Resource Partitioning by Stream-dwelling Crayfish: The Influence of Body Size

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Abstract
The crayfishes Orconectes luteus and O. punctimanus are sympatric in over 200 km of the Jacks Fork and Current rivers in south-central Missouri [USA]. The combinations of substrate particle size, current velocity, depth and macrophyte cover used by young-of-the-year and adults of each species tended to be distinct. O. punctimanus was more restricted in its use of habitats. Young-of-the-year were strongly associated with macrophyte cover and shallow water, and adults with larger substrate particles; both life stages preferred slow current velocities. O. luteus was more evenly distributed over most of the habitats. Young-of-the-year used current velocities and substrate particle sizes somewhat more in proportion to their availability than did adults, which were concentrated within the higher ranges of both of these variables. Laboratory choice experiments involving combinations of species and size classes indicated that crayfish dominance was a function of body size and that habitat use by one size class could be modified when larger individuals of the same or a different species were present. The young of O. punctimanus hatched earlier and grew faster than did those of O. luteus. O. punctimanus therefore maintained a distinct size advantage throughout its life cycle. O. punctimanus probably inhabited its preferred set of microhabitat conditions and excluded O. luteus from these areas. Because much of the remaining stream habitat was unsuitable for O. punctimanus, competitive exclusion did not occur. Other factors, most notably fish predation, probably complement physical factors and species interactions in determining the distributional patterns of the crayfishes.