Density-dependent mortality and the latitudinal gradient in species diversity

Ecologists have long postulated that density-dependent mortality maintains high tree diversity in the tropics(1-6). If species experience greater mortality when abundant, then more rare species can persist(1,2,7-9). 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 factors(1-5). If so, decreasing density-dependent mortality with increasing latitude could partially explain the observed latitudinal gradient in tree diversity(4-6). 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 forests(6,10-16), 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.