Biology Reference
In-Depth Information
(pp. 41-44, 76). They are likely to apply only to those species that are
indeed closely packed, i.e., use up much of the resources required by
them, thereby competing for them. These taxa have large SES values, as
neutral theory of biodiversity is applicable only to species high in the
ecological hierarchy.
A summary, and prospects for an ecology of the future
There is evidence for equilibrium conditions at all levels. Thus, there has
been evolutionary stasis over long periods (see, for instance, Ordovician
benthos); some communities appear to be saturated and in apparent
dynamic equilibrium, at least for certain periods (e.g., island communities
of birds); and equilibrium conditions are common in populations (e.g.,
some insect populations). However, at all of these levels, periods of
equilibrium are often interrupted by even longer periods of nonequilib-
rium. Thus, in evolutionary history, periods of increasing diversity are
more pronounced than periods of stasis, and there has been a significant
increase in diversity over evolutionary time. Evidence for nonsaturation
and nonequilibrium in ecological communities is more convincing than
evidence for saturation, as shown, for example, by very large differences
in species numbers of parasites infecting different fish species, and by
temporary reductions in species richness due to disturbances. At the level
of populations, environmental disruptions are prevalent and so common
that most populations probably cannot persist in equilibrium for pro-
longed periods, and re-establishment of equilibrium conditions may take
many years, as shown by some well documented examples of insect
populations. Nonequilibrium at the different levels is due to different
mechanisms: at the population level, nonequilibria are mainly due to
environmental (biotic and abiotic) disturbances; at the community level,
they are mainly due to the existence of vast numbers of vacant niches but
also disturbances; on an evolutionary scale, they are due to mass extinc-
tions, numerous vacant niches in communities, and as yet uncolonized
vast habitats, where whole ecosystems can still be established. For exam-
ple, the muddy ocean floor was first colonized in the early Ordovician
leading to rapid diversification of benthic animals, and invasion of land led
to an explosion in the numbers of species of various taxa. Diversity was
reduced several times as a result of crashes caused by external factors (e.g.,
mass extinctions due to asteroid impacts or other events), and periods
of long evolutionary stasis were apparently due to a loss of evolutionary
