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

Подождите немного. Документ загружается.

Germany) were disturbed at intervals of 2–12 days by disrupting the normal

stratiﬁcation in the water column with bubbles of compressed air. Again, both

species richness and Shannon’s diversity index were highest at intermediate

frequencies of disturbance (Figure 10.11c).

10.4.3 Environmental age: evolutionary time

It has also often been suggested that communities that are ‘disturbed’ only on very

extended time scales may nonetheless lack species because they have yet to reach

an ecological or an evolutionary equilibrium. Thus communities may differ in

species richness because some are closer to equilibrium and are therefore more

saturated than others (see Figure10.3d).

For example, many have argued that the tropics are richer in species than

temperate regions at least in part because the tropics have existed over long and

uninterrupted periods of evolutionary time, whereas the temperate regions are

still recovering from the Pleistocene glaciations when temperate biotic zones

shifted in the direction of the tropics. It now seems, however, that tropical areas

were also disturbed during the ice ages, not directly by ice but by associated

climatic changes that saw tropical forest contracting to a limited number of small

refuges surrounded by grassland. Thus, although it seems likely that some com-

munities, by virtue of disturbances in their distant past, are less saturated than

others, we cannot pinpoint these communities with conﬁdence.

Chapter 10 Patterns in species richness

339

Disturbance interval (days)

Frequency of disturbance Intensity of disturbance (mean percentage of bed moved)

Species richness

(a)

(b)

(c)

Mean species richness

F R 0 20406080I

2 6 10 und. 2 6 10 und.

3.0

2.5

2.0

1.0

1.5

Diversity index

Figure 10.11

(a) Pattern in species richness

(± SE) on rocky-shore boulders in

each of three classes categorized

according to the frequency with

which they are disturbed: frequently

disturbed (F), disturbed at an

intermediate rate (I), and rarely

disturbed (R). Species richness is

highest at the intermediate level

of disturbance. (b) Relationship

between insect species richness

and the intensity of disturbance,

measured as the average percentage

of the bed that moves during

successive 2-month periods, in each

of 54 stream sites in the Taieri River,

New Zealand. Species richness is

again highest at intermediate levels

of disturbance. (c) Both species

diversity (Shannon index) and

species richness of phytoplankton

communities are highest at

intermediate frequencies of

disturbance in controlled ﬁeld

experiments in Lake Plußsee in north

Germany. ‘und’ represents species

richness in the undisturbed state.

(a) AFTER SOUSA, 1979B; (b) AFTER TOWNSEND ET AL., 1997; (c) AFTER FLÖDDER & SOMMER, 1999

9781405156585_4_010.qxd 11/5/07 14:58 Page 339

An alternative explanation for lower species richness in temperate than tropical

areas invokes the idea that species evolve faster in the tropics because of higher rates

of mutation in these warmer climes. Wright et al. (2006) compared the rates of evolu-

tion of pairs of woody plant species, one each from tropical areas (e.g. Eucalyptus

deglupta, Clematis javana, Banksia dentate and 42 others) and temperate areas

(Eucalyptus coccifera, Clematis paniculata, Banksia marginata, etc., respectively).

Evolution, as assessed by the rate of nucleotide substitution in a particular region

of DNA, turns out to be more than twice as fast in the tropical species.

10.5 Gradients of species richness

Sections 10.3 and 10.4 have demonstrated how difﬁcult explanations for varia-

tions in species richness are to formulate and test. It is easier to describe patterns,

especially gradients, in species richness. These are discussed next. Explanations

for these, too, however, are often very uncertain.

10.5.1 Habitat area and remoteness: island

biogeography

It is well established that the number of species on islands decreases as island area

decreases. Such a species–area relationship is shown in Figure 10.12a for plants

on small islands east of Stockholm, Sweden.

‘Islands’, however, need not be islands of land in a sea of water. Lakes are

islands in a ‘sea’ of land, mountaintops are high-altitude islands in a low-altitude

ocean, gaps in a forest canopy where a tree has fallen are islands in a sea of trees,

Part III Individuals, Populations, Communities and Ecosystems

340

0.5

200

Species richness

(a)

Island area (ha)

160

120

1.2

1.0

0.8

0.6

0.4

0.2

50201051 3 0.01

100

10.00.1 1.0

(b)

Species richness

34210

Log number of species

(c)

10 100 1000 10,000 100,000

(d)

Species richness

Lake surface area (km

)

Area of source pool (m

)Log cave area (m

)

Figure 10.12

Species–area relationships: in

each case the number of species

increases with ‘island’ area. (a) For

plants on small islands off the east

coast of Sweden in 1999. (b) For

birds inhabiting lakes (‘islands’ of

water in a ‘sea’ of land) in Florida.

caves in Mexico. (d) For ﬁsh living in

Australian desert springs connected

to pools of different sizes. All

regression lines are signiﬁcant at

P < 0.05; no line is shown in (b)

because the regression is not

signiﬁcant.

(a) AFTER LOFGREN & JERLING, 2002; (b) AFTER HOYER

& CANFIELD, 1994; (c) AFTER BRUNET & MEDELLÍN,

2001; (d) AFTER KODRIC-BROWN & BROWN, 1993

species–area relationships on

oceanic islands

habitat islands and areas of

mainland

9781405156585_4_010.qxd 11/5/07 14:58 Page 340

and there can be islands of particular geological types, soil types or vegetation types

surrounded by dissimilar types of rock, soil or vegetation. Species–area relationships

can be equally apparent for these types of islands (Figure 10.12b–d).

The relationship between species richness and habitat area is one of the most

consistent of all ecological patterns. However, the pattern raises an important

question: ‘Is the impoverishment of species on islands more than would be

expected in comparably small areas of mainland?’ In other words, does the char-

acteristic isolation of islands contribute to their impoverishment of species? These

are important questions for an understanding of community structure since there

are many oceanic islands, many lakes, many mountaintops, many woodlands

surrounded by ﬁelds, many isolated trees and so on.

Probably the most obvious reason why larger areas should contain more species

is that larger areas typically encompass more different types of habitat. However,

MacArthur and Wilson (1967) believed this explanation to be too simple. In their

equilibrium theory of island biogeography they argued that island size and isola-

tion themselves played important roles: that the number of species on an island

is determined by a balance between immigration and extinction; that this balance

is dynamic, with species continually going extinct and being replaced (through

immigration) by the same or by different species; and that immigration and

extinction rates may vary with island size and isolation (Box 10.3).

Chapter 10 Patterns in species richness

341

‘island effects’ and community

structure

10.3 HISTORICAL LANDMARKS

10.3 Historical landmarks

Taking immigration ﬁrst, imagine an island that as

yet contains no species at all. The rate of immigra-

tion of species will be high, because any colonizing

individual represents a species new to that island.

However, as the number of resident species rises,

the rate of immigration of new, unrepresented species

diminishes. The immigration rate reaches zero when

all species from the source pool (i.e. from the main-

land or from other nearby islands) are present on the

island in question (Figure 10.13a).

The immigration graph is drawn as a curve,

because immigration rate is likely to be particularly

high when there are low numbers of residents and

many of the species with the greatest powers of dis-

persal are yet to arrive. In fact, the curve should really

be a blur rather than a single line, since the precise

curve will depend on the exact sequence in which

species arrive, and this will vary by chance. In this

sense, the immigration curve can be thought of as the

‘most probable’ curve.

The exact immigration curve will depend on the

degree of remoteness of the island from its pool of

potential colonizers (Figure 10.13a). The curve will

always reach zero at the same point (when all mem-

bers of the pool are resident), but it will generally have

higher values on islands close to the source of immi-

gration than on more remote islands, since colonizers

have a greater chance of reaching an island the closer

it is to the source. It is also likely that immigration rates

will generally be higher on a large island than on a

small island, since the larger island represents a larger

‘target’ for the colonizers (Figure 10.13a).

The rate of species extinction on an island

(Figure 10.13b) is bound to be zero when there are no

species there, and it will generally be low when there

are few species. However, as the number of resident

MacArthur and Wilson’s equilibrium theory of island biogeography

9781405156585_4_010.qxd 11/5/07 14:58 Page 341

MacArthur and Wilson’s theory makes several predictions:

1 The number of species on an island should eventually become roughly

constant through time.

2 This should be a result of a continual turnover of species, with some

becoming extinct and others immigrating.

3 Large islands should support more species than small islands.

4 Species number should decline with the increasing remoteness of an island.

On the other hand, a higher richness on larger islands would be expected simply

as a consequence of larger islands having more habitat types. Does richness increase

with area at a rate greater than could be accounted for by increases in habitat

Part III Individuals, Populations, Communities and Ecosystems

342

species rises, the extinction rate is assumed by the

theory to increase, probably at a more than proportion-

ate rate. This is thought to occur because with more

species, competitive exclusion becomes more likely,

and the population size of each species is on average

smaller, making it more vulnerable to chance extinc-

tion. Similar reasoning suggests that extinction rates

should be higher on small than on large islands –

population sizes will typically be smaller on small islands

(Figure 10.13b). As with immigration, the extinction

curves are best seen as ‘most probable’ curves.

In order to see the net effect of immigration and

extinction, their two curves can be superimposed

(Figure 10.13c). The number of species where the

curves cross (S*) is a dynamic equilibrium and

should be the characteristic species richness for

the island in question. Below S*, richness increases

(immigration rate exceeds extinction rate); above S*,

richness decreases (extinction exceeds immigration).

The theory, then, makes a number of predictions,

described in the text.

Immigration rate

(a)

Number of resident species

Extinction rate

(b)

Number of resident species

Small island

Large island

(c)

Number of resident species

Close, large

Close, small

Distant, small

Distant, large

Small

Large

Close or large island

Distant or small

island

Size of

species pool

Immigration rate ( )

or extinction rate ( )

Figure 10.13

MacArthur and Wilson’s (1967) equilibrium theory of island biogeography. (a) The rate of species immigration on to an island,

plotted against the number of resident species on the island, for large and small islands and for close and distant islands.

(b) The rate of species extinction on an island, plotted against the number of resident species on the island for large and small

islands. (c) The balance between immigration and extinction on small and large islands and on close and distant islands. In each

case, S* is the equilibrium species richness; C, close; D, distant; L, large; S, small.

partitioning variation between

habitat diversity and area itself

9781405156585_4_010.qxd 11/5/07 14:58 Page 342

diversity alone? Some studies have attempted to partition species–area variation on

islands into that which can be entirely accounted for in terms of habitat diversity,

and that which remains and must be accounted for by island area in its own right.

For beetles on the Canary Islands, the relationship between species richness and

habitat diversity (as measured by plant species richness) is much stronger than

that with island area, and this is particularly marked for the herbivorous beetles,

presumably because of their particular food plant requirements (Figure 10.14a).

Contrasting with the Canary Island results, in a study of a variety of animal groups

living on the Lesser Antilles islands in the West Indies, the variation in species

richness from island to island was partitioned, statistically, into that attributable

to island area alone, that attributable to habitat diversity alone, that attributable to

correlated variation between area and habitat diversity (and hence not attributable

to either alone) and that attributable to neither (Figure 10.14b). For reptiles and

amphibians, like the beetles of the Canary Islands, habitat diversity was far more

important than island area. But for bats, the reverse was the case; and for birds

and butterﬂies, both area itself and habitat diversity had important roles to play.

Overall, therefore, studies like this suggest a separate area effect (larger islands are

larger targets for colonization; populations on larger islands have a lower risk of

extinction) beyond a simple correlation between area and habitat diversity.

An example of species impoverishment on more remote islands can be seen in

Figure 10.15 for non-marine, lowland birds on tropical islands in the southwest

Paciﬁc. With increasing distance from the large ‘source’ landmass of Papua New

Guinea, there is a decline in the number of species, expressed as a percentage of

the number present on an island of similar area but close to Papua New Guinea.

Chapter 10 Patterns in species richness

343

Species richness

(a)

(b)

250

200

150

100

250

200

150

100

0 500 1000 1500 2000 2500

Island area (km

)

0 200 400 600 800 1000

Number of plant species

1.0

0.8

0.6

0.4

0.2

Proportion of variance

Bats Reptiles and

amphibians

Birds Butterflies

Neither

Habitat diversity

Both

Island area

Figure 10.14

(a) The relationships between

species richness of herbivorous

(circles) and carnivorous (triangles)

beetles of the Canary Islands

and both island area (left) and

plant species richness (right).

(b) Proportion of variance in species

richness, for four animal groups,

among islands in the Lesser Antilles

related uniquely to island area (blue),

related uniquely to habitat diversity

(orange), related to correlated

variation between area and habitat

diversity (green) and unexplained

by either (maroon). Regression lines

are signiﬁcant at P < 0.05; no lines

are shown in the left panel of (a)

because the regression are not

signiﬁcant.

(A) AFTER BECKER, 1992; (B) AFTER RICKLEFS & LOVETTE, 1999

bird species richness on

Paciﬁc islands decreases

with remoteness

9781405156585_4_010.qxd 11/5/07 14:58 Page 343

A more transient but nonetheless important reason for the species impoverish-

ment of islands, especially remote islands, is the fact that many lack species that

they could potentially support, simply because there has been insufﬁcient time

for the species to colonize. An example is the island of Surtsey, which emerged

in 1963 as a result of a volcanic eruption. The new island, 40 km southwest of

Iceland, was reached by bacteria and fungi, some sea birds, a ﬂy and seeds of

several beach plants within 6 months of the start of the eruption. Its ﬁrst established

vascular plant was recorded in 1965, the ﬁrst moss colony in 1967 and the ﬁrst

bush (a dwarf willow, Salix herbacea) in 1998. An earthworm was found in 1993

and slugs in 1998, probably carried in by birds (Hermannsson, 2000). By 2004,

more than 50 species of vascular plant, 53 mosses, 45 lichens and 300 species of

invertebrate had been recorded, though not all persisted (Surtsey Research Society,

website www.surtsey.is). Colonization by new species occurred both above and

below the water line, with marine invertebrates, which disperse as larval stages in

the ocean, accumulating faster than terrestrial plants (Figure 10.16).

Finally, it is important to reiterate that no aspect of ecology can be fully under-

stood without reference to evolutionary processes (see Chapter 2), and this is

particularly true for an understanding of island communities. On isolated islands,

Part III Individuals, Populations, Communities and Ecosystems

344

1965

1963

1970 1975 1980

1985

1990 20001995

Species richness

Year

Terrestrial vascular plants

Coastal marine invertebrates

0 2,000 4,000 6,000 8,000 10,000

Distance from Papua New Guinea (km)

100

12.5

6.25

Degree of saturation (%)

Figure 10.15

The number of resident, non-marine, lowland

bird species on islands more than 500 km

from the large ‘source’ landmass of Papua

New Guinea expressed as a percentage

of the number of species on an island of

equivalent area but close to Papua New

Guinea – this can be thought of as the

‘degree of saturation’ of the bird community.

It is plotted against island distance from

Papua New Guinea.

AFTER DIAMOND, 1972

Figure 10.16.

Regular surveys of species richness of animals and plants have

occurred since the emergence in 1963 of the volcanic Surtsey

Island, near Iceland. Shown here are the results of standard

surveys of coastal marine invertebrates up to 1992 (barnacles,

isopods, decapods, mollusks, starﬁsh, brittlestars, sea urchins

and sea squirts; maroon circles) and of terrestrial vascular plants

up to 2004 (open circles).

AFTER HERMANNSON, 2000; SURTSEY RESEARCH SOCIETY, WEBSITE WWW.SURTSEY.IS

species missing because of

insufﬁcient time for colonization

evolution rates on islands may be

faster than colonization rates

9781405156585_4_010.qxd 11/5/07 14:58 Page 344

the rate at which new species evolve may be comparable to or even faster than

the rate at which they arrive as new colonists. Clearly, the communities of these

islands will be incompletely understood by reference only to ecological processes.

Take the remarkable numbers of Drosophila species (fruitﬂies) found on the

remote volcanic islands of Hawaii. There are probably about 1500 Drosophila

species worldwide but at least 500 of these are found on the Hawaiian Islands;

they have evolved, almost entirely, on the islands themselves. The communities

of which they are a part are clearly much more strongly affected by local evolution

and speciation than by the processes of invasion and extinction.

10.5.2 Latitudinal gradients

One of the most widely recognized patterns in species richness is the increase that

occurs from the poles to the tropics. This can be seen in a wide variety of groups,

including trees, marine invertebrates, butterﬂies and lizards (Figure 10.17). The

pattern can be seen, moreover, in terrestrial, marine and freshwater habitats.

A number of explanations have been put forward for the general latitudinal

trend in species richness, but not one of these is without problems. In the ﬁrst place,

the richness of tropical communities has been attributed to a greater intensity of

predation and to more specialized predators. More intense predation could reduce

the importance of competition, permitting greater niche overlap and promoting

higher richness (see Figure 10.3c), but predation cannot readily be forwarded as

Chapter 10 Patterns in species richness

345

Species richness

100

10 40 50 7020 30 60

Species richness

Latitude (degrees)

(a) Marine bivalves

500

400

300

200

100

90 70 50 30 10 10 30 50 70 90

N S Latitude (degrees)

70 60 50 5040 4030 3020 2010 0 10

(b) Butterflies

160

120

25 35 45 55 65 75

(d) Trees

Latitude (°N) Latitude (°N)

Figure 10.17

Latitudinal patterns in species

richness for: (a) marine bivalves,

(b) swallowtail butterﬂies,

(d) trees in North America. In each

case there is a decline from low

latitudes (the equator is at 0°) to

high latitudes (the poles are at 90°).

(a) AFTER FLESSA & JABLONSKI, 1995; (b) AFTER

SUTTON & COLLINS, 1991; (c) AFTER ROSENZWEIG &

SANDLIN, 1997; (d) AFTER CURRIE & PAQUIN, 1987

9781405156585_4_010.qxd 11/5/07 14:58 Page 345

the root cause of tropical richness, since this begs the question of what gives rise

to the richness of the predators themselves.

Second, increasing species richness may be related to an increase in productivity

as one moves from the poles to the equator. Certainly, on average, there is more

heat and more light energy in increasingly tropical regions, and, as discussed in

Section 10.3.1, both of these have tended to be associated with greater species

richness, though increased productivity in at least some cases has been associated

with reduced richness.

Moreover, light and heat are not the only determinants of plant productivity.

Tropical soils tend, on average, to have lower concentrations of plant nutrients than

temperate soils. The species-rich tropics might therefore be seen, in this sense, as

reﬂecting their low productivity. In fact, tropical soils are poor in nutrients because

most of the nutrients are locked up in the large tropical biomass. A productivity

argument might therefore have to run as follows. The light, temperature and water

regimes of the tropics lead to high biomass communities but not necessarily to diverse

communities. This, though, leads to nutrient-poor soils and perhaps a wide range of

light regimes from the forest ﬂoor to canopy far above. These in turn lead to high

plant species richness and thus to high animal species richness. There is certainly

no simple ‘productivity explanation’ for the latitudinal trend in richness.

Some ecologists have invoked the climate of low latitudes as a reason for their

high species richness. Speciﬁcally, equatorial regions are generally less seasonal than

temperate regions, and this may allow species to be more specialized (i.e. have

narrower niches, see Figure 10.3b). The greater evolutionary ‘age’ of the tropics has

also been proposed as a reason for their greater species richness, and another line

of argument suggests that the repeated fragmentation and coalescence of tropical

forest refugia promoted genetic differentiation and speciation, accounting for much

of the high richness in tropical regions. And in a related context, we have already

noted that the rate of evolution may be faster in the tropics (see Section 10.4.3).

All these ideas are plausible too, but far from proven generalizations.

Overall, therefore, the latitudinal gradient lacks an unambiguous explanation.

This is hardly surprising. The components of a possible explanation – trends with

productivity, climatic stability and so on – are themselves understood only in an

incomplete and rudimentary way, and the latitudinal gradient intertwines these

components with one another, and with other, often opposing forces – isolation,

harshness and so on.

10.5.3 Gradients with altitude and depth

A decrease in species richness with altitude, analogous to that observed with latitude,

has frequently been reported in terrestrial environments (e.g. Figure 10.18a, b). On

the other hand, some have reported an increase with altitude (e.g. Figure 10.18c),

while about half the studies of altitudinal species richness have described hump-

shaped patterns (e.g. Figure 10.18d) (Rahbek, 1995).

At least some of the factors instrumental in the latitudinal trend in richness are

also likely to be important as explanations for altitudinal trends (though the prob-

lems in explaining the latitudinal trend apply equally to altitude). For example,

declines in species richness have often been explained in terms of decreasing

productivity associated with lower temperatures and shorter growing seasons

at higher altitude, or physiological stress associated with climatic extremes near

Part III Individuals, Populations, Communities and Ecosystems

346

productivity as an explanation?

climatic variation or evolutionary

age as explanations?

9781405156585_4_010.qxd 11/5/07 14:58 Page 346

mountaintops. Indeed, the explanation for the converse, positive relationship

between ant diversity and altitude in Figure 10.18c, is that precipitation increased

with altitude in this case, resulting in higher productivity and less physiologically

extreme conditions at higher altitude. In addition, high-altitude communities

almost invariably occupy smaller areas than lowlands at equivalent latitudes, and

they will usually be more isolated from similar communities than lowland sites.

Therefore the effects of area and isolation are likely to contribute to observed

decreases in species richness with altitude.

In aquatic environments, the change in species richness with depth shows

some strong similarities to the terrestrial gradient with altitude. In larger lakes, the

cold, dark, oxygen-poor abyssal depths contain fewer species than the shallow

surface waters. Likewise, in marine habitats, plants are conﬁned to the photic

zone (where light penetrates and they can photosynthesize), which rarely extends

below 30 m. In the open ocean, therefore, there is a rapid decrease in richness

Chapter 10 Patterns in species richness

347

Species richnessSpecies richness

140

1500 1900 2100 2300 2500

(b)

1700

120

100

500

1000

1500

2000

2500

3000

14(c)

500

1000

4000

2000

5000

6000

1200

(d)

1000

800

600

400

200

3000

Shrubs

Herbs

Epiphytes

Altitude (m)

Altitude (m) Altitude (m)

Vines

Trees

(a)

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

7000

7500

8000

Species richness

100

200

300

400

Altitude (m)

Figure 10.18

Relationships between species richness and altitude for:

(a) breeding birds in the Nepalese Himalayas, (b) plants in the

Sierra Manantlán, Mexico, (c) ants in Lee Canyon in the Spring

Mountains of Nevada, USA, and (d) ﬂowering plants in the

Nepalese Himalayas. Species richness decreases with altitude

in (a) and (b), increases with altitude in (c) and shows a

hump-backed relationship in (d).

(a) AFTER HUNTER & YONZON, 1992; (b) AFTER VÁZQUEZ & GIVNISH, 1998; (c) AFTER SANDERS ET AL., 2003; (d) AFTER GRYTNES & VETAAS, 2002

9781405156585_4_010.qxd 11/5/07 14:58 Page 347

with depth, reversed only by the variety of bizarre animals living on the ocean

ﬂoor. Interestingly, however, in coastal regions the effect of depth on the species

richness of benthic (bottom-dwelling) animals is to produce not a single gradient,

but a peak of richness at about 1000 m, possibly reﬂecting higher environmental

predictability there (Figure 10.19). At greater depths, beyond the continental

slope, species richness declines again, probably because of the extreme paucity of

food resources in abyssal regions.

10.5.4 Gradients during community succession

Section 9.4 described how, in community successions, if they run their full course,

the number of species ﬁrst increases (because of colonization) but eventually

decreases (because of competition). This is most ﬁrmly established for plants, but

the few studies that have been carried out on animals in successions indicate, at

the least, a parallel increase in species richness in the early stages of succession.

Figure 10.20 illustrates this for birds following the cessation of shifting cultiva-

tion in tropical rain forest, and for insects associated with an old-ﬁeld succession

in a temperate region.

To a certain extent, the successional gradient is a necessary consequence of

the gradual colonization of an area by species from surrounding communities

that are at later successional stages; that is, later stages are more fully saturated

with species (see Figure 10.3d). However, this is a small part of the story, since

succession involves a process of the replacement of species and not just the mere

addition of new ones.

Indeed, as with the other gradients in species richness, there is something of

a cascade effect with succession: one process that increases richness kick-starts a

second, which feeds into a third, and so on. The earliest species will be those that

are the best colonizers and the best competitors for open space. They immedi-

ately provide resources (and introduce heterogeneity) that were not previously

present. For example, the earliest plants generate resource depletion zones (see

Section 3.3.2) in the soil that inevitably increase the spatial heterogeneity of plant

nutrients. The plants themselves provide a new variety of microhabitats, and for

the animals that might feed on them they provide a much greater range of food

resources (see Figure 10.3a). The increase in herbivory and predation may then

Part III Individuals, Populations, Communities and Ecosystems

348

1008040

Species richness

Depth (km)

Figure 10.19

Depth gradient in species richness

of bottom-dwelling vertebrates and

invertebrates (ﬁsh, decapods,

holothurians, asteroids) in the ocean

southwest of Ireland.

AFTER ANGEL 1994

a cascade effect?

9781405156585_4_010.qxd 11/5/07 14:58 Page 348