TROUT PREDATION AND SIZE COMPOSITION OF STREAM DRIFT

Nocturnal drift of stream [in Colorado, USA] invertebrates is examined as a predator avoidance adaptation via the hypothesis that those taxa or growth stages which are large, and subject to intense predation relative to smaller forms, should be most strongly constrained to nocturnal drift activity. Smaller taxa or stages may be aperiodic or day active. Data from an alpine stream document for the abundant and widespread mayfly Baetis bicaudatus that, on a continuum from small to large size, the risk of predation increases while the propensity to drift during the day decreases. There is supporting and some contrary evidence from the literature for the generality of this pattern.