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therefore mediate the equilibrium number of
species found in a particular environment.
The strength of interactions between species is
known to differ. Some species appear to be
disproportionately important in determining how
their communities are structured. Terborgh (1986)
has shown how only twelve plant species in a flora
of approximately 2000 species at Cocha Cashu
reserve in Peru sustain nearly the entire fruit-
eating animal community for three months of the
year. Although the climate is tropical, flowering
and fruiting are strongly seasonal and there are
periods of the year when there is very little food
available for frugivorous animals. Such species have
become known as 'keystone' species. A keystone
is the wedge-shaped stone at the apex of an arch
that locks the whole structure together. If this
stone is removed the whole arch will fall. If such
species exist, clearly they should be a conservation
priority.
One prediction that stems from the
competition hypothesis concerns the nature of
changes that take place in a vegetation community
in response to disturbance. Catastrophes such as
storms, floods and fires kill off the dominant plants
and create opportunities for pioneer plants to
colonise. As succession proceeds, increasingly
competitive plants invade and grow to dominate
the community while the less competitive
organisms are driven to local extinction.
Ultimately, the community will consist of a stable
mixture of the most competitive species, each with
its own unique niche. A hypothesis put forward
by J.H.Connell (1978) suggested that this
equilibrium condition rarely, if ever, occurs.
Connell pointed out that most natural
environments are repeatedly disturbed by events
of a range of magnitudes and at a variety of time-
scales. A high magnitude or frequency of
disturbance will result in a community with a large
proportion of pioneer and early successional
plants. In contrast, the most competitive plants will
dominate environments with rare or low-
magnitude disturbance. Intermediate levels of
disturbance will allow opportunities for both
guilds of plant to coexist and are therefore likely
to maximise ecological diversity. Natural
environmental disturbance may act in a very
similar way to predation in preventing
interspecific competition from leading to local
extinctions.
Regional-scale processes
Many ecologists are uncomfortably aware that
the standard ecological models assume that local-
scale processes (such as competition and local
disturbance regimes) determine the level of
species diversity. However, as Ricklefs (1987) has
pointed out, if these models were accurate areas
that have very similar physical environments
should support similar number of species.
Community composition should correspond to
limits set by local conditions alone, but there is a
worrying lack of evidence for local determinism.
Ricklefs argues that local diversity is strongly
governed by much larger-scale biogeographical
processes. Ecology, he claims, has ignored these
scales because they are less amenable to
experimentation and analysis. Preoccupation
with trophic and competitive interactions has
misled many ecologists into assuming that closed
population processes alone can explain local
population structure. A biogeographical
perspective is required before population
dynamics can be properly understood.
Ricklefs and Schulter (1993) have proposed
that species diversity is influenced by a hierarchy
of processes, each one acting at a different
temporal and spatial scale (Figure 15.1). At the
very largest scale, global and regional processes
both play a fundamental role in generating the
pool of species from which regional communities
are drawn. Regional processes determine how
species disperse over space. All environments are
patchy. As a direct consequence, the abundance of
individuals is not constant across a species range. It
is likely that there will be peaks of abundance
where conditions are particularly favourable, and
the population will decline or even disappear in
less suitable habitat. Space, therefore is not
continuously occupied by individuals; rather, they
exist in a series of small, interacting sub-
populations. Andrewartha and Birch (1954)
