Download Volume 124, Issue 11, pages 1417–1426, November 2015

Mechanisms of coexistence in diverse
herbivore–carnivore assemblages:
demographic, temporal and spatial
heterogeneities affecting prey vulnerability
1. Norman Owen-Smith
Article first published online: 29 JUL 2015
DOI: 10.1111/oik.02218
© 2015 The Author
Oikos
Volume 124, Issue 11, pages 1417–1426, November 2015
Owen-Smith, N. (2015), Mechanisms of coexistence in diverse herbivore–carnivore
assemblages: demographic, temporal and spatial heterogeneities affecting prey
vulnerability. Oikos, 124: 1417–1426. doi: 10.1111/oik.02218
Simple models coupling the dynamics of single predators to single prey populations tend to
generate oscillatory dynamics of both predator and prey, or extirpation of the prey followed
by that of the predator. In reality, such oscillatory dynamics may be counteracted by prey
refugia or by opportunities for prey switching by the predator in multi-prey assemblages.
How these mechanisms operate depends on relative prey vulnerability, a factor ignored in
simple interactive models. I outline how compositional, temporal, demographic and spatial
heterogeneities help explain the contrasting effects of top predators on large herbivore
abundance and population dynamics in species-rich African savanna ecosystems compared
with less species-diverse northern temperate or subarctic ecosystems. Demographically,
mortality inflicted by predation depends on the relative size and life history stage of the
prey. Because all animals eventually die and are consumed by various carnivores, the
additive component of the mortality inflicted is somewhat less than the predation rate. Prey
vulnerability varies annually and seasonally, and between day and night. Spatial variation
in the risk of predation depends on vegetation cover as well as on the availability of food
resources. During times of food shortage, herbivores become prompted to occupy more
risky habitats retaining more food. Predator concentrations dependent on the abundance of
primary prey species may restrict the occurrence of other potential prey species less
resistant to predation. The presence of multiple herbivore species of similar size in African
savannas allows the top predator, the lion, to shift its prey selection flexibly dependent on
changing prey vulnerability. Hence top–down and bottom–up influences on herbivore
populations are intrinsically entangled. Models coupling the population dynamics of
predators and prey need to accommodate the changing influences of prey demography,
temporal variation in environmental conditions, and spatial variation in the relative
vulnerability of alternative prey species to predation.
Synthesis
While re-established predators have had major impacts on prey populations in northern
temperate regions, multiple large herbivore species typically coexist along with diverse
carnivores in African savanna ecosystems. In order to explain these contrasting outcomes,
certain functional heterogeneities must be recognised, including relative vulnerability of
alternative prey, temporal variation in the risk of predation, demographic differences in
susceptibility to predation, and spatial contrasts in exposure to predation. Food shortfalls
prompt herbivores to exploit more risky habitats, meaning that top–down and bottom–up
influences on prey populations are intrinsically entangled. Models coupling the interactive
dynamics of predator and prey populations need to incorporate these varying influences on
relative prey vulnerability.