Contribution of conspecific negative density dependence to species diversity is increasing towards low environmental limitation in Japanese forests

Pavel Fibich,Jan Altman,Jiří Doležal,Satoshi N Suzuki,Masae I Ishihara

doi:10.1038/s41598-021-98025-5

Abstract

Species coexistence is a result of biotic interactions, environmental and historical conditions. The Janzen-Connell hypothesis assumes that conspecific negative density dependence (CNDD) is one of the local processes maintaining high species diversity by decreasing population growth rates at high densities. However, the contribution of CNDD to species richness variation across environmental gradients remains unclear. In 32 large forest plots all over the Japanese archipelago covering > 40,000 individual trees of > 300 species and based on size distributions, we analysed the strength of CNDD of individual species and its contribution to species number and diversity across altitude, mean annual temperature, mean annual precipitation and maximum snow depth gradients. The strength of CNDD was increasing towards low altitudes and high tree species number and diversity. The effect of CNDD on species number was changing across altitude, temperature and snow depth gradients and their combined effects contributed 11–18% of the overall explained variance. Our results suggest that CNDD can work as a mechanism structuring forest communities in the Japanese archipelago. Strong CNDD was observed to be connected with high species diversity under low environmental limitations where local biotic interactions are expected to be stronger than in niche-based community assemblies under high environmental filtering.

Highlights

Species coexistence is a result of biotic interactions, environmental and historical conditions
We explored three hypotheses: (1) the strength of conspecific negative density dependence (CNDD) is increasing along environmental gradients towards low environmental limitation, (2) species diversity is correlated with the strength of CNDD, and (3) the effect of CNDD on species diversity is affected by environmental gradients
We found support for species richness patterns driven by local biotic interactions; their strength was changing along environmental gradients

Summary

Introduction

Species coexistence is a result of biotic interactions, environmental and historical conditions. Conspecific negative density dependence (CNDD) is the process that decreases population growth rates at high densities, because of natural enemies and competition for resources, and favors locally less common species over common o nes[3]. Several mechanisms were suggested to explain the shift in strength of CNDD across latitudes (e.g. stronger intra-specific competition or pressure from natural enemies in the tropics than in the temperate latitudes, strong dispersal limitation for rare species and their enemies in the tropics). Janzen[18] suggested that species in the tropics experience a narrower range of temperature than those in temperate latitudes leading to higher specialization in the tropics In addition to such local processes (e.g. biotic interactions), regional effects (i.e. evolutional, geographic and geology history) influence global patterns of tree diversity[16,19,20]. To what extend CNDD contributes to species richness variation across a wider resource or environmental gradients remains to be tested

Methods

Results

Discussion

Conclusion