A geometric model describing a quasi-equilibrium of energy flow in populations of stream insects.
Studies of growth, population structure, and metabolism for the mayfly nymph Ephemerella dorothea suggest that in natural streams this species maintains a nearly uniform weight-specific metabolic rate from the time of hatching (0.002 mg) to maturity (4.0 mg) as average stream temperature increases from 0.5 degrees to 15 degrees . The observed homeostasis represents a central tendency between two opposing factors that affect metabolism (size and temperature) and is achieved because any increase in metabolic rate due to warming of streams from winter to late spring is almost completely compensated by the increase in nymphal size from hatching to emergence. Results indicate that natural selective processes have favored individuals whose growth rates are proportional to seasonal changes in temperature. The model suggests a possible mechanism for explaining synchronous larval growth, pulsed adult emergence, and progressive decreases in adult weight of individuals emerging after the main cohort.
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Stroud Water Research Center of the Academy of Natural Sciences of Philadelphia, Avondale, Pennsylvania 19311.