Geographic Analysis of Thermal Equilibria: A Conceptual Model for Evaluating the Effect of Natural and Modified Thermal Regimes on Aquatic Insect Communities

Adult body size and fecundity of several species of hemimetabolous aquatic insects were shown to depend largely on thermal conditions during larval growth. We suggest that an "optimum" thermal regime exists where adult size and fecundity are maximized; temperature regimes warmer or cooler than the "optimum'' result in small and less fecund adults. Two hypotheses concerning river water temperatures and size variation of adult insects are described. First, maximum adult size reflects an equilibrium between several developmental processes that appear highly temperature dependent, viz., (i) the rate and duration of larval growth, and (ii) the specific time in larval development that adult structures begin maturing and the rate of this maturation process. Second, a species distribution both locally within drainage systems and over a large geographic area is limited, in part, by lowered fecundity as adult size gradually diminishes in streams of increasingly cold or warm temperature cycles. The importance of river water temperatures to insect community development is discussed.