Stochastic species turnover and stable coexistence in a species-rich, fire-prone plant community

Abstract

Understanding the mechanisms that maintain diversity is important for managing ecosystems for species persistence. Here we used a long-term data set to understand mechanisms of coexistence at the local and regional scales in the Cape Floristic Region, a global hotspot of plant diversity. We used a dataset comprising 81 monitoring sites, sampled in 1966 and again in 1996, and containing 422 species for which growth form, regeneration mode, dispersal distance and abundances at both the local (site) and meta-community scales are known. We found that species presence and abundance were stable at the meta-community scale over the 30 year period but highly unstable at the local scale, and were not influenced by species' biological attributes. Moreover, rare species were no more likely to go extinct at the local scale than common species, and that alpha diversity in local communities was strongly influenced by habitat. We conclude that stochastic environmental fluctuations associated with recurrent fire buffer populations from extinction, thereby ensuring stable coexistence at the meta-community scale by creating a "neutral-like" pattern maintained by niche-differentiation.

Figure 1. Frequency distribution of the temporal species turnover in the 81 sites sampled in 1966 and 1996.

Figure 2. Relationship between alpha diversity measured in 1966 and in 1996.

Figure 3. Dominance–diversity curves for both sampling dates (1966 and 1996).

These plots display logarithmic species abundances against species rank order in the community .

Figure 4. (a) Relationship between the local species abundance (populations of each species within each plot at the time considered) in 1966 and 1996.

Data shown are for one set of estimated actual abundances, a range of r² values for 100 estimates are reported [r² = 0.037–0.058, p<0.001]. (b) Relationship between the meta-community spread (number of plots where a species is present) in 1966 and in 1996. (c) Relationship between the meta-community species abundance (total number of individuals for each species across all sites) in 1966 and 1996; The plots b and c are shown on a logarithmic scale.

Figure 5. Relative (a and b, w_pi) and absolute weights of evidence (c and d, Δw_pi) of five derived predictors [dispersal model (Disp), regeneration type (Reg), growth form (Gf) and meta-community spread (Spread) and meta-community species abundance (Abundance) for explaining frequency of extinction (a and c) and frequency of colonisation (b and d).

W_pi are considered as relative, not absolute because they will be >0 even if the predictor has no explanatory importance due to random factors , . Absolute weight of evidence from random permutation gives an unbiased estimation of the weight of evidence after accounting for random artefacts.