Mammalian herbivores affect leafhoppers associated with specific plant functional types at different timescales

Theory predicts that mammalian herbivores affect the quantity and quality of plants on which they preferentially feed in the short term. In the longer term, they can promote either preferred or less preferred plants, depending on whether preferred plants are adapted or sensitive to grazing. Less clear are the short- and long-term responses of herbivorous insects to mammalian herbivory, and how these responses depend on the specific plants or plant functional types on which the insects feed. We progressively excluded large, medium and small mammals for five growing seasons in two subalpine vegetation types with long-term differences in mammalian grazing intensity. Short-grass vegetation has a history of intensive grazing, while tall-grass vegetation has been grazed less intensively. We tested whether mammals altered the abundance and body size of leafhoppers specialized on specific plant functional types (grasses, sedges, forbs, or legumes/forbs), distinguishing between short-term (exclosures) and long-term (vegetation types) differences in mammalian grazing pressure. Furthermore, we assessed whether leafhoppers’ responses were explained by changes in biomass or quality of the plant functional types on which they feed. In the short term, mammal exclosures increased the abundance of grass- and forb-feeding leafhoppers via increases in the biomass of grasses and forbs, regardless of vegetation type. Both grasses and forbs are preferred food plants of mammals. In the long term, the biomass of sedges, which are less preferred by mammals, increased in the less intensively grazed tall-grass vegetation. This resulted in a higher abundance of sedge-feeding leafhoppers. The small size of these sedge feeders lowered the average leafhopper body size in the tall-grass vegetation. Plant nutritional quality did not explain any effects of exclusions or vegetation types. Our results demonstrate that both short- and long-term effects of mammalian herbivores on the biomass of specific plant functional types caused concurrent changes in the abundance of specialized herbivorous insects, which scaled up to community-wide shifts in insect body size, a key life-history trait. A plant-functional-type approach can thus help to predict how overabundance or extinction of mammalian herbivores impacts on other components of the food web at various timescales. A plain language summary is available for this article.