VARIABLE DOMINANCE IN POND COMMUNITIES: ASSESSING SPATIOTEMPORAL VARIATION IN COMPETITION AND PREDATION INTENSITY

Abstract

Historically, fish predation has been viewed as a primary determinant of zooplankton community size structure and variable dominance by large-bodied Daphnia species. However, recent models have emphasized the relative carbon, nitrogen, and phosphorus content of algae as a potential determinant of these patterns. In this research, I utilized five fishless ponds that varied in Daphnia pulex relative abundance and patterns of seasonal succession to explore the factors influencing Daphnia population performance in these systems. D. pulex population growth rates were measured in three treatments: (1) predators/competitors excluded, (2) competitors present and predators excluded, and (3) competitors and the predator Chaoborus present. The experiment was conducted twice in the growing season. In contrast to the top-down view, effects of competition outweighed effects of predation but were variable among ponds (being strongest in those ponds/times in which Daphnia relative abundance was low, and weakest when Daphnia were dominant). Competitor interaction strength was also temporally variable, increasing later in the season. Several potential determinants (biotic and abiotic) of variable competition intensity were explored using regressions. Only the C:P ratio of edible algae showed a significant relationship; negative effects of competition increased as algae became more phosphorus limited, in keeping with recent stoichiometric models. However, significant collinearities in several biotic/abiotic variables among ponds suggest that this interpretation may not be the only explanation for variable competition intensity. Corresponding Editor: J. E. Havel.