OF J. V. LAKE

3.2 TOPICAL ECONCERNS

3.2 Topical ECOncerns

Carbon dioxide is one of several global ‘greenhouse

gases’ (see Section 13.3.1), whose increasing con-

centrations are believed by most scientists to be lead-

ing to rises in global mean temperatures, to a growth

in the number of ‘extreme’ and ‘record’ weather events,

and to the threat of the major biomes of Earth sub-

stantially changing their distribution (see Box 4.1).

The Intergovernmental Panel on Climate Change

(IPCC) was established in 1988 by the World Meteoro-

logical Organization (WMO) and the United Nations

Environment Programme (UNEP). Each report pro-

duced by the IPCC is written by some 200 independ-

ent scientists and other experts in approximately 120

countries around the world and is reviewed by another

400 independent experts.

A recent report (IPCC, 2001) describes the current

state of understanding of the climate system, provid-

ing estimates of future change and highlighting areas

of uncertainty. It concludes that an increasing body of

observations point to a warming world – temperatures

have risen during the past four decades in the lowest

8 km of the atmosphere, global average sea level has

risen, snow cover and ice extent have decreased.

These changes have occurred at the same time as

atmospheric greenhouse gases continue to increase

due to human activities and the panel points to new

and stronger evidence that most of the observed

warming over the past half century is attributable to

human activities. Scientists now have greater conﬁd-

ence in the ability of models to project future climate,

and all reasonable scenarios point to a substantial

temperature increase. Globally averaged surface tem-

perature is expected to increase by between 1.4°C

and 5.8°C in the period 1990 to 2100, with complex

consequences for weather patterns and sea level.

Policy-makers and law-makers are faced with dif-

ferent groups of scientiﬁc ‘experts’ offering different

projections into the future, and with many interest

groups, including a number of industries resisting

attempts to force them to change their behavior in

order to reduce emissions of greenhouse gases. Even

though the majority of scientists believe the problem

to be a very real one, the truth is that predictions of

the future can never be made with absolute certainty.

Global warming? Can we risk it?

9781405156585_4_003.qxd 11/5/07 14:44 Page 93

Plants have responded to even larger ﬂuctuations of carbon dioxide over geological

history. During the Triassic, Jurassic and Cretaceous periods, atmospheric con-

centrations of carbon dioxide were four to eight times greater than at present.

Concentrations can also vary in space and over short time scales. In a terrestrial

community (Figure 3.21a), carbon dioxide concentration is highest (up to around

1800 µll

−1

) very close to the ground in summer, where it is released rapidly

from decomposing organic matter in the soil. Diffusion alone guarantees that the

concentration quickly declines with increasing height, but in the daytime photo-

synthesizing plants also actively remove carbon dioxide from the air, whereas at night

Part II Conditions and Resources

0400 20001200

Jul 4

0400

255

(a)

(b)

455

405

355

305

255

455

405

355

305

20001200

Nov 21

July

August

0123456789

180

170

160

150

140

130

120

110

100

190

200

210

concentration (µl l

–1

)

Time of day

concentration (µmol l

–1

)

Depth (m)

Put yourself in the position of a politician. Would it

be reasonable of you to demand major changes of

signiﬁcant sectors of the national economy, in order to

avert a disaster that may never happen in any case?

Or, since the consequences of the ‘worst case’ and

even some of the ‘middle of the road’ scenarios are so

profound, is the only responsible course of action to

minimize risk – to behave as if disaster is certain if we

do not change our collective behavior, even though it

is not? One alternative might be to wait for better data.

But suppose that by the time better data are available

it is too late ...

Figure 3.21

(a) Average carbon dioxide

concentrations for each hour of

the day in a mixed deciduous forest

(Harvard Forest, Massachusetts, USA)

on November 21 and July 4 at three

heights above the ground: , 0.05 m;

, 1 m; , 12 m. (b) Variation in carbon

dioxide concentration with depth in Lake

Grane Langsø, Denmark in early July,

and also in late August after the lake

has become stratiﬁed with little mixing

between the warm water at the surface

and the colder water beneath.

(a) AFTER BAZZAZ & WILLIAMS, 1991; (b) AFTER RIIS & SAND-JENSEN, 1997

variations beneath a canopy

9781405156585_4_003.qxd 11/5/07 14:44 Page 94

concentrations increase as plants respire and there is no photosynthesis. During

the winter, when low temperatures mean that rates of photosynthesis, respiration

and decomposition are all slow, concentrations remain virtually constant through

day and night at all heights. Thus, plants growing in different parts of a forest will

experience quite different carbon dioxide environments: the lower leaves on a

forest shrub will usually experience higher carbon dioxide concentrations than its

upper leaves, and seedlings will live in environments richer in carbon dioxide than

mature trees. In aquatic environments, variations in carbon dioxide concentration

can be just as striking, especially when water mixing is limited, for example during

the summer ‘stratiﬁcation’ of lakes, with layers of warm water towards the surface

and colder, carbon dioxide-rich layers beneath (Figure 3.21b).

Are higher concentrations of carbon dioxide better for plant growth? When

other resources are present at adequate levels, additional carbon dioxide scarcely

inﬂuences the rate of photosynthesis of C4 plants but increases the rate of C3 plants.

Indeed, artiﬁcially increasing the carbon dioxide concentration in greenhouses

is a commercial technique to increase crop (C3) yields. We might reasonably

predict dramatic increases in the productivity of individual plants and of whole

crops and natural communities as atmospheric concentrations of carbon dioxide

continue to increase. However, there is also much evidence that the responses

may be complicated. For example, when six species of temperate forest tree were

grown for 3 years in a carbon dioxide-enriched atmosphere in a glasshouse, they

were generally larger than controls, but the enhanced growth effect declined even

within the relatively short time scale of the experiment (Bazzaz et al., 1993).

Moreover, there is a general tendency for carbon dioxide enrichment to reduce

the nitrogen concentration in above-ground plant tissues (Cotrufo et al., 1998),

which may induce insect herbivores to eat 20–80% more foliage to maintain their

nitrogen intake, effectively negating any growth enhancement.

3.4 Animals and their resources

Green plants are autotrophs: their resources are quanta of radiation, ions and

simple molecules. Plants assemble them into complex molecules (carbohydrates, fats,

proteins) and then package them into cells, tissues, organs and whole organisms.

It is these packages that form the food resources for virtually all other organisms,

the heterotrophs (decomposers, predators, grazers, parasites). These consumers

unpack the packages, metabolize and excrete some of the contents, and reassemble

the remainder into their own bodies. They in turn may be consumed, unpacked

and reconstituted in a chain of events in which each consumer becomes, in turn,

a resource for some other consumer.

Heterotrophs can generally be grouped as follows:

1 Decomposers, which feed on already dead plants and animals.

2 Parasites, which feed on one or a very few host plants or animals while they

are alive but do not (usually) kill their hosts, at least not immediately.

3 Predators, which, during their life, eat many prey organisms, typically (and

in many cases always) killing them.

4 Grazers, which, during their life, consume parts of many prey organisms,

but do not (usually) kill their prey, at least not immediately.

Chapter 3 Physical conditions and the availability of resources

what will be the consequences of

current rises?

autotrophs and heterotrophs

9781405156585_4_003.qxd 11/5/07 14:44 Page 95

The usual mental image of a predator–prey relationship is something akin

to a lion eating a gazelle, but the relationship encompasses a much wider array

of consumer–resource interactions. For example, a squirrel is a predator when it

eats an acorn (it kills the acorn embryo); a whale is a predator as it feeds on krill;

and even a fungus can be regarded as a predator when it feeds on and kills a

growing seedling. In each case, the predator kills its food resource as it consumes

all or part of it. Here, we concentrate on animal consumers (and take the subject

further still in Chapter 7).

An important distinction between animal consumers is whether they are spe-

cialized or generalized in their diet. Generalists (polyphagous species) take a wide

variety of prey species though they very often have clear preferences and a rank

order of what they will choose when there are alternatives available. Specialists,

on the other hand, may specialize on particular parts of their prey but range over

a number of species. This is most common among herbivores because, as we shall

see, different parts of plants are quite different in their composition. Thus, many

birds specialize on eating seeds though they are seldom restricted to a particular

species. Finally, a consumer may specialize on a single species or a narrow range

of closely related species (when it is said to be monophagous). Examples are cater-

pillars of the cinnabar moth (which eat the leaves, ﬂower buds and very young

stems of a species of ragwort, Senecio) and many species of host-speciﬁc parasites.

3.4.1 Nutritional needs and provisions

The various parts of a plant have very different compositions (Figure 3.22a) and

so offer quite different resources. Bark, for example, is largely composed of dead

Part II Conditions and Resources

monophagy and polyphagy

plants as (a variety of) foods

Goose

Cow

(liver)

Seeds

(Brazil nut)

Fat

Fiber

Wood

(softwood)

Bark

(softwood)

Fruit

(plum)

Phloem sap

(Yucca flaccida)

Leaf

(cabbage)

Fish

(catfish)

Cow

(heart)

Shrimp

Minerals

Carbohydrate

Protein

Xylans and other

wood chemicals

(a) (b)

Figure 3.22

The composition of various plants

(a) and animals (b) that may serve

as food resources for herbivores or

carnivores. Note that the different

parts of a plant have very different

compositions, whereas different

species of animal (and their parts)

are remarkably similar.

9781405156585_4_003.qxd 11/5/07 14:44 Page 96

cells with corky and ligniﬁed walls, packed with defensive phenolics, and is quite

useless as a food for most herbivores (even species of ‘bark beetle’ specialize on

the nutritious cambium layer just beneath the bark, rather than on the bark itself).

The richest concentrations of plant proteins (and hence of nitrogen) are in the

meristem in the buds at shoot tips and in leaf axils. Not surprisingly, these are usu-

ally heavily protected with bud scales and defended from herbivores by prickles

and spines. Seeds are usually dried, packaged reserves rich in starch or oils as well

as specialized storage proteins. And the very sugary and ﬂeshy fruits are resources

provided by the plant as ‘payment’ to animals that disperse the seeds. Very little

of the plants’ nitrogen is spent on these rewards.

The diversity of different food resources offered by plants is matched by the

diversity of specialized mouthparts and digestive tracts that have evolved to con-

sume them. This diversity is especially developed in the beaks of birds and the

mouthparts of insects (Figure 3.23).

For a consumer, the body of a plant is a quite different package of resources

from the body of an animal. First, plant cells are bounded by walls of cellulose, lignin

and other structural carbohydrates that give plants their high ﬁber content and

contribute to their high ratio of carbon to other elements. These large amounts

of ﬁxed carbon mean that plants are potentially rich sources of energy. Yet the

overwhelming majority of animal species lack cellulolytic and other enzymes that

Chapter 3 Physical conditions and the availability of resources

(a) (b)

Figure 3.23

Examples of the variety of specialized mouthparts in herbivorous insects. (a) Honeybee with a long ‘tongue’ (glossia) for sucking. (b) Hawkmoth

with an even longer sucking proboscis. (c) Leichhardt’s grasshopper with large, plate-like chewing mandibles. (d) Acorn weevil with chewing

mouthparts at the very end of its long rostrum. (e) Rose aphid with a piercing stylet.

(a) © DOUG SOKELL, VISUALS UNLIMITED; (b) © VISUALS UNLIMITED; (c) © MANTIS WILDLIFE FILMS/OXFORD SCIENTIFIC FILMS IOR320MWF001; (d, e) © OXFORD SCIENTIFIC FILMS ICO340OSF00501, IHE120PRR00101

from plants into animals

9781405156585_4_003.qxd 11/5/07 14:44 Page 97

can digest these compounds: they are quite useless as a direct energy resource for

most herbivores. Moreover, the cell wall material of plants hinders the access of

digestive enzymes to the plant cell contents. The acts of chewing by the grazing

mammal, cooking by humans, and grinding in the gizzard of birds are necessary

precursors to digestion of plant food because they allow digestive enzymes access

to the cell contents. The carnivore, by contrast, can more safely gulp its food.

Many herbivores have made up for their own lack of cellulolytic enzymes by

entering into a mutualistic (beneﬁcial to both parties) association with cellulolytic

bacteria and protozoa in their guts that do have the appropriate enzymes. The

rumen (or sometimes the cecum) of many herbivorous mammals is a temperature-

regulated culture chamber for these microbes into which already partially frag-

mented plant tissues ﬂow continually (Figure 3.24). The microbes receive a home

and a supply of food. The herbivorous ‘host’ beneﬁts by absorbing many of the

major byproducts of this microbial fermentation, especially fatty acids.

Unlike plants, animal tissues contain no structural carbohydrate or ﬁber com-

ponent but are rich in fat and protein. The C : N ratio of plant tissues commonly

exceeds 40 : 1, in contrast to ratios that rarely exceed 10 : 1 in bacteria, fungi

and animals. Thus herbivores, which undertake the ﬁrst stage of making animal

Part II Conditions and Resources

20 0

(a) (b)

Figure 3.24

The digestive tracts of herbivores

are commonly modiﬁed to provide

fermentation chambers inhabited by

a rich fauna and ﬂora of microbes.

The ﬁgure shows the digestive

tracts of four different herbivorous

mammals with the fermentation

chambers highlighted in a darker

shade. (a) Rabbit, with a

fermentation chamber in the

expanded cecum. (b) Zebra, with

fermentation chambers in both

cecum and colon. (c) Sheep, with

foregut fermentation in an enlarged

portion of the stomach, rumen and

reticulum. (d) Kangaroo, with an

elongate fermentation chamber in

the proximal portion of the stomach.

AFTER STEVENS & HUME, 1998

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