G. Evelyn Hutchinson

Ecological Aspects of Succession in Natural Populations

THE striking changes that occur in both experimental mixed cultures and in natural populations have, as Dr. Woodruff has reminded us, been observed for a long time. In attempting a modern explanation of such changes it is first desirable to call attention to certain controlled laboratory experiments, that will provide a key to the more complex situation in nature. These experiments are due to Gause and are doubtless generally familiar; they are explicable by a relatively complete mathematical theory, provided by Lotka (1925), by Volterra (.1926), and by Gause himself (1934, 1935). The particular experiments which are fundamental from the standpoint of the present contribution show that: (1) If two species live in an identical niche, competing for the same food supply, maintained at a constant level, one species will entirely displace the other. This has been demonstrated with Paramecium aurelia and Glaucoma scintillans; the latter, having the higher coefficient of multiplication, alone remains. (2) Dominance in competition is dependent on the environment conditions. This is strikingly shown in experiments with mixed cultures of P. aurelia and P. caudatum, in cultures that have, or have not, received biologically conditioned medium. If metabolic products of Paramecium, Bacillus pyocyaneus, etc., are not added, caudatum starts growing faster; if such products have been added, the reverse is true. In general, if the ecological conditions are such that utilization of food is the controlling factor, caudatum is dominant, if resistance to metabolic products, either heteroor homotypical, is the controlling factor, aurelia is dominant (Gause, Nastukova and Alpatov, 1934).

Thoughts on Aquatic Insects

Aquatic adults seldom have terrestrial juveniles. Respiratory problems may lead to tropical species being smaller than related temperate ones. Suctorial feeding on higher plants seems unknown. Cryptic coloration is common, aposematic, and epigamic rare. Flightlessness is common, but potentially flying forms must be generally available; this restricts tracheal gills to juveniles. (Accepted for publication 12 January 1981)