Theory predicts that higher biodiversity in the tropics is maintained by specialized interactions among plants and their natural enemies that result in conspecific negative density dependence (CNDD). By using more than 3000 species and nearly 2.4 million trees across 24 forest plots worldwide, we show that global patterns in tree species diversity reflect not only stronger CNDD at tropical versus temperate latitudes but also a latitudinal shift in the relationship between CNDD and species abundance. CNDD was stronger for rare species at tropical versus temperate latitudes, potentially causing the persistence of greater numbers of rare species in the tropics. Our study reveals fundamental differences in the nature of local-scale biotic interactions that contribute to the maintenance of species diversity across temperate and tropical communities.
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We thank everyone involved in the collection of the vast quantity of data in the CTFS-ForestGEO network (see table S20 for site-specific acknowledgments). This work was carried out during the 2016 CTFS-ForestGEO Workshop in Hainan, China, and was supported by NSF grant DEB-1545761 to S.J.D. This work was also supported by NSF grants DEB-1256788 and -1557094 to J.A.M., NSF grant DEB-1257989 to S.A.M., and the Tyson Research Center. We thank K. Harms, R. Chisholm, T. Fung, members of the Myers Lab, and three anonymous reviewers for helpful comments and discussions. We declare no conflicts of interest. The data used in the primary analyses will be available at the Smithsonian Institution's CTFSForestGEO database portal: www.forestgeo.si.edu/group/Data/ Access+the+data.
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