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

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bodies out of plants, are involved in a massive burning off of carbon as the C : N

ratio is lowered. The main waste products of herbivores are therefore carbon-rich

compounds (carbon dioxide and ﬁber). Carnivores, on the other hand, get most

of their energy from the protein and fats of their prey, and their main excretory

products are consequently nitrogenous.

Even if the cell wall is excluded from consideration, the C : N ratio is high

in plants compared with other organisms. Aphids, which gain direct access to cell

contents by driving their stylets into the phloem transport system, gain a resource

that is rich in soluble sugars (see Figure 3.22a). In their search for valuable

nitrogen, they use only a fraction of this energy resource and excrete the rest

in sugary rich honeydew that may drip as a rain from an aphid-infested tree. For

most herbivores and decomposers, the body of a plant is a superabundant source

of energy and carbon; it is other components of the diet, especially nitrogen, that

are more usually limiting.

The bodies of different species of animal have remarkably similar composition

(see Figure 3.22b). In terms of protein, carbohydrate, fat, water and minerals

per gram, there is very little to choose between a diet of caterpillars or cod, or of

earthworms, shrimps or venison. The packages may be differently assembled (and

the taste may be different), but the contents are essentially the same. Moreover,

the different parts of an animal have very similar nutritional content. Unlike

herbivores, carnivores are not faced with difﬁcult problems of digestion (and they

vary very little in their digestive apparatus) but rather with difﬁculties in ﬁnding,

catching and overcoming the defenses of their prey.

3.4.2 Defense

The value of a resource to a consumer is determined not only by what it contains

but by how well its contents are defended. Not surprisingly, organisms have evolved

physical, chemical, morphological and behavioral defenses against being attacked.

These serve to reduce the chance of an encounter with a consumer and/or increase

the chance of survival in such encounters. The spiny leaves of holly are not eaten

by larvae of the oak eggar moth, but if the spines are removed the leaves are eaten

quite readily. No doubt similar results would be achieved in equivalent experi-

ments with foxes as predators and de-spined hedgehogs or porcupines as prey.

On a smaller scale, many plant surfaces are clothed in epidermal hairs (trichomes)

that may keep the smaller predators (such as thrips and mites) away from the leaf

surface (Figure 3.25; see also Figure 3.27a).

Any feature of an organism that increases the energy spent by a consumer in

discovering or handling it is a defense if, as a consequence, the consumer eats less

of it. The thick shell of a nut increases the time that an animal spends extracting

a unit of effective food, and this may reduce the number of nuts that are eaten. We

have already seen that most green plants are relatively overprovided with energy

resources in the form of cellulose and lignin. It may therefore be cheap to build

husks and shells around seeds (and woody spines on stems) if these defense tissues

contain rather little protein, and if what is protected is far more valuable.

Both plants and animals have a battery of chemical defenses. The plant

kingdom, in particular, is very rich in ‘secondary’ chemicals that apparently play

no role in normal plant biochemical pathways. A defensive function is gener-

ally ascribed to these chemicals and a defensive role has been demonstrated

Chapter 3 Physical conditions and the availability of resources

animals as food

some resources are protected...

. . . or defended

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Part II Conditions and Resources

100

Figure 3.25

A mite trapped in the protective trichomes (hairs) on the surface of a

Primula leaf. The trichomes themselves support capsules of irritant

volatile oils at their tip. Each white bar towards the foot of the image

represents 10 µm.

COUTESTY OF C.J. VELTKAMP

Table 3.1

Slugs (Agriolimax reticulatus) graze on the leaves of clover (Trifolium repens). There are forms of

clover that release hydrogen cyanide when the cells are damaged. Slugs nibble clover leaves and reject

cyanogenic forms but continue to consume the leaves of non-cyanogenic forms. Two plants, one of each

form, were grown together in plastic containers and slugs were allowed to graze for seven successive

nights. The table shows the numbers of leaves in different conditions after slug grazing. +/− indicate

deviation from random expectation; the difference from random expectation is signiﬁcant at P < 0.001.

AFTER DIRZO & HARPER, 1982

CONDITIONS OF LEAVES AFTER GRAZING

UP TO MORE THAN

NOT 50% OF LEAF 50% OF LEAF

DAMAGED NIBBLED REMOVED REMOVED

Cyanogenic plants 160 (+) 22 (+) 38 (–) 9 (–)

Non-cyanogenic plants 87 (–) 7 (–) 30 (+)65 (+)

unequivocally in some cases. Populations of white clover, for example, commonly

contain some individuals that release hydrogen cyanide when their tissues are

attacked (cyanogenic forms) and others that do not; those that cannot are eaten

by slugs and snails. The cyanogenic forms, however, are nibbled but then rejected

(Table 3.1).

Noxious plant chemicals have been classiﬁed into two broad types. The ﬁrst

are quantitative chemicals (so-called because they are most effective at relatively

high concentrations), which make the tissues that contain them, such as mature

oak leaves, relatively indigestible. They are also often called constitutive chemicals,

since they tend to be produced even in the absence of herbivore attack. The

second type are toxic or qualitative chemicals, which are poisonous even in small

quantities but can be produced relatively rapidly and are therefore commonly

inducible: only produced in response to damage itself, and hence with lower ﬁxed

costs to the plants.

Plants differ in their chemical defenses from species to species and also

from tissue to tissue within an individual plant. Broadly, relatively short-lived,

optimal defense theory:

constitutive and inducible

defenses

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ephemeral plants gain a measure of protection from consumers because of the

unpredictability of their appearance in space and time. They therefore need to

invest less in defense than predictable, long-lived species like forest trees. The

latter, precisely because they are apparent for long periods to a large number of

herbivores, tend to invest in constitutive chemicals that, while costly, afford them

broad protection; whereas ephemeral plants tend to produce inducible toxins as

required. Moreover, it may be predicted that, within an individual plant, the

more important plant parts should be protected by costly, constitutive chemicals,

whereas less important parts should rely on inducible toxins (McKey, 1979;

Strauss et al., 2004). This is conﬁrmed, for example, by a study of wild radish,

in which plants were either attacked by caterpillars of the butterﬂy, Pieris rapae,

or left as unmanipulated controls (Figure 3.26). Flower petals are known to be

highly important to ﬁtness in this insect-pollinated plant, and concentrations of

toxic glucosinolates were twice as high in petals as in undamaged leaves: levels

that were maintained constitutively, irrespective of whether the petals were

damaged by the caterpillars. Leaves, on the other hand, have a much less direct

inﬂuence on ﬁtness: high levels of leaf damage can be sustained without any

measurable effect on reproductive output. Constitutive levels of glucosinolates,

as already noted, were low; but if leaves were damaged, the (induced) con-

centrations were even higher than in the petals.

Animals have more options than plants when it comes to defending them-

selves, but some still make use of chemicals. For example, defensive secretions of

sulfuric acid of pH 1 or 2 occur in some marine gastropod groups, including

the cowries. Other animals, which can tolerate the chemical defenses of their

plant food, actually store the plant toxins and use them in their own defense.

A classic example is the monarch butterﬂy, whose caterpillars feed on milkweeds,

which contain cardiac glycosides, which are poisonous to mammals and birds.

These caterpillars can store the poison, and it is still present in the adult. Thus, a

bluejay will vomit violently after eating one, and, once it recovers, will reject all

others on sight. In contrast, monarchs reared on cabbage are edible.

Chemical defenses are not equally effective against all consumers. Indeed,

what is unacceptable to some animals may be the chosen, even unique, diet of

others. Many herbivores, particularly insects, specialize on one or a few plant

species whose particular defense they have overcome. For example, females of the

cabbage root ﬂy, with eggs to lay, home in on a brassica crop from distances as

far as 15 m downwind of the plants. It is probably hydrolyzed glucosinolates

(toxic to many other species) that provide the attractive odor.

Chapter 3 Physical conditions and the availability of resources

101

AFTER STRAUSS ET AL., 2004

Leaf

Petal

Undamaged

Damaged

Total glucosinolates

Figure 3.26

Concentrations of glucosinolates (µgmg

−1

dry mass) in the petals

and leaves of wild radish, Raphinus sativus, either undamaged or

damaged by caterpillars of Pieris rapae. Bars are standard errors.

chemical defense in animals

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An animal may be less obvious to a predator if it matches its background, or

possesses a pattern that disrupts its outline, or resembles an inedible feature of its

environment. A straightforward example of such crypsis is the green coloration

of many grasshoppers and caterpillars (Figure 3.27). Cryptic animals may be

highly palatable, but their morphological traits and color (and their choice of the

appropriate background) reduce the likelihood that they will be used as a resource.

In contrast, noxious or dangerous animals often seem to advertize the fact by bright,

conspicuous colors and patterns (aposematism; Figure 3.27b). The monarch

butterﬂy (see earlier), for example, is aposematically colored. One attempt by a

bird to eat an adult monarch is so memorable that others are subsequently avoided

for some time. The adoption of memorable body patterns by distasteful prey,

moreover, immediately opens the door for deceit by other species – there will be

a clear advantage to a palatable prey if it mimics an unpalatable species. Thus, the

palatable viceroy butterﬂy mimics the distasteful monarch, and a bluejay that has

learned to avoid monarchs will also avoid viceroys.

By living in holes, animals (millipedes, moles) may avoid stimulating the

sensory receptors of predators, and by ‘playing dead’ (opossum, African ground

squirrel) animals may fail to stimulate a killing response. Animals that withdraw

to a prepared retreat (rabbits and prairie dogs to their burrows, snails to their

shells) or roll up and protect their vulnerable parts by a tough exterior (armadillos,

hedgehogs) reduce their chance of capture. Other animals seem to try to bluff

themselves out of trouble by threatening or startling displays (Figure 3.27d). Moths

and butterﬂies that suddenly expose eye spots on their wings are one example. No

Part II Conditions and Resources

102

(a)

(b)

Figure 3.27

Lepidopterous caterpillars illustrate a

range of defense strategies. (a) The

irritating hairs of the gypsy moth.

(b) Aposematism (advertizing

distastefulness) in the black

swallowtail. (c) A cryptic

(camouﬂaged) noctuid, looking like

bark. (d) Another swallowtail rearing

and hence possibly startling a

potential predator.

crypsis, aposematism,

and mimicry

behavior

9781405156585_4_003.qxd 11/5/07 14:44 Page 102

doubt the most common behavioral response of an animal in danger of becoming

a used resource is to run away.

3.5 Effects of intraspeciﬁc competition

for resources

Resources are consumed. The consequence is that there may not be enough of a

resource to satisfy the needs of a whole population of individuals. Individuals may

then compete with each other for the limited resource. Intraspeciﬁc competition

is competition between individuals of the same species.

In many cases, competing individuals do not interact with one another directly.

Rather, they deplete the resources that are available to each other. Grasshoppers

may compete for food, but a grasshopper is not directly affected by other grass-

hoppers so much as by the level to which they have reduced the food supply.

Two grass plants may compete, and each may be adversely affected by the

presence of close neighbors, but this is most likely to be because their resource

depletion zones overlap – each may shade its neighbors from the incoming

ﬂow of radiation, and water or nutrients may be less accessible than they would

otherwise be around the plants’ roots. The data in Figure 3.28, for example, show

the dynamics of the interaction between a single-celled aquatic plant, a diatom,

and one of the resources it requires, silicate. As diatom density increases over

time, silicate concentration decreases: there is then less available for the many

than there had been previously for the few. This type of competition – in which

competitors interact only indirectly, through their shared resources – is termed

exploitation.

On the other hand, competing individual vultures may ﬁght one another

over access to a newly found carcass. Individuals of other species may ﬁght for

ownership of a ‘territory’ and access to the resources that a territory brings with

it. A barnacle that settles on a rock denies the space to another barnacle. This is

called interference competition.

Whether competition occurs through exploitation, interference or a combina-

tion of the two, its ultimate effect is on the vital rates of the competitors – their

survival, growth and reproduction – compared with what they would have been

if resources had been more abundant. Competition typically leads to decreased

Chapter 3 Physical conditions and the availability of resources

103

0 10203040 50

Population density (cells ml

–1

) ( )

Time (days)

Silicate (µmol l

–1

) ( )

Figure 3.28

A population of the freshwater diatom Asterionella formosa was grown in ﬂasks of

culture medium. The diatom consumes silicate during growth and the population of

diatoms stabilizes when the silicate has been reduced to a very low concentration.

AFTER TILMAN ET AL., 1981

exploitation: competitors

depleting each other’s resources

direct interference

competition and vital rates

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rates of resource intake per individual, and thus to decreased rates of individual

growth or development, perhaps to decreases in amounts of stored reserves or to

increased risks of predation. Figure 3.29a shows how the mortality rate of steel-

head trout increases, as the number of competing ﬁsh rises, at a range of food

levels; Figure 3.29b shows how the birth rate of the sand dune grass Vulpia declines

as individuals become increasingly crowded.

In practice, intraspeciﬁc competition is often a very one-sided affair: a strong

early seedling will shade and suppress a stunted, late one; a large vulture is likely

to ﬁght off a smaller one. Some of the competitive strength of individuals is related

to timing (the early seedling) or to random events (one seed may germinate in a

depression where it obtains more water than its neighbors). Sometimes the winner

and loser may be genetically different and then competition will be playing a role

in natural selection.

The effects of intraspeciﬁc competition on any individual are typically greater

the more crowded the individual is by its neighbors – the more the resource

depletion zones of other individuals overlap its own. This often translates into

saying that the greater the density of a population of competitors the greater

is the effect of competition. Hence, the effects of intraspeciﬁc competition are

often said to be density-dependent. But it is doubtful that any organism has a way

of detecting the density of its population. Rather, it responds to the effects of

being crowded.

On the other hand, at low densities in the case of Vulpia (Figure 3.29b), the

per capita birth rate or fecundity was independent of density (where per capita

means literally ‘per head’ or ‘per individual’). That is, the fecundity was effectively

the same at a density of 1000 plants/0.25 m

as it was at a density of 500/0.25 m

Thus, there is no evidence at these densities that individuals are affected by

the presence of other individuals and hence no evidence of intraspeciﬁc com-

petition. But as density increases further, the per capita birth rate progressively

decreases. These effects are now density-dependent, and this may be taken as

an indication that at these densities, individuals are suffering as a result of

intraspeciﬁc competition.

The patterns in Figure 3.29 make the point that as crowding (or density)

increases, the fecundity per individual is likely to decline and the mortality per

individual likely to increase (which would mean that the survival rate per indi-

vidual would decrease). But what can we expect to happen to the total number

Part II Conditions and Resources

104

(b)

Low

Stocking density

Medium

(a)

High

Probability of mortality

1.0

0.8

0.6

0.4

0.2

Number of flowering plants/0.25 m

–1

Number of seeds/plant

Figure 3.29

(a) The rate of mortality among steelhead

trout (Oncorynchus mykiss) reared

at a range of densities (32, 63 and

127 per m

) and at a range of food

levels (1.4, 2.9 and 5.8 g of food pellets

per day: yellow, maroon and blue lines,

respectively). (b) The average number

of seeds produced per plant of the dune

grass Vulpia fasciculata growing at a

range of densities.

(a) AFTER KEELEY, 2001; (b) AFTER WATKINSON & HARPER, 1978

density dependence

competition and the total number

of survivors

9781405156585_4_003.qxd 11/5/07 14:44 Page 104

of seeds or eggs produced by populations at different densities – or to the total

number of survivors? In some cases, although the rate per individual declines

with increasing density, the total fecundity or total number of survivors in the

population continues to increase. This can be seen (Figure 3.30a) to have been

the case for the plant populations in Figure 3.29b – at least over the range of

densities examined. In other cases, the rate per individual declines so rapidly

with increasing density that the total fecundity or total number of survivors in

the population actually gets smaller the greater the number of contributing indi-

viduals. This can be seen in Figure 3.30b for the highest densities of a bacterial

parasite of the planktonic crustacean Daphnia magna.

In yet further cases, the mortality risk or fecundity per individual declines with

increasing density such that the total number of survivors or total fecundity is

the same irrespective of the number of contributing individuals. This is referred

to as exactly compensating density dependence, and the competition leading to

Chapter 3 Physical conditions and the availability of resources

105

(a) AFTER WATKINSON & HARPER, 1978; (b) AFTER EBERT ET AL., 2000; (c) AFTER LE CREN, 1973; (d) AFTER IRVINE ET AL., 2001

(a)

10010

Number of spores (in millions)

100,000

(b)

(c)

1,000 10,000

0 18,000

(d)

6,000 12,000

Total number of seeds/0.25 m

Number of flowering plants/0.25 m

Log

final trout density (m

–2

)

Log

initial trout density (m

–2

)

1.5

1.0

1.0 2.52.0

0.5

Dose (spores ml

–1

)

M. marshalli fecal egg count

M. marshalli adult worm burden

Figure 3.30

Undercompensating, overcompensating and exactly compensating effects of intraspeciﬁc competition.

(a) An undercompensating effect on fecundity: the total number of seeds produced by Vulpia fasciculata

continues to rise as density increases. (b)When the planktonic crustacean Daphnia magna was infected

with varying numbers of spores of the bacterium Pasteuria ramosa, the total number of spores produced

per host in the next generation was independent of density (exactly compensating) at the lower densities,

but declined with increasing density (overcompensating) at the higher densities. Standard errors are

shown. (c) An exactly compensating effect on mortality: the number of surviving trout fry is independent

of initial density at higher densities. (d) The total number of eggs of the parasitic nematode Marshallagia

marshalli produced by infected reindeer (eggs per gram of feces) increased in direct proportion to

the number of adult nematodes in the reindeer: there was no evidence of competition between the

nematodes.

9781405156585_4_003.qxd 11/5/07 14:44 Page 105

it is sometimes referred to as ‘contest-like’, since this is the pattern you would

expect to see if there were a ﬁxed number of winners and all the other competitors

were doomed to lose. Examples are shown for fecundity in Figure 3.30b (at the

lower densities) and for survivors in Figure 3.30c. Finally, of course, birth or

mortality rates may be density-independent (no competition) throughout the range

examined, in which case the total number of births or survivors will simply con-

tinue to rise in direct proportion to the original density (e.g. Figure 3.30d).

3.6 Conditions, resources and the

ecological niche

Finally, many of the ideas in this chapter can be brought together in the concept

of the ecological niche. The term niche, though, is frequently misunderstood

and misused. It is often used loosely to describe the sort of place in which an

organism lives, as in the sentence ‘Woodlands are the niche of woodpeckers’.

However, strictly, where an organism lives is not its niche but its habitat. A niche

is not a place but an idea: a summary of the organism’s tolerances and require-

ments. The habitat of a gut microorganism would be an animal’s alimentary canal;

the habitat of an aphid might be a garden; and the habitat of a ﬁsh could be

a whole lake. Each habitat, however, provides many different niches: many other

organisms also live in the gut, the garden, or the lake – and with quite different

lifestyles. The niche of an organism describes how, rather than just where, an

organism lives.

The modern concept of the niche was proposed by Hutchinson in 1957 to

address the ways in which tolerances and requirements interact to deﬁne the con-

ditions and resources needed by an individual or a species in order to practice its

way of life. Temperature, for instance, is a condition that limits the growth and

reproduction of all organisms, but different organisms tolerate different ranges

of temperature. This range is one dimension of an organism’s ecological niche:

Figure 3.31a shows how different species of plants vary in the temperature dimen-

sion of their niche. But there are many such dimensions for the niche of a species:

its tolerance of various other conditions (relative humidity, pH, wind speed,

waterﬂow, and so on), and its need for various resources (nutrients, water, food,

and so on). Clearly the real niche of a species must be multidimensional.

It is easy to visualize the early stages of building such a multidimensional niche.

Figure 3.31b illustrates the way in which two niche dimensions (temperature and

salinity) together deﬁne a two-dimensional area that is part of the niche of a sand

shrimp. Three dimensions, such as temperature, pH and the availability of a

particular food, may deﬁne a three-dimensional niche volume (Figure 3.31c). It

is hard to imagine (and impossible to draw) a diagram of a more realistic, multi-

dimensional niche. (Technically, we now consider a niche to be an n-dimensional

hypervolume, where n is the number of dimensions that make up the niche.) But

the simpliﬁed three-dimensional version nonetheless captures the idea of the

ecological niche of a species. It is deﬁned by the boundaries that limit where it

can live, grow and reproduce, and it is very clearly a concept rather than a place.

The concept has become a cornerstone of ecological thought, as we shall see in

later chapters.

Part II Conditions and Resources

106

the niche of an organism is

deﬁned by its needs and

tolerances

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Chapter 3 Physical conditions and the availability of resources

107

Temperature (°C)

510

2600

2500

1900

900

600

550

530

250

240

Ranunculus glacialis

Oxyria digyna

Geum reptans

Pinus cembra

Picea abies

Betula pendula

Larix decidua

Leucojum vernum

Betula pendula

Fagus sylvatica

Taxus baccata

Abies alba

Prunus laurocerasus

Quercus ilex

Olea europaea

Quercus pubescens

Citrus limonum

100% mortality

50% mortality

Zero mortality

0 5 10 15 20 25 30 35 40 45

Salinity (%)

(a)

(b)

(c)

Temperature

Food available

Figure 3.31

(a) A niche in one dimension. The range of

temperatures at which a variety of plant species

from the European Alps can achieve net

photosynthesis at low intensities of radiation

(70 W m

−2

). (b) A niche in two dimensions for

the sand shrimp (Crangon septemspinosa)

showing the fate of egg-bearing females in aerated

water at a range of temperatures and salinities.

an aquatic organism showing a volume deﬁned by

the temperature, pH and availability of food.

(a) AFTER PISEK ET AL., 1973; (b) AFTER HAEFNER, 1970

Conditions and resources

Conditions are physicochemical features of the envir-

onment such as its temperature and humidity. They

may be altered but are not consumed. Environmental

resources are consumed by living organisms in the

course of their growth and reproduction.

Environmental conditions

There are three basic types of response curve to

conditions. Extreme conditions may be lethal with,

between the two extremes, a continuum of more

favorable conditions; or a condition may be lethal only

at high intensities; or a condition may be required by

organisms at low concentrations but become toxic at

high concentrations.

These responses are accounted for, in part, by

changes in metabolic effectiveness. But at extremely

high temperatures, for example, enzymes and other

proteins become unstable and break down, and

the organism dies; and at high environmental

SUMMARY

Summary

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Part II Conditions and Resources

108

temperatures, terrestrial organisms may encounter

serious, perhaps lethal, problems of dehydration. At

temperatures a few degrees above zero, organisms

may be forced into extended periods of inactivity,

or ice may form between cells and draw water from

within them. The timing and duration of temperature

extremes, however, may be as important as absolute

temperatures.

In practice, the effects of conditions may be deter-

mined largely by the responses of other community

members, through food consumption, disease or

competition.

Many conditions are important stimuli for growth

and development and prepare an organism for con-

ditions that are to come.

Plant resources

Solar radiation, water, minerals and carbon dioxide

are all critical resources for green plants. The shape of

the curve that relates the rate of photosynthesis to the

intensity of radiation varies greatly among species.

The radiation that reaches a plant is forever changing;

the plant integrates the diverse exposures of its vari-

ous leaves.

Most variation in leaf shape has probably evolved

under selection to optimize the photosynthesis achieved

per unit of water transpired. Any mechanism or pro-

cess that slows the rate of water loss, such as closing

of the stomata, reduces the photosynthetic rate. If the

rate of water uptake falls below the rate of release, the

body of the plant starts to wilt. If the deﬁcit accumu-

lates, the whole plant may die. Plants may avoid or

tolerate water shortage. Specialized biochemical pro-

cesses may increase the amount of photosynthesis

that can be achieved per unit of water lost in C4 and

CAM (as opposed to C3) plants.

The primary water-absorbing zone on roots is

covered with root hairs that make intimate contact

with soil particles. Roots create water depletion zones

around themselves. Root architectures are much less

tightly programmed than those of shoots, and those

established early in a plant’s life can determine its

responsiveness to later events. Roots also extract key

minerals from the soil. Root architecture is particularly

important here because different nutrients are held in

the soil by different forces.

Animals and their resources

Green plants are autotrophs. Decomposers, pre-

dators, grazers and parasites are heterotrophs. The

various parts of a plant have very different com-

positions and so offer quite different resources. This

diversity is matched by the diversity of mouthparts

and digestive tracts that have evolved to consume

them. The body of a plant is a quite different package

of resources from the body of an animal. To make

better use of plant material, many herbivores enter into

a mutualistic association with cellulolytic bacteria and

protozoa in their alimentary canal.

The C : N ratio of plant tissues greatly exceeds

those of bacteria, fungi and animals. Thus, herbivores

typically have a superabundant source of energy

and carbon, but nitrogen is often limiting; their main

waste products are carbon dioxide and ﬁber. The

bodies of different species of animal have remark-

ably similar compositions. Carnivores are not faced

with problems of digestion, but rather with difﬁculties

in ﬁnding, catching and overcoming the defenses of

their prey. Carnivores’ main excretory products are

nitrogenous.

Effects of intraspeciﬁc competition for

resources

Individuals may compete indirectly, via a shared

resource, through exploitation, or directly, through inter-

ference. The ultimate effect of competition is on survival,

growth and reproduction of individuals. Typically,

the greater the density of a population of competi-

tors, the greater is the effect of competition (density

dependence). As a result, though, the total number of

survivors, or of offspring, may increase, decrease or

stay the same as initial densities increase.

Conditions, resources and the ecological niche

Where an organism lives is its habitat. A niche is a

summary of an organism’s tolerances and require-

ments. The modern concept, proposed by Hutchinson

in 1957, is an n-dimensional hypervolume.

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