Determinants of bryophyte species richness on the Zhoushan Archipelago, China

Abstract

Bryophytes of continental islands have received little attention in biogeography. To quantify the contributions of island attributes to the species richness (SR) of different bryophyte groups on continental islands, we investigated the species number of fifteen bryophyte groups and seven island attributes (area, elevation, isolation degree, human disturbance intensity, rainfall, vegetation coverage and island shape) on 66 continental islands of the Zhoushan Archipelago in the East China Sea. By using generalized linear mixed models (GLMMs), we found that all the focal environmental variables exerted effects on bryophyte SR to different extents. According to the marginal R2 and the ratios of marginal R2 to conditional R2 of the GLMMs, area (0.776/0.956) and elevation (0.741/0.981) were the two most important determinants of bryophyte SR, followed by disturbance intensity (0.497/0.755), vegetation coverage (0.478/0.708) and isolation (0.234/0.311). Island shape irregularity (0.049/0.068) and rainfall (0.045/0.06) exerted weak but positive effects on bryophyte SR. The effects of island attributes on bryophyte SR were different, with positive effects on the SR of all or most of the focal bryophyte groups for island area, elevation, rainfall and shape irregularity, and a negative effect for isolation degree. The relationships of bryophyte SR with disturbance intensity and vegetation coverage followed unimodal models for most of the focal bryophyte groups. The responses of different bryophyte groups to a given environmental variable are taxon-specific. With the increase of island area and elevation, SR increased faster in mosses than liverworts, and faster in pleurocarpous mosses than in acrocarpous mosses. With the increase of island shape irregularity, SR increased for most of the focal bryophyte groups, but decreased for pleurocarpous mosses. Our results imply that a forest reserve for conservation of bryophyte diversity should be designated with a large area and a higher elevation, moderate disturbance to produce irregularly―shaped fragments with different types of vegetation.