Insect herbivores increase plant species richness in a long‐term grassland experiment

Abstract Understanding how herbivory influences plant diversity is central to plant community ecology. Although much is known about the effects of large‐bodied mammals, such as cattle or bison, the long‐term impact of small‐bodied herbivores, such as insects and small mammals, on plant diversity rarely has been examined. The paucity of such studies probably stems from the hypothesis that small herbivores are unlikely to alter plant diversity because they remove little plant tissue per capita and avoid consuming coarse tissues. However, few long‐term experiments have manipulated small herbivore densities under different ecological conditions and quantified their chronic effects. Over 12 years, we experimentally manipulated densities of individual herbivore groups (insects, small mammals and larger mammals (i.e. rabbits and deer)) using insecticide and fencing in a Kansas USA tallgrass prairie. Herbivore reduction treatments were applied in factorial combination with fertilizer addition to test whether herbivore effects varied with soil fertility. We quantified effects of each herbivore group and their interactions with soil fertilization on plant species richness. Reducing insect densities produced sustained decreases in total plant species richness beginning in year five of the experiment. The magnitude of insects' effect exceeded the magnitude of the negative effect of soil fertilization on species richness. Reducing access by small mammals and larger mammals rarely affected total plant species richness. Reducing insects lowered plant species richness consistently across functional and life‐history groups, although effects on annuals and forbs developed more rapidly (2–4 years) than for perennials and graminoids (8–9 years). Small mammal reduction temporarily lowered annual plant species richness for 3 years in the middle of the experiment. Evidence for interaction effects between herbivore density reduction and soil fertilization upon plant species richness was weak. Synthesis . In grasslands that lack large grazing mammals, chronic insect herbivory can promote plant species richness. Indirect mechanisms, such as alleviating light competition, likely underlie insects' diversity‐enhancing effects. Herbivores' effects were largely consistent across different soil nutrient/ecosystem productivity conditions, contrary to theory for large grazers. The time required for treatment divergence suggests that short‐term experiments often underestimate insects' impact on plant communities.