Biology Reference
In-Depth Information
11
.
What explains the differences
found? A summary, and
prospects for an ecology
of the future
What explains the differences between communities?
Rohde (
1980a
) suggested that animal communities can be arranged in a
continuum from random and unstructured to highly structured, depend-
ing on ecological characteristics of species in the communities. Animals
with little vagility and/or small population or individual size live in largely
empty niche space. They are less subject to structuring mechanisms, in
particular competition, than are large animals or animals that live in large
populations with much vagility (although they may be nonrandom to a
degree because of nonrandom colonization events). The latter have filled
extant niche space to a greater degree, i.e., they are closer to saturation,
and include the predominantly large mammals and birds, and free-living
vagile insects occurring in large populations. (Saturation, however, does
not exclude the possibility of further increases in diversity by subdivision
of niches.) Gotelli and Rohde (
2002
) tested this hypothesis using null-
model analysis to check for nonrandomness in the structure of metazoan
ectoparasites of 45 species of marine fish, and compared the results with
those for herps, birds, and mammals. In parasites, co-occurrence patterns
could not be distinguished from those that might arise by random colon-
ization and extinction. Presence-absence matrices for small-bodied taxa
(parasites, herps) with low vagility and/or small population size were
mostly random, whereas presence-absence matrices for large-bodied
taxa with high vagility and/or large population size (birds, mammals)
were highly structured, supporting Rohde's hypothesis. For Figure
11.1
,
some data from Gotelli and McCabe (
2002
) were also used, which
support the hypothesis even more strongly. It must be realized, however,
that the null-model analysis, even if it reveals structure in communities,
says nothing about the mechanisms responsible for structuring: interspe-
cific competition - suggested to be responsible by Diamond (
1975
)-is
