|dc.description.abstract||Aim Our aim is to test if long-term patterns in α alpha and ß beta diversity along an elevational transect on two aspects of a mountain are consistent through time using spiders as model organisms, quantify the role of elevation and time (seasonal and inter-annual) in explaining these patterns and partition the relative contribution of nestedness, species turnover and species loss in explaining these diversity patterns.
Location The transect is across the Cederberg mountains in the Cape Floristic
Kingdom, Western Cape, South Africa and is constituted by 17 sites with an elevational range of 1900 m on two aspects of the mountain (east and west). Methods Spider assemblages were sampled biannually (wet and dry seasons) over 6 years. Four replicates per site, each consisting of a 5 x 2 pitfall grid, were sampled for 5 days sessions. Generalized linear mixed models with Poisson
error structure for species richness (alpha diversity), binomial error structure for beta diversity (Jaccard dissimilarity ßcc and its partitioned components, ß-3 and ßrich), and Gaussian error structure for beta diversity values standardized by a null model (SES) were used to model the effects of elevation and season on these two indices respectively. Results Although varying considerably between years, spider alpha diversity had a hump-shaped pattern on the western aspect and U-shaped on the eastern
aspect. However, season interacted with elevation to produce more complex patterns during the dry season. There was no significant nestedness except for two instances on the western aspect. Replacement accounted for 60–70% of beta diversity between sites, and elevational distance decay in beta diversity was the result of increased turnover on the western aspect and increased species loss on the eastern aspect. Standardized patterns suggest that there are no effects of season on beta diversity except for a decreased distance decay during the dry season on the western aspect. Main conclusions Large-scale predictors of spider alpha diversity explained small amounts of variation in spider diversity, pointing to the importance of local and stochastic processes. Regional turnover of spider diversity is mainly
the result of niche processes, suggesting localized adaptation of taxa, further
supported by the lack of nestedness in assemblages.||en