Biology Reference
In-Depth Information
species were larger than in the second phase: some species were elimi-
nated by interspecific competition (competitive exclusion), not evident
in the first phase.
The experiment described by Simberloff andWilson was conducted
on six very small mangrove islets (11-18m diameter) in the Florida
Keys. The nearest islet was about 2m, and the most distant one, 533m
from the mainland. The regional species pool of arthropods consisted
of about 1000 species, and 20-40 species occurred on each of the islets.
Arthropods were extinguished by fumigation, and recolonization was
monitored over two years (later extended by Simberloff (
1976
)to
three years). Species reached equilibrium numbers (i.e., numbers
before the experiment) in about 200 days, although on two of the
islets species numbers remained slightly lower. Importantly, not long
after the beginning of recolonization there were slightly higher peaks
of species numbers, later levelling off to the equilibrium. The kind of
species continued to change, but the total numbers remained more or
less constant over the two-year period. The lower numbers on the two
islets are unexplained, but may be due to the kinds of species newly
acquired, which may have been less well adapted to the habitat, or had
different competitive abilities. The islet nearest to the mainland had a
distinctly higher equilibrium than the most distant one, the intermedi-
ate ones had intermediate equilibrium values. Even after three years,
species composition had not converged to the original one (Simberloff
1976
).
Moulton and Pimm (
1987
) studied the effects of the 49 bird species
introduced to the Hawaiian islands between 1869 and 1983, and found
convincing evidence for competitive exclusion. Pielou (
1975
) has
pointed out that in the interactive phase the equilibrium is dynamic:
extinction is continuously balanced by immigration, sometimes by
different species. A static quasi-equilibrium will be reached only when
an island at the interactive equilibrium level is prevented from further
colonization by a suddenly erected barrier.
Also, in contrast to Gilbert's verdict, Rosenzweig (
1995
) has con-
cluded that the theory of island biogeography has held up well. Island
diversity is self-regulating, and competition as well as predation probably
helps to increase extinction rates with increasing diversity by negative
feedback. However, evidence for the theory comes in the main from
some plants and birds.
One of the most important assumptions of the equilibrium theory of
island biogeography is that the distance of an island from the nearest
