Biology Reference
In-Depth Information
8
.
Some detailed examples
at the community level
Tropical rainforests: how is diversity maintained?
General considerations
Tropical rainforests are among the most diverse ecosystems on land.
Amazonian rainforests, for example, can have more than 280 species of
trees in one hectare (with a diameter at breast height of at least 10 cm), and
a 0.52 km
2
plot in Borneo had 1175 species (with a diameter at breast
height of at least 1 cm) (references in Wright
2002
). Diversity further
down in the hierarchy is considerably greater, and it is much more
difficult to assess: many if not most species have not yet even been
described. For these reasons, most detailed long-term ecological studies
that have been conducted in rainforests are concerned with the larger
species, i.e., trees. Wright (
2002
) reviews much of the work done on
mechanisms that permit coexistence of so many species in tropical rain-
forests. Connell and various co-workers have made long-term studies of
rainforests and equally diverse marine systems, i.e., coral reefs in eastern
Australia, extending over more than 30 years. I discuss their work on
tropical and subtropical rainforests in northern and southeastern
Queensland in some detail, because it tests clearly formulated hypotheses
using long-term extensive and intensive data sets from habitats little
disturbed by man (Connell
1978
,
1979
; Connell et al.
1984
; Connell
and Green
2000
).
The main problem Connell addresses is the degree to which equili-
brium or nonequilibrium conditions contribute to the great diversity in
tropical rainforests. Nonequilibrium may, for example, be due to fre-
quent and reasonably strong disturbances that prevent the establishment
of equilibrium, and thus lead to the survival of many competitively
inferior species. Equilibrium will develop when such disruptions are
absent, or weak and infrequent, and when compensatory mechanisms
that prevent replacement of rare species by one or a few dominant ones
