When predators unexpectedly decrease instead of increase plant production

Conservation of predators often is promoted on their ability to provide important ecosystem services. One of these services is when predators decrease herbivore numbers, and thereby, increase plant production. In many grasslands, grasshoppers are the dominant herbivore, even when large herbivores are abundant, and birds are their main predator. We examined when this service emerges using a multi-site and -year grassland field experiment that excluded birds. When birds reduce grasshopper abundance, plant production only increases if grasshoppers also decrease the limiting nutrient (nitrogen) for plant production. Grasshoppers decrease nitrogen if they selectively feed on plant species that decompose more rapidly, which increases the litter from plant species that decompose more slowly. Therefore, when there are fewer grasshoppers, nitrogen availability for plants and their production increases. However, across sites, grasshoppers increase nitrogen availability more often than they decrease it; therefore, this ecosystem service of predation is not consistently observed.