Diel Activity Patterns in Snowfield Foraging Invertebrates on Mount Rainier, Washington

Abstract

An invertebrate fauna consisting of carabid and staphylinid beetles, phalangids, gryllablattids, Collembola, and iceworms forages for insect fallout on the snowfields of Mount Rainier. Among these animals, large body size is associated with nocturnalism and small size with diurnalism probably because of the effects of body size on bird predation and radiative heating during the day. The diel activity patterns of the nocturnally active species are unusual, in comparison to other alpine arthropods previously studied, in that microclimate conditions are of very little importance in controlling its duration on most summer nights. Instead, the occurrence of food and reproductive satiation appear to be the major controller of activity duration. It is suggested that the evolutionary success of the snowfield foraging arthropods is due to their exploitation of a temporal and physiological niche where the environmental restraints typical of alpine habitats can be avoided.