untitled

island. Thus, for islands of equal size the expected number of species for distant islands,

Sfar, should be lower than that for near islands, Snear.

The extinction rate is determined by several factors such as the size of the popu-

lation and the number of competing and predatory species. All of these are related

to the area of the ecosystem such that islands of larger sizeshould have lower extinc-

tion rates. Thus, the expected number of species for a large island, Slarge, should be

larger than that for a small island, Ssmall. This leads to the classic species–area equa-

tion that we explored in Section 18.5.3.

Some classic studies provided circumstantial evidence for this theory of island bio-

geography. Diamond (1969) showed that bird species on the California Channel Islands

maintained similar numbers of species over a 50-year period. There was a turnover

of species because some went extinct in the interval but others arrived to replace

them. Simberloff and Wilson (1970) showed that small mangrove islands, artificially

denuded of their invertebrates, regained a similar number of species as occurred before

the removal but the types of species differed to some extent. These results concur

with the theory.

We should recognize that this is an idealized concept and there are several factors

that could distort it. The complexity of the habitat is one such factor (Lack 1971),

but several of the other factors determining biodiversity that we discussed in Section

21.12.1 also apply. The important point is to understand that communities are dynamic,

with species coming and going, and not to place too much emphasis on looking for

an equilibrium.

One important prediction from this theory concerns what happens when a

piece of a large mainland ecosystem is suddenly isolated. Isolation means that the

immigration rate should be reduced and hence the species number should decline

to a lower value. This process, called faunal relaxation(Diamond, 1972), was

documented for islands that were isolated from the mainland by the 100 m rise in

seawater level after the ice age, 10,000 years ago. Examples of such islands, called

landbridge islands, are those in the Malay archipelago (the Sunda Islands), amongst

others (MacArthur 1972).

380 Chapter 21

Number of species

Rate

IMMIGRATION EXTINCTION

Near

Far

Small

Large

Sfar Snear

Ssmall

Slarge

Fig. 21.3Hypothetical
relationship between the
rates of immigration or
extinction of species and
the number of species in
a habitat patch or island.
Where the immigration
and extinction curves cross
we obtain an equilibrium
number of species, S. For
a given extinction rate,
islands closer to the source
of immigrants have a higher
number (Snear) than islands
further away (Sfar). For a
given immigration rate,
larger islands have a higher
number of species (Slarge)
than smaller islands (Ssmall).