Geoscience Reference
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on ungulates in the greater Yellowstone area (Boyce 1992b), I explored the
dynamics of an age-structured model for elk and wolves. However, qualita-
tively the dynamics did not appear very different from those emerging from
the model without age structure. My model was designed to encourage users
to experiment with management alternatives, so speed of computation was an
important consideration. The age-structured model was much slower, so I
decided to abandon age structure to enhance the user interface. Dixon et al.
(1997) questioned my conclusion that the dynamics were not altered much,
partly because of the strong age-specificity of wolf predation (Mech 1995). But
after reconstructing an age-structured model, Dixon et al. (1997) ultimately
came to the same conclusion that the dynamic patterns were similar in the
structured and unstructured models.
Functional response
The shape of the functional response has major influence on the ultimate
dynamics between predator and prey; therefore, accurately characterizing the
functional response is one of the most important steps in developing a useful
predator-prey model.
Functional responses generally have been viewed too simply. The focus has
mostly been on characterizing the relationship between rate of prey capture
and the density of prey or, in the case of ratio-dependent models, the ratio of
prey to predators. In nature, however, the rate at which killing occurs is as
much a function of the conditions for predation and the vulnerability of prey
as it is a function of the abundance of prey. For example, wolves prey on indi-
viduals in poor condition or those rendered more vulnerable because of deep
snow (Mech 1995).
Spatial structure
Spatial heterogeneity is argued to stabilize predator-prey interactions (Huf-
faker 1959), but this generalization is overly simplistic. Recent attempts to
model parasitoid-host interactions in a cellular automaton revealed that dis-
persal can result in complex spatial patterns in the abundance of parasitoids
and their prey (Commins et al. 1992). Similar results have been described for
integrodifference equations for predator and prey (Neubert et al. 1995).
Predators can be responsible for sinks in source-sink systems (Pulliam 1988),
and dispersal capability of predator and prey can have strong implications for
their dynamics. Raptors with high dispersal capability tend to show strong
regional fluctuations in abundance, whereas species that disperse less show
