Background/Question/Methods Ecological theory has shown that spatial or temporal stochasticity can allow competing species to coexist on a single resource, but there are very few empirical examples that demonstrate a mechanism by which this can occur in nature. Here we show for a benthic marine community that small-scale spatial variability in the settlement of larvae can enhance the persistence of an inferior competitor. We use a combination of observational and experimental data to quantify competition for space between recruiting juveniles of a dominant solitary ascidian and a poorly competing bryozoan. We also quantify the spatial distribution of significant small-scale variability in the settlement of dispersing larvae. We show how the intrinsic nonlinearity of competition for space implies that this spatial variability enhances recruitment of the inferior competitor. We use an individual-based model parameterized with field data to show how this mechanism can significantly affect long-term competitive outcomes.
Results/Conclusions We find that competition is highly asymmetric during recruitment, and that the effect of the dominant competitor is intrinsically nonlinear because at high density space is saturated and mortality of the inferior competitor is total. This nonlinearity implies that any spatial variation in the dominant will benefit the inferior competitor, because some areas will be relatively open while others have clusters of conspecific competitors. For the range of spatial variation we observe (CV ~ 50%), this mechanism can yield an order of magnitude increase in recruitment of the inferior competitor. We also present experimental evidence from a similar benthic community that shows a 2- to 3-fold increase in recruitment of inferior competitors under spatially variable settlement of a dominant. The mechanism we describe should apply broadly to communities where competition over an individual’s lifetime occurs at a scale that is small relative to the scale of dispersal of offspring. Our results add a different perspective to the debate on the role of niche vs. neutral processes in structuring communities, because the mechanism we describe may or may not involve trait differences that are typically considered as niche differences. However, regardless of the mechanism behind the spatial variability we describe, the effect is to stabilize community dynamics by increasing intraspecific competition and giving a relative advantage to species when rare.