Biology Reference
In-Depth Information
Wright (
2002
), in his review, concludes that of all the hypotheses listed
above, three are strongly supported. First, there is strong support for the
importance of niche differences: plants are not randomly distributed along
micro-topographical gradients, and there is a tradeoff between survivor-
ship and growth during regeneration. Second, host-specific pests reduce
recruitment near conspecific adults ( Janzen-Connell effect). Third,
negative density dependence over large spatial scales may regulate popu-
lations of abundant species. Finally, Wright suggests that a fourth hypoth-
esis may be important, i.e., that competition is reduced because of
suppressed understorey plants.
The hypotheses considered to be the best supported do not include gap
formation due to disturbances (the intermediate disturbance hypothesis of
Connell), although Wright does not rule out its limited applicability. He
also does not include the possibility that gradual climatic change may be
important. Indeed, such a contribution will be difficult to prove since
effects can only be expected over long periods. Even for the hypotheses
considered to be the most important, Wright concedes that ''implications
for species coexistence and plant diversity remain conjectural. Large size,
low population density, and long generation times may well preclude
experimental evaluation of mechanisms of plant species coexistence in
tropical forests.''
Nonequilibrium in small animals and plants?
This discussion has been concerned entirely with large trees, for the very
simple reason that these are the ones which have been most thoroughly
studied because of their large size, and because they lend themselves
relatively easily to manipulative experiments. However, the vast majority
of tropical rainforest species are much smaller: fungi, algae, microorgan-
isms of various groups among the plants, and a vast array of invertebrates
among the animals, most of them not yet described. There are no long-
term quantitative studies comparable to those made on trees. We can only
speculate on mechanisms responsible for their diversity. Assuming that
the intermediate disturbance hypothesis applies to them, we could pos-
tulate that high diversity would be largely maintained by a high degree
of ''openness'' and ''nonequilibrium'' that is even greater than in the large
tree communities. The strength of disturbances is scale-dependent.
A lightning strike may clear an area of some square meters and enable
early tree colonizers to invade. Much less is needed to create a ''gap'' for
some forest invertebrates, for example land planarians. For such species,
