Deterministic and stochastic processes of biodiversity change with altitude in northern forests of Japan

Background/Question/Methods

Understanding the spatial pattern of biodiversity along environmental gradients is a central theme in ecology. Several hypotheses have been proposed to explain underlining mechanisms determining the distribution and abundance of species, including niche-based vs. neutral processes that control the changes in species turnover (beta diversity) along an altitudinal gradient. However, the relative importance of stochastic/deterministic processes to beta diversity has been less studied. For northern hardwood and conifer forests of Japan (Shiretoko UNESCO World Heritage Site), we conducted a field survey to record distribution and abundance of woody plants and oribatid mites in seventy 0.01-ha plots at altitudes from 50 to 1200 m above sea level.

Results/Conclusions

Beta diversity of plant and mite communities decreased with altitude. We found that this was attributed to not only the local species pool size (gamma diversity) at each elevation but also changes in the resource distribution. Interestingly, the beta deviation (deviation of observed beta diversity from that predicted for the local species pool) of plants significantly changed with altitude, as a result of the altitudinal changes in stand structure that determine the within-stand resource distribution. Beta deviation of mites did not vary along the altitudinal gradient, as a result of spatial variations of the resource amount independent of altitude. These results imply niche-based mechanisms (resource partitioning) of the community assembly. Thus, conservation of biodiversity should consider not only the preservation of local species richness but also the maintenance of landscape-level heterogeneity of environmental conditions to conserve natural community processes. Our results re-emphasize the importance of a coarse-filter approach for forest biodiversity conservation at a regional scale.