Cougar (Puma concolor) appear to be having an inverse density-dependent effect,
destabilizing bighorn sheep (Ovis canadensis) populations in the Sierra Ladron of New
Mexico, USA. These effects occur because cougar prey primarily on domestic cattle,
which therefore subsidize the cougar population in this area (Rominger et al. 2004).
The introduction of exotic predators and their exotic primary prey in Australia and
New Zealand has caused declines and extinctions of endemic marsupials and birds.
Thus, red foxes that depend on European rabbits and sheep carrion are able to drive
black-footed rock-wallabies (Petrogale penicillata) and other marsupials to extinc-
tion in Australia (Kinnear et al. 1998; Sinclair et al. 1998). In New Zealand, stoats
(Mustela erminea), black rats (Rattus rattus), and brush-tailed possums (Trichosurus
vulpecula) that depend upon exotic house mice (Mus domesticus), a variety of exotic
passerine birds, and fruits are driving endemic birds such as kokako (Callaeas
cinerea) and yellowheads (Mohoua ochrocephala) to extinction (King 1983; Murphy
and Dowding 1995); experimental reductions of these predators have allowed an increase
in the endemic birds (Elliott 1996; Innes et al. 1999).We have seen how the behavior of predators can influence the nature and degree of
predation. We will now examine how the behavior of prey affects predation rates.If a prey species can migrate beyond the range of its main predators, then their popu-
lations can escape predator regulation (Fryxell and Sinclair 1988a). This has been
shown theoretically (Fryxell et al. 1988a) and there are some examples supporting
this idea. The explanation for this escape from predator regulation is that predators,
with slow-growing, non-precocial young, are obliged to stay within a small area to
breed. In contrast, ungulate prey, with precocial young, do not need to stay in one
place because the young can follow the mother within an hour or so of birth. Thus,
the prey can follow a changing food supply while the predators cannot. For example,
the wildebeest migrations in Serengeti can follow seasonal changes in food and are
regulated by food abundance; meanwhile their lion and hyena predators, although
commuting up to 50 km from their territories, cannot move nearly as far as the wilde-
beest. Other examples from Africa are reported for wildebeest migrations in Kruger
National Park, South Africa (Smuts 1978), and white-eared kob (Kobus kob) in Sudan
(Fryxell and Sinclair 1988b). In North America a similar escape from predation is
suggested for migrating caribou herds – the George River herd in Quebec (Messier
et al. 1988), the barren-ground caribou (Heard and Williams 1991), the Wells Gray
Park mountain caribou through altitudinal migration (Seip 1992), and possibly the
“forty-mile” caribou before hunting reduced the herd (Urquhart and Farnell 1986).Theoretical studies propose that animals can reduce their risk of predation by form-
ing groups, herds, or flocks (Hamilton 1971), and that group sizes should increase
with increasing predator densities (Alexander 1974). The benefit from avoiding
predators, however, is counteracted by the cost of intraspecific competition within
the group. There should be some group size where the benefit–cost ratio is optimized
(Terborgh and Janson 1986).
The effect of predators on herding behavior is illustrated in Fig. 10.11. The rela-
tionship between muskox (Ovibos moschatus) group size and wolf density suggests
that predators are the most likely explanation for differences in group size in differ-
ent populations (Heard 1992).176 Chapter 10
10.8 Behavior of the prey
10.8.1Migration
10.8.2Herding and
spacing