Biodiversity, exotic plant species, and herbivory: The good, the bad, and the ungulate Marty Vavra * , Catherine G. Parks, Michael J. Wisdom Pacific Northwest Research Station, USDA, Forest Service, La Grande Forestry and Range Sciences Laboratory, 1401 Gekeler Lane, La Grande, OR 97850, United States Abstract Invasion of natural ecosystems by exotic plant species is a major threat to biodiversity. Disturbance to native plant communities, whether natural or management induced, is a primary factor contributing to successful invasion by exotic plant species. Herbivory by both wild and domestic ungulates exerts considerable impact on structure and composition of native plant communities. Intensive herbivory by ungulates can enhance exotic plant invasion, establishment, and spread for three reasons: (1) many exotic plants are adapted to ground disturbances such as those caused by ungulate feeding, trampling, and movements; (2) many exotic plants are adapted for easy transport from one area to another by ungulates via endozoochory and epizoochory; (3) many exotic plants are not palatable or are of low palatability to ungulates, and consequently, their survival is favored as ungulates reduce or eliminate palatable, native plants. Ungulate herbivory is a chronic, landscape-scale disturbance capable of influencing plant communities as much as episodic events such as fire. Consequently, ungulate herbivory has the potential to facilitate the invasion and establishment of exotic plants in the interior Pacific Northwest where ungulates occupy nearly every ecosystem. Moreover, ungulate herbivory has intensified in many ecosystems, owing to the addition of domestic ungulates with that of existing, wild ungulates, coupled with the reduction or elimination of migratory movements and predators that previously regulated wild ungulate populations and influenced their distributions. Despite the observational evidence for ungulate herbivory as a strong facilitator of exotic plant invasion and establishment, current knowledge of cause– effect relations is severely limited by a lack of manipulative experiments. Most studies have been observational, unreplicated, and lack the experimental controls needed to eliminate or account for confounding sources of variation. Heightened attention to conservation of biodiversity will increase the importance of managing ungulates in balance with the plant communities that support them. Published by Elsevier B.V. Keywords: Disturbance; Competition; Grazing; Exotic plants; Biological diversity; Ungulates; Herbivory 1. Introduction Invasion of natural ecosystems by exotic plant species is recognized as a major threat to biodiversity. The conifer- dominated forest communities of the interior Pacific Northwest (described in detail by Franklin and Dyrness, 1973) differ in their relative susceptibility to exotic plant invasion with respect to landscape ecology and land uses (Parks et al., 2005). The lower and mid-elevation communities of the ponderosa pine (Pinus ponderosa), Douglas-fir (Pseudotsuga menziesii), and grand fir (Abies grandis) zones (Franklin and Dyrness, 1973) are the most impacted because of human induced (grazing, logging) disturbance. Disturbance of interior Pacific Northwest forest communities, whether natural or management induced, is often considered a contributing factor to successful invasion by exotic plant species and to a concomitant decline in biodiversity (Parks et al., 2005). Ungulate herbivory exerts considerable impact on structure and composition of native plant communities (Hobbs, 1996). Livestock particularly have been recognized as agents of detrimental change in the composition, structure, and devel- opment of plant communities (Fleischner, 1994). Herbivory by wild ungulates was once considered to have no impact, but now is recognized by scientists as an ecological force in ecosystems (Augustine and McNaughton, 1998; Riggs et al., 2000; Kie and Lehmkuhl, 2001). One subtle aspect of ungulate herbivory is its potential role in the spread and establishment of non-native invasive plants. At least 17 million acres of western Federal lands were reportedly infested by invasive plants in 1996, more than quadrupling their range between 1985 and 1995 (Westbrooks, 1998). Effects of chronic herbivory, especially coupled with www.elsevier.com/locate/foreco Forest Ecology and Management 246 (2007) 66–72 * Corresponding author. Tel.: +1 541 962 6561; fax: +1 541 962 6504. E-mail address: mvavra@fs.fed.us (M. Vavra). 0378-1127/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.foreco.2007.03.051