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The demographic value of existing habitat can be affected by changes in predator populations. In Finland, wader populations increased when the Peregrine Falcon (Falco peregrinus) declined, due to poisoning by agricultural chemicals during the 1960-1970. Following regulation ofDDTusage, nesting Peregrinenumbers have increasedmore than tenfold, while wader populations concurrently declined. The "protection" hypothesis states that although top predators may negatively affect populations of primary prey species, they may benefit other species populations by suppressing activities of mesopredators, such as egg and chick predation.We studied diet preferences of Peregrine Falcons in south-west Finnish Lapland during a period of falcon population growth, based on indices of species consumption versus availability. Preferred and optimal sized prey species, excluding ducks, were equally abundant on bogs with and without Peregrines, and neither did the abundance of non-preferred or non-optimal sized birds differ between bogs with and without Peregrines. Thus, the protection hypothesis was supported for ducks. We found that preferred prey species significantly declined in numbers over time, while populations of non-preferred and non-optimal sized prey species declined less or remained stable, as expected if Peregrines would have direct negative effects on prey species. However, local effects were apparent when comparing local versus national trends of suitable and less suitable sized prey for the Peregrines.While the raptors' role in the population declines of wetland speciesmust have increased after their recovery in the 1970s, this effect might be difficult to separate from other concurrent effects of habitat loss.
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1. Introduction
Predation is a strong interaction in animal communities that can affect prey population sizes (Krebs et al. 2004).Whether predation has consequences on a prey population depends on the ratio between prey and predator numbers and the importance of the prey species in the predator's diet (e.g., Korpimäki et al. 1991). The simplest way of evaluating prey importance for a given predator is by estimating proportions of prey species in a predator's diet. Central place foragers i.e., animals that rear their young in a fixed place,must find food within a limited area, in contrast to free-moving, non-nesting foragers, which are not bound to any specific site within a home range. Raptors that specialize on birds generally have relatively low prey densities around their nests, which may be...