Abstract
Neutral community models have shown that limited migration can have a pervasive influence on the taxonomic composition of local communities even when all individuals are assumed of equivalent ecological fitness. Notably, the spatially implicit neutral theory yields a single parameter I for the immigration-drift equilibrium in a local community. In the case of plants, seed dispersal is considered as a defining moment of the immigration process and has attracted empirical and theoretical work. In this paper, we consider a version of the immigration parameter I depending on dispersal limitation from the neighbourhood of a community. Seed dispersal distance is alternatively modelled using a distribution that decreases quickly in the tails (thin-tailed Gaussian kernel) and another that enhances the chance of dispersal events over very long distances (heavily fat-tailed Cauchy kernel). Our analysis highlights two contrasting situations, where I is either mainly sensitive to community size (related to ecological drift) under the heavily fat-tailed kernel or mainly sensitive to dispersal distance under the thin-tailed kernel. We review dispersal distances of rainforest trees from field studies and assess the consistency between published estimates of I based on spatially-implicit models and the predictions of the kernel-based model in tropical forest plots. Most estimates of I were derived from large plots (10–50 ha) and were too large to be accounted for by a Cauchy kernel. Conversely, a fraction of the estimates based on multiple smaller plots (1 ha) appeared too small to be consistent with reported ranges of dispersal distances in tropical forests. Very large estimates may reflect within-plot habitat heterogeneity or estimation problems, while the smallest estimates likely imply other factors inhibiting migration beyond dispersal limitation. Our study underscores the need for interpreting I as an integrative index of migration limitation which, besides the limited seed dispersal, possibly includes habitat filtering or fragmentation.
Highlights
Community ecology underwent a sea-change in 2001 with the advent of the neutral theory of biodiversity [1]
The range of dispersal distances from 1 to 600 m encompasses the largest values observed in tropical forests as reported in Table 2 and Figure S1
Our study of ~I found two contrasted situations: in the case of the Cauchy kernel, ~Iwas mainly sensitive to variation in community size, Rc, except for very small R9d5, while in the case of the Gaussian kernel, ~I was mainly sensitive to variation in the dispersal distances, as embodied by. We denote the former situation as a ‘size dependent’ (SD) regime, because in this case the outcome of the migration-drift dynamics on community composition depends far more on a change in the size of the community than on a change in the dispersal parameter R9d5
Summary
Community ecology underwent a sea-change in 2001 with the advent of the neutral theory of biodiversity [1]. In his seminal contribution, Hubbell [2] assumed that species biodiversity at local and regional scales is maintained by a stochastic interplay of birth, death, migration and speciation processes involving individuals of equivalent fitness. Hubbell [2] assumed that species biodiversity at local and regional scales is maintained by a stochastic interplay of birth, death, migration and speciation processes involving individuals of equivalent fitness He argued that propagule dispersal limitation is a universal and strong enough constraint on migration which generates the complex observed patterns of local species coexistence. The original SINM, along with some of its later variants have been used to infer
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