Size, weapons, and armor as predictors of competitive outcomes in fossil and contemporary marine communities

Abstract

Inter- and intraspecific competition are usually observed over a few generations but their patterns and consequences are seldom tractable in natural systems over longer timescales relevant to macroevolutionary change. Here, we use win-draw-lose competitive overgrowths for a marine benthic community of encrusting bryozoans that have evolved together in New Zealand for at least 2.3 million years to investigate battles for substrate space, a resource that is limiting for these colonial organisms. Using more than 6,000 fossilized and contemporary battles, we explored what combination of traits—including relative zooid (module) size, weapons, armor, and relative abundance—best predict battle outcomes, and if these are time varying. In simpler models, where we disregard trait–trait interactions, we find that the effects of larger zooid sizes and three-dimensional growth on battle outcomes are both positive, while that of relative abundance is negative, such that more common species are more often overgrown by less common species. When we include trait–trait and trait–time interactions in our models, we confirm that a larger zooid size is advantageous for successful overgrowth but infer that it is time varying. In these complex models, we also detect interactions between combinations of traits, where more armored and weaponized bryozoans seem to be at a disadvantage in overgrowth battles. Our best models do perform statistically better than the null model, but we find that there is low predictability for overgrowth outcomes for our New Zealand data set of fossil and contemporary battles, suggesting unmeasured variables and/or high stochasticity in a system that is otherwise well characterized. A best model, developed using our New Zealand data, is applied to three ecologically similar systems described in previous studies to investigate its general predictive power, with the expectation that it would perform better than null models. Surprisingly, we find that the best model developed within the New Zealand system cannot be extrapolated to other encrusting cheilostome bryozoan communities and that these three communities often even have opposite signs for trait coefficients. We conclude that there is much to learn about multi-species marine communities where biotic interactions such as competition may have long-lasting consequences for ecological and evolutionary dynamics.