Over several years, the same population might change from one state to another, as its status with respect to available resources changed. Several species of geese increased during the latter half of the twentieth century in response to reduced hunting pressure in their wintering areas, but then came up against food shortage in the breeding areas, as growing numbers competed for favoured food plants. This increased competition resulted in reduced chick survival in Lesser Snow Geese Chen caerulescens in the central Canadian arctic (Francis et al. 1992), and in reduced summer survival among adult Pink-footed Geese Anser brachyrhynchus on Svalbard (Madsen et al. 2002). The major constraint to further population growth thus shifted from the wintering to the breeding areas as populations grew. In some other goose populations, studied in less detail, increasing competition was manifest chiefly in declining proportions of young in wintering flocks (Figure 26.5). In Brent Geese Branta bernicla wintering in western Europe, total numbers fluctuated from year to year around the long-term upward trend, according to annual variations in breeding success, as discussed above (Summers & Underhill 1987).
The pattern in which breeding density fluctuated from year to year in parallel with the previous year's breeding success was recorded only in short-lived species, in which individuals breed in their first year of life (passerines and dabbling ducks). It would not be expected in longer lived species, in which individuals do not breed until they are two or more years old, and in which annual recruitment rates are naturally low. In such species, breeding success would need to be poor over several years before any effect on breeding numbers became obvious, unless accompanied by a simultaneous increase in mortality or emigration (for Arctic Tern Sterna paradisaea, see Suddaby & Ratcliffe 1997).
Was this article helpful?