Biology Reference
In-Depth Information
plant communities are stable entities consisting of competing species -
and that species coexist because each has a ''niche.'' This paradigm, in its
extreme, has been dead for some time. Nevertheless, it has yet to be
replaced by a credible ''non-equilibrium paradigm.''
Colwell (
1984
) doubts that community models can be precise, and he
emphasizes the need to acknowledge the very great differences in the
biologies of various kinds of organisms. Importantly, he states that ''obser-
vation, logical inference, and plausibility arguments are sometimes as
capable of scientific revelation as experiments and statistics.'' Schoener
(
1986a
) has made a detailed attempt to establish a theory of community
ecology based on six organismic and six environmental axes, but Lawton
(
1999
) expresses scepticism about its usefulness and about the possibility
of ever establishing in general a useful model for even small communities
comprising say 10 or 20 species, with few exceptions, of which he
mentions lake communities, because of the latter's simple trophic struc-
ture and few key species.
Equilibrium, and disturbance leading to nonequilibrium
We saw earlier that apparent equilibria may well exist in populations, and
that nonequilibria may arise due to disturbances and other factors. The
same holds for communities. We begin the discussion with two examples,
one providing evidence for apparent equilibrium, the other for non-
equilibrium after disturbance. We then discuss some theoretical consid-
erations and provide more experimental evidence, as well as evidence
from invasions.
A particularly impressive example of insect populations controlled by
a parasitoid, and kept in apparent equilibrium over long periods, was
studied by Murdoch and collaborators. Over many years, they studied
the California red scale, Aonidiella aurantii, an insect pest of citrus, and its
biological control agent, the parasitoid wasp Aphytis melinus (Murdoch
1994
). The wasp controls populations of the scale very effectively. Over
decades, densities were less than 1% of those present before the wasp was
introduced. In one example, 10-year sampling (20-30 generations of
the scale, 50-80 generations of the wasp) showed only minor fluctuations
around a mean density that was apparently constant. There was
no evidence for local extinction. The constancy of population size
contradicts theoretical predictions, according to which, populations
suppressed far below their resource limits, should be unstable with
major fluctuations. How can this be explained?
