Biology Reference
In-Depth Information
Island biogeography: evidence for equilibrium
conditions?
One of the most widely discussed and controversial issues in biogeo-
graphy/macroecology is that of species numbers and species dynamics
onislands.Islandsdifferinsuchfeaturesassize,distancefromthenearest
continent or other colonizing sources, and habitat heterogeneity, and are
excellent models to study the effects of these features on diversity and
species turnover. Simberloff (
1974
,
1976
), Pielou (
1979
), Pianka (
1983
),
and Gilbert (
1980
), among others, give discussions of the equilibrium
theory of island biogeography, to which the reader is referred. The main
points of the theory are as follows. According to MacArthur and Wilson
(
1963
,
1967
), because of balanced immigration and extinction, numbers
of species on islands tend towards a dynamic equilibrium; species num-
bers depend on the area of the island and distance from the source of
colonization, with there being a turnover of species once equilibrium
has been reached. As pointed out by Gilbert (
1980
,seealsoSimberloff
1978
and Connor and Simberloff
1978
),thetheoryasformulatedby
MacArthur and Wilson does not consider competition between species.
When such interactions are incorporated, there would be a noninteractive
phase at equilibrium, leading to super-saturation, followed by an interactive
phase which either ''relaxes'' equilibrium or leads to oscillations around
the equilibrium and finally to equilibrium (Simberloff ). According to
Gilbert (
1980
), demonstration that the theory applies requires demonstration
of the following: (1) there must be a close relationship between area
and species number; (2) the number of species must remain constant over
time; (3) there must be a turnover of species, i.e., some species must be
replaced by others over time. The best evidence for this comes from birds.
Gilbert is not convinced that the theory describes natural situations
well: ''many, if not all, insular continental situations are at best badly
described by the theory''. He believes that a main reason for this is the
''extreme oversimplification'' of the model, paying no attention to internal
habitat diversity and to the differences between species. Therefore, ''quanti-
tatively, ... it would seem that the model has little evidence to support its
application to any situation''.
However, others have found support for its main assumptions.
Wilson and Simberloff (
1969
)andSimberloffandWilson(
1970
)pro-
vided experimental evidence for the distinction of noninteractive and
interactive equilibrium on islands, by defauning islands and monitoring
their recolonization. In the first, noninteractive phase, numbers of
