How do Brent Geese (Branta b. bernicla) cope with evil? Complex relationships between predators and prey

Abstract

Actual predation is rarely observed in the field, and therefore the role of predators is often severely underestimated. Species are limited in their distribution, which is caused not only by predation but also by the anti-predator behaviour that prey-species have developed under the continuous selection pressure of predators. How finely tuned this behaviour can be, is illustrated in this paper by the way in which Dark-bellied Brent Geese play one predator against another. Snowy Owls keep Arctic Foxes at bay, and so in years when lemmings are abundant Brent Geese can safely nest in association with nesting Snowy Owls. However, this is dangerous in years when lemmings are scarce and Snowy Owls revert to a predatory species that can even take adult Brent Geese on the nest. Another, more common, nesting strategy of Brent Geese is to nest inside Herring Gull colonies on small islands. Herring gulls are not a threat to adult geese, but they can take goose eggs as well as taking a particularly high toll by preying on newly hatched goslings. On the other hand, Herring Gulls are probably useful to Brent Geese as indicators of places which will become fox-free islands after the ice break-up. Moreover the gulls provide additional protection against Snowy Owls as well as against other gulls. Furthermore, through fertilisation by their droppings, the gulls provide the nesting geese with high quality grass during the incubation period. Predator activity and abundance in the arctic is largely a factor of lemming abundance. This paper highlights the ideas we have developed about the impact of predators by comparing data on the breeding success of Brent Geese nesting in the same study area during two complete lemming cycles between 1990 and 1995.