species–area relationship - evolution and ecology and schoener 2009... · relationship,...

FURTHER READING

Coleman, P. J. . Stratigraphical and structural notes on the British

Solomon Islands with reference to the fi rst geological map. British Solo-mon Islands Geological Record (1959–1962) : –.

Hughes, G. W., P. M. Craig, and R.A. Dennis. . Geology of the outer

Eastern Islands. Geological Survey Division Solomon Islands Bulletin .

Kroenke, L. W. . Solomon Islands: San Cristobal to Bougainville

and Buka, in Cenozoic tectonic development of the southwest Pacifi c. L.

W. Kroenke, ed. Technical Bulletin . Suva, Fiji: Committee for Co-

ordination of Joint Prospecting for Mineral Resources in South Pacifi c

Offshore Areas, ch .

Mahoney, J., G. Fitton, P. Wallace, and the Leg Scientifi c Party. .

ODP Leg : basement drilling on the Ontong Java Plateau. JOIDESJournal .: – and covers.

Reagan, A. J., and H. M. Griffi n. . Bougainville before the confl ict. Canberra: Pandanus Books/ANU.

Vedder, J. G., and T. S. Bruns. . The geology and offshore resources of Pacifi c island arcs: Solomon Islands and Bougainville. Earth Science Series . Hous-

ton: Circum-Pacifi c Council for Energy and Mineral Resources.

Vedder, J. G., K. S. Pound, and S. Q. Boundy, eds. . The geology and offshore resources of Pacifi c island arcs: central and western Solomon Islands. Earth Science Series . Houston: Circum-Pacifi c Council for

Energy and Mineral Resources.

REFERENCES

Davies, H. L., et al. . Geology of Oceania, in Encyclopedia of Geol-ogy, vol. 4. R. C. Selley, L. R. M. Cocks, and I. R. Plimer, eds. Oxford:

Elsevier, –.

Smith, W. H. F., and D. T. Sandwell. . Global seafl oor topography from

satellite altimetry and ship depth soundings. Science : –.

SOUTH GEORGIA

SEE ATLANTIC REGION

SOUTH SANDWICH ISLANDS

SEE ATLANTIC REGION

SPECIES–AREA RELATIONSHIP

DAVID A. SPILLER AND THOMAS W. SCHOENER

University of California, Davis

For over a century, ecologists have been captivated by the

tendency for number of species within a taxonomic group

to increase with island area. This “species–area relation-

ship” has been found for a broad range of organisms in

numerous archipelagoes around the world. A partial list of

studies demonstrating the relationship includes land plants

on the Galápagos (Fig. ) and Aleutian Islands; insects on

the Tuscan Islands and on subantarctic islands; reptiles on

islands in the Gulf of California and in the West Indies;

birds on the Canary, Solomon, and Aegean Islands; and

mammals on islands in the Philippines and North Ameri-

can Great Lakes. Although most studies are of higher

organisms, even protozoans and diatoms have shown the

relationship. To explain the occurrence of the species–area

relationship, ecologists have proposed several hypotheses

FIGURE 1 A good example of the species–area relationship is the num-

ber of land plant species on the Galápagos Islands. Top: Map of the

Galápagos Islands showing the number of land plant species. Note that

the largest island contains the most species and that numbers tend to

be higher on large islands than on small islands. However, several devi-

ations from this pattern exist, such as the “unexpected” high number

of species (319) on the medium-sized island at the southern end of the

archipelago, suggesting that factors other than area may sometimes

be important in determining the number of species on islands. Bottom:

Relationship between the number of species and island area. Regres-

sion line is the least-squares estimate: log(number of species) = 1.46 +

0.33 log(area). Data taken from Hamilton et al. (1964).

S P E C I E S – A R E A R E L AT I O N S H I P 857

Gillespie08_S.indd 857Gillespie08_S.indd 857 4/23/09 12:58:39 PM4/23/09 12:58:39 PM

on causal mechanisms, which shed light on the processes

that structure biological communities. Conservation biolo-

gists have applied the relationship to the design of nature

reserves.

STATISTICAL MODELS

The species-area relationship can be estimated statistically

using the power equation

S = k Az ()

where S = number of species, A = area of island, and kand z are fi tted parameters. Logarithmic transforma-

tion of both sides of the power equation yields the linear

equation

log S = log k + z log A ()

in which z is the slope of the estimated species–area rela-

tionship (i.e., the rate of increase of log S with log A).

Because the scales are logarithmic, the z-value is inde-

pendent of scale, allowing comparisons between differ-

ent studies even when the units for area are different. A

second equation, with S instead of log S in Equation , is

often used instead.

Log-transformed data on number of land plant species

and area for the Galápagos Islands (Fig. , bottom) show

a positive linear relationship with a slope (z-value) of ..

Similarly, many other studies have found that the data fi t

the log-log model with z-values usually ranging from .

to .. Preston developed a mathematical explanation

for the prevalence of the linear log-log relationship, with

specifi c assumptions about the distribution of species

abundances and other biological processes, in which the

z-value is expected to be approximately .. However,

a detailed analysis of studies by Connor and McCoy

showed that the data often fi t other statistical models just

as well or better, suggesting that biological interpreta-

tion of parameters in statistical models should be made

cautiously.

WHY DOES THE NUMBER OF SPECIES

INCREASE WITH ISLAND AREA?

Although the species–area relationship is one of the surest

generalizations in ecology, there has been much debate

on the causal factors and processes. There are six major

hypotheses, as follows.

Random Sampling

Assuming that the individuals on a given island are a

random sample from a nearby mainland or other source

containing all species, and larger islands contain more indi-

viduals, then the number of species should increase with

island area. An analogy would be a bowl of colored marbles

with ten marbles of each of ten different colors. The larger

the handful of marbles you take, the more different colors

you will get on average. This simple explanation may serve

as a null model to compare with the other fi ve hypotheses,

all of which incorporate biological processes.

Larger Populations and Less Extinction on Larger

Islands

The hypothesis that populations are larger on larger

islands, implying lower extinction rates, is an integral part

of the equilibrium theory of island biogeography devel-

oped by MacArthur and Wilson in . According to

this theory, the number of species present on an island

is determined by the dynamic equilibrium between the

rate of immigration of species not already on the island

from a source pool and the rate of extinction of species

already on the island (Fig. A). Because larger islands

FIGURE 2 Graphical representations of the MacArthur–Wilson equi-

librium model of island biogeography. (A) The effect of island area;

S = equilibrium number of species on a small island, L = equilibrium

number of species on a large island, P = number of species in the pool

on the mainland (source). (B) The effect of distance of the island from

the source of immigration; N = equilibrium number of species on a near

island, F = equilibrium number of species on a far island.

858 S P E C I E S – A R E A R E L AT I O N S H I P


contain more individuals (larger population size) in each

species, and extinction rate is inversely proportional to

population size, extinction rate of species on small islands

is higher than on large islands, so the equilibrium number

of species is positively related to island area. In Sim-

berloff provided experimental evidence for this hypothesis

by showing that the number of species on mangrove islets

decreased when he reduced the area of islets. In addition

to the area effect, the equilibrium theory contains a dis-

tance effect; immigration rates are higher for islands near

the source pool than for those farther away (Fig. B).

Larger Interception Area and More Immigration

on Larger Islands

In addition to the lower extinction rate in the equilib-

rium model, a larger island may also be a bigger “target”

for colonists; this causes the immigration curve of a large

island to be higher than that of a small island, thereby

increasing the equilibrium for the large island.

Higher Habitat Diversity on Larger Islands

Because the number of different types of habitats may

increase with island area and different species often occur

in different habitats, more species occur on larger islands.

Hence, the species-area relationship may be indirect via the

positive effect of area on habitat diversity. This hypothesis

is most popular among ecologists familiar with the natu-

ral histories of the species in their studies. Early on, Wat-

son found that habitat diversity was a better predictor of

Aegean bird species number than by area. Very recently,

Morrison found that the number of ant species on islets in

the Bahamas was predicted by number of land plants spe-

cies better than was area; in this case, different plant species

may serve as different types of habitats or provide different

types of food resources for ants. Returning to Galápagos

land plants, Hamilton and collaborators showed in

that elevation (a proxy for habitat diversity) was a better

predictor of number of species than was area.

Lower Abiotic Disturbance on Larger Islands

The impact of abiotic disturbances, such as hurricanes

and tsunamis, may be more devastating on small islands

than on large islands, exterminating species mostly on the

former and thereby causing the positive species–area rela-

tionship. Such an effect could go well beyond the effect

of population size on extinction, as equivalently sized

populations could be more vulnerable on smaller islands.

Whittaker pointed out that such disturbances can affect

island communities for decades or even centuries, during

which time species would not be at a dynamic equilib-

rium as in the MacArthur–Wilson model. In addition to

the effect of episodic major disturbances, chronic non-

catastrophic disturbances, such as wind and salt spray,

may affect small islands more than large ones because of

the greater perimeter-area ratio on small islands. Only

species that can tolerate these harsh conditions could per-

sist on small islands.

Greater Speciation on Larger Islands

The larger the island, the more likely that geographic bar-

riers exist that cause isolation between viable subpopula-

tions of a species; this can lead to allopatric speciation.

The effect may be geometric, as the opportunity for spe-

ciation is itself proportional to species number, causing an

especially high species–area slope.

Lomolino’s Combined Model

Lomolino proposed in a general model for the species–

area relationship that integrates disturbance and equilibrium

dynamics along with speciation on islands. In this model the

species–area relationship has three regions (Fig. ): () On

small islands, the stochastic effects of disturbances are pre-

dominant, making the relationship between number of spe-

cies and island area variable and unpredictable (an example

is given in the later discussion of spiders in the Bahamas). ()

On medium to large islands, the deterministic effects of area

on extinction/immigration and habitat diversity predomi-

nate, and number of species increases at a decreasing rate as

the number of species in the source pool is approached. ()

The largest islands contain barriers isolating species popu-

lations, leading to allopatric speciation. Losos and Schluter

assessed the speciation component for Anolis lizards of the

West Indies. For these relatively poor dispersers, the sharp

increase in the species–area slope due to speciation obliter-

ates the fl attening portion (region ) of Lomolino’s model.

FIGURE 3 Lomolino’s general species–area model containing three

different regions (see text for explanation). Modifi ed from Lomolino

and Weisen (2001).

S P E C I E S – A R E A R E L AT I O N S H I P 859


ECOLOGICAL CORRELATES OF VARIATION IN

THE SPECIES–AREA SLOPE

Comparisons among different kinds of organisms have

revealed substantial variation in the slope of the spe-

cies–area relationship. Wright showed that on islands

in the West Indies, the slope was greater for non-fl ying

mammals than for bats or birds, suggesting that dispersal

ability infl uences the relationship. A plausible explana-

tion is that islands are more isolated for species with low

dispersal ability, so most of them do not occur on small

islands, whereas species with high dispersal ability are fre-

quently found on small islands because their immigration

rates are high even though they may not persist for long.

Schoener suggested that z-values should be greater for spe-

cies occurring at low density (e.g., territorial carnivores)

than for those occurring at high density because the low-

density species can only persist on larger islands. Holt and

collaborators developed a mathematical model predicting

that the z-value for species at the top of the food chain is

higher than for species in lower trophic levels.

SPIDERS ON BAHAMIAN ISLANDS

Studies of web spiders occurring on islands in the Baha-

mas by Schoener and Spiller illustrate the species–area

relationship and address several of the issues discussed

above (Fig. ). Complete censuses of all web spiders

occurring over the entire areas of islands were con-

ducted annually over a ten-year period. Number of

species (mean over time) was positively correlated with

island area for four reasons. First, larger islands tended to

have larger populations, which in turn had lower extinc-

tion rates than did small populations, increasing their

number of species. Second, larger islands have a higher

immigration rate: the “target effect.” Third, some species

are habitat generalists (e.g., Argiope argentata), occurring

in areas with high or low vegetation, whereas others are

more specialized (e.g., Gasteracantha cancriformis), living

in only high vegetation (Fig. ). Small islands tend to

have only low vegetation, whereas large islands contain

areas with both low and high vegetation. Therefore, only

FIGURE 4 Aerial photograph showing a portion of the Bahamian spi-

der study area.

generalists occur on small islands, whereas both general-

ists and specialists occur on large islands with more types

of habitats. Fourth, tropical storms can affect smaller

islands more drastically: During several recent hurri-

canes, smaller Bahamian islands, which are lower, were

completely inundated by high water, apparently killing

all spiders. Note that variation in the numbers of spider

species on small islands is relatively high as in Lomolino’s

model. Another factor that can affect number of species

is the presence of lizards, which are major predators of

spiders and which occurred on about half of the study

islands: Islands with lizards tended to have fewer spider

species (Fig. ). This “lizard effect” is more apparent for

larger islands than for smaller islands, and the slope of

the species-area relationship is greater for islands without

lizards (Fig. ). Hence, the z-value is greater for islands

without lizards, on which spiders occupy a higher level in

the food web, as in Holt’s model.

FIGURE 5 (A) Argiope argentata. (B) Gasteracantha cancriformis.

Photographs by D. Spiller.

FIGURE 6 Illustration of a lizard eating a spider by G. Dan.

860 S P E C I E S – A R E A R E L AT I O N S H I P


THE DESIGN OF NATURE RESERVES

In addition to “true islands” (bodies of land surrounded

by water), the species–area relationship has been well doc-

umented for many types of “island analogues” (patches of

suitable habitat for species isolated by unsuitable habitat)

on mainlands. Knowledge of the factors that shape the

species–area relationship can be used to evaluate differ-

ent strategies for designing nature reserves that maintain

the highest number of species. Of course, the ideal strat-

egy would be to have many huge preserves, but this may

not be feasible. Hence, the problem that conservation

biologists have pondered is whether one large reserve or

several small reserves with the same total area is better.

The answer to this question, sometimes referred to as the

SLOSS (single large or several small) debate, is not obvi-

ous: A single large reserve will have a lower per-species

extinction rate than will any smaller reserve, but the more

reserves there are, the less chance there will be for a spe-

cies to disappear simultaneously from all of them. In a

review of land plants, insects, and vertebrates on islands,

Quinn and Harrison showed that total numbers of spe-

cies on groups of several small islands were higher than

on a comparable area consisting of only one or a few large

islands. Among other explanations, they suggest that the

several small islands were distributed over a larger region

and contained more types of habitats, enabling more spe-

cies to exist on the entire set of islands. Similarly, genetic

diversity within a species, which reduces extinction likeli-

hood, may be increased with a certain amount of reserve-

area fragmentation. In conclusion, several small preserves

distributed over a large or varied region may sometimes

be the better conservation strategy.

SEE ALSO THE FOLLOWING ARTICLES

Extinction / Fragmentation / Galápagos Islands, Biology / Island

Biogeography, Theory of / Spiders

FURTHER READING

Connor, E. F., and E. D. McCoy. . The statistics and biology of the

species-area relationship. American Naturalist : –.

Hamilton, T. H., R. H. Barth, and I. Rubinoff. . The environmen-

tal control of insular variation in bird species abundance. Proceedings of the National Academy of Sciences of the United States of America :

–.

Lomolino, M. V., and M. D. Weisen. . Toward a more general spe-

cies-area relationship. Journal of Biogeography : –.

MacArthur, R. H., and E. O. Wilson. . The theory of island biogeogra-phy. Princeton, NJ: Princeton University Press.

Quinn, J. P., and S. P. Harrison. . Effects of habitat fragmentation and

isolation on species richness: evidence from biogeographic patterns.

Oecologia : –.

Whittaker, R. J. . Disturbed island ecology. Trends in Ecology and Evolution : –.

SPIDERS

MIQUEL A. ARNEDO

University of Barcelona, Spain

The ability to produce silk is a distinctive feature of spi-

ders. Silk-mediated airborne dispersal has allowed spiders

to colonize even the most remote archipelagoes. Spiders

on islands have served as models for the study of the evo-

lutionary and ecological underpinnings of biodiversity.

Because of their generalist predatory habits, introduced

spiders may pose a serious threat to islands’ native fauna.

DEFINING A SPIDER

Spiders (order Araneae) comprise a megadiverse group of

arthropods that includes close to , species distributed

in families. The origin of spiders can be traced back to

the Devonian, about million years ago, representing

some of the earliest evidence of terrestrial life on Earth. As

the dominant non-vertebrate predators in most terrestrial

ecosystems, spiders have enormous ecological importance.

ACCESSING ISLANDS

Dispersal capabilities and generalist predatory habits

make spiders formidable pioneers. Spiders have a unique

FIGURE 7 Species–area relationship for Bahamian orb spiders on 37

islands without lizards and 27 islands with lizards. Regression lines

for islands without and with lizards are respectively: log(number of

species) = -0.85 + 0.41log(area), log(number of species) = -0.28 +

0.16log(area). Unpublished data (T. Schoener and D. Spiller).

S P I D E R S 861


species–area relationship - evolution and ecology and schoener 2009... · relationship,...

Documents