Species abundance distribution and its scaling in trait space in a stochastic niche model

Background/Question/Methods

Whether and how species abundance patterns reflect the dynamic processes underlying the formation and maintenance of a community remains an open question in ecology. Recent studies focus on the relative roles of neutral and niche-based mechanisms in shaping these patterns. Neutral dynamics are based on demographic stochasticity and immigration, whereas niche dynamics are generated by trait differences that affect the fitnesses of species interacting through competition.  One recent study has shown that when species in different niches are essentially independent of one another, abundance patterns produced under niche and neutral dynamics are very similar, especially when diversity is relatively high compared with the number of niches. However, the expected differences in abundance patterns are unknown when potential species interactions are considered. Furthermore, since system-specific complexities can influence abundance patterns even under neutrality, it is unclear how to robustly distinguish the influence of niches even if they lead to different abundance patterns. To investigate these issues, we compare the patterns resulting from a stochastic Lotka-Volterra competition model in which the strength of competitive interactions can depend on the distance between species on a trait axis. This model generates either neutral communities (when competition does not depend on distance) or communities with niches (when competition declines with distance). In particular it generates niche communities in which species do not fall distinctly into separate non-interacting niches. 

Results/Conclusions

We show that the differences in species abundance distributions of these two types of communities are more substantial than in the recent study mentioned above, suggesting that mechanisms that produce niches also influence abundance patterns. We observe that species abundance data for a rainforest community on Barro Colorado Island, Panama exhibit a characteristic found in the niche communities in our model. We also show that the species abundance distribution differs across scales on the trait axis in niche communities in a way that would not be expected in neutral communities. This finding suggests that comparing abundance distributions across trait scales may be a robust way of identifying the presence of niches in a community when the expected distribution is not known a priori.