COMPETITION AMONG SMALL, FREE-FLOATING, AQUATIC PLANTS

The surface cover of three species of small, free-floating, aquatic pl ants in a beaver swamp was monitored for 1 yr. Simultaneous experiment s at the same site quantified competitive relationships among these sp ecies in different seasons. In the summer competition experiment, Salv inia minima grew rapidly and had negative effects on the relative chan ge in cover of both Azolla caroliniana and Spirodela punctata. Relativ e change in cover of S. minima increased when it was grown with S. pun ctata, but only when A. caroliniana was not present. A negative correl ation in the swamp between cover of S. minima and S. punctata, the two most abundant species during the summer, is consistent with the stron g competitive effects of S. minima in the summer competition experimen t. During the autumn competition experiment, S. minima had a negative effect on A. caroliniana's relative increase in cover. This effect, ho wever, was not competitive and was caused by an herbivore of S. minima that switched to A. caroliniana as S. minima's growth slowed in the a utumn. Total cover of small, floating, aquatic plants was lowest in th e autumn. Salvinia minima, the best competitor for surface space durin g the summer experiment, is also the largest species, the most difficu lt to sink and the most buoyant once submerged. Competition has an imp ortant, but limited, role in this community. The effects of competitio n on surface area were seasonal, and the striking gains in cover of Sa lvinia minima during the summer were reversed by its relative intolera nce of winter conditions and its higher reduction in cover during floo ds. Apart from late summer, cover of S. punctata was by far the highes t throughout the study period, despite S. minima's competitive ability .