A new look at predator prey relationships

A new paper by University of Notre Dame biologist Gary Belovsky appearing in the latest issue of journal Ecology Letters drives home the point that predator-prey relationships are much more complex than originally thought.

Belovsky studied   how behavioral responses of grasshoppers to avian predators affected grasshopper survival and reproduction at different grasshopper population densities. He arranged a series of cages containing grasshoppers. These cages were enclosed within a tent constructed of aviary netting. He designated it as a "no threat" area because its design prevented birds from approaching the cages and "scaring" the grasshoppers. A second set of cages were provided that were not enclosed in tent. Belvosky designated it as a "threat" area .This allowed birds to feed around the cages, perch on top consuming grasshoppers caught outside the cages and "scare" the grasshoppers inside the cages.

The research demonstrated that grasshopper behavior changed with the threat of predators. The behavioral changes with the threat of predation increased survival at low grasshopper densities. Reduced feeding made food available to more individuals. While the changes decreased survival at higher densities, food shortages were made worse by reduced feeding. The behavioral changes decreased per capita reproduction over all grasshopper population densities.  Behavioral changes increased survival at low grasshopper densities and then decreased survival at high densities. At low grasshopper densities, the total reproductive output of the grasshopper population remained unchanged with predation threat, but declined at higher densities.

Usually this type of variable response is overlooked when prey predation relationships are considered. The researcher says resource availability may need to be considered when assessing how prey behavior changes with predation and how the threats affect population and food web dynamics.

Prey change behaviour with predation threat, but demographic effects varies with prey density: Experiments with grasshoppers and birds

Gary E. Belovsky, Angela Nardoni Laws. Jennifer B. Slade

Ecology Letters (2011) 14: 335–340