Context matters: The strength of distance-dependent mortality varies with light, species, seed size, and adult abundance

Background/Question/Methods

Density or distance-dependent mortality by host-specific natural enemies is thought to be a powerful mechanism affecting the abundance and coexistence of tree species. Indeed, many studies demonstrate that this process is common across temperate and tropical forests, but few examine how the strength of this process varies between locations, across biotic and abiotic environments, or with species traits. This is a critical shortcoming, as it impedes the ability to understand how and when distance-dependent mortality influences forest diversity. Moreover, understanding the context-dependence of distance-dependent mortality can allow us to predict how the loss of dispersal agents, as is happening in forests around the world, can alter communities. In this study, we experimentally measured the strength of distance-dependent mortality in the Mariana Islands, asking how this process varied by species and light conditions. Because one island has lost all avian seed dispersers, we also tested whether the distance-dependent mortality is stronger in the absence of dispersers, given that seed density underneath conspecifics is likely to be higher. We planted 4,330 seedlings from five bird-dispersed tree species and one bat-dispersed species in plots near and far from conspecific adults at 14 sites across four islands, then measured seedling survival, seedling height, number of leaves, and leaf herbivory while accounting for the local light environment.

Results/Conclusions

We found negative distance-dependent mortality in five out of six species, but the strength of this effect varied with light levels and changed in direction and degree by species. High light exacerbated the effects of distance on seedling survival in one species, moderated it in another, and reversed it in two species. The strength of distance-dependent mortality across species was inversely related to seed size and adult species abundance. Surprisingly, distance-dependent mortality was not stronger on Guam, where dispersers are absent, suggesting that natural enemy densities near parent trees have not increased on this island. In summary, we found that the strength of distance-dependent mortality varies greatly across species and the environment, implying a complex role for this mechanism in determining the abundance and distribution of species, and species response to the loss of dispersers.