Biology Reference
In-Depth Information
A further possible compensatory mechanism mentioned above is that
of circular networks (which involves at least two different competitive
mechanisms between species). Connell points out that rainforest trees in
the tropics are too similar for such a mechanism to be likely, although it
remains an as yet unproven possibility.
Connell et al.(
1984
) concluded from the evidence available at the time
that compensatory mechanisms seem to be operative at some spatial
scales, but not at others. Over the whole area, seedling recruitment of
subcanopy and understorey, but not of canopy species was indeed higher
for rarer species. At the scale between nearest neighbors, there was
compensatory growth soon after establishment of seedlings which
stopped after about 6 years. Growth was slower close to conspecifics.
There was compensatory mortality for most size classes. All of this may
prevent local displacement of rarer species by the more common ones.
Concerning the mechanisms responsible, the authors discuss intra- and
interspecific competition, and indirect effects due to microorganisms and
other natural enemies, but a decision as to which mechanisms are respon-
sible is impossible. Concerning the two hypotheses that were tested, these
apply to some species but not to others.
Gap dynamics (intermediate disturbance hypothesis)
In spite of evidence for the existence of compensatory mechanisms, Connell
(
1979
) concluded that the most likely explanation for the high diversity in
tropical rainforests is the occurrence of frequent small disturbances keeping
the system open and in local nonequilibrium. Regional equilibrium may
(or may not) be brought about permitting the persistence of less shade-
tolerant species among the more tolerant ones. Greatest diversity can be
expected in open-nonequilibrium systems that are at an intermediate stage
after a severe disturbance, or which are exposed to smaller and neither very
frequent nor very infrequent disturbances. Connell names this the ''inter-
mediate disturbance hypothesis.'' However, as stressed by Connell et al.
(
1984
),thereisnowayatthisstagetodeterminetherelativeimportance
of compensatory mechanisms, environmental heterogeneity, intermediate
disturbances, gradual climatic change, random variation in conditions affect-
ingreproductiveandmortalityrates,amongothers.
Moreover, Wright (
2002
) has pointed out that very few of the vast
number of species of tropical rainforest trees have been examined with regard
to gap dynamics. Criticism of the hypothesis is based on experimental
evidence and theoretical considerations. Since adults of species with different
