PREDATION DANGER IN THE EVOLUTIONARY ECOLOGY OF MIGRATION

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Summary of proposal Predators kill and consume prey, and prey use defensive behaviour, physiology and morphology to reduce the danger. The mortality that predators inflict (termed `direct killing') impacts prey populations, but the defenses induced in prey also have impacts, termed `risk effects'. Over the past two decades ecologists have made the surprising discovery that risk effects can be as big or bigger than direct killing. The reason is that defenses (`safety-seeking') requires prey to forage less, be more cautious, or to invest in defensive morphology, all of which require time and energy that could otherwise be devoted to acquiring the resources for survival and reproduction. Although an active area of investigation in ecology, it appears that half or more of the total impact of predators can typically be attributed to risk effects. The impacts on ecosystems when top predators are introduced (or removed) therefore extend far beyond the number of prey individuals killed (or spared), because prey respond by raising (or lowering) the level of their defenses, which in turn affects other trophic levels. All of this is exemplified by the now-famous mid-1990s re-introduction of wolves to Yellowstone National Park. Raptor (eagles, hawks, falcons) recovered strongly in North America after DDT was banned in the early 1970s. Falcon numbers, for example, are now at least 3 - 7-fold higher. At the same time, the numbers in other avian groups have declined strongly, which has alarmed conservationists and sent them searching for causes. Generally, no strong evidence points to any of the suspected reasons. In contrast to other areas in ecology, avian ecology and conservation has been slow to embrace the concept of risk effects, and the possibility that raptor recovery has initiated a continental-scale version of the Yellowstone wolf reintroduction, with risk effects (partially) responsible for the strong declines in bird populations, has not been considered. This is a plausible hypothesis, as the few studies on large-scale systems suggest that the risk effects alone are large enough to send populations into decline. I focus on falcons and shorebirds, and propose to estimate the relative contributions of direct killing and risk effects to the 70% declines registered in North American shorebird census numbers since the 1970s. My studies of falcons and long-distance migratory shorebirds of the past two decades have identified a number of candidate behaviours that could generate risk effects, some of which in theory appear large enough to be able to account for a substantial portion of the decline. These possibilities are almost entirely unappreciated and remain uninvestigated by the avian conservation research community. The results will help conservation work by assessing the potentially strong influence of risk effects