UC San Diego

Trade-offs, or negative functional associations among traits that affect fitness, are fundamental to ecological and evolutionary processes and are the basis for non- neutral species diversity through niche differentiation. Recent studies indicate that trade-offs occur in multiple dimensions involving population limitation by alternative resources or abiotic constraints and may drive species interactions and structure ecological communities. I tested for multivariate trade-offs among 32 natural populations of the freshwater algae Scenedesmus dimorphus in response to light, phosphorus, salinity, nitrogen source (nitrate vs. ammonium) and the presence of pathogenic fungi. Surprisingly, no evidence of trade-offs was found and positive or neutral correlations were observed among strains in response to the treatments. Strains that grew the fastest and reached the highest density at steady state tended to be least affected by light limitation, phosphorus limitation, increased salinity, and switching to ammonium as the nitrogen source. This may suggest an absence of functional constraints in S. dimorphus that limit intraspecific variation in response to several important environmental factors, nutrients and energy availability. Trade-offs may occur with other potential limiting factors such as grazers or temperature. Further research is needed to identify important axes of differentiation that constrain diversification and coexistence among Scenedesmus genotypes