A gloomy future for light-bellied brent geese in Tusenøyane, Svalbard, under a changing predator regime

Abstract

The endangered population of light-bellied brent geese (Branta bernicla hrota) breeding in Svalbard and north-east Greenland used to have its core breeding area in the archipelago of Tusenoyane in south-east Svalbard. Studies carried out during 1987–1991 showed that the Tusenoyane population was subject to heavy egg predation by polar bears and, in one year, Arctic foxes. Revisiting some key nesting islands in August 2018, we found few nests used by brent geese and no families. The high density of common scurvygrass (Cochlearia officinalis), a food favoured by brent geese and therefore formerly depleted by them, indicates that the geese have been absent for some time. Among other bird species, such as barnacle goose and common eider, very few young were observed as well. As potential predators, polar bears, or signs of their recent presence, were observed on most islands, and great skuas occurred on almost all islands, with 60 individuals on Luroya, formerly an important island for geese. In contrast, only a single pair of great skuas was observed 30 years ago. The observations suggest that recent expansion of great skuas in the North Atlantic, including Svalbard, has led to a novel extreme predation pressure, additional to that caused by mammalian predators. Despite the loss of Tusenoyane as a breeding ground, the population of brent geese has increased in recent decades; so we can infer that the population now recruits from remote but mainly unknown breeding grounds.