Quantifying multiple-site compositional turnover in an Afrotemperate forest, using zeta diversity

Abstract

Background: Species turnover is typically measured by partitioning diversity components into alpha and pairwise beta diversity. However, alpha and beta components cannot express the full spectrum of multiple-site compositional turnover. To this end, zeta diversity has been proposed as an extended framework to allow complete biodiversity partitioning and to measure multiple-site species turnover. We use a zeta-diversity framework to explore the turnover and potential community assembly processes of an African Montane Forest. Methods: Using a 20 m grid, we explore the species turnover in a 4.55 ha forest plot located in the Garden Route National Park of South Africa, with 47 and 27 canopy and sub-canopy tree species in the regional pool. We first calculate how zeta diversity declines and how the probability of retention of species with particular occupancies changes with increasing zeta orders (i.e. the number of sites [grid cells] involved in the calculation). Using null models with row sums and column sums constrained respectively, we explore whether species turnover is driven by mechanisms of ecological differences (species-specific occupancies) or habitat heterogeneity (site-specific alpha diversity and thus environmental filters). Results: The decline of zeta diversity with zeta order followed a power law; that is, the probability of retention increased with species occupancies, suggesting common species being more likely to be discovered in extra sites. The null model retaining row sums (species' occupancy) of the species-by-site matrix recreated perfectly the decline of zeta diversity, while the null model of habitat heterogeneity (retaining column sums) was rejected. This suggests that mechanisms driving species-specific occupancies (i.e. ecological differences between species) dictate the multi-site species turnover in the community. The spatial patterns of zeta diversity revealed little spatial structuring forces, supporting a fine-grain structure in these southern Cape forests. Conclusions: The framework of zeta diversity revealed mechanisms driving the large discrepancies in the occupancy among species that are behind the species turnover in the African Montane forest plot. Future studies could further link species turnover to spatial distance decay. Environmental filters and temporal turnover from landscape demography could bring a cohesive understanding of community assembly in these unique forest ecosystems.

Using alpha, beta, and zeta diversity in describing the health of stream‐based benthic macroinvertebrate communities

“…Our observations, whereby f diversity decays via a power-law with respect to f order, support a model of stream community composition being determined more by niche differentiation than by a stochastic assembly process (Scheiner et al 2011, Hui and McGeoch 2014, Hui et al 2018. Both our measures of turnover, b and f 10 diversity, are more strongly associated with the standard deviation of land use than altitude within both low and high altitude watersheds (Appendix S1: Tables S3-S4).…”

“…, Hui and McGeoch , Hui et al. ). Both our measures of turnover, β and ζ 10 diversity, are more strongly associated with the standard deviation of land use than altitude within both low and high altitude watersheds (Appendix : Tables S3–S4).…”

Species richness and turnover among indigenous and introduced plants and insects of the Southern Ocean Islands

“…Zeta diversity values for orders greater than 2 provide information on the turnover of widespread species in the community (i.e., those shared by more than two sites), whereas ζ 2 and other pairwise turnover metrics are biased toward turnover in rare and endemic species with limited distributions (Hui et al. ). Zeta diversity values for each focal group were calculated for ζ orders 1 to 5 (ζ 1 –ζ 5 ) using the zeta.decline.mc function in the zetadiv package in R (version 1.1.1, Latombe et al.…”

“…Where ζ i decline takes an exponential form, stochastic assembly processes could potentially be expected to dominate turnover among communities, whereas its power‐law form might reflect predominance of niche differentiation processes in driving turnover (see Hui and McGeoch , Hui et al. ). In an exponential model, the chance of observing a species at an additional site does not depend on the species’ current occupancy (scale independency), whereas a power‐law model best fits data where species have different probabilities of occupying an additional site (e.g., commonly occurring species are more likely to occur in a new site than rare species; see Hui and McGeoch ).…”

“…Thus, zeta diversity reconciles several biodiversity patterns, offers potential insights into the relative contributions of stochastic and niche assembly processes, and can provide a means to understand biodiversity responses to environmental change (Hui et al. ). Yet its potential has not yet been examined in the context of understanding differences in both the form and likely processes underlying community spatial variation among indigenous and introduced species.…”