Functional redundancy of multiple forest taxa along an elevational gradient: predicting the consequences of non‐random species loss

Abstract 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 β‐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 existing climate‐change scenarios, with possible consequences for ecosystem functioning.