Who Limits Whom: Predators or Prey?
Roger A. Powell
Department of Zoology, North Carolina State University, Raleigh, North Carolina 27695-7617
Animal populations can be limited by the availability of food (limited from the bottom of the food
chain up), by predators (limited from the top of the food chain down), or by the interaction of
these two processes. Whether carnivores, in particular, limit the populations of their prey, or are
limited by those prey, has long been controversial and is critical to conservation of prey and
predatory species. I return to the question because it is a good question that we wish to have
answered and, in part, because it has no simple answer. Our knowledge of ecological communities
has matured to the point that we can tease the question apart, look at its pieces, and find
conditional answers. Predator-prey models suggest that predators may limit prey populations on
one scale while food limits prey, and prey limit predators on another scale. Predator populations
are always limited by the availability of their prey. Data from the literature supports action on
two time scales.