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 (b-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 b-diversity of woody plants and grounddwelling oribatid mites.Location Shiretoko National Park in Hokkaido, Japan. MethodsThe diversity of plants and oribatids was investigated in seven plots (each containing 10 subplots) at different altitudes, and the b-diversity of the two organism groups was calculated for each altitude. The dependence of b-diversity on the size of the species pool (g-diversity) is an issue of long-standing importance. We therefore used null modelling, which randomly shuffles individuals among subplots while preserving the g-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 b-diversity deviates from the expected b-diversity under stochastic assembly processes. Environmental data were collected to evaluate the possible effects of habitat condition/heterogeneity on community processes. ResultsIn 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 b-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 b-diversity. Indeed, local-scale environmental heterogeneity favoured a more deterministic assembly of oribatids at a given altitude. Main conclusionThe biogeographical patterns of b-diversity are not independent of community processes and, in reality, are shaped by local stochastic/ deterministic factors that change within a landscape.

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Various local processes simultaneously shape ecological assemblages. β-diversity is a useful metric for inferring the underlying mechanisms of community assembly. However, β-diversity is not independent of γ-diversity, which may mask the local mechanisms that govern community processes across regions. Recent approaches that rely on an abundance-based null model could solve this sampling issue. However, if abundance varies widely across a region, the relative roles of deterministic and stochastic processes may be substantially misestimated. Furthermore, there is additional uncertainty as to whether null models used to correct γ-dependence in β-diversity should be independent of the observed patterns of species abundance distributions or whether the models should reflect these patterns. Here, we aim to test what null models with various constraints imply about the underlying processes shaping β-diversity. First, we found that an abundance-driven sampling effect could substantially influence the calculation of γ-corrected β-diversity. Second, we found that the null models that preserve the species abundance patterns could better reflect empirical patterns of spatial organization of individuals. The different implications generated from different applications of the null model approach therefore suggest that there are still frontiers regarding how local processes that shape species assemblages should be quantified. Carefully exploring each facet within different assembly processes is important.

Distorted plant diversity patterns due to ungulate herbivory could be explained by changes in community assembly processes, but the effects of ungulate herbivory on plant community assembly remain unclear. Here, we examined the role of deer herbivory in the regulation of the assembly processes of a forest floor plant community by assessing species and functional diversity in over-and no-grazing plots (control and exclosure plots, respectively) in Shiretoko National Park in Japan. Compared with the exclosure plot, vegetation coverage was considerably lower, and species richness and diversity were higher in the control plot. Functional traits associated with competitive ability (leaf area and chlorophyll content) were significantly higher in the exclosure plot. The pattern of functional diversity changed from overdispersion to clustering with an increase in local crowdedness. This trait clustering indicates that the local communities that were free from ungulate disturbance gradually became dominated by some competitively superior plant species, which led to low species diversity and biotic homogenization. In contrast, the reduction in vegetation due to overgrazing by deer resulted in an increase in the relative importance of stochastic assembly processes, which enabled the coexistence of various species, including less competitive ones. Our results emphasize that although deer overabundance is of concern, their complete exclusion has a negative consequence from an ecological perspective.Because deer herbivory is an inherent process that affects the biodiversity of plants on the forest floor, the establishment of fences requires careful consideration to ensure the conservation of ecological processes and their associated biodiversity.

Aims Climate change can substantially alter ecological communities. However, we hypothesized that, even if novel communities emerge, those communities may not be novel in terms of functional composition. To infer the processes associated with rising temperatures, we evaluated elevational taxonomic/functional turnover of plant and invertebrate communities. Because climate change interacts synergistically with other environmental factors, and therefore is not the sole driver of change in ecological communities, we also considered how the taxonomic/functional composition of the communities would be affected by mammalian overgrazing/browsing, which has become prominent in the study region.Location Shiretoko National Park, Shiretoko Peninsular, Hokkaido, Japan.Methods We investigated the diversity of eight groups of organisms (taxa) in forests of northern Japan, and calculated the distance decay of taxonomic/functional similarity (Sørensen's b-diversity) along an elevational gradient. A null model was used to separate functional turnover from taxonomic turnover. We then simulated how taxonomic/functional turnover along the gradient would be changed after non-random loss of species sensitive to mammalian herbivory.Results We found that each group showed elevational decay in taxonomic similarity. Along an elevational gradient, species groups structured by stronger dispersal limitation showed faster species turnover. This suggested differences in the process of climate-induced species reassembly among the groups. We also found that elevational turnover of communities based on functional traits tended to be lower than that based on taxonomic identity for the majority of the groups, supporting our hypothesis of functional redundancy across the elevational gradient. We thus speculated that climate-induced emergence of taxonomically novel communities may have limited influence on critical ecosystem processes supported by functional diversity. Furthermore, while random species loss did not change functional turnover, non-random loss of species attributable to mammalian herbivory substantially accelerated elevational functional turnover of the taxa. This suggested a possible loss of the functional redundancy of communities.Main conclusions Future communities may be novel not simply because climates are changing at unprecedented rates but also because of the synergetic influences of other environmental changes. Thus ecological processes may be more seriously affected in the future than is generally anticipated based on ª 2015 John Wiley