Abstract
Tropical forests are mega-diverse ecosystems that display complex and non-equilibrium dynamics. However, theoretical approaches have largely focused on explaining steady-state behaviour and fitting snapshots of data. Here we show that local and niche interspecific competition can realistically and parsimoniously explain the observed non-equilibrium regime of permanent plots of nine tropical forests, in eight different countries. Our spatially-explicit model, besides predicting with accuracy the main biodiversity metrics for these plots, can also reproduce their dynamics. A central finding is that tropical tree species have a universal niche width of approximately 1/6 of the niche axis that echoes the observed widespread convergence in their functional traits enabling them to exploit similar resources and to coexist despite of having large niche overlap. This niche width yields an average ratio of 0.25 between interspecific and intraspecific competition that corresponds to an intermediate value between the extreme claims of the neutral model and the classical niche-based model of community assembly (where interspecific competition is dominant). In addition, our model can explain and yield observed spatial patterns that classical niche-based and neutral theories cannot.
Highlights
The classical competition niche theory (CCNT) based on the Lotka-Volterra competition equations and Hutchinson’s multidimensional niche [1,2] generally predicts the coexistence of a relatively large number of species with disjoint niches at equilibrium
Our results suggest that this pattern can be understood in terms of local competition and is related to another finding obtained from our model: the relative species abundances (RSA) distribution represented on the niche axis displays a pattern with clumps and gaps (Fig. 3a) that is qualitatively similar to the one obtained using Lotka-Volterra competition equations [23]
We found that local competitive interactions coupled with limited, stochastic dispersal can give rise to the non-equilibrium dynamics for a seemingly universal range of niche widths identical to all tree species that may be interpreted as ‘‘neutral’’
Summary
The classical competition niche theory (CCNT) based on the Lotka-Volterra competition equations and Hutchinson’s multidimensional niche [1,2] generally predicts the coexistence of a relatively large number of species with disjoint niches at equilibrium. The neutral theory of biodiversity (NTB) [4] assuming that ecological dynamics stem from the ecological drift of functionally equivalent species with identical niches, has been able to predict the relative species abundances (RSA), the species-area relationship (SAR) and main biodiversity indices in tropical forests with only four parameters with an accuracy and consistency that have eluded the classical niche-based theory [4,5]. In the same vein as the classical nichebased theories, the NTB predicts the (stochastic) equilibrium of community metrics with the species loss due to ecological drift being balanced by dispersal and/or speciation [4]. Not being a spatially-explicit theory, the original, standard NTB cannot predict distinctive features of the spatial dynamics of megadiverse tropical forests [6]. The NTB has been criticized for ignoring the strong evidence of functional [7] and fitness differences [8] among tree species and for incorrectly predicting the observed rates of species turnover [9] and of spatial differentiation at biogeographic scale [10]
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