Do fitness-equalizing tradeoffs lead to neutral communities?

Life history complementarity and the maintenance of biodiversity

Life history, the schedule of when and how fast organisms grow, die and reproduce, is a critical axis along which species differ from each other^1-4. In parallel, competition is a fundamental mechanism that determines the potential for species coexistence^5-8. Previous models of stochastic competition have demonstrated that large numbers of species can persist over long timescales, even when competing for a single common resource^9-12, but how life history differences between species increase or decrease the possibility of coexistence and, conversely, whether competition constrains what combinations of life history strategies complement each other remain open questions. Here we show that specific combinations of life history strategy optimize the persistence times of species competing for a single resource before one species overtakes its competitors. This suggests that co-occurring species would tend to have such complementary life history strategies, which we demonstrate using empirical data for perennial plants.

Dispersal limitation favors more fecund species in the presence of fitness-equalizing demographic trade-offs

Demographic equivalence is the central assumption of the neutral theory of species diversity and has attracted much criticism, since species clearly differ from each other in many traits. Two simple answers--that is, dispersal limitation and demographic trade-offs--have been suggested to resolve this problem. Both processes are considered to be capable of making interspecific differences in fitness smaller on their own, thus potentially reconciling neutrality with reality. However, when the two mechanisms operate simultaneously, as they must do in natural communities, we are surprised to find that they interfere with each other in such a way that dispersal limitation favors more fecund species. Fitness equivalence is no longer guaranteed by a perfect trade-off, and contrary to popular belief, dispersal limitation is found to impede rather than facilitate the stochastic coexistence of species. Still, more species can coexist than allowed through demographic equivalence, providing a potentially alternative explanation for biodiversity maintenance in nature.

Thirty years of forest census at Barro Colorado and the importance of immigration in maintaining diversity

The neutral theory of community ecology can predict diversity and abundances of tropical trees, but only under the assumption of steady input of new species into the community. Without input, diversity of a neutral community collapses, so the theory's predictions are not relevant unless novel species evolve or immigrate. We derive analytically the species input needed to maintain a target tree diversity, and find that a rate close to per recruit would maintain the observed diversity of 291 species in the Barro Colorado 50-ha tree plot in Panama. We then measured the rate empirically by comparing species present in one complete enumeration of the plot to those present five years later. Over six census intervals, the observed rate of input was to species per recruit, suggesting that there is adequate immigration of novel species to maintain diversity. Species interactions, niche partitioning, or density-dependence, while they may be present, do not appear to enhance tree species richness at Barro Colorado.