moose densities go up (Gasaway et al. 1992). Other studies in Alaska and Yukon
have repeated these wolf removals and show similar increasing moose and caribou
populations (Boertje et al. 1996; Hayes and Harestad 2000). In a natural experiment
where red foxes were removed for several years by an epizootic of sarcoptic mange,
prey numbers increased, particularly those of hares and several grouse species
(Lindstrom et al. 1994). In general, predator removal experiments show that the prey
population increases or that some index such as calf or fledgling survival increases.
The observations in Table 10.1 and Fig. 10.1 appear to go in opposite directions.
No interpretation of cause and effect is possible because they represent correlations.
We cannot tell, for example, whether the predators are truly regulating the prey at
levels well below that allowed by the food supply or whether predators are catching
those that are suffering from malnutrition (so that predators are not regulating), or
whether both processes are occurring. Prey availability is influenced by a number of
factors: (i) whether there are alternative, more preferred, prey in the area; (ii) the
size of this and other prey populations; (iii) the vulnerability of different age and
sex classes; (iv) whether the predators specialize on particular prey; and (v) how the
environment effects the efficiency of the predators in catching prey. To understand
these processes we need to understand the behavior of predators.In order to interpret predator–prey interactions we must first understand how pred-
ators respond to their prey. We ask three questions. How do predators respond to:
(i) changes in prey density; (ii) changes in predator density; and (iii) differences in
the degree of clumping of prey? We look at these in the following three sections.The response of predators to different prey densities depends on: (i) the feeding
behavior of individual predators, which is called the functional response(see also
Sections 12.4 and 12.5); and (ii) the response of the predator population through
reproduction, immigration, and emigration, which is called the numerical response
(see also Section 12.5) (Solomon 1949). We deal with the functional response first.
Understanding of the functional response was developed by Holling (1959). If we
imagine a predator that: (i) searches randomly for its prey; (ii) has an unlimited appetite;
and (iii) spends a constant amount of time searching for its prey, then the number
of prey found will increase directly with prey density as shown in Fig. 10.2a. This
is called a Type Iresponse. For the lower range of prey densities some predators
may show an approximation to a Type I response, such as reindeer feeding on lichens
(Fig. 10.3), but for the larger range of densities these assumptions are unrealistic.
For one thing, no animal has an unlimited appetite. Furthermore, a constant search
time is also unlikely. Each time a prey is encountered, time is taken to subdue, kill,
eat, and digest it (handling time, h). The more prey that are eaten per unit time (Na),
the more total time (Tt) is taken up with handling time (Th) and the less time there
is available for searching (Ts) (i.e. search time declines with prey density, N).
Thus, handling time is given by:Th=hNa (10.1)and total time is:Tt=Th+Ts (10.2)PREDATION 165
10.5 The behavior of predators
10.5.1The functional
response of
predators to prey
density