OOS 20-6
How demographic, temporal and spatial heterogeneities contribute to the coexistence of large mammalian herbivores with large carnivores in African savanna ecosystems
African savanna ecosystems commonly support five large mammalian carnivores along with 20 or more species of large herbivore. While body size distinctions structure the predator-prey linkages, explaining how these predators and their prey coexist requires understanding the interplay of demographic composition, seasonal and annual variation in conditions, and spatial variation in risks. The lags that are inherent in the responses of carnivore populations to changing prey availability tend to generate oscillatory dynamics, which can be suppressed by these heterogeneities, coupled with shifting prey selection by the carnivores. Moreover, carnivores can shift facultatively between hunting and scavenging to varying degrees. To exemplify these relationships, I draw primarily from studies on the dynamics of large herbivore populations in South Africa’s Kruger National Park, where more information is available than for other African savanna ecosystems. Moreover, distinct herbivore population responses have been observed there, including climatically entrained oscillations, narrowly regulated abundance levels, resilient recovery and progressive declines towards local extirpation.
Results/Conclusions
A narrow margin separates levels of predation that can be sustained from those inducing herbivore population declines. While juveniles are killed by multiple carnivores, the largest ungulates are subject to predation once adult only by lions, while megaherbivores are vulnerable to predation only while juvenile. Because all animals are eaten sooner or later, predation merely advances mortality. The extent to which mortality incurred via predation is additive or compensatory depends on whether carnivores hunt by stalking or coursing. Risk is affected by how rainfall variation influences vegetation growth, and hence both food production for herbivores and cover for stalking carnivores. Accordingly, susceptibility to predation varies diurnally, seasonally and annually, but with distinctions in the responses of particular herbivore species to these conditions. Herbivore population responses depend additionally on whether the species is primary or secondary prey for particular carnivores. Drawing on Kruger Park data, I illustrate how these heterogeneities enable the persistence of particular herbivore species, while the viability of other species may be prejudiced by circumstances suppressing the effectiveness of these mechanisms. Most fundamental for facilitating persistence is spatial variation in risk, by providing refugia conferring sufficient security even under the worst conditions.