Community assembly processes shape an altitudinal gradient of forest biodiversity

Aim Spatial patterns in biodiversity along environmental gradients are a central theme in ecology. However, the ways in which local assembly processes control changes in species turnover (-diversity) along broader gradients have been less well documented. In this study, we aimed to elucidate factors and processes governing the altitudinal gradients in the -diversity of woody plants and ground-dwelling oribatid mites. Location Shiretoko National Park in Hokkaido, Japan. Methods The diversity of plants and oribatids was investigated in seven plots (each containing 10 subplots) at different altitudes, and the -diversity of the two organism groups was calculated for each altitude. The dependence of -diversity on the size of the species pool (-diversity) is an issue of long-standing importance. We therefore used null modelling, which randomly shuffles individuals among subplots while preserving the -diversity, the relative abundance of each species per plot and the number of individuals per subplot. This approach enabled us to estimate how much the observed -diversity deviates from the expected -diversity under stochastic assembly processes. Environmental data were collected to evaluate the possible effects of habitat condition/heterogeneity on community processes. Results In plants, deterministic processes dominated in the low-productivity, high-altitude stands because of the finer-scale niche partitioning seen among small individuals within less-stratified stands. In the structurally developed, low-altitude stands, the community structure was more strongly affected by stochasticity, probably resulting from one-sided competition such that the canopy trees intercept the majority of light, a primary resource for plants, and therefore the small understorey individuals had limited access to light. Among the oribatids, the altitudinal gradient of -diversity was less evident than among the studied plants. However, this nonlinearity does not support the notion that local assembly processes contribute little to the spatial pattern of -diversity. Indeed, local-scale environmental heterogeneity favoured a more deterministic assembly of oribatids at a given altitude. Main conclusion The biogeographical patterns of -diversity are not independent of community processes and, in reality, are shaped by local stochastic/deterministic factors that change within a landscape.