Shared predators and indirect trophic interactions:: lemming cycles and arctic-nesting geese

1. We investigated the hypothesis that cyclic lemming populations indirectly affect arctic nesting greater snow geese (Anser caerulescens atlanticus L.) through the behavioural and numerical responses of shared predators. 2. The study took place on Bylot Island in the Canadian High Arctic during two lemming cycles. We recorded changes in foraging behaviour and activity rate of arctic foxes, parasitic jaegers, glaucous gulls and common ravens in a goose colony during one lemming cycle and we monitored denning activity of foxes for 7 years. We also evaluated the total response of predators (i.e. number of eggs depreciated). 3. Arctic foxes were more successful in attacking lemmings than goose nests because predators were constrained by goose nest defence. Predators increased their foraging effort on goose eggs following a lemming decline. 4. Activity rates in the goose colony varied 3.5-fold in arctic foxes and 4.8-fold in parasitic jaegers. and were highest 2 and 3 years after the lemming peak. respectively. The breeding output of arctic foxes appeared to be driven primarily by lemming numbers. 5. Predators consumed 19-88% of the annual goose nesting production and egg predation intensity varied 2-7-fold, being lowest during peak lemming years. Arctic foxes and parasitic jaegers were the key predators generating marked annual variation in egg predation. 6. Our study provides strong support for short-term. positive indirect effects and long-term. negative indirect effects of lemming populations on arctic-nesting geese. The outcome between these opposing indirect effects is probably an apparent competition between rodents and many terrestrial arctic-nesting birds.