Asymmetric neighborhood dissimilarity effects and growth rank reversals facilitate species coexistence and shape species abundances

Background/Question/Methods

The question that why rare species are rare but will not be excluded from communities is fundamental in ecology but remained unresolved. In this study, we focused on the effects of neighborhood dissimilarities (ND) in trait and phylogeny on tree growth, and verified a situation that can simultaneously explain species coexistence and species relative abundances in a tropical forest by considering species variation both in ND strengths and tree growth rate. Specifically, we hypothesized that:(1)Tree growth rate is positively related to ND due to niche portioning among species; (2)Common species grow faster than rare species under community average states (e.g. under community average crowding and ND). This relative growth advantage makes common species common. (3)Common species are subjected to stronger ND effects than rare species, which reduces the growth advantage for common species and makes rare species persistence possible (i.e. there is asymmetric ND effects); (4)Tree growth rates at zero ND for rare species are equal to or faster than that for common species (i.e. growth ranks reverse from community average ND to zero ND), which enables rare species to recover when extremely rare and makes stable coexistence possible. To test these hypotheses, we conducted individual-based Bayesian models to assess the existences of growth rank reversals between average-ND and zero-ND states, and asymmetric ND effects on growth.

Results/Conclusions

We found community-scale positive effects on tree growth for all the five ND indices: maximum height (H_max), wood density (WD), leaf mass per area (LMA), multi-trait and phylogeny based NDs. Tree growth rates conditioned at community average NDs were positively related to species abundance for all the five ND indices. Species abundance was positively related to the coefficients of H_max, LMA, multi-trait and phylogeny based ND indices. Rare species had equal or faster growth rate conditioned at zero ND for the H_max, LMA, multi-trait and phylogeny based ND indices. Thus, we found empirical supports for the hypothesized situation which can simultaneously explain species coexistence and species relative abundances.