Causes of change

The increase in the Dark-bellied Brent Goose population between the mid-1950s and late 1980s has been attributed to a high survival rate for individuals over six months old and a pattern of high breeding success once or twice every three years [Summers & Underhill 1991]. Ebbinge et al. [2002] argue that availability of suitable nesting habitat is also an important factor influencing reproductive potential and that while density-dependent factors were significant in influencing reproduction, they do not impact adult survival. The subsequent decline in the Dark-bellied Brent Goose population has been attributed to poor breeding in years of low lemming abundance on the breeding ground; lemmings are the main food source for the potential predators of goslings [Ebbinge et al. 2013].

The Svalbard Light-bellied Brent Goose population numbered 50,000 before a disease of its principal food source (Zostera marina) and/or human over-exploitation caused numbers to crash in the first half of the 20th century [Denny et al. 2004]. Numbers were further reduced to c.2,000 individuals in the 1960s, likely as a result of overhunting on the Danish wintering grounds; since the population was protected in Denmark in 1972, numbers increased to 8–10,000 during 2010–16 [Madsen et al. 2019]. At its height, at least 20,000 birds wintered in Britain but this number had declined to c.1,600 by the mid 1960s, after which numbers increased [Denny et al. 2004].

The global Canadian Light-bellied Brent Goose population increased rapidly in the early 1980s, thanks to a series of successful breeding seasons; low productivity in the 1990s caused the population to crash but several years of high productivity saw the population increase markedly by the early 2000s [Robinson & Colhoun 2006]. In Ireland, in addition to changes in productivity rates, the apparent increase and decrease in numbers in mid 1980s and mid 1990s respectively may, in part, have been the result of changes in counter effort [Robinson et al. 2004].