Biology Reference
In-Depth Information
regeneration requirements often occur close together, application of the
hypothesis may be limited. Reasons for the closeness of such species may
be that environmental gradients close to each other in gaps may be quite
different, and that new gaps may be superimposed on already existing ones.
Experimental and observational evidence given byWright against the import-
ance of the hypothesis are that, in one experiment, more than 80%of shade-
tolerant saplings survived after a gap hadbeenformed,andthatregeneration
of most trees is so slow that they do not reach the canopy in a single gap cycle.
Environmental heterogeneity
The discussion so far has ignored the effect of environmental heterogen-
eity. Species coexistence might be possible in heterogeneous habitats
even in the absence of compensatory mechanisms. Theoretically, each
species in a rich assemblage of species may be adapted to a different
component of the heterogeneous habitat. For example, one species
might prefer a slightly more acid soil than the others, another species
might prefer a harder over a softer substrate, etc. But heterogeneity may
not only be spatial, i.e., exist at a local site, it may also be temporal, due to
climate change. The crucial question is: how important is heterogeneity
in determining diversity relative to environmental disturbances? Connell
(
1979
) argues that it is impossible to test the hypothesis. Among the
difficulties are the practical impossibility of determining the degree of
specialization required, and the impossibility of being certain about the
niche axes relevant for an evaluation. Furthermore and importantly, it is
highly unlikely that the 100 tree species coexisting on 1 hectare of
tropical rainforest have niches narrow enough to permit coexistence, in
particular since tropical rainforest trees of many species often live inter-
mingled in small areas. Nevertheless, environmental heterogeneity will
permit coexistence of some species without the requirement for any
additional mechanism. Concerning spatial and temporal variations in
the environment, Wright (
2002
) emphasizes that, among biotic
resources, it is unlikely that pollinators and seed dispersal agents are
important in promoting plant diversity, because most of these agents are
generalized, and very few, such as wasps pollinating figs, are strictly host-
specific (see above, Novotny et al.
2002
). In contrast, spatial heterogen-
eity is marked and the distribution of many plant species responds to
micro-topographic gradients. However, since many plant species are
distributed across such gradients, it is highly unlikely that the very great
diversity can be explained by spatial heterogeneity alone.
