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The effects of goose herbivory and loss of vegetation on ground beetle and spider assemblages in an Arctic supratidal marsh

Abstract During the last two decades loss of vegetation in a supratidal marsh as a result of the foraging activities of lesser snow geese has led to considerable habitat heterogeneity at La Pérouse Bay, Manitoba. Most sites within the marsh are devoid of vegetation, and organic-rich saline sediments are exposed. The loss has affected soil invertebrate assemblages, particularly spiders and beetles that are an important food source for shorebirds and passerines. Above-ground terrestrial invertebrates were collected in the summer of 1997 from pan traps at three sites established within a supratidal marsh. Only one area of relatively undamaged salt marsh remained within this supratidal marsh. The selected 3 sites were a degraded area, an undamaged salt marsh, and a brackish, riverine marsh. In the degraded area, plant cover was less than 2%, surface soil temperatures were high, and sediments were less aerobic and were dried out in summer compared to those from the undamaged area and the riverine marsh. Spiders were most numerous in the undamaged area and least abundant in the degraded area; numbers declined at all locations in late summer. Beetles showed the same seasonal trend, but the highest and lowest densities were recorded in the riverine marsh and in degraded area, respectively. With the exception of a few species, most of these invertebrates were boreal rather than Arctic in their distribution ranges. Overall, the results clearly show that spiders and beetles are sensitive to loss of vegetation and changes in environmental conditions. There was no evidence that increased bird predation in the open, degraded areas accounted for the recorded changes in assemblages.