Environmental Engineering Reference
In-Depth Information
cent (within the last eighty years). Caughley instead proposed that elephants
actually undergo stable limit cycle dynamics. Using tree ring counts of long-
lived baobab trees, Caughley reconstructed the age-structure of the tree
population. He noted that the age-structure was sharply bell-shaped and
that the most abundant trees were 140 years old.This age-class of baobabs
may have arisen when elephants were low in abundance. By corollary, when
elephants were at the peak of their cycle, baobabs would be rare because
they would have been highly exploited by elephants.Thus, if elephants and
baobabs undergo limit cycle dynamics, the time for one complete cycle
(peak to peak) would be, on average, 280 years. Allowing for some asyn-
chrony between elephants and baobabs, a more conservative estimate of the
cycle time is on the order of two hundred years.
Caughley was careful to note that this hypothesis, although plausible
based on the data, would require further testing before ascribing a high de-
gree of reliability to it. Nevertheless, the study illustrates that one can rea-
son through alternative hypotheses using simple ecological principles. More
importantly, if the limit cycle hypothesis is correct, then undertaking a cull
(solution under alternative hypotheses) could seriously perturb the long-
term dynamics of the system, perhaps leading to collapse of the managed
population. Caughley's point is that one can and should take precautions
to think through the alternatives before undertaking management actions
that could have serious irreversible effects if they are based on misguided
presumptions.
Competitors and Predators
The preceding discussion of population dynamics assumed that the only
factors limiting the maximum size of a population at some location was
competition for fixed supplies of resources or space.That is, I assumed that
the factors influencing the dynamics of the population were
intrinsic
to the
population itself. However, this assumption may not always be true. In re-
ality, all species populations face
extrinsic
limiting factors because they are
embedded in food webs in which they must compete with individuals of
other species for resources or space. Moreover, they also may be subject to
predation.
We can deduce the effect of competitors and predators on a focal species
population using the fitness principles underlying figure 4.2a. In essence,
individuals of a competitor species can have the same qualitative effect on
individuals within a focal species population as individuals of the same
