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

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and so forth) in all communities, since this will depend on the inherent stability of

the community combined with the variability of the environment. One study tend-

ing to support this investigated 10 small streams in New Zealand that differ in the

intensity and frequency of ﬂow-related disturbances to their beds (Figure 9.26).

Food webs in the more disturbed, ‘unstable’ streams were characterized by less

complex communities: fewer species and fewer links between species.

Chapter 9 From populations to communities

319

0.4

0.8

0.6

0.2

0.4

0.8

0.6

0.2

0.4

0.8

0.6

0.2

0.4

0.8

0.6

0.2

0.80.40 0.60.2 0.80.40 0.60.2 0.80.40 0.60.2 0.80.40 0.60.2

Least connected species removed firstRandom species removalMost connected species removed first

Cumulative secondary extinctions / S

Species removed / S

Grassland

(S = 61)

Scotch Broom

(S = 85)

Ythan 1

(S = 124)

Ythan 2

(S = 83)

El Verde

(S = 155)

Canton

(S = 102)

Stony

(S = 109)

Chesapeake

(S = 31)

St Marks

(S = 48)

St Martin

(S = 42)

Little Rock

(S = 92)

Lake Tahoe

(S = 172)

Mirror

(S = 172)

Bridge Brook

(S = 25)

Chachella

(S = 29)

Skipwith

(S = 25)

Figure 9.25

The results of a simulation study. The effect of sequential species removal on the number of consequential (secondary) species extinctions, as a

proportion of the total number of species originally in the web, S, for each of 16 previously described food webs. The three different rules for

species removal are described in the lower panel. Robustness of the webs (the tendency not to suffer secondary extinctions) was usually lowest

when the most connected species were removed ﬁrst and highest when the least connected were removed ﬁrst.

AFTER DUNNE ET AL., 2002

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Part III Individuals, Populations, Communities and Ecosystems

320

120

100

020406080100

Intensity of disturbance

(a)

(b)

Web size (no. of species)Mean number of feeding links

Figure 9.26

In New Zealand streams, less disturbed sites support more

‘complex’ communities, with (a) more species (greater web size)

and (b) greater connectance between species. The average number

of feeding links per animal species (number of prey species in the

diet) increases with the intensity of ﬂow-related disturbances to

the streambed.

AFTER TOWNSEND ET AL., 1998

Multiple determinants of the dynamics of

populations

To understand the factors responsible for the popula-

tion dynamics of even a single species in a single

location, it is necessary to have a knowledge of

physicochemical conditions, available resources, the

organism’s life cycle, and the inﬂuence of competi-

tors, predators and parasites on rates of birth, death,

immigration and emigration.

There are contrasting theories to explain the abund-

ance of populations. At one extreme, researchers

emphasize the apparent stability of populations and

SUMMARY

Summary

This line of argument, moreover, carries a further, very important implication

for the likely effects of unnatural perturbations caused by humans on communities.

We might expect these to have their most profound effects on the dynamically

fragile, complex communities of stable environments, which are relatively

unaccustomed to perturbations, and least effect on the simple, robust communities

of variable environments, which have previously been subjected to repeated

(albeit natural) perturbations.

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Chapter 9 From populations to communities

321

point to the importance of forces that stabilize (density-

dependent factors). At the other extreme, those who

place more emphasis on density ﬂuctuations may

look at external (often density-independent) factors

to explain the changes. Key factor analysis is a

technique that can be applied to life table studies to

throw light both on determination and on regulation of

abundance.

Dispersal, patches and metapopulation

dynamics

Movement can be a vital factor in determining and/or

regulating abundance. A radical change in the way

ecologists think about populations has involved

focusing attention less on processes occurring within

populations and more on patchiness, the colonization

and extinction of subpopulations within an overall meta-

population, and dispersal between subpopulations.

Temporal patterns in community composition

Disturbances that open up gaps (patches) are com-

mon in all kinds of community. Founder-controlled

communities are those in which all species are

approximately equivalent in their ability to invade gaps

and are equal competitors that can hold the gaps

against all comers during their lifetime. Dominance-

controlled communities are those in which some

species are competitively superior to others so that

an initial colonizer of a patch cannot necessarily main-

tain its presence there.

The phenomenon of dominance control is respon-

sible for many examples of community succession.

Primary successions occur in habitats where no

seeds or spores remain from previous occupants

of the site: all colonization must be from outside the

patch. Secondary successions occur when existing

communities are disturbed but some at least of their

seed, etc. remain. It can be very difﬁcult to identify

when a succession reaches a stable climax com-

munity, since this may take centuries to achieve and in

the meantime further disturbances are likely to occur.

The exact nature of the colonization process in an

empty patch depends on the size and location of that

patch. Many communities are mosaics of patches at

different stages in a succession.

Food webs

No predator–prey, parasite–host or grazer–plant pair

exists in isolation. Each is part of a complex food web

involving other predators, parasites, food sources

and competitors within the various trophic levels of a

community.

The effect of one species on another (its herbivor-

ous prey) may be direct and straightforward. But

indirect effects may also be felt by any of the myriad

species linked more remotely in the food web. One of

the most common is a ‘trophic cascade’, in which,

say, a predator reduces the abundance of a herbivore,

thus increasing the abundance of plants.

Top-down control of a food web occurs in situ-

ations in which the structure (abundance, species

number) of lower trophic levels depends on the effects

of consumers from higher trophic levels. Bottom-up

control, on the other hand, occurs in a community

structure dependent on factors, such as nutrient

concentration and prey availability, that inﬂuence

a trophic level from below. The relative importance

of these forces varies according to the trophic level

under investigation and the number of trophic levels

present.

Some species are more tightly woven into the food

web than others. A species whose removal would

produce a signiﬁcant effect (extinction or a large

change in density) in at least one other species may

be thought of as a strong interactor. Removal of some

strong interactors leads to signiﬁcant changes that

spread throughout the food web; we refer to these as

keystone species.

The relationship between food web complexity and

stability is uncertain (and care is needed in deciding

what is meant by stability). Mathematical and empir-

ical studies agree in suggesting that, if anything,

population stability decreases with complexity, whereas

the stability of aggregate properties of whole com-

munities increases with complexity, especially species

richness.

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Part III Individuals, Populations, Communities and Ecosystems

322

REVIEW QUESTIONS

Review questions

Asterisks indicate challenge questions

1* Construct a ﬂow diagram (boxes and arrows)

with a named population at its center to

illustrate the wide range of abiotic and biotic

factors that inﬂuence its pattern of abundance.

2 Population census data can be used to

establish correlations between abundance and

external factors such as weather. Why can such

correlations not be used to prove a causal

relationship that explains the dynamics of the

population?

3 Distinguish between the determination and

regulation of population abundance.

4* Imagine a number of species with patchy

distributions: a plant, an insect and a mammal

– or consider examples of such species with

which you are familiar. How would you identify

‘habitable patches’ of these species that are

not currently occupied by them?

5 What is meant by a ‘metapopulation’ and how

does it differ from a simple ‘population’?

6 Deﬁne founder control and dominance control

as they apply to community organization.

In a mosaic of habitat patches, how would

you expect communities to differ if they were

dominated by founder or dominance control?

7 What factors are responsible for changes

in species composition during an old-ﬁeld

succession?

8* Draw up a food web of, say, six or seven

species with which you are familiar and

which spans at least three trophic levels.

Take each species in turn and suggest the

kind of community organization that would be

necessary for this to be a keystone species.

9 What are meant by bottom-up and top-down

control? How is the importance of each likely

to vary with the number of trophic levels in a

community?

10 Discuss what is understood about the

relationship between the complexity and

stability of food webs.

9781405156585_4_009.qxd 11/5/07 14:56 Page 322

323

Chapter 10

Patterns in species

richness

CHAPTER CONTENTS

10.1 Introduction

10.2 A simple model of species richness

10.3 Spatially varying factors that inﬂuence species richness

10.4 Temporally varying factors that inﬂuence species richness

10.5 Gradients of species richness

10.6 Patterns in taxon richness in the fossil record

10.7 Appraisal of patterns in species richness

Chapter contents

KEY CONCEPTS

In this chapter you will:

understand the meanings of species richness, diversity indices and

rank–abundance diagrams

appreciate that species richness is limited by available resources, the

average portion of the resources used by each species (niche breadth)

and the degree of overlap in resource use

recognize that species richness may be highest at intermediate levels of

productivity, predation intensity or disturbance but tends to increase

with spatial heterogeneity

appreciate the importance of habitat area and remoteness in

determining richness, especially with reference to the equilibrium

theory of island biogeography

understand richness gradients with latitude, altitude and depth, and

during community succession, and the difﬁculties of explaining them

appreciate how theories of species richness can also be applied to the

fossil record

Key concepts

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10.1 Introduction

Why the number of species varies from place to place, and from time to time,

are questions that present themselves not only to ecologists but to anybody

who observes and ponders the natural world. They are interesting questions in

their own right – but they are also questions of practical importance. It is clear

that if we wish to conserve or restore the planet’s biological diversity, then we

must understand how species numbers are determined and how it comes about

that they vary. We will see that there are plausible answers to the questions we

ask, but these answers are not always clearcut. Yet this is not so much a dis-

appointment as a challenge to ecologists of the future. Much of the fascination

of ecology lies in the fact that many of the problems are blatant, whereas the

solutions can be difﬁcult to ﬁnd. We will see that a full understanding of patterns

in species richness must draw on our knowledge of all the areas of ecology dis-

cussed so far in this book.

The number of species in a community is referred to as its species richness.

Counting or listing the species present in a community may sound a straight-

forward procedure, but in practice it is often surprisingly difﬁcult, partly because

of taxonomic inadequacies, but also because only a proportion of the organisms

in an area can usually be counted. The number of species recorded then depends

on the number of samples that have been taken, or on the volume of the habitat

that has been explored. The most common species are likely to be represented in

the ﬁrst few samples, and as more samples are taken, rarer species will be added

to the list. At what point does one cease to take further samples? Ideally, the

investigator should continue to sample until the number of species reaches a

plateau. At the very least, the species richness of different communities should

be compared only if they are based on the same sample sizes (in terms of area of

habitat explored, time devoted to sampling or, best of all, number of individuals

included in the samples).

An important aspect of the structure of a community is completely ignored,

though, when its composition is described simply in terms of the number of species

present – namely, that some species are rare and others common. Intuitively, a

community of 10 species with equal numbers in each seems more diverse than

another, again consisting of 10 species, with 91% of the individuals belonging to

the most common species and just 1% in each of the other nine species. Yet, each

community has the same species richness. Diversity indices are designed to com-

bine both species richness and the evenness or equitability of the distribution

Part III Individuals, Populations, Communities and Ecosystems

324

determining species richness

diversity indices and

rank–abundance diagrams

An accurate appreciation of the world’s biological diversity is becoming

increasingly important. For our conservation efforts to be effective we must

understand why species richness varies widely across the face of the Earth. Why

do some communities contain more species than others? Are there patterns or

gradients in this biodiversity? If so, what are the reasons for these patterns?

9781405156585_4_010.qxd 11/5/07 14:58 Page 324

of individuals among those species (Box 10.1). Moreover, attempts to describe

a complex community structure by one single attribute, such as richness, or even

diversity, can still be criticized because so much valuable information is lost.

A more complete picture of the distribution of species abundance in a community

is therefore sometimes provided in a rank–abundance diagram (Box 10.1).

Chapter 10 Patterns in species richness

325

1860 1900

Year

1940

Control

Fertilized

Species diversity (H)

10.1 QUANTITATIVE ASPECTS

10.1 Quantitative aspects

The measure of the character of a community that

is most commonly used to take into account both

species richness and the relative abundances of those

species is known as the Shannon or the Shannon–

Weaver diversity index (denoted by H). This is calculated

by determining, for each species, the proportion of

individuals or biomass (P

for the ith species) that that

species contributes to the total in the sample. Then,

if S is the total number of species in the community

(i.e. the richness), diversity (H) is:

H =−∑P

ln P

where the summation sign ∑ indicates that the

product (P

ln P

) is calculated for each of the S species

in turn and these products are then summed. As

required, the value of the index depends on both

the species richness and the evenness (equitability)

with which individuals are distributed among the

species. Thus, for a given richness, H increases with

equitability, and for a given equitability, H increases

with richness.

An example of an analysis using diversity indices is

provided by the unusually long-term study that com-

menced in 1856 in an area of pasture at Rothamsted

in England. Experimental plots received a fertilizer

treatment once every year, and control plots did not.

Figure 10.1 shows how species diversity (H) of grass

species changed between 1856 and 1949. While the

unfertilized area remained essentially unchanged,

the fertilized area showed a progressive decline in

diversity. This ‘paradox of enrichment’ is discussed

in Section 10.3.1.

Rank–abundance diagrams, on the other hand,

make use of the full array of P

values by plotting P

against ‘rank’; i.e. the most abundant species takes

rank 1, the second most abundant rank 2, and so

on, until the array is completed by the rarest species

of all. The steeper the slope of a rank–abundance

diagram, the greater the dominance of common

species over rare species in the community (a steep

slope means a sharp drop in relative abundance,

, for a given drop in rank). Thus, in the case of the

Rothamsted experiment, Figure 10.2 shows how the

dominance of commoner species steadily increased

(steeper slope) while species richness decreased

over time.

Diversity indices and rank–abundance diagrams

Figure 10.1

Species diversity (H) declined progressively in a plot of

pasture that regularly received fertilizer in an experiment

commencing in 1856 at Rothamsted in England. In

contrast, species diversity remained constant in a

control plot that received no fertilizer.

AFTER TILMAN, 1982

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Nonetheless, for many purposes, the simplest measure, species richness,

sufﬁces. In the following sections, therefore, we examine the relationships between

species richness and a variety of factors that may, in theory, inﬂuence richness in

ecological communities. It will become clear that it is not always easy to come up

with unambiguous predictions and clean tests of hypotheses when dealing with

something as complex as a community.

10.2 A simple model of species richness

To try to understand the determinants of species richness, it will be useful

to begin with a simple model (Figure 10.3). Assume, for simplicity, that the

resources available to a community can be depicted as a one-dimensional con-

tinuum, R units long. Each species uses only a portion of this resource continuum,

and these portions deﬁne the niche breadths (n) of the various species: the

average niche breadth within the community is n¯. Some of these niches overlap,

and the overlap between adjacent species can be measured by value o. The

average niche overlap within the community is then o¯. With this simple back-

ground, it is possible to consider why some communities should contain more

species than others.

First, for given values of n¯ and o¯, a community will contain more species

the larger the value of R, i.e. the greater the range of resources (Figure 10.3a).

Second, for a given range of resources, more species will be accommodated

if n¯ is smaller, i.e. if the species are more specialized in their use of resources

(Figure 10.3b). Alternatively, if species overlap to a greater extent in their use of

resources (greater o¯), then more may coexist along the same resource continuum

(Figure 10.3c). Finally, a community will contain more species the more fully

saturated it is; conversely, it will contain fewer species when more of the resource

continuum is unexploited (Figure 10.3d).

Part III Individuals, Populations, Communities and Ecosystems

326

1.0

1949

1919

1903

1872 1862

1856

Relative abundance

Species rank

–1

–2

–3

–4

Figure 10.2

Change in the rank–abundance pattern of plant

species in the Rothamstead fertilized plot from 1856

to 1949. Note how the slope of the regression line

becomes progressively steeper with time since

commencement of fertilizer addition. A steeper plot

indicates that the commoner species comprise a

greater proportion of the total community – in other

words, this pasture community gradually became

dominated by just a few species.

AFTER TOKESHI, 1993

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We can now consider the relationship between this model and two important

kinds of species interactions described in previous chapters: interspeciﬁc com-

petition and predation. If a community is dominated by interspeciﬁc competition

(see Chapter 6), the resources are likely to be fully exploited. Species richness

will then depend on the range of available resources, the extent to which species

are specialists and the permitted extent of niche overlap (Figure 10.3a–c). We

will examine a range of inﬂuences on each of these three.

Predation, on the other hand, is capable of exerting contrasting effects (see

Chapter 7). First, we know that predators can exclude certain prey species; in

the absence of these species the community may then be less than fully saturated,

in the sense that some available resources may be unexploited (Figure 10.3d). In

this way, predation may reduce species richness. Second though, predation may

tend to keep species below their carrying capacities for much of the time, reducing

the intensity and importance of direct interspeciﬁc competition for resources.

This may then permit much more niche overlap and a greater richness of species

than in a community dominated by competition (Figure 10.3c).

The next two sections examine a variety of factors that inﬂuence species richness.

To organize these, Section 10.3 focuses on factors that often vary spatially (from

place to place): productivity, predation intensity, spatial heterogeneity and envir-

onmental ‘harshness’. Section 10.4 then focuses on factors reﬂecting temporal

variation: climatic variation, disturbance and evolutionary age.

Chapter 10 Patterns in species richness

327

(a)

(b)

(c)

(d)

More species because

greater range of resources

(larger R

)

More species because

each is more specialized

(smaller n

)

More species because

each overlaps more with

its neighbors (larger o)

More species because

resource axis is more fully

exploited (community

more fully saturated)

Figure 10.3

A simple model of species richness.

Each species utilizes a portion n

of the available resources (R),

overlapping with adjacent species

by an amount o. More species may

occur in one community than in

another because: (a) a greater range

of resources is present (larger R),

(b) each species is more specialized

(smaller average n), (c) each species

overlaps more with its neighbors

(larger average o), or (d) the

resource dimension is more fully

exploited.

AFTER MACARTHUR, 1972

competition and predation may

inﬂuence species richness

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10.3 Spatially varying factors that inﬂuence

species richness

10.3.1 Productivity and resource richness

For plants, the productivity of the environment can depend on whichever nutri-

ent or condition is most limiting to growth (dealt with in detail in Chapter 11).

Broadly speaking, the productivity of the environment for animals follows the

same trends as for plants, mainly as a result of the changes in resource levels at

the base of the food chain.

If higher productivity is correlated with a wider range of available resources, then

this is likely to lead to an increase in species richness (Figure 10.3a). However,

a more productive environment may have a higher rate of supply of resources

but not a greater variety of resources. This might lead to more individuals per

species rather than more species. Alternatively again, it is possible, even if the

overall variety of resources is unaffected, that rare resources in an unproductive

environment may become abundant enough in a productive environment for

extra species to be added, because more specialized species can be accommodated

(Figure 10.3b).

In general, though, we might expect species richness to increase with

productivity – a contention that is supported by an analysis of the species

richness of trees in North America in relation to a crude measure of available

environmental energy, potential evapotranspiration (PET). This is the amount

of water that under prevailing conditions would evaporate or be transpired from

a saturated surface (Figure 10.4a). However, while energy (heat and light) is

Part III Individuals, Populations, Communities and Ecosystems

328

increased productivity might be

expected to lead to increased

richness...

. . . and often does

160

120

0 600 1200 1800

(a)

(b)

600

100

200

300

400

500

200

400

1400

600

800

1000

1200

Species richness

Tree species richness

Potential evapotranspiration (mm yr

–1

)

Rainfall (mm yr

–1

)

Potential evapotranspiration (mm yr

–1

)

Figure 10.4

(a) Species richness of trees in North America (north of the Mexican border) in relation to potential

evapotranspiration. For this analysis the continent was divided into 336 quadrats following lines of latitude

and longitude. (b) Species richness of southern African trees (each dot represents a 25,000 km

map

quadrat) in relation to both rainfall and potential evapotranspiration. The three-dimensional surface

describes the regression relationship of species richness with rainfall and potential evapotranspiration.

The surface is divided into zones of increasing depth of color representing increasing species richness.

(a) AFTER CURRIE & PAQUIN, 1987; CURRIE, 1991; (b) DATA FROM O’BRIEN, 1993; AFTER WHITTAKER ET AL., 2003

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