Dispersal, phenology and habitat preference of subalpine whirligig beetles (Coleoptera: Gyrinidae: Gyrinus)

Abstract

Whirligig beetle populations from five temporary kettle ponds and one marsh near the Rocky Mountain Biological laboratory, Gunnison County, western Colorado U.S.A. were studied during summer 2010. The purpose of the study was to measure the effects of extreme fluctuating environmental conditions including seasonal habitat drying, and disturbance by cattle grazing on the phenology, habitat preference and dispersal of two species of Gyrinidae. The phenology of these beetles was compared among the ponds by observing the timing of emergence from pupa to imago as determined by elytral condition. In general, beetles were observed to emerge in early July, earlier than previously recorded in North America, possibly accelerated as an adaptation to the high elevation conditions. Beetles varied in emergence times among ponds, and two sites had no adult beetles until they were populated by emergent beetles in 8, July and 15, July. Relative population size of beetles in the ponds was estimated by comparing the total numbers of beetles collected biweekly and correcting those totals by sampling effort (people hours collecting beetles). Total pond area, area of aquatic vegetation, average surface water temperature and change in temperature range from the beginning to the end of the study did not explain the observed variation in relative population size. However, relative population size decreased with increasing habitat loss among ponds, suggesting that beetles preferred more permanent ponds. Active dispersal was observed and recorded using a mark-recapture system that involved small brands on the elytra using a tungsten needle. Over 600 beetles were marked and released in their natal ponds, and 82 were recaptured in the same pond. However, three marked beetles were recaptured in different ponds, demonstrating that they flew as far as 800 meters. Mass dispersal was also observed when cattle disturbed one pond, and another pond dried. Adults also dispersed en masse from a pond just before a second generation of beetles emerged, suggesting that dispersal may a natural life history phenomenon.

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