Geography Reference
In-Depth Information
biologically diverse communities arise or are
maintained any attempts at their conservation are
likely to be largely haphazard.
Ideas abound on the causes of high species
diversity. To a considerable extent other levels of
variation have been ignored. This undoubtedly
reflects the comparative ease with which species
diversity can be quantified and not that this level
of biological diversification is of greater
importance. The development of a rigorous
classification of the functional attributes of an
ecosystem presents a significant challenge for the
environmental sciences.
Species richness depends on the number of
species that are able to maintain populations and
reflects the balance between processes increasing
and decreasing the number of individuals per
species. Some of these processes have been the
focus of a great deal of ecological research. They
can be crudely characterised as falling into two
groups: theories that describe ecological processes
that occur at a local scale, and those that invoke
biogeographical processes occurring at a regional
scale or larger.
hypothesised that the more species that live in a
particular habitat the narrower their individual
resources bases will be. They argued that there
would be a limit to how narrow niches could
become. Beyond a certain point, species
populations would become so small that they
would be highly likely to go locally extinct. This
limit to how similar species can be in their
resource use means that the total amount of
resource available in a particular habitat sets the
upper limit to the number of species that can
coexist. In summary, local conditions, ecologists
argue, determine the maximum number of
species that are found there. Phillips
et al
. (1994)
concluded, from a study of twenty-five tropical
forest sites, that the more dynamic forests are
(measured from their mortality and recruitment
rates) the greater their species diversity. They
hypothesised that high levels of spatial and
temporal variability in dynamic forests provided
opportunities for a wide range of species.
Clinebell
et al
. (1995) have recently demonstrated
that annual rainfall and rainfall seasonality alone
account for over 60 per cent of the variation in
species richness in sixty-nine neotropical forests
spread across the continent.
The idea of ecosystems existing in equilibrium
with environmental conditions has been
elaborated to include the effects of interactions
between species. One species population may
become a resource for another to exploit, for
example. Additionally, predator-prey relationships
may prevent competition between prey species
leading to extinctions. Janzen (1970) proposed that
predators might prevent one species from driving
competing species to extinction by feeding
predominantly on those species that are most
abundant. The more potential prey of a particular
species there are around, the more likely it is that
they will be eaten. This is termed a density-
dependent process, since the rate of loss to
predators depends directly on the abundance of
prey. Competitively superior species populations
may therefore be limited in size by losses to
predators. Predation may thus help to maintain
species diversity by preventing extinctions due to
interspecific competition. Trophic interactions can
Local-scale processes
Research by the Russian zoologist G.F.Gause
during the 1930s demonstrated experimentally
that a population would grow exponentially
until it exhausted the supply of an important
resource. The rate of supply of this resource
would then set the equilibrium population size.
Furthermore, Gause showed that if the
populations of two or more species were limited
by the same resource they would inevitably
compete for supplies. The species that was better
able to capture the scarce resource would increase
its population at the expense of the poorer
competitor. Gause concluded that this weaker
species would be driven to local extinction and
therefore that species could not coexist if they
had a common limiting resource. The corollary
of this idea was that of the 'niche', that all
coexisting organisms must have a unique
combination of resource requirements in order
to avoid competitive interactions. Ecologists
