Biology Reference
In-Depth Information
colonizing source determines species numbers and rate of species turn-
over on the island. Obviously, this distance should not be measured in
absolute terms but relative to the dispersal abilities and/or vagility of
species concerned. Hence, as pointed out above, most evidence for the
theory comes from birds and some plants which have great vagility or
disperse well. It is unlikely that it will apply to most invertebrates which
disperse only sporadically and over short distances. For these taxa, disturb-
ances on islands some distance from a colonizing source that lead to
species reductions, will lead to long-lasting nonequilibria. The theory
has been extended to make predictions about metapopulation dynamics,
host-parasite systems, etc. For example, Nowak and May (
1994
)and
Levin and Pimentel (
1981
) have used features of the theory and applied
them to host-parasite systems. Price (
1980
), in particular, has provided
evidence that most parasite communities are commonly not in equilibrium.
It is therefore unlikely that they can be described by the equilibrium
theory of island biogeography.
Hubbel's (
2001
) neutral theory of biodiversity is a generalization of
MacArthur and Wilson's theory of island biogeography. Volkov et al.
(
2003
) have shown that the relative species abundance of tree species on
an island in Panama is better descibed by the neutral theory than a log
normal distribution (but see McGill
2003
). As shown in Chapter
4
(The
packing rules based on fractal geometry and competition) and p. 76, trees
(and herbivorous mammals), in their ecological characteristics, are quite
different from many other groups, and care should therefore be taken
not to generalize these findings.
In conclusion, there is evidence for regulation leading to equilibrium
in some island communities (such as birds and some arthropods), but
application of the island theory of biogeography to other systems (such as
parasites) is controversial.
Inter- and intraoceanic patterns: historical events
and centers of diversity are important
Some authors have claimed that similar numbers of taxa have evolved in
different large regions, such as continents and oceans, indicative of
equilibrium conditions. In the following, I show that species numbers,
at least for certain groups, are quite different in large habitats of compar-
able size and complexity, and that historical events, and in particular
evolutionary time available for acquisition of species, are important
factors determining extant diversity in particular habitats.
