Abstract
The functional response of a predator describes the change in per capita kill rate to changes in prey density. This response can be influenced by predator densities, giving a predator-dependent functional response. In social carnivores which defend a territory, kill rates also depend on the individual energetic requirements of group members and their contribution to the kill rate. This study aims to provide empirical data for the functional response of wolves Canis lupus to the highly managed moose Alces alces population in Scandinavia. We explored prey and predator dependence, and how the functional response relates to the energetic requirements of wolf packs. Winter kill rates of GPS-collared wolves and densities of cervids were estimated for a total of 22 study periods in 15 wolf territories. The adult wolves were identified as the individuals responsible for providing kills to the wolf pack, while pups could be described as inept hunters. The predator-dependent, asymptotic functional response models (i.e. Hassell-Varley type II and Crowley-Martin) performed best among a set of 23 competing linear, asymptotic and sigmoid models. Small wolf packs acquired >3 times as much moose biomass as required to sustain their field metabolic rate (FMR), even at relatively low moose abundances. Large packs (6-9 wolves) acquired less biomass than required in territories with low moose abundance. We suggest the surplus killing by small packs is a result of an optimal foraging strategy to consume only the most nutritious parts of easy accessible prey while avoiding the risk of being detected by humans. Food limitation may have a stabilizing effect on pack size in wolves, as supported by the observed negative relationship between body weight of pups and pack size.
Highlights
The predator functional response was originally defined as the change in per capita kill rate in response to changing prey density (Solomon 1949; Holling 1959a)
This study aims to provide empirical data for the functional response of wolves Canis lupus to the highly managed moose Alces alces population in Scandinavia
We explored prey and predator dependence, and how the functional response relates to the energetic requirements of wolf packs
Summary
The predator functional response was originally defined as the change in per capita kill rate in response to changing prey density (Solomon 1949; Holling 1959a). In its simplest form, the functional response model may be represented by a linear relationship between per capita kill rate and prey density. The dependence of the per capita kill rate on per capita prey availability seems more realistic than dependence on absolute prey density Such ratio-dependent models (RD) (Arditi & Ginzburg 1989) are the least mechanistic among different predator-dependent functional response models and suggest that attack rates will decrease and handling time will increase with increasing predator densities (Arditi et al 2004). Potential mechanisms of predator dependence are (i) lowered individual encounter rates between predators and prey (Beddington 1975; Arditi & Ginzburg 1989); (ii) higher rates of unsuccessful individual attacks or prolonged individual prey handling time due to intraspecific interference (Hassell & Varley 1969; Beddington 1975; DeAngelis, Goldstein & O’Neill 1975; Arditi & Akßcakaya 1990); and (iii) intensified antipredator behaviour of the prey (Crowley & Martin 1989; Skalski & Gilliam 2001; Lima 2009)
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