Print PageThe impact of predators on wildlife populations has been hotly debated for decades. Predator effects on prey demography have traditionally been ascribed solely to direct killing. An emerging alternative is that the impact of predators may be far greater than that due to direct killing if the costs of anti-predator responses reduce prey reproduction and increase deaths from other causes. This alternative remains rarely considered in vertebrate conservation and management because it has yet to be experimentally demonstrated in a wild bird or mammal that predation risk alone can affect any of the three determinants of population growth: number of offspring produced per year, juvenile survival or adult survival. The principal challenges include, eliminating direct predation to quantify risk effects alone, and manipulating risk for long enough over a large enough spatial scale. We tested the effect of predation risk alone on the number of offspring produced per year by free-living female song sparrows. Direct predation was actively eliminated by protecting nests using electric fencing and seine netting, while predation risk was manipulated by broadcasting playbacks of either predator- or non-threatening- calls and sounds, every few minutes, throughout the 130 day breeding season, over a 16 ha area.
Results/Conclusions
Predation risk alone reduced the number of offspring produced per year by almost 40 %. Females exposed to predator playbacks throughout the breeding season fledged a total of 3.8 ± 0.4 offspring (mean ± SE) whereas those exposed to non-threatening playbacks fledged 6.0 ± 0.4 (P < 0.001). Continuous video surveillance of nests verified that we successfully eliminated direct predation. Predator playback females produced fewer offspring because they laid fewer eggs, fewer of their eggs hatched, and more of their nestlings expired prior to fledging. Multiple measures indicated that these effects were associated with effects on parental anti-predator behaviours. Predator playback broods were relatively lighter evidently due to the impairment of parental provisioning. As this may be expected to reduce subsequent juvenile survival the effect on population growth is likely to be even greater than that resulting from the 40 % reduction in offspring fledged. We suggest predation risk effects of this magnitude may not be atypical since those reported here correspond with previous correlational results comparing sites that naturally varied in predator abundance. We conclude that the impact of predators on wildlife populations may often be significantly underestimated if the costs of predation risk are not duly considered.
