Biology Reference
In-Depth Information
mortality due to an increase in predation at high prey population dens-
ities. One could add frequency-dependent mortality due to greater risk
of disease caused by parasites or microorganisms. Connell mentions
another mechanism, that of ''circular networks.'' A circular network
exists, for example, if species 1 out-competes species 2, species 2 out-
competes species 3, but species 3 out-competes species 1. A different
competitive mechanism must be involved for at least one species in the
chain. Co-occurrence in a single cell is not possible, because one species
will finally win unless the various species are exactly balanced in their
chances to survive, a highly unlikely event. If species occur in pairs in
different cells in an open, locally nonequilibrial system, the whole
system will stay in equilibrium if dispersal and extinction rates are the
same for the species, even though in single cells only one species will
survive. In other words, these compensatory mechanisms work only in
open systems.
Connell (
1979
) summarizes these considerations as follows. Early
colonizers (which cannot invade occupied sites) are abundant and dom-
inate the cover if moderate to large disturbances occur frequently; they
are not replaced and die out when disturbances are small and frequent, or
when infrequent disturbances occur at an intermediate time after a large
disturbance; early colonizers are absent when disturbances are very small
and infrequent, or when disturbances occur a long time after a large
disturbance. Late colonizers (which cannot invade open, exposed sites)
are absent or rare (near gap edges) when disturbances are moderate to
large and frequent; they are common and invade in gaps or shade, when
disturbances are small and frequent, or when infrequent disturbances
occur at an intermediate time after a large disturbance; late colonizers
occur in open nonequilibrium or open equilibrium if compensatory
mechanisms permit coexistence, or they occur in closed equilibrium
with a single dominant species, if compensatory mechanisms are not
operative.
Connell (
1979
) then evaluates evidence from tropical rainforests, based
on his own studies and those of others, to explain how these theoretical
considerations conform to what is happening in nature. Firstly, he points
out - referring to work by Eggeling on a tropical rainforest in Uganda,
and by Jones on a tropical rainforest in Nigeria - that evidence supports
the view that a single species will become dominant if disturbances are
rare: the canopy in both rainforests was dominated by large, old trees of
a few species. In the Uganda rainforest, colonizing, mixed, and climax
stands could be distinguished. In the colonizing stand spreading into
