What Explains Forest Grouse Mortality: Predation Impacts of Raptors, Vole Abundance, or Weather Conditions?

Risto Tornberg,Erkki Korpimäki,Vitali Reif

doi:10.1155/2012/375260

Risto Tornberg, Erkki Korpimäki + Show 1 more

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https://doi.org/10.1155/2012/375260

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Abstract

We investigated predation rates of black grouse chicks during 1985–2007 in two localities in western Finland in light of three predation hypothesis: The Alternative Prey Hypothesis (APH) stating that vole-eating generalist predators cause a collapse in grouse reproduction after voles’ decline, the Main Prey Hypothesis (MPH), where grouse specialised predators by a lagged response cause an inversely density dependent predation for prey and the Predation Facilitation Hypothesis (PFH), where generalist and specialist predators act in concert. We also studied the effect of weather on grouse reproduction. We found that buzzard predation alone did not support APH, but did so when combined with goshawk predation. Kill rate by goshawks showed a linear response for black grouse chicks but was not density dependent. It, however, explained the losses of chicks but not their autumn density. Combined density of chicks with adults correlated with vole index in the latter study period (since 1994), thus, giving some support for APH. Weather seemed to have no effect on black grouse reproduction. Although buzzards and goshawks took, on average, only 10% of hatched grouse chicks we conclude that the among-year survival pattern of juvenile forest grouse may largely be determined by raptor predation.

Highlights

Predation mortality has been suggested as one of the forces that drive population cycles of small game animals
We investigated predation rates of black grouse chicks during 1985–2007 in two localities in western Finland in light of three predation hypothesis: The Alternative Prey Hypothesis (APH) stating that vole-eating generalist predators cause a collapse in grouse reproduction after voles’ decline, the Main Prey Hypothesis (MPH), where grouse specialised predators by a lagged response cause an inversely density dependent predation for prey and the Predation Facilitation Hypothesis (PFH), where generalist and specialist predators act in concert
The weight proportion of black grouse hens in the goshawk diet tended to increase in SO but slightly tended to decrease in Oulu, whereas proportions of grouse chicks tended to increase in both areas (Figure 1), but none of the trends were statistically significant

Summary

Introduction

Predation mortality has been suggested as one of the forces that drive population cycles of small game animals (forest grouse and hares). The predators hunting on small game can be tentatively divided into two groups: mammal-eaters, which mainly subsist on small mammals (voles, mice, and shrews), and bird-eaters, which prey on small mammals only when vole abundance is high. Impacts of the former group on reproduction of small game have been described by the alternative prey hypothesis (hereafter APH). The impact of such predators on small game depends on their functional and numerical responses to fluctuations of small mammal populations. But connecting fluctuations of snowshoe hares (Lepus americanus) and ruffed grouse (Bonasa umbellus), was discovered in Canada [14]

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