densities relative to controls. The clear implication is that both bottom-up and
top-down processes are important to the natural regulation of snowshoe hares.
Despite these results, however, none of the treatments dismantled the hare cycle. This
result may have arisen from the use of semi-permeable fencing in the experimental
treatments, allowing hare populations within the exclosures to be driven by dynamics
generated outside, via immigration.
The best interpretation of the existing information is that the snowshoe hare–lynx
cycle is a complex tri-trophic interaction synchronized to some degree by the
exogenous environmental rhythm of the sunspot cycle. These results suggest that
coupled resource–consumer models can be a vital step in understanding complex
patterns of population dynamics that can occur in natural ecosystems.
A resource is something an animal needs and whose consumption diminishes its avail-
ability to other consumers. Consumers and their resources often form a system in
which the rate of increase of the resources is determined by the density of the
animals eating them, and the rate of increase of the animals is determined by the
density of the resources. Such a complex system can be studied only by breaking
it down to its dynamic components, of which three dominate. First, there is the
functional response of the animal, the rate of resource intake by a single consumer
as a function of resource abundance. Second, there is the numerical response of the
consumer, the rate at which its population increases as a function of the resource
abundance. Finally, we require supplementary information on the growth rate of
resources in relation to resource abundance. On the basis of these functional rela-
tionships, the full dynamic behavior of the system can be described. We illustrate
this approach with two well-studied consumer–resource systems: kangaroos and
their plants in Australia, and wolves, moose, and woody plants in North America.
Interactive systems with these components can be deterministically stable (such as
the Australian plant–kangaroo system) or unstable (such as the wolf, moose, and woody
plant system). Deterministic instability is evident in the repetitive population fluctu-
ations (stable limit cycles) or non-repetitive fluctuations (deterministic chaos). Even
stable food chain models can show pronounced long-term fluctuations in response
to stochastic environmental variability (centripetal systems). Two well-documented
cyclical populations (voles in northern Europe and snowshoe hares in North
America) have dynamics consistent with predictions of coupled consumer–resource
models.
CONSUMER–RESOURCE DYNAMICS 215
12.8 Summary
