Death and danger at migratory stopovers: problems with “predation risk”

Abstract

related terms, including, ‘‘escape performance’’, ‘‘predation hazard’’, ‘‘vulnerability’’, ‘‘falling victim to’’, ‘‘exposure to predators’’, and ‘‘perceived predation risk’’ has further complicated matters. Exact definitions are lacking, which has allowed a variety of different meanings to march under the common term of ‘‘predation risk’’. To illustrate the definitional problem, imagine a situation in which predators hunt ground-feeding birds. The birds avoid attacks by cautious behavior, such as repeatedly scanning the environment for predators. A field ecologist studying this situation, even for a long time, might observe no or very few predation events, and could conclude that there was little ‘‘risk of predation’’. But if the ground-feeding birds did not scan, the predators could catch them easily, and another field ecologist might conclude that the situation is very ‘‘risky’’. This meaning refers to the probability of mortality that would occur if the birds did not scan. Thus, ‘‘predation risk’’ can mean quite different things. Which meaning is used has a great effect on the interpretation of what is ‘‘biologically’’ or ‘‘ecologically significant’’. Our primary purpose here is to outline possible meanings of terms relating to ‘‘predation risk’’, and suggest a general scheme that relates them one to another. We show that the two hypotheses Dierschke considers are not contradictory alternatives, as he suggests. Rather, they address different meanings of ‘‘predation risk’’ that are in our view not only compatible, but complementary. We suggest that recognizing four concepts will help to clarify thinking and analysis of issues relating to ‘‘predation risk’’: ‘‘mortality’’, ‘‘danger’’, ‘‘escape performance’’, and ‘‘anti-predator behavior’’. To help avoid ambiguity, we will henceforward avoid the term ‘‘predation risk’’ when describing what we mean.