As scientists become more certain that evolutionary processes can occur at the same rate as ecological processes, the role of intraspecific trait variation has been increasingly important in community ecology. Variation in body size has been used as a proxy for a wide array of vital population/community functional traits due to their linkage to life-history strategies. While extrinsic factors, like competition and predation, have been well-studied, intrinsic mechanisms shaping size distributions are less well-known. This study aimed to look at the effects of intrinsic mechanisms, specifically population composition, intraspecific competition, and density-dependence, on shaping size distributions using the keystone aquatic herbivore, Daphnia, as our model organism. We predicted that body size distributions would reflect dominance by individual genotypes (clones) and small individuals would be favored under high densities. In order to test these effects on size-distributions, clonal populations, mixed clones, and mixed species treatments were set-up in microcosms. Size-distribution shifts as well as clonal composition changes using electrophoretic markers were monitored.
Results/Conclusions
Body size distributions remained consistent throughout the experiment, with species differentiating by median size and IQR (MANOVA, F = 4.035, P = 0.009). The largest species tested, Daphnia magna should have complete competitive dominance based on traditional niche and allometric theory; however it appears intraspecific competition may be inhibiting dominance at the species level (mixed clones richness = 2.09 ± 0.83, mixed species richness = 2.88 ± 0.94). Intraspecific competition may explain weak trends between niche overlap and coexistence among clones (r2 = 0.04, P = 0.343). In light of these findings, intrinsic factors may play a larger role in shaping zooplankton composition than previously recognized.