The genetic dimension of pest pressure in the tropical rainforest.

Wet tropical forests are among the most diverse ecosystems on Earth and can host several hundreds of tree species per hectare. To maintain such diversity, the community must contain large numbers of relatively rare species rather than be dominated by a few very common trees, as is often the case in temperate forests. Explaining the mechanisms preventing dominance by common species has been a major task of tropical forest ecology. One of the most promising mechanisms is negative density dependence (NDD) of tree abundance driven by pests, including fungal diseases ('pest pressure'). NDD entails that the chance of survival of a sapling increases with the distance from a mature tree of the same species, thus preventing species from becoming locally dominant. Curiously, the strength of NDD is negatively correlated with abundance, meaning that tree species that are more common generally show weaker NDD (Comita et al. ). Interactions between plants and soil pathogens have been shown to play an important role in NDD (Klironomos ), and rare species are apparently more strongly affected (Mangan et al. ). However, the genetic mechanisms underlying this phenomenon have remained obscure. In this issue of Molecular Ecology, Marden et al. () suggest that reduced diversity of the genes involved in pathogen recognition (Resistance genes or R genes) could explain why NDD is stronger in locally rare species.