JEFFERIES, R.L.; ROCKWELL, R.F.; UNIVERSITY OF TORONTO; AMERICAN MUSEUM OF NATURAL HISTORY: An Overabundance of Lesser Snow Geese and an Apparent Trophic Cascade

In recent decades migratory geese that breed in the Arctic but overwinter in temperate regions of Northwestern Europe and North America have shown dramatic increases in numbers that appear to be linked to their increasing use of agricultural crops as a food resource. The foraging behaviour of the mid-continent population of lesser snow geese is an example of this change in resource acquisition that has led to a relaxation of the density-dependent regulation which occurred earlier in their traditional wintering area, the coastal marshes of the Gulf States. The agricultural nutrient subsidy has led to an apparent trophic cascade in coastal marshes of the Hudson Bay Lowlands and at other locations where the increased numbers of birds breed. Direct and indirect biotic and abiotic processes mediated by feedback mechanisms and initiated by goose foraging have led to sustained change resulting in the development of alternative stable states. These non-linear changes, that display threshold responses, include loss of vegetation, irreversible changes in soil properties and a decline in microbial activity, and decreases in densities of invertebrate and passerine species. The coastal systems display low resilience, and the coalesence of local disturbed areas has led to a highly fragmented landscape where there is a marked temporal asymmetry between a possible decline in goose numbers and revegetation of coastal areas.