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Density-dependent mortality and the latitudinal gradient in species diversity


Ecologists have long postulated that density-dependent mortality maintains high tree diversity in the tropics1,2,3,4,5,6. If species experience greater mortality when abundant, then more rare species can persist1,2,7,8,9. Agents of density-dependent mortality (such as host-specific predators, and pathogens) may be more prevalent or have stronger effects in tropical forests, because they are not limited by climatic factors1,2,3,4,5. If so, decreasing density-dependent mortality with increasing latitude could partially explain the observed latitudinal gradient in tree diversity4,5,6. This hypothesis has never been tested with latitudinal data. Here we show that several temperate tree species experience density-dependent mortality between seed dispersal and seedling establishment. The proportion of species affected is equivalent to that in tropical forests6,10,11,12,13,14,15,16, failing to support the hypothesis that this mechanism is more prevalent at tropical latitudes. We further show that density-dependent mortality is misinterpreted in previous studies. Our results and evidence from other studies suggest that density-dependent mortality is important in many forests. Thus, unless the strength of density-dependent mortality varies with latitude, this mechanism is not likely to explain the high diversity of tropical forests.

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Figure 1: The observed (a) and predicted (b) relationship between conspecific seed density and seed germination for Acer rubrum.
Figure 2: The proportion of tree taxa experiencing density-dependent mortality at different latitudes.
Figure 3: Five analyses used to test for density-dependent mortality are applied to seed bank incorporation, germination, establishment and over-winter survival.


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Funding was provided by Sigma Xi and National Science Foundation grants. We thank M. Dietze, K. Harms, I. Ibanez, S. Ladeau, J. Lynch, J. McLachlan, J. Mohan, A. Pringle and M. Rocca for discussion of data and comments on drafts of the manuscript. Support in the field was provided by R. Hille Ris Lambers and H. Passmore.

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Hille Ris Lambers, J., Clark, J. & Beckage, B. Density-dependent mortality and the latitudinal gradient in species diversity. Nature 417, 732–735 (2002).

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