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Competitors and predators alter settlement patterns and reproductive success of an intra-guild prey
​A pygmy owl in winter

Where do ​Pygmy owls breed?

We found that Pygmy Owls strongly avoided breeding close to conspecifics but did not avoid Tengmalm’s Owl nests. Nest-box occupation of Pygmy Owls was positively correlated to the proportion of old spruce, mature and old pine forests, and farmlands, and occupation probability was higher at high vole abundance and in areas occupied in previous years. Pygmy Owl avoidance of conspecifics decreased when voles were abundant. At high conspecific density, Pygmy Owls showed lower hatching success and delayed hatching date, in agreement with the observed spatial avoidance. Finally, even though breeding Pygmy Owls did not spatially avoid Tengmalm’s Owls, the density of hetero-specifics correlated with low hatching and fledging success in Pygmy Owls. This suggests short term fitness costs when living close to competitors, even when lethal effects of intra-guild interactions are subtle. The negative impact of Tengmalm’s owl densities on pygmy owl breeding success was however partially mitigated by high vole abundance, suggesting reduced competitive and intra-guild predation costs when food is abundant.

Competition

Our results suggest that joint costs of exploitative and interference competition by Tengmalm’s Owls, as well as intra-guild predation, were lower than those induced by intraspecific competition only. This result might be due to the fairly similar body size between the species considered, Tengmalm’s Owls being only twice as large as Pygmy Owls, whereas in lethal interactions among predators the killer species is an average three times larger than the victim species. Interactions among con- and hetero-specifics can therefore modify the spatial settlement and reproductive success of individuals on a landscape scale, also within the predator guild. Our work highlights the fact that to comprehensively understand the interactions within a guild, it is necessary to account for both spatial overlap and year-to-year fluctuations in resources that potentially modulate the strength of the relationships between species. In this sense, the system constituted by vole-eating predators and their high-amplitude cyclic main prey offers a unique opportunity to study intra-guild predation in nature, in pseudo-experimental settings.

Our study was published in Ecological Monographs.
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-17.02.2017
Chiara Morosinotto
Alexandre Villers
Robert L. Thomson
Rauno Varjonen
Erkki Korpimäki

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