Only in Africa by Owen-Smith Norman

Author:Owen-Smith, Norman [Owen-Smith, Norman]
Language: eng
Format: epub
ISBN: 9781108832595
Publisher: CambridgeUP
Published: 2021-08-23T00:00:00+00:00

What Difference Does Predation Make?

All animals ultimately die, but their deaths may occur somewhat sooner through the agency of a carnivore and reduce their reproductive contribution to maintaining a population. However, with fewer mouths left more food is available to feed the animals that remain and promote their reproductive success. This mechanism compensates to some extent for the losses of animals killed by predators. Hence the additive effect of predation on prey abundance is somewhat less than commonly surmised. The indirect effects of the risk of predation on where herbivores can safely feed and rest can be much greater.20

Moreover, predation usually interacts with resource limitations.21 While food remains plentiful, herbivores can restrict their movements to places where they are most secure from predation. When food runs short, in the dry seasons of dry years, they are forced to take greater risks while seeking food, increasing their exposure to predation. Hence although almost all herbivores die through the agency of a predator, the mortality incurred by the population depends on the prevailing rainfall conditions.7 The main effect of predation is to restrict habitat occupation to more secure places, most of the time. For example, in Kruger NP wildebeest herds remain settled within open glades with short grass providing little cover for predators for most of the year, day and night, until little grass remains, before venturing out into more densely wooded areas.22,23

The reason why wildebeest are orders of magnitude more numerous in Serengeti than in Kruger NP is due not simply to better-quality grazing, but also to the vast extent of open grassland where they are relatively secure from being ambushed by lions. The huge concentrations of migratory ungulates overwhelm the capacity of lions to have much additive impact. About 15 percent of wildebeest deaths and 30 percent of zebra deaths result directly from starvation, and many of the wildebeest eaten by lions are scavenged from hyena kills.24 Nevertheless, the densities of resident ungulates, including topi, impala and warthog, increased in northern Serengeti after lion numbers there were reduced as a consequence of illegal hunting concentrated on buffalo, despite the fact that the species responding positively were not the primary prey species of lions.25

However, there are circumstances in which predation can have a substantial impact on prey abundance, via a nexus of interactions among alternative prey species. In Kruger NP, several of the less-common herbivore species declined to 20 percent or less of their former abundances, shortly following a severe drought (Figure 12.2). Their downward trends were associated with a two- to threefold increase in adult mortality, implicating increased predation specifically by lions.26,27 Lions had increased following a doubling in zebra numbers. Zebras increased because park managers had provided additional waterholes, aimed at alleviating the effects of droughts. Lions turned to hunting the less-common ungulate species, which became more vulnerable to predation than their preferred prey, i.e. wildebeest, zebra and buffalo, as a consequence of the drought for various reasons. Notably, it was the alternative prey species, rather than the primary prey species, that were most susceptible to elevated predation.

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