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

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in northeast Brazil. However, the applicability of the idea of recolonization

following a genuine extinction is questionable in any plant species that has a

buried seed bank (see Section 5.2.2). In E. paniculata, for instance, the heavy

seeds almost always drop in the immediate vicinity of the parent rather than

being dispersed to other patches. ‘Extinctions’, then, are typically the result of the

catastrophic loss of habitat (note in Figure 9.11 that the chance of extinction has

effectively nothing to do with the previous population size); and ‘recolonizations’

are almost always simply the result of the germination of seeds following habitat

restoration. Recolonization by dispersal, a prerequisite for a true metapopulation,

is extremely rare.

9.4 Temporal patterns in community

composition

9.4.1 Founder-controlled and dominance-controlled

communities

From the perspective of environmental patchiness, the metapopulation concept

is important for our understanding of population dynamics, but when community

organization is the focus of attention we usually refer to the patch dynamics concept.

The concepts are closely related. Both accept that a combination of patchiness and

dispersal between patches can give rise to dynamics quite different from those

that would be observed if there was just one, homogeneous patch.

Disturbances that open up gaps are common in all kinds of community. Gaps are

simply patches within which many species suffer local extinction simultaneously.

In forests, high winds, elephants or simply the death of a tree through old age may

all create gaps. In grassland, agents include frost, burrowing animals and cattle

dung. On rocky shores, gaps may be formed as a result of severe wave action during

hurricanes, battering by moored boats or the action of predators.

Two fundamentally different kinds of community organization can be recognized

(Yodzis, 1986). When all species are good colonists and essentially equal competitors,

communities are described as founder controlled; when some species are strongly

superior competitively, communities can be described as dominance controlled.

The dynamics of the two are quite different, and we deal with them in turn.

In founder-controlled communities, species are approximately equivalent

in their ability to invade gaps and can hold the gaps against all comers during

their lifetime. Hence, the probability of competitive exclusion in the community

as a whole may be much reduced where gaps are appearing continually and

randomly. This can be referred to as a ‘competitive lottery’. On each occasion

that an organism dies (or is killed) a gap is opened for invasion. All conceivable

replacements are possible, and species richness is maintained at a high level

in the system as a whole. For example, three species of ﬁsh co-occur on the

upper slope of Heron Reef, part of the Great Barrier Reef off eastern Australia:

Eupomacentrus apicalis, Plectroglyphidodon lacrymatus and Pomacentrus wardi.

Within rubble patches, the available space is occupied by a series of non-

overlapping territories, which individuals hold throughout their juvenile and

adult life, defending them against individuals of their own and other species.

Chapter 9 From populations to communities

299

disturbances and the patch

dynamics concept of

community organization

founder-controlled communities:

competitive lotteries

9781405156585_4_009.qxd 11/5/07 14:56 Page 299

But there seems to be no particular tendency for space initially held by one species

to be taken up, following mortality, by the same species. Nor is any sequence

of ownership evident (Table 9.4). Pomacentrus wardi both recruited and lost

individuals at a higher rate than the other two species, but all three species appear

to have recruited at a sufﬁcient level to balance their rates of loss and maintain a

resident population of breeding individuals.

Indeed, communities of tropical reef ﬁsh in general may often conform to

the founder-controlled model (Sale & Douglas, 1984). They are extremely rich

in species. The number of ﬁsh species on the Great Barrier Reef ranges from

900 in the south to 1500 in the north, and more than 50 resident species may

be recorded on a single patch of reef 3 m in diameter. Only a proportion of this

species richness is likely to be attributable to resource partitioning of food and space

– indeed the diets of many of the coexisting species are very similar. It is vacant

living space that seems to be a crucial limiting factor, generated unpredictably

in space and time when a resident dies or is killed. The lifestyles of the species

match this state of affairs. They breed often, sometimes year-round, and produce

Part III Individuals, Populations, Communities and Ecosystems

300

Table 9.4

For three species of reef ﬁsh, the numbers of each species observed occupying sites, or parts of sites,

that had been vacated during the immediately prior period between censuses through the loss of residents

of each species. The sites vacated through loss of 120 residents were reoccupied by 131 ﬁsh; the species

of the new occupant is not dependent on the species of the previous resident (χ

= 5.88; P > 0.1).

AFTER SALE, 1979

REOCCUPIED BY:

RESIDENT LOST E. APICALIS P. LACRYMATUS P. WARDI

Eupomacentrus apicalis 9319

Plectroglyphidodon lacrymatus 12 5 9

Pomacentrus wardi 27 18 29

The Great Barrier Reef, Australia.

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numerous clutches of dispersive eggs or larvae. The species compete in a lottery

for living space in which larvae are the tickets, and the ﬁrst arrival at the vacant

space wins the site, matures quickly and holds the space for its lifetime.

In dominance-controlled communities, by contrast, some species are com-

petitively superior to others, and an initial colonizer of a patch cannot necessarily

maintain its presence there. In these cases, disturbances that open up gaps lead

to reasonably predictable sequences of species, because different species have

different strategies for exploiting resources – early species are good colonizers

and fast growers, whereas later species can tolerate lower resource levels and

grow to maturity in the presence of early species, eventually outcompeting them.

Such sequences are examples of community successions. An idealized view of a

succession is shown in Figure 9.12. Open space is colonized by one or more of a

group of opportunistic, early-succession species (p

, p

, etc., in Figure 9.12). As time

passes, more species invade, often those with poorer powers of dispersal. These

eventually reach maturity, dominating mid-succession (m

, m

, etc.) and many

or all of the pioneer species are driven to extinction. Later still, the community

reaches a climax stage when the most efﬁcient competitors (c

, c

, etc.) oust their

neighbors. In this sequence, if it runs its full course, the number of species ﬁrst

increases (because of colonization) then decreases (because of competition).

Some disturbances are synchronized over extensive areas. A forest ﬁre may

destroy a huge tract of a climax community. The whole area then proceeds

through a more or less synchronous succession. Other disturbances are much

smaller and produce a patchwork of habitats. If these disturbances are out of

phase with one another, the resulting community comprises a mosaic of patches

at different stages of succession.

9.4.2 Community succession

If an opened-up gap has not previously been inﬂuenced by a community, the

sequence of species is referred to as a primary succession. Lava ﬂows caused by

volcanic eruptions, substrate exposed by the retreat of a glacier and freshly

Chapter 9 From populations to communities

301

High

Richness

Low

Mid-successional Climax

Soon after a disturbance Long after a disturbance

Pioneer and early successional communities

Time

Figure 9.12

Hypothetical succession in a gap –

an example of dominance control.

The occupancy of gaps is reasonably

predictable. Richness begins at a low

level as a few pioneer (p

) species

arrive; reaches a maximum in

midsuccession when a mixture of

pioneer, mid-successional (m

) and

climax (c

) species occur together;

and drops again as competitive

exclusion by climax species takes

place.

dominance-controlled

communities and community

succession

primary and secondary

successions

9781405156585_4_009.qxd 11/5/07 14:56 Page 301

formed sand dunes are all examples. But where the species of an area has been

partially or completely removed but seeds and spores remain, the subsequent

sequence is termed a secondary succession. The loss of trees locally as a result of

high winds may lead to secondary successions, as can cultivation followed by the

abandonment of farmland (so-called old-ﬁeld successions).

Primary successions often take several hundreds of years to run their course.

However, on recently denuded rocks in the marine subtidal zone a primary

succession may take only a decade or so. The research life of an ecologist is

sufﬁcient to encompass a subtidal succession but not that following glacial retreat.

Fortunately, however, information can sometimes be gained over the longer time

scale. Successional stages in time may be represented by community gradients

in space. The use of historical maps, carbon dating or other techniques may

enable the age of a community since initial exposure to be estimated. A series of

communities currently in existence – a ‘chronosequence’ – can then be inferred

to reﬂect succession.

An extensive chronosequence of dune-capped beach ridges occurs on the coast

of Lake Michigan in the USA. Thirteen ridges of known age (30–440 years old)

show a clear pattern of primary succession to forest. The dune grass Ammophila

breviligulata dominates the youngest, still mobile, dune ridge. Within 100 years,

these are replaced by evergreen shrubs such as Juniperus communis and by prairie

bunch grass Schizachyrium scoparium. Conifers begin colonizing the dune ridges

after 150 years, and a mixed forest of pine species develops between 225 and

400 years. Deciduous trees such as the oak and maple do not become important

components of the forest until 440 years.

Experimental seed addition and seedling transplants have shown that later

species are nonetheless capable of germinating in young dunes (Figure 9.13a).

The more developed soil of older dunes may improve the performance of late-

successional species, but their successful colonization of young dunes is mainly

prevented by limited seed dispersal, together with seed predation by rodents

(Figure 9.13b). Eventually, however, the early species are competitively excluded

as trees establish and grow.

Successions on old ﬁelds have been studied primarily in the eastern United

States, where many farms were abandoned by farmers who moved west after

the frontier was opened up in the 19th century. Most of the pre-colonial mixed

conifer–hardwood forest had been destroyed, but regeneration was swift after

the ‘disturbance’ caused by farmers came to an end. The early pioneers of the

American West left behind exposed land that was colonized by pioneers of a very

different kind. The typical sequence of dominant vegetation is: annual weeds →

herbaceous perennials → shrubs → early successional trees → late successional

trees. A particularly detailed study of old-ﬁeld succession has been performed

at the Cedar Creek Natural History Area in Minnesota on well-drained and

nutrient-poor soil. This study is discussed in detail in Section 1.3.2.

Old-ﬁeld succession has also been studied in the productive Loess Plateau

in China, which for millennia has been affected by human activities so that few

areas of natural vegetation remain. One study examined the vegetation at four

plots abandoned by farmers for known periods of time: 3, 26, 46 and 149 years.

Of a total of 40 plant species identiﬁed, different species were dominant (in

terms of relative abundance and relative ground cover) in the different aged plots

(Figure 9.14). The early-successional species were annuals and biennials with

Part III Individuals, Populations, Communities and Ecosystems

302

a primary succession in

duneland

secondary succession on ﬁelds

abandoned by farmers

9781405156585_4_009.qxd 11/5/07 14:56 Page 302

Chapter 9 From populations to communities

303

Seedling emergence (proportion of viable seed)

40015030

0.5

Dune age (years)

(a)

0.4

0.3

0.2

0.1

Seedling emergence (proportion of viable seed)

PrAb

0.5

Species

(b)

0.4

0.3

0.2

0.1

Ammophila

Schizachyrium

Pinus strobus

Pinus resinosa

Seed predation

No predation

P < 0.0001

Figure 9.13

(a) Seedling emergence (means + SE) from added seeds of species

typical of different successional stages on dunes of four ages.

(b) Seedling emergence of the four species (Ab, Ammophila;

Ss, Schizachyrium; Ps, Pinus strobes; Pr, Pinus resinosa) in the

presence and absence of rodent predators of seeds.

Importance value

0.7

Successional stages (years)

0.6

0.5

0.4

0.3

0.2

0.1

14946263

Artemisia scoparia

Lespedeza davurica

Artemisia gmelinii

Seraria viridis

Stipa bungeana

Bothriochloa ischaemun

Figure 9.14

Variation in the relative importance of six species during an old-ﬁeld

succession on the Loess Plateau in China.

AFTER WANG, 2002 AFTER LICHTER, 2000

9781405156585_4_009.qxd 11/5/07 14:56 Page 303

high seed production. By 26 years, the perennial herb Lespedeza davurica, with

its ability to spread laterally by vegetative means and a well-developed root

system, had replaced Artemisia scoparia. The 46-year-old plot was characterized

by the highest species richness and diverse life history strategies, dominated by

perennial lifestyles. The dominance of the grass Bothriochloa ischaemun at

149 years was related to its perennial nature, ability to spread clonally and high

competitive ability. Unlike the abandoned ﬁelds of the eastern USA, the climax

vegetation of the Loess Plateau appears to be steppe grassland rather than forest.

But as in the idealized succession of Figure 9.12, an initial increase in species

number as a result of colonization and a subsequent decrease as a result of

competition are both apparent.

Early-succession plants have a fugitive lifestyle. Their continued survival depends

on dispersal to other disturbed sites. They cannot persist in competition with later

species, and thus they must grow and consume the available resources rapidly.

High growth and photosynthetic rates are crucial properties of the fugitive. Those

of later successional plants are much lower (Table 9.5).

In contrast to the pioneer annuals, seeds of later successional plants can

germinate in the shade, for example, beneath a forest canopy. They can continue

to grow at these low light intensites, too – quite slowly but faster than the species

they replace (Figure 9.15).

The early colonists among the trees usually have efﬁcient seed dispersal; this

in itself makes them likely to be early on the scene. They are usually precocious

reproducers and are soon ready to leave descendants in new sites elsewhere.

The late colonists are those with larger seeds, poorer dispersal and long juvenile

phases. The contrast is between the lifestyles of the ‘quickly come, quickly gone’

and ‘what I have, I hold’.

Part III Individuals, Populations, Communities and Ecosystems

304

Table 9.5

Some representative photosynthetic rates (mg CO

−2

−1

) of plants in a successional sequence.

Late-successional trees are arranged according to their relative successional position.

AFTER BAZZAZ, 1979

PLANT RATE PLANT RATE

Summer annuals Early-successional trees

Abutilon theophrasti 24 Diospyros virginiana 17

Amaranthus retroﬂexus 26 Juniperus virginiana 10

Ambrosia artemisiifolia 35 Populus deltoides 26

Ambrosia triﬁda 28 Sassafras albidum 11

Chenopodium album 18 Ulmus alata 15

Polygonum pensylvanicum 18

Setaria faberii 38 Late-successional trees

Liriodendron tulipifera 18

Winter annuals Quercus velutina 12

Capsella bursa-pastoris 22 Fraxinus americana 9

Erigeron annuus 22 Quercus alba 4

Erigeron canadensis 20 Quercus rubra 7

Lactuca scariola 20 Aesculus glabra 8

Fagus grandifolia 7

Herbaceous perennials Acer saccharum 6

Aster pilosus 20

early and late successional

species have different properties

9781405156585_4_009.qxd 11/5/07 14:56 Page 304

The fact that plants dominate most of the structure and succession of com-

munities does not mean that the animals always follow the communities that

plants dictate. This will often be the case, of course, because the plants provide

the starting point for all food webs and determine much of the character of the

physical environment in which animals live. But it is also sometimes the animals

that determine the nature of the plant community, for example, through heavy

grazing or trampling (Box 9.4). More often, though, animals are passive followers

of successions among the plants.

Figure 9.12 was described as an idealized succession, and one respect in which

it was idealized was in arriving at a climax community at the end. Do real succes-

sions reach a climax? Some may. The succession of seaweeds on an overturned

boulder may reach a climax in only a few years. Old-ﬁeld successions, on the other

hand, might take 100–300 years to reach a climax, but in that time the probabil-

ities of ﬁre or severe hurricanes, which occur every 70 or so years in New England,

are so high that a process of succession may never go to completion. Bearing in

Chapter 9 From populations to communities

305

AFTER BAZZAZ, 1996

PAR

Early successional

Mid successional

Late successional

Figure 9.15

Idealized light saturation curves

(photosynthetic rate, Ps, plotted

against the quantity of

photosynthetically active radiation,

PAR) for early-, mid-, and late-

successional plants.

animals are often affected by,

but may also affect, plant

successions

the concept of a climax

community

9781405156585_4_009.qxd 11/5/07 14:56 Page 305

mind that forest communities in northern temperate regions, and probably also

in the tropics, are still recovering from the last glaciation, it is questionable

whether the idealized climax vegetation is often reached in nature.

In fact, the perception of whether a climax has been reached, like so much

else in ecology, is likely to be a matter of scale. As mentioned previously, many

successions take place in a mosaic of patches, with each patch, having been

Part III Individuals, Populations, Communities and Ecosystems

306

9.4 TOPICAL ECONCERNS

9.4 Topical ECOncerns

Some endangered animal species are associated

with particular stages of a succession. Their con-

servation then depends on a full understanding of

the successional sequence, and intervention may be

required to maintain their habitat at an appropriate

successional stage.

An intriguing example is provided by a giant New

Zealand insect, the weta Deinacrida mahoenuiensis

(Orthoptera, Anostostomatidae). This species, which

is believed to have been formerly widespread in forest

habitat, was discovered in the 1970s in an isolated

patch of gorse (Ulex europaeus). Ironically, in New

Zealand gorse is an introduced weed that farmers

spend much time and effort attempting to control.

However, its dense, prickly sward provides a refuge

for the giant weta against other introduced pests,

A giant weta on a gorse branch.

COURTESY OF GREG SHERLEY, DEPARTMENT OF CONSERVATION, WELLINGTON, NEW ZEALAND

particularly rats but also hedgehogs, stoats and

possums, which could readily capture wetas in their

original forest home. Mammalian predation is believed

to be responsible for weta extinction elsewhere.

New Zealand’s Department of Conservation pur-

chased this important patch of gorse from the

landowner, who insisted that his cattle should still be

permitted to overwinter in the reserve. Conservationists

were unhappy about this, but the cattle subsequently

proved to be part of the weta’s salvation. By opening

up paths through the gorse, cattle provided entry for

feral goats that browse the gorse, producing a dense

hedge-like sward and preventing the habitat from

succeeding to a stage inappropriate to the wetas.

This story involves a single, endangered, endemic

insect together with a whole suite of introduced pests

(gorse, rats, goats, etc.) and introduced domestic

animals (cattle). Before the arrival of people in New

Zealand, the island’s only land mammals were bats,

and New Zealand’s endemic fauna has proved to be

extraordinarily vulnerable to the mammals that arrived

with people. However, by maintaining gorse succes-

sion at an early stage, the grazing goats provide a

habitat in which the wetas can escape the attentions

of rats and other predators.

Because of its economic cost to farmers, ecologists

have been trying to ﬁnd an appropriate biological con-

trol agent for gorse, ideally one that would eradicate it.

How would you weigh up the needs of a rare insect

against the economic losses associated with gorse

on farms?

Conservation sometimes requires manipulation of a succession

successions in a patchwork – the

size and shape of gaps

9781405156585_4_009.qxd 11/5/07 14:56 Page 306

disturbed independently, at a different successional stage. Boulders on a rocky

shore are a good example. Climax communities in such cases can then only occur,

at best, on a very local scale. Moreover, when successions occur in a patchwork,

the nature of the succession, both locally and overall, is likely to depend on the

size and shape of the patches (gaps). The centers of very large gaps are most

likely to be colonized by species producing propagules that travel relatively great

distances. Such mobility is less important in small gaps, since most recolonization

will be by propagules from, or simply lateral movement by, established individuals

around the periphery.

Intertidal beds of mussels provide excellent opportunities to study the pro-

cesses of formation and ﬁlling-in of gaps. In the absence of disturbance, mussel beds

may persist as extensive monocultures. More often, they are an ever-changing

mosaic of many species that inhabit gaps formed by the action of waves. The

size of these gaps at the time of formation ranges from a single mussel space to

hundreds of square meters. Gaps begin to ﬁll as soon as they are formed. An

experimental study of mussel beds of Brachidontes solisianus and B. darwinianus

in Brazil aimed to determine the effects of patch size and location within a patch

on the dynamics of succession (Figure 9.16).

High densities of the limpet Collisella subrugosa occurred in the smallest

gaps in the ﬁrst 6 months after gap formation, but not in medium or large gaps

(Figure 9.16a). It was also much quicker to colonize the periphery than the center

of the large gaps (Figure 9.16b). This association of the limpets with patch edges

(and hence small patches) probably occurs because they are less vulnerable there to

visually hunting predators. Small gaps were also most quickly colonized by lateral

migration of the two mussel species (Figure 9.16a), but from around 6 months,

B. darwinianus increasingly predominated and also built up its numbers in the

medium and large gaps. In the absence of further disturbance, B. darwinianus

would seem likely to outcompete B. solisianus. After around 6 months, too, the

Brachiodontes mussels, which cannot be identiﬁed to species when they are small,

recruited signiﬁcantly from settled larvae in the central areas of the large gaps

(Figure 9.16b). Finally, the barnacle Chthamalus bisinuatus also recruited from

settled larvae, largely as a pulse after around 6 months, especially in the largest

gaps (Figure 9.16a) and more in the center than at the periphery of the large gaps

(Figure 9.16b).

Thus, the smaller the gap, the more the succession within it was dominated by

lateral movement than by true migration, and even within a large gap, succession

proceeded differently at the center and at the periphery. On the shore as a whole,

as in any patchy and disturbed habitat, there was a mosaic of patches in different

successional states – those states being determined by patch size, the time since

the last disturbance and even on location within a patch.

9.5 Food webs

No predator–prey, parasite–host or grazer–plant pair exists in isolation. Each is part

of a complex web of interactions with other predators, parasites, food sources and

competitors within its community. Ultimately, it is these food webs that ecologists

wish to understand. However, it has been useful to isolate groups of competitors

as we did in Chapter 6, of predator–prey and parasite–host pairs as in Chapter 7,

Chapter 9 From populations to communities

307

9781405156585_4_009.qxd 11/5/07 14:56 Page 307

and of mutualists as in Chapter 8, simply because we have little or no hope of

understanding the whole unless we have some understanding of the component

parts. Toward the end of Chapter 7 (Section 7.6), our ﬁeld of view was expanded

to include the effects of predators on groups of competitors and to show, for

example, the importance of predator-mediated coexistence.

Part III Individuals, Populations, Communities and Ecosystems

308

Chthamalus bisinuatus

Collisella subrugosa

0.2

0.4

0.6

Percent coverPercent cover

Brachidontes solisianus

1994

Brachidontes darwinianus

SepJulyJanSep Nov Mar May

1995

Chthamalus bisinuatus

0.1

0.2

0.3

0.1

0.2

0.3

0.4

Collisella subrugosa

(a) (b)

Date

1994

SepJulyJanSep Nov Mar May

1995

Date

Density (individuals cm

–2

)Density (individuals cm

–2

)

Density (individuals cm

–2

)Density (individuals cm

–2

) Density (individuals cm

–2

)

Brachidontes recruits

Small

Medium

Large

Center

Periphery

Figure 9.16

(a) Mean abundances (± SE) of four colonizing species in experimentally cleared small, medium and large gaps in intertidal mussel beds.

(b) Recruitment of three species at the periphery (within 5 cm of the gap edge) and in the centre of 400 cm

square gaps.

AFTER TANAKA & MAGALHAES, 2002

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