Biology Reference
In-Depth Information
So, the general conclusion from the discussion at this time is that several
compensatory mechanisms, as well as environmental heterogeneity, may
play some role, as may disturbance at intermediate levels. ''Gaps'' are
commonly produced in tropical rainforests by lightning, strong winds,
and landslips, and these gaps are invaded by early colonizers followed by
other species, thus maintaining high-diversity communities, since the
potentially dominant species never have the chance to completely dis-
place all or most of the other, less dominant species.
Coevolution
Concerning evolutionary factors potentially responsible for the great
diversity, Connell (
1980
)discussedtheroleofcoevolution in rain-
forest trees and concluded that it is highly unlikely to have any
great importance, because there are too many species of nearest
neighbors. In a Queensland rainforest, the mean number of tree
species with a height greater than 0.5m per 10
10m plot had
57 other trees among the nearest neighbors. Among the nearest
neighbors of less common species, not a single species appeared
more often than once. Hence, coevolution between any of these
tree species is extremely unlikely.
Some recent work
Wright (
2002
) reviews much of the work done on mechanisms that
permits coexistence of so many species in tropical rainforests, including
much work done after Connell's studies were published. He discusses the
following, not mutually exclusive, main hypotheses:
(1) Rare species are favoured demographically by compensatory mortality,
caused by allelopathy, intraspecific competition, and/or reduction of
recruitment near conspecific adults by host-specific pests (the latter
referred to as ''Janzen-Connell hypothesis'');
(2) Species do not have the opportunity to compete because of recruit-
ment limitation, and/or because of low understorey densities;
(3) The environment is neither temporally nor spatially constant, leading
to regeneration niches or gaps;
(4) Time for competitive exclusion is insufficient because of intermediate
disturbances (Connell) or chance population fluctuations (Hubbell), or
because communities are in dynamic equilibrium (Huston);
