Biology Reference
In-Depth Information
even a branch or a few leaves fallen from a tree may create ''clearings''
large enough for species requiring a small disturbed area. A bird dropping
may represent an empty gap for some species of insects and microorgan-
isms. Such small disruptive events are much more common than larger
disturbances, they are daily or even hourly events. So, it seems likely that
nonequilibrium conditions are the rule for communities of small organ-
isms: there will always be an oversupply of empty gaps ready for coloniza-
tion, and there will always be numerous local communities at different
stages of succession, resulting in great diversity. Future studies should
attempt to analyse such events quantitatively. Advantages for such studies
include the relatively small areas necessary for assessment, and the shorter
time scales. Difficulties include the taxonomic diversity and scanty
knowledge of the flora and fauna.
In conclusion, agreement about the relative importance of various
factors contributing to the maintenance of the great species diversity of
trees in tropical rainforests has not been reached. However, it seems likely
that a range of mechanisms is important, including repeated environ-
mental disruptions, spatial and temporal heterogeneity, operation of
compensatory mechanisms that prevent displacement of rarer species,
and gradual climatic change, among others. It may well be that commu-
nities of small species, whether plant or animal, which are vastly more
speciose than trees, are controlled by a different mix of factors. Quantitative
studies are necessary to provide evidence. Finally, with the exception of
the brief consideration of coevolution (which is not likely to be of any
importance, at least for trees), the discussion was entirely concerned with
the maintenance of diversity, but what about its evolution? Is it possible that a
factor not even mentioned in the various studies, i.e., direct temperature
effects on evolutionary speed, may be the most important? This aspect is
discussed in Chapter
9
(pp. 159-165).
Ectoparasites of marine fish: non-interactive unsaturated
communities
As mentioned in the section ''Species nonsaturation and nonequilibria'',
parasites of fishes are very speciose and represent a major component of
the Earth's fauna. They are ideal objects for ecological studies because they
live in well defined habitats, the number of possible replicas is practically
unlimited, and the distribution of parasites in their microhabitats can be
easily mapped and quantitatively evaluated. Rohde and collaborators
have studied extensively the ecological and zoogeographical aspects of a
