Environmental Engineering Reference
In-Depth Information
4.2) and the population should remain at or near thirty individuals.The fact
that the population can rebound to a hundred individuals within a short time
span indicates that thirty is not the carrying capacity for the deer herd. One
might counterargue that the population rebounds because of lag effects (fig-
ure 4.3). If this were true, then the population would eventually crash down
in size on its own, because deaths would exceed births when populations
exceeded their carrying capacity.Thus, culling would not be warranted—
natural processes would take care of the overabundance problem.
There are a couple of reasons why an estimated carrying capacity of
thirty is probably wrong. First, management obtained this number from
other regions containing deer, not the particular preserve itself. As pointed
out above, carrying capacity (equilibrium) can vary over space. So there is
no guarantee that an estimate derived at one location applies exactly at an-
other. Second, managing a public nature preserve is tricky business because
it requires reconciling conflicting values of society.The choice of thirty for
the carrying capacity in this case is more than likely based on management's
implicit
value to protect those many services of the preserve (e.g., songbird
abundances or mature stands of forest) that are sought by society.This im-
plicit value is in large measure the cause for the thinning of the herd.
Value Trade-offs and Ecological Implications
To disentangle science and values, consider the following set of concepts
from Sinclair (1997). For the sake of argument, and to keep the concepts rel-
atively straightforward, let's suppose that management must reconcile only
two competing values: a large deer herd versus a densely wooded forest (im-
plicitly carrying with it the attendant services of the forest such as song-
bird and wildflower species diversity). These competing interests are
illustrated graphically where the axes represent the abundance of deer and
forests in figure 4.4.
This graph immediately makes explicit the source of the management
problem. Namely, that there is a trade-off in that management, and society
for that matter, cannot have maximum values of all possible services within
the preserve. At one extreme (the bold circle) we have maximum forest
cover, but this can only be achieved if the deer herd is eliminated entirely.
This is, effectively, a carrying capacity of the forest itself. If we allow deer to
enter the preserve, then we must forego some forest cover because deer must
utilize forest vegetation for sustenance. At the other extreme (the bold
square) we have a maximum deer population size that the reserve can
