Predator-prey interactions of raptors in an arctic environment

Abstract: This thesis concerns the predator-prey interactions of three raptor species in a Swedish arctic community: the gyrfalcon (Falco rusticolus), the rough-legged buzzard (Buteo lagopus) and the golden eagle (Aquila chrysaetos).The gyrfalcon behaved like a highly specialised ptarmigan (Lagopus spp.) predator. Gyrfalcon’s functional response to ptarmigan was close to density independent, and ptarmigan remained the dominating prey even in areas with the lowest ptarmigan density. The gyrfalcon did not respond functionally to microtine rodents (i.e. lemmings and voles) and it was clear that the gyrfalcon did not use microtines as an alternative prey category to ptarmigan. As the gyrfalcons did not switch to any alternative prey when ptarmigan was scarce, their reproductive success seemed to be directly dependent on the amount of ptarmigan available in the breeding territories. Of the two ptarmigan species in the study area, rock ptarmigan (L. mutus) dominated gyrfalcon’s diet. Locally, the proportion of rock ptarmigan in gyrfalcons’ diets showed a positive relationship to the expected availability of rock ptarmigan in the breeding territories, indicating a density dependent utilisation.The rough-legged buzzard behaved like a highly specialised microtine rodent predator and Norwegian lemming (Lemmus lemmus) was its preferred microtine species. The buzzards showed a type 2 functional response to lemmings. Surprisingly though, they also had a type 3 functional response to grey-sided voles (Clethrionomus rufocanus). We present an optimal diet model where a central place forager, during good food conditions, benefits from partial prey preference, which renders separate functional responses to each prey category. We discuss how the double functional responses of the buzzard affect the population dynamics of sympatric vole species, on both temporal and spatial scales.The golden eagle behaved like a generalist predator, and it preyed on all major prey categories in the study area: microtines, ptarmigan, mountain hare, (Lepus timidus) and reindeer (Rangifer tarandus). It seemed to respond functionally to microtine rodent fluctuations with an increased consumption of lemmings during a peak year in the microtine rodent cycle. The golden eagle showed a numerical response to its main prey, the ptarmigan.Ptarmigan, microtine rodents and hares seemed to have synchronized population fluctuations in the study area. Such synchronized population fluctuations are believed to be generated by predation. Although the three raptors are the main predators of their community, their predation patterns fail to explain the observed prey population dynamics in the study area.