Coexistence mechanisms and the paradox of the plankton: quantifying selection from noisy data

Many species of phytoplankton typically co-occur within a single lake, as do many zooplankton species (the "paradox of the plankton"). Long-term co-occurrence suggests stable coexistence. Coexistence requires that species be equally "fit" on average. Coexistence mechanisms can equalize species' long-term average fitnesses by reducing fitness differences to low levels at all times, and by causing species' relative fitness to fluctuate over time, thereby reducing differences in time-averaged fitness. We use recently developed time series analysis techniques drawn from population genetics to estimate the strength of net selection (time-averaged selection over a year) and fluctuating selection (an index of the variation in selection throughout the year) in natural plankton communities. Analysis of 99 annual time series of zooplankton species dynamics and 49 algal time series reveals that within-year net selection generally is statistically significant but ecologically weak. Rates of net selection are -10 times faster in laboratory competition experiments than in nature, indicating that natural coexistence mechanisms are strong. Most species experience significant fluctuating selection, indicating that fluctuation-dependent mechanisms may contribute to coexistence. Within-year net selection increases with enrichment, implying that among-year coexistence mechanisms such as trade-offs between competitive ability and resting egg production are especially important at high enrichment. Fluctuating selection also increases with enrichment but is independent of the temporal variance of key abiotic factors, suggesting that fluctuating selection does not emerge solely from variation in abiotic conditions, as hypothesized by Hutchinson. Nor does fluctuating selection vary among lake-years because more variable abiotic conditions comprise stronger perturbations to which species exhibit frequency-dependent responses, since models of this mechanism fail to reproduce observed patterns of fluctuating selection. Instead, fluctuating selection may arise from internally generated fluctuations in relative fitness, as predicted by models of fluctuation-dependent coexistence mechanisms. Our results place novel constraints on hypotheses proposed to explain the paradox of the plankton.