Natural enemies and the diversity of plant communities

The processes that determine the structure of plant communities are of considerable practical and theoretical interest. Natural enemies such as herbivores, seed predators and pathogens provide one potentially important influence on plant diversity. I investigated the effects of natural enemies on plant diversity in two contrasting, species-rich plant communities (tropical forests in Panama and temperate grasslands in the UK), focusing on pre-dispersal seed predation by insects, and the mortality of seeds and seedlings caused by soil fungi. In Panama I found that pre-dispersal insect seed predators generate significant levels of mortality in multiple tropical tree species, with high heterogeneity in predation rates among individuals and at different forest sites. Insect seed predators were highly host-specific, consistent with a role in enhancing plant diversity. At Upper Seeds, a calcareous grassland site in the UK, I used manipulative experiments to show that soil fungi increase the diversity of plants propagating from soil seed banks. A parallel experiment in Panama, mimicking germination under light gap conditions, revealed differential effects of fungi among sites, with fungicide treatment appearing to increase the diversity of propagated seedlings at some sites but reducing it at others. These results suggest that the influence of soil fungi on pre-emergence mortality can alter plant diversity, even when post-emergence mortality from fungal pathogens is limited. In Panama, I also tested whether enemy-mediated mortality increases with rainfall, potentially contributing to the positive regional correlations widely observed between precipitation and plant diversity. In contrast to predictions, neither pre-dispersal insect seed predation nor the influence of soil fungi on seedling recruitment were affected significantly by site humidity, or (for soil fungi) with experimentally manipulated soil moisture levels. Overall, my results provide evidence that pre-dispersal seed predators and soil fungi can affect plant recruitment and diversity at early life stages, with potential consequences for the community structure of adult plants.