Oecologia (1995) 103:1-9 9 Springer-Verlag 1995 Russell B. Rader - J.g. McArthur The relative importance of refugia in determining the drift and habitat selection of predaceous stoneflies in a sandy-bottomed stream Received: 12 April 1994 / Accepted: 30 January 1995 Abstract Patch structure in sandy, compared to rocky streams, is characterized by isolated snags that can only be colonized by drifting. By measuring drift from patch- es (snags) of various quality we determined the factors that influence habitat selection and drift of the predace- ous stoneflies Acroneuria abnormis and Paragnetina fu- mosa. The presence of refugia (loose bark and leaf packs) was more important than hunger level and modi- fied the effects of increased predator densities and ag- gressive interactions. Stoneflies concentrated to 8x natu- ral densities with access to refugia remained longer on snags than a single stonefly without access to refugia. During periods of activity, refugia were defended with larger stoneflies always displacing smaller nymphs. Dur- ing long periods of inactivity, two and sometimes three nymphs would rest side-by-side sharing the same refuge. Hunger level (starved versus satiated stonefiies), an indi- rect measure of a predator's response to prey availability, had no significant effect on drift or habitat selection re- gardless of the presence of refugia. Stonefly predators had a uniform distribution while their prey were clumped. Drift was deliberate and almost always delayed until night, usually at dusk or dawn. An examination of previous research plus the results of this study suggest that non-predatory intra- and interspecific interactions can be an important mechanism causing drift in streams. Key words Stonefly dispersion - Drift and habitat selection 9 Refugia 9 Aggressive interference 9 Hunger Introduction The patch dynamics model provides an important ap- proach to understanding the mechanisms that control R.B. Rader (E~i~) USDA, Forest Service, RockyMountainExperiment Station, Laramie, WY, 82070, USA Fax: 307-742-0397 J.V. McArthur Savannah River EcologyLaboratory,Drawer E, Aiken, SC, USA spatial and temporal patterns of distribution and diversity (Pickett and White 1985). Disturbances and environmen- tal variation are emphasized as important processes in creating a mosaic of patches with temporally varying re- source profitability's and microhabitat characteristics (Clements 1916; Connell 1978; Sousa 1979). This view, when extended to streams, has been modified to include faunal mobility characteristics and the continuous redis- tribution of benthic invertebrates (Townsend and Hildrew 1976, Townsend 1989; Lancaster et al. 1991). Patch structure, which differs between streams in different geo- graphical areas, will determine the tradeoffs associated with moving between patches. In rocky streams, a mosa- ic of patches at various scales are connected across a continuous bottom. Macroinvertebrates can crawl be- tween patches without risking hazards associated with the drift (e.g., fish predation; Flecker 1992). In sandy streams and most large rivers, the primary habitat for predatory stoneflies and their prey is disconnected, iso- lated patches (snags) separated by uninhabitable stretch- es of soft sediments (e.g., sand). As a consequence, drift- ing is the only means of moving between patches or snags. Therefore, factors that determine drift in sandy streams also determine habitat selection (the decision to stay or leave). By measuring drift from patches of vari- ous quality we can determine the relative importance of factors that influence habitat utilization in sandy streams and rivers. Intra- and inter-specific aggressive interactions are a common behavioral characteristic of predaceous stone- flies in at least two families prominent in flowing waters, Perlidae and Perlodidae (Walde and Davies 1984; Peck- arsky and Penton 1985; Sj6str6m 1985; Peckarsky 1988a). The propensity of predatory stoneflies to drift may be influenced by aggressive interactions. As stone- fly density increases, the number of aggressive encoun- ters increase, potentially resulting in an increase in drift. Differences in the hunger state of individual predators can be used to estimate the importance of prey availabili- ty in determining habitat selection and drift (Wiley and Kohler 1984). Starved compared to satiated foragers