Community assembly processes shape an altitudinal gradient of forest biodiversity
Correspondence: Akira S Mori, Graduate School of Environment and Information Sciences, Yokohama National University, 79-7 Tokiwadai, Hodogaya, Yokohama 240-8501, Japan.
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 (β-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 β-diversity of woody plants and ground-dwelling oribatid mites.
Shiretoko National Park in Hokkaido, Japan.
The diversity of plants and oribatids was investigated in seven plots (each containing 10 subplots) at different altitudes, and the β-diversity of the two organism groups was calculated for each altitude. The dependence of β-diversity on the size of the species pool (γ-diversity) is an issue of long-standing importance. We therefore used null modelling, which randomly shuffles individuals among subplots while preserving the γ-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 β-diversity deviates from the expected β-diversity under stochastic assembly processes. Environmental data were collected to evaluate the possible effects of habitat condition/heterogeneity on community processes.
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 β-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 β-diversity. Indeed, local-scale environmental heterogeneity favoured a more deterministic assembly of oribatids at a given altitude.
The biogeographical patterns of β-diversity are not independent of community processes and, in reality, are shaped by local stochastic/deterministic factors that change within a landscape.