Why are rare species rare, and how do they persist despite their rarity? Yenni et al. (2012) proposed that those two fundamental questions have the same answer. Rare species (meaning, rare compared to others in the same community) are rare-yet-persistent because they experience stronger negative frequency dependence that common species. Stronger negative frequency dependence means that their per-capita growth rates decline more steeply with a unit increase in their frequency (relative abundance). All else being equal, strengthening negative frequency dependence makes species rarer because it reduces the "break even" relative abundance at which they exhibit zero per-capita growth (equal birth and death rates). But strengthening negative frequency dependence also promotes persistence because it implies that a species that drops below its "break even" relative abundance will experience a large increase in per-capita growth rate, increasing the odds that it bounces back rather than declining to extinction. We used long-term time series data for zooplankton communities in over 50 temperate lakes to test this hypothesis. We estimated the strength of negative frequency dependence for every species in every lake and regressed strength of negative frequency dependence on mean frequency. We also analyzed the relationship between negative frequency dependence and mean frequency in a multispecies Lotka-Volterra competition model incorporating environmental stochasticity and seasonality.
Results/Conclusions
Preliminary analyses indicate that rarer zooplankton species experience stronger negative frequency dependence in most lakes. The relationship is statistically significant in a randomization test in approximately half the lakes. In these respects, our results mirror those of Yenni et al. (2017), who tested the relationship between rarity and strength of negative frequency dependence in terrestrial communities sampled on an annual timescale. Exceptional lakes in which common species exhibit stronger negative frequency dependence than rare ones appear to be those exhibiting long-term trends in species' relative abundances. Interestingly, a simple stochastic Lotka-Volterra model can predict various relationships between negative frequency dependence and rarity, with no consistent effect of seasonality on the direction or strength of the relationship. These theoretical results contrast with those of Yenni et al. (2012). Our results confirm stronger negative frequency dependence in rare species as a general pattern that demands an explanation, and identify a need for new theory to explain this pattern.