Academic journal article The American Midland Naturalist

Effects of Stream Permanence of Crayfish Community Structure

Academic journal article The American Midland Naturalist

Effects of Stream Permanence of Crayfish Community Structure

Article excerpt

ABSTRACT.-Little is known regarding the impact of stream drying on community structure and habitat selection of lotic crayfish. Crayfish community structure was measured through quantitative sampling of riffle, run and pool habitats in 15 intermittent and 21 permanent streams in the Spring River watershed in the Ozark Plateau region of northern Arkansas and southern Missouri. MANOVA was used to determine whether stream permanence, habitat and crayfish size affect crayfish density and ANOVA was used to determine the effect of stream permanence and habitat type on species richness. Canonical Correspondence Analysis was used to determine the relationship between environmental variables and crayfish relative densities. Four crayfish species were collected: Orconectes marchandi, O. punctimanus, O. ozarkae and Cambarus hubbsi. Overall, crayfish densities were significantly greater in intermittent streams than in permanent streams with density of O. marchandi and O. punctimanus significantly greater in intermittent streams than in permanent streams. Densities of O. ozarkae and C. hubbsi did not differ significantly between the two stream types. Large individuals of O. punctimanus and O. ozarkae were found in significantly greater densities than small individuals. There was a significant relationship between crayfish relative density and abiotic environmental variables for permanent streams, but not for intermittent streams. In permanent streams, percent gravel, substrate diversity and mean current velocity were among the most important factors in determining crayfish density. Taxa mobility and predation risk are likely explanations for observed patterns of crayfish density.

INTRODUCTION

The effect of stream drying on community structure and habitat selection of lotic crayfish is unclear due to species-specific responses to drying and few studies examining the question. Most studies examining the effects of stream permanence on benthic communities have focused on short-lived aquatic insects, many of which have a terrestrial life-history stage, whereas few have examined wholly aquatic, and relatively long-lived crayfish. Momot (1966) reported that the crayfish, Orconectes nais, migrated to pools to escape the effects of stream drying, whereas Williams and Hynes (1976) found that the burrowing crayfish, Cambarus fodiens, retreated into the hyporheic zone as the stream bed dried. Taylor (1983) found that drought differentially affected two resident crayfish species. The burrowing species (Cambarus latimanus) showed no significant changes due to drought, whereas populations of the non-burrowing species (Procambarus spiculifer) experienced reductions in adult and overall population densities, reduced adult body size, a shift in reproductive timing and increased juveniles within the population. In addition, these drought-induced changes were maintained in one population for the duration of an 8-y study and resulted in the extinction of two sub-populations (Taylor, 1988).

More is known about the effect of stream permanence on other benthic invertebrates. Researchers have shown that intermittent streams can support benthic communities different from those found in permanent systems (Clifford, 1966; Casey and Ladle, 1976; Williams and Hynes, 1976, 1977; Ladle and Bass, 1981; Wright et al., 1984; Delucchi and Peckarsky, 1989; Feminella 1996). Differences in community structure related to stream permanence should be expected if some aquatic invertebrate species are unable to withstand drought conditions or have reduced competitive or predatory abilities as streams dry. However, because much of the data on the effects of stream drying on benthic communities is derived from a single site (habitat) or from a series of sites on a single stream (Williams, 1987), differentiating between the effects of stream drying and the spatial variability inherent in stream communities is difficult. Often cited reasons for type-specific community differences include a greater range of physical-chemical conditions in intermittent streams that often exceed those seen in permanent streams (Williams, 1996), proximity to sources of colonists (Boulton, 1989) and species-specific tolerances and adaptations to drought conditions (Williams, 1998). …

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