Biology Reference
In-Depth Information
events were superimposed on a ''walk'' to ever increasing diversity. The
overall picture then is one far from saturation and far from evolutionary
equilibrium. Each new species opens new possibilities for others, i.e., for
parasites, hyperparasites, symbionts, predators, etc., and this must lead to
an ever increasing trend towards higher diversity. Rosenzweig's (
1995
)
claim that the resource base is still the same and therefore sets limits to
species numbers with the implication that, even if newly evolved species
provide opportunities for others, the system as a whole cannot ''get out of
control'' by ever-increasing species numbers, is not supported by the
overwhelming evidence that many niches are vacant, i.e., that resources
are not fully exploited. Also, there is no reason to assume that diversity
cannot increase by subdivision of niches. There may well be a limit to
diversity determined by the minimum possible body size and population
size of species, and by the limited space, energy, minerals etc. available on
Earth, but no evidence indicates that we are even close to that limit.
The common occurrence of nonequilibrium conditions in populations
and communities has important implications for the significance of inter-
specific competition. As shown in Chapters
3
and
4
, much of the
evidence for competition is faulty, although there can be no doubt that
it plays an important role in structuring many communities. But, as
discussed in Chapter
5
, many (and perhaps most) examples given for
competition can also be explained, or are indeed better explained, by
non-competitive mechanisms. Thus, niche restriction and segregation
may simply be a consequence of random selection of niches (habitats,
microhabitats, food resources, etc.) in largely empty niche space. The fact
that species do not expand into adjacent niche space may be due to the
need to specialize. In other words, species need to be adapted to particular
niches in order to survive: species that use too many niches may hang on
for a while in all of them, but they will be pushed over the rim when
environmental conditions become less favourable. Niche restriction may
be further enhanced by the necessity to find mating partners, which may
be impossible if niches are too wide. Finally, segregation may be the result
of reinforcement of reproductive barriers: segregated species cannot
hybridise and produce unfit or less fit offspring.
Much effort has gone into ecological modelling; according to some
authors, it now appears feasible that supercomputers can be used to develop
''a single theoretical model which would be able to describe the entire
dynamics of an ecosystem since the first appearance of life in it up till
