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

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In a broader sense, however, the signiﬁcance of interspeciﬁc competition rests

not on a limited number of high proﬁle effects, but on an answer to the question

‘How widespread are the ecological and evolutionary consequences of interspeciﬁc

competition in practice?’ We address this question in two ways. In the ﬁrst, dealt

with in Section 6.5.1, we ask ‘How prevalent is current competition in natural

communities?’. To demonstrate current competition requires experimental ﬁeld

manipulations, in which one species is removed from or added to the community

and the responses of the other species are monitored. It is important to answer this

question, because where current competition is demonstrable, neither the ghost

of competition past nor spatial and temporal variation are likely to have a crucial

role. And, if current competition is prevalent, then interspeciﬁc competition is likely

to be an important structuring force in nature. However, even if current competi-

tion is not prevalent, past competition, and therefore competition generally, may

still have played a signiﬁcant role in structuring communities.

The second problem, dealt with in Section 6.5.2, is to distinguish between inter-

speciﬁc competition (past or present) and ‘mere chance’: species differ not as a

reﬂection of interspeciﬁc competition but because they are different species. The

many studies in which experimental ﬁeld manipulations have not been possible

can be examined to determine whether observed patterns provide strong evidence

for a role for competition, or are open to alternative interpretations.

6.5.1 The prevalence of current competition

There have been two classic surveys of ﬁeld experiments on interspeciﬁc com-

petition. Schoener (1983) examined the results of all the experiments he could

Chapter 6 Interspeciﬁc competition

209

were vast areas left untouched. Now, he said,

the grasses are virtually everywhere and soon

will be in concentrations large enough to fuel

massive ﬁres. ‘This is not an easy problem to

solve’, he said.

. . . Despite the harsh conditions, a rainbow

of wildﬂowers blooms regularly in the desert,

sometimes carpeting the ground with blossoms

after a rainstorm. Zebra-tailed lizards,

rattlesnakes, desert tortoises and kangaroo rats

are able to get by for long periods without water

and bear up under the sun. But the innocuous-

looking grasses threaten all these species by

choking out wildﬂowers and killing off shelter

and food that they rely on.

. . . Esque [of the US Geological Survey]

has roped off 12 experimental sites, six of which

he burned in 1999 to see how quickly invasive

species re-establish themselves. But the result

only showed the unpredictability of the desert:

the ﬁrst year, the invasive red brome took hold,

but this year, native wildﬂowers came back in

force.

. . . Esque said ‘It’s not black and white with

what is going on. We don’t know if we are looking

at coexistence or competition.’

Creek, CA) and may not be republished without

permission.)

1 Some people have suggested bringing sheep

into the desert to graze the invading grasses.

Do you think this is a sensible idea? What further

information would help you make a decision?

2 The US Geological Survey scientist found that

red brome grass appeared to be outcompeting

native ﬂowers one year but not the next. Suggest

some factors that may have changed the

competitive outcome.

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ﬁnd – 164 studies in all. He found that approximately equal numbers of studies

had dealt with terrestrial plants, terrestrial animals and marine organisms, but

that studies of freshwater organisms amounted to only about half the number

in the other groups. Amongst the terrestrial studies, however, he found that

most were concerned with temperate regions and mainland populations, and

that there were relatively few dealing with phytophagous (plant-eating) insects.

Any conclusions were therefore bound to be subject to the limitations imposed

by what ecologists had chosen to look at. Nevertheless, Schoener found that

approximately 90% of the studies had demonstrated the existence of interspeciﬁc

competition, and that the ﬁgures were 89%, 91% and 94% for terrestrial, fresh-

water and marine organisms, respectively. Moreover, even if he looked at single

species or small groups of species (of which there were 390) rather than at whole

studies, which may have dealt with several groups of species, he found that

76% showed effects of competition at least sometimes, and 57% showed effects

in all the conditions under which they were examined. Once again, terrestrial,

freshwater and marine organisms gave very similar ﬁgures.

Connell’s (1983) review was less extensive than Schoener’s: 72 studies, dealing

with a total of 215 species and 527 different experiments. Interspeciﬁc competi-

tion was demonstrated in most of the studies, more than half of the species and

approximately 40% of the experiments. In contrast to Schoener, Connell found

that interspeciﬁc competition was more prevalent in marine than in terrestrial

organisms, and also that it was more prevalent in large than in small organisms.

Taken together, Schoener’s and Connell’s reviews certainly seem to indicate

that active, current interspeciﬁc competition is widespread. Its percentage occur-

rence amongst species is admittedly lower than its percentage occurrence amongst

whole studies, but this is to be expected, since, for example, if four species were

arranged along a single niche dimension and all adjacent species competed with

each other, this would still be only three out of six (or 50%) of all possible pairwise

interactions.

Connell also found, however, that in studies of just one pair of species, inter-

speciﬁc competition was almost always apparent, whereas with more species

the prevalence dropped markedly (from more than 90% to less than 50%). This

can be explained to some extent by the argument outlined above, but it may

also indicate biases in the particular pairs of species studied, and in the studies

that are actually reported (or accepted by journal editors). It is highly likely that

many pairs of species are chosen for study because they are ‘interesting’ (because

competition between them is suspected) and if none is found this is simply not

reported. Judging the prevalence of competition from such studies is rather like

judging the prevalence of debauched clergymen from the ‘gutter press’. This is a

real problem, only partially alleviated in studies on larger groups of species when

a number of ‘negatives’ can be conscientiously reported alongside one or a few

‘positives’. Thus the results of surveys, such as those by Schoener and Connell,

exaggerate, to an unknown extent, the frequency of competition.

As previously noted, phytophagous insects were poorly represented in Schoener’s

data, but reviews of this group alone have tended to suggest either that competition

is relatively rare in this group overall (Strong et al., 1984) or rare in at least certain

types of phytophagous insects, for example ‘leaf-biters’ (Denno et al., 1995). On

a more general level, it has been suggested that herbivores as a whole are seldom

food-limited, and are therefore not likely to compete for common resources

Part III Individuals, Populations, Communities and Ecosystems

210

surveys of published studies of

competition indicate that current

competition is widespread...

. . . but these surveys exaggerate

to an unknown extent the true

frequency of competition

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(Hairston et al., 1960; Slobodkin et al., 1967). The bases for this suggestion are

the observations that green plants are normally abundant and largely intact, they

are rarely devastated, and most herbivores are scarce most of the time. Schoener

found the proportion of herbivores exhibiting interspeciﬁc competition to be

signiﬁcantly lower than the proportions of plants, carnivores or detritivores.

Taken overall, therefore, current interspeciﬁc competition has been reported

in studies on a wide range of organisms and in some groups its incidence may be

particularly obvious, for example amongst sessile organisms in crowded situations.

However, in other groups of organisms, interspeciﬁc competition may have little

or no inﬂuence. It appears to be relatively rare among herbivores generally, and

particularly rare amongst some types of phytophagous insect.

6.5.2 Competition or mere chance?

There is a tendency to interpret differences between the niches of coexisting

species as conﬁrming the importance of interspeciﬁc competition. But the theory

of interspeciﬁc competition does more than predict ‘differences’. It predicts

not simply that the niches of competing species differ, but that they differ more

than would be expected from chance alone. A more rigorous investigation of

the role of interspeciﬁc competition, therefore, should address itself to the

question: ‘Does the observed pattern, even if it appears to implicate competition,

differ signiﬁcantly from the sort of pattern that could arise in the community

even in the absence of any interactions between species?’ This question has been

the driving force behind analyses that seek to compare real communities with

so-called neutral models. These are hypothetical models of actual communities

that retain certain of the characteristics of their real counterparts, but reassemble

or reconstruct some of the community components in a way that speciﬁcally

excludes the consequences of interspeciﬁc competition. In fact, the neutral model

analyses are attempts to follow a much more general approach to scientiﬁc

investigation, namely the construction and testing of null hypotheses. The idea is

that the data are rearranged into a form (the neutral model or null hypothesis)

representing what the data would look like in the absence of interspeciﬁc com-

petition. Then, if the actual data show a signiﬁcant statistical difference from

the null hypothesis, the null hypothesis is rejected and the action of interspeciﬁc

competition is strongly inferred.

In fact, the approach has been applied to three different predictions of what a

community structured by interspeciﬁc competition should look like: (i) potential

competitors that coexist in a community should exhibit niche differentiation; (ii) this

niche differentiation will often manifest itself as morphological differentiation; and

(iii) within a community, potential competitors with little or no niche differentia-

tion should not coexist, so each should tend to occur only where the other is

absent (‘negatively associated distributions’). The application of null hypotheses

to community structure – that is, the reconstruction of natural communities with

interspeciﬁc competition removed – has not been achieved to the satisfaction of

all ecologists. But a brief examination of a study of niche differentiation in lizard

communities shows the potential and rationale of the neutral model approach

(Box 6.3). For these lizard communities, niches are more spaced out than would

be expected by chance alone and interspeciﬁc competition therefore appears to

play an important role in community structure.

Chapter 6 Interspeciﬁc competition

211

neutral models

niche differentiation,

morphological differentiation

and negatively associated

distributions

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Part III Individuals, Populations, Communities and Ecosystems

212

6.3 QUANTITATIVE ASPECTS

6.3 Quantitative aspects

Lawlor (1980) investigated differential resource

utilization in 10 North American lizard communities,

consisting of four to nine species. For each com-

munity, there were estimates of the amounts of each

of 20 food categories consumed by each species.

This pattern of resource use allowed the calculation,

for each pair of species in a community, of an index

of resource use overlap, which varied between 0 (no

overlap) and 1 (complete overlap). Each community

was then characterized by a single value: the mean

resource overlap for all pairs of species present.

A number of ‘neutral models’ of these commun-

ities were then created. They were of four types. The

ﬁrst type, for example, retained the minimum amount

of original community structure. Only the original num-

ber of species and the original number of resource

categories were retained. Beyond that, species were

allocated food preferences completely at random,

such that there were far fewer species completely

ignoring food in particular categories than in the

real community. The niche breadth of each species

was therefore increased. The fourth type, on the

other hand, retained most of the original community

structure: if a species ignored food in a particular cat-

egory, then that was left unaffected, but among those

categories where food was eaten, preferences were

reassigned at random. These neutral models were

then compared with their real counterparts in terms

of their patterns of resource use overlap. If competi-

tion is a signiﬁcant force in determining community

structure, then the niches should be spaced out, and

resource use overlap in the real communities should

be less – and statistically signiﬁcantly less – than that

in the neutral models.

The results (Figure 6.19) were that in all commun-

ities, and for all four neutral models, the model mean

overlap was higher than that observed for the real

community, and that in almost all cases this was

statistically signiﬁcant. For these lizard communities,

therefore, the observed low overlaps in resource use

suggest that niches are more segregated than would

be expected by chance alone, and that interspeciﬁc

competition plays an important role in community

structure.

Neutral models of lizard communities

A desert lizard of the southwestern United States.

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Where niche differentiation is manifested as morphological differentiation,

the spacing out of niches can be expected to have its counterpart in regularity

in the degree of morphological difference between species belonging to a guild.

One example is shown in Figure 6.20 for four species of fossil strophomenide

brachiopod (so-called ‘lamp shells’ that resemble bivalve mollusks) that appear

from the fossil record to have coexisted. If successively sized species are

compared, they have a consistent ratio for body outline length of around 1.5.

Moreover, when Hermoyian et al. (2002) built 100,000 null models that each

drew four species at random from the complete strophomenide brachiopod

fossil fauna (74 taxa) and calculated size ratios between adjacent species, they

rejected the null hypothesis that the observed ratios could have arisen from

randomly selected taxa (P < 0.03), supporting the hypothesis that competition

had played a key role in structuring this community.

The null model approach to the analysis of distributional differences involves

comparing the pattern of species co-occurrences at a suite of locations with what

would be expected by chance. An excess of negative associations would then be

consistent with a role for competition in determining community structure. Gotelli

Chapter 6 Interspeciﬁc competition

213

0.9

0.8

0.7

0.6

0.5

0.4

0.3

0.2

0.1

RA1

975468975

0.6

0.5

0.4

0.3

0.2

0.1

RA3

RA2

RA4

Number of species in the community

Mean overlap in resource use, α

Figure 6.19

The mean indices of resource use overlap for each of 10 North American lizard communities are shown as solid circles. These can

be compared, in each case, with the mean (horizontal line), standard deviation (vertical rectangle), and range (vertical line) of mean

overlap values for the corresponding set of 100 randomly constructed communities. The analysis was performed using four different

types of reorganization algorithms (RA1 to RA4).

AFTER LAWLOR, 1980

morphological patterns

negatively associated

distributions

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and McCabe (2002) carried out a ‘meta-analysis’: an analysis of all the analyses of

others that they could ﬁnd (96 data sets in all) that had examined the distribu-

tion of species assemblages across sets of replicated sites. For every real data set,

a ‘checkerboard score’, C, was computed. This is highest when every species-pair

in a community forms a perfect checkerboard: sites are either ‘black’ or ‘white’

– the species never co-occur. It takes its lowest value when all species-pairs always

co-occur. Next, 1000 randomized versions of each data set were simulated and

C calculated each time. The observed C-value for each data set was then expressed

as the number of standard deviations it was, C

, from the mean of the simulations.

The null hypothesis is that C

should be zero (real communities not different from

simulated communities), but in particular that a C

-value greater than 2 indicates

a signiﬁcant negative association between species in the data set. The results,

classiﬁed by taxonomic group, are shown in Figure 6.21. There was a signiﬁcant

excess of negative associations for plants and homeothermic vertebrates and for

ants, but the excess was not signiﬁcant for invertebrates (other than ants), ﬁsh,

amphibians and reptiles.

This kind of pattern – sometimes a role for competition is conﬁrmed, sometimes

not – has been the general conclusion from the neutral model approach. What then

should be our verdict on it? Perhaps most fundamentally, its aim is undoubtedly

worthy. It concentrates the minds of investigators, stopping them from jumping

to conclusions too readily; it is important to guard against the temptation to see

competition in a community simply because we are looking for it. On the other

hand, the approach can never take the place of a detailed understanding of the

ﬁeld ecology of the species in question, or of manipulative experiments designed

to reveal competition by increasing or reducing species abundances. It, like so

many other approaches, can only be part of the community ecologist’s armory.

Part III Individuals, Populations, Communities and Ecosystems

214

Frequency

In SOL (ln mm)

2.521.510.50

Figure 6.20

Distributions of strophomenide body outline length (SOL) of samples

of four coexisting species of brachiopods collected from a late

Ordovician (ca. 448–438 million years before present) marine

sediment in Indiana, USA. The species shown, from left to right, are

Eochonetes clarksvillensis, Leptaena richmondensis, Strophomena

planumbona and Raﬁnesquina alternata.

Figure 6.21

An analysis of data sets of species distributions across sites,

classiﬁed by taxonomic group (mean ± SE) seeking evidence of an

excess of negative associations, as measured by the standardized

‘checkerboard score’ (see text). The dashed line indicates an

effects size of 2.0, which is the approximate 5% signiﬁcance level.

AFTER HERMOYIAN ET AL., 2002AFTER GOTELLI & MCCABE, 2002

Standardized checkerboard score

Birds

Bats

Mammals

Ants

Invertebrates

Fish

Amphibians and

reptiles

Plants

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Chapter 6 Interspeciﬁc competition

215

Ecological effects of interspeciﬁc competition

The essence of interspeciﬁc competition is that indi-

viduals of one species suffer a reduction in fecundity,

survivorship or growth as a result of exploitation of

resources or interference by individuals of another

species.

Species are often excluded by interspeciﬁc competi-

tion from locations at which they could exist perfectly

well in the absence of interspeciﬁc competition.

With exploitation competition, the more suc-

cessful competitor is the one that more effectively

exploits shared resources. Two species exploiting

two resources can compete but still coexist when

each species holds one of the resources at a level

that is too low for effective exploitation by the other

species.

A fundamental niche is the combination of con-

ditions and resources that allow a species to exist

when considered in isolation from any other species.

Whereas its realized niche is the combination of

conditions and resources that allow it to exist in the

presence of other species that might be harmful to its

existence – especially interspeciﬁc competitors.

The Competitive Exclusion Principle states that

if two competing species coexist in a stable environ-

ment, then they do so as a result of differentiation

of their realized niches. If, however, there is no such

differentiation, or if it is precluded by the habitat,

then one competing species will eliminate or exclude

the other. However, whenever we see coexisting

species with different niches it is not reasonable to

jump to the conclusion that this is the principle in

operation.

The only true test for whether competition occurs

between species is to manipulate the abundance

of each competitor and observe the response of its

counterparts.

Environments are usually a patchwork of favor-

able and unfavorable habitats; patches are often

only available temporarily; and patches often appear

at unpredictable times and in unpredictable places.

Under such variable conditions, competition may only

rarely ‘run its course’.

Evolutionary effects of interspeciﬁc

competition

Although species may not be competing now, their

ancestors may have competed in the past. We can

expect species to have evolved characteristics that

ensure that they compete less, or not at all, with mem-

bers of other species. Coexisting present-day com-

petitors, and coexisting species that have evolved an

avoidance of competition, can look, at least super-

ﬁcially, the same.

By invoking something that cannot be observed

directly – ‘the ghost of competition past’ – it is impos-

sible to prove an evolutionary effect of interspeciﬁc

competition. However, careful observational studies

have sometimes revealed patterns that are difﬁcult to

explain in any other way.

Interspeciﬁc competition and community

structure

Interspeciﬁc competition tends to structure com-

munities by acting within guilds – groups of species

exploiting the same class of resource in a similar

fashion.

Niche complementarity can be discerned in some

communities, where coexisting species that occupy

a similar position along one niche dimension tend to

differ along another dimension.

Niches can be differentiated through differential

resource utilization. In many cases, however, differ-

ential resource utilization expresses itself as either a

microhabitat differentiation between the species or

a difference in geographic distribution. Alternatively,

differential resource utilization may express itself as

a temporal separation between species. Niches can

also be differentiated on the basis of conditions. This

too can express itself as either a microhabitat differ-

entiation, or a difference in geographic distribution, or

a temporal separation.

SUMMARY

Summary

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Part III Individuals, Populations, Communities and Ecosystems

216

How signiﬁcant is interspeciﬁc competition

in practice?

Surveys of published studies of competition indicate

that current competition is widespread but these

exaggerate to an unknown extent the true frequency

of competition.

The theory of interspeciﬁc competition predicts that

the niches of competing species should be arranged

regularly rather than randomly in niche space, that as

a reﬂection of this they should be more distinct

morphologically than expected by chance, and that

competitors should be negatively associated in their

distributions. Neutral models have been developed

to determine what the community pattern would

look like in the absence of interspeciﬁc competition.

Real communities are sometimes structured in a

way that makes an inﬂuence of competition difﬁcult

to deny.

REVIEW QUESTIONS

Review questions

Asterisks indicate challenge questions

1 Some experiments concerning interspeciﬁc

competition have monitored both the

population densities of the species involved

and their impact on resources. Why is it

helpful to do both?

2 Interspeciﬁc competition may be a result

of exploitation of resources or of direct

interference. Give an example of each and

compare their consequences for the species

involved.

3 Deﬁne fundamental niche and realized niche.

How do these concepts help us to understand

the effects of competitors?

4 With the help of one plant and one animal

example, explain how two species may coexist

by holding different resources at levels that

are too low for effective exploitation by the

other species.

5* Deﬁne the Competitive Exclusion Principle.

When we see coexisting species with different

niches is it reasonable to conclude that this is

the principle in action?

6 Explain how environmental heterogeneity may

permit an apparently ‘weak’ competitor to

coexist with a species that might be expected

to exclude it.

7* What is the ‘ghost of competition past’?

Why is it impossible to prove an evolutionary

effect of interspeciﬁc competition?

8 Provide one example each of niche

differentiation involving physiological,

morphological and behavioral properties

of coexisting species. How may these

differences have arisen?

9 Deﬁne ‘niche complementarity’ and, with the

help of an example, explain how it may help to

account for the coexistence of many species

in a community.

10* Discuss the pros and cons of the neutral model

approach to evaluating the effects of

competition on community composition.

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217

Chapter 7

Predation, grazing

and disease

CHAPTER CONTENTS

7.1 Introduction

7.2 Prey ﬁtness and abundance

7.3 The subtleties of predation

7.4 Predator behavior: foraging and transmission

7.5 Population dynamics of predation

7.6 Predation and community structure

Chapter contents

KEY CONCEPTS

In this chapter you will:

distinguish the similarities and differences among ‘true predators’,

grazers and parasites

understand the subtleties of predation, including the capacity of prey

to compensate

appreciate the value of the optimal foraging approach for analyzing

predator choices

recognize the underlying tendency of populations of predators and

prey to cycle and the ‘damping’ effect of crowding and patchy

distributions

understand the consequences of predation for community

composition

Key concepts

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7.1 Introduction

Ask most people to name a predator and they are almost certain to say something

like lion, tiger or grizzly bear – something big, ferocious, instantly lethal. However,

from an ecological point of view, a predator may be deﬁned as any organism that

consumes all or part of another living organism (its prey or host) thereby bene-

ﬁting itself, but reducing the growth, fecundity or survival of the prey. Thus, this

deﬁnition extends beyond the likes of lions and tigers by including organisms that

consume all or part of their prey and also those that merely reduce their prey’s

growth, fecundity or survival. Predators are not all large, aggressive or instantly

lethal – they need not even be animals. Here we examine these consumers

together and try to understand the part they play in determining the structure and

dynamics of ecological systems.

Within the broad deﬁnition, three main types of predator can be distinguished.

1 ‘True’ predators:

invariably kill their prey and do so more or less immediately after

attacking them;

consume several or many prey items in the course of their life.

True predators therefore include lions, tigers and grizzly bears, but also

spiders, baleen whales that ﬁlter plankton from the sea, zooplanktonic

animals that consume phytoplankton, birds that eat seeds (each one an

individual organism) and carnivorous plants.

2 Grazers:

attack several or many prey items in the course of their life;

consume only part of each prey item;

do not usually kill their prey, especially in the short term.

Grazers therefore include cattle, sheep and locusts, but also, for example,

blood-sucking leeches that take a small, relatively insigniﬁcant blood meal

from several vertebrate prey over the course of their life.

3 Parasites:

consume only part of each prey item (usually called their host);

do not usually kill their prey, especially in the short term;

attack one or very few prey items in the course of their life, with which

they therefore often form a relatively intimate association.

Part III Individuals, Populations, Communities and Ecosystems

218

predator: a term extending

beyond the obvious examples

‘true’ predators, grazers

and parasites

Every living organism is either a consumer of other living organisms, or is

consumed by other living organisms, or – in the case of most animals – is both.

We cannot hope to understand the structure and dynamics of ecological

populations and communities until we understand the links between

consumers and their prey.

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