Recently, plant ecologists have increasingly focused on the role of soil organisms in determining plant community processes. Below-ground herbivores, such as worms, tend to promote plant diversity when they feed on dominant plant species. However, van Ruijven et al. show that the combined effects of above- and below-ground herbivores cannot be predicted from their separate effects. Different combinations of invertebrate herbivores (nematodes and wireworms below ground, and grasshoppers above ground) were added to experimental species-rich grassland plant communities. When added separately, the nematodes and wireworms had positive effects on diversity, whereas the grasshoppers had neutral effects. When added together, however, the combined effect on diversity was negative. The different feeding preferences of the two groups of herbivores appeared to alter the competitive interactions among the plant species within the communities, eventually producing the nonadditive effects observed. Differential distributions of above- and below-ground herbivores may well contribute to locally hetero-geneous diversity levels. -- AMS
Ecol. Lett. 8, 30 (2005).
