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