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Abstract Aims Both ecological drift and environmental heterogeneity can produce high beta diversity among communities, but only the effect of drift is expected to be enhanced in communities of small size. Few studies have explicitly tested the influence of community size on patterns of beta diversity. Here we applied a series of analyses aimed at testing the influence of drift versus environmental heterogeneity on beta diversity among tree communities on islands of variable size.
Location Thousand Island Lake, Zhejiang Province, China.
Methods We used data on mapped tree communities and environmental conditions for 20 small islands (<1 ha) and nine large islands (>1 ha) created via the construction of a hydroelectric dam in 1959. Beta diversity was calculated using abundance‐based multiple‐site dissimilarity based on the Bray–Curtis index. On the basis of the hypothesis of ecological drift among small islands, we tested for higher beta diversity among small than large islands using: (a) raw data (b) controlling for the number of individual sampled on a given island, and (c) controlling for the contiguous sampling area and thus for intra‐island environmental heterogeneity. We also tested the prediction that the relationship between species composition and environmental variables should be weaker on small islands using canonical correspondence analyses.
Results Using raw data and controlling for the number of individuals, community dissimilarity was significantly greater among small islands than among large islands. However, when controlling for contiguous sampling area this difference disappeared. Contrary to the prediction based on ecological drift, the strength of overall composition–environment relationships was not significantly weaker for small islands in any of the analyses, and environmental heterogeneity increased faster with area among small islands than among large islands.
Main Conclusions Despite a result using raw data that was consistent with the hypothesis of ecological drift, our full set of results clearly indicated the high beta diversity among small islands was more likely due to environmental heterogeneity rather than ecological drift. This result points to a clear need to control for sampling area among habitats of different size when testing for statistical signatures of drift.
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null (Ed.)Abstract Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity.more » « less
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Abstract Aim Small, old‐growth forest fragments generally have more small‐seeded plants than large patches, due to the disappearance of large seed dispersing vertebrates. This pattern may differ for secondary forest fragments where differential migration ability rather than persistence of seed dispersers may be driving plant community assembly. In this study, we investigated the effect of habitat fragmentation on seed dispersers and plant community structure in regenerating forests.
Location The Thousand Island Lake, China.
Taxon Plants, birds and mammals.
Methods We compiled diversity and abundance data for birds and mammals on islands in the Thousand Island Lake, China. We also surveyed the secondary plant communities and measured seed dispersal traits.
Results Community‐weighted mean seed size of woody plants decreased with island size. This pattern was related to compositional difference of the dispersers. We found that mammal diversity and abundance was only weakly or not related to island size; whereas bird diversity and abundance increased strongly with island size. Density of bird‐dispersed plants was significantly positively related with island size. Since birds tend to disperse smaller seeds than mammals, the trend in seed size may have been a consequence of the shift in relative abundance of the two disperser guilds.
Main Conclusions Differential responses of seed dispersers to habitat fragmentation may lead to pervasive shifts in the plant community structure of regenerating forest fragments. Our study highlights the importance of keeping large continuous forests in order to retain mammals and their dispersal capabilities.
