Variable Response Of Butterflies And Vegetation To Elk Herbivory: An Exclosure Experiment In Ponderosa Pine And Aspen-Mixed Conifer Forests

摘要

Herbivory by wild ungulates influences the structure and composition of plant communities, but less is known about how such changes affect species richness and abundance of insect herbivores. In Bandelier National Monument, New Mexico, 1999 to 2002, we used ungulate exclosures and reference sites to measure the response of butterflies to herbivory by high numbers of elk (Cervus elaphus) in ponderosa pine (Pinus ponderosa) grassland and aspen-mixed conifer forests. At each site, we measured butterfly abundance, species richness, and selected environmental variables. Butterfly abundance and species richness significantly differed among dates in ponderosa pine-grassland sites and differed significantly for date, treatment, and date*treatment interaction in aspen-mixed conifer sites in 2001, the year of greatest butterfly abundance and richness. Environmental variables differed little between treatments in either habitat, with the exception of the distribution of aspen stem densities, which significantly differed among height classes and treatments in 2001 and 2002. Results of nonmetric, multidimensional scaling ordination indicated that butterfly community composition in late summer was best explained by non-woody biomass, blooming forb species richness and abundance, and aspen saplings excluded from elk in the aspen-mixed conifer sites. Although elk were only excluded for 4 y, our results indicated that, at high densities, they browse non-woody vegetation and young aspen, which in turn has a cascading affect on adult butterfly community composition in some habitats during some years.