Effects of Exotic Plants on Native Ungulate Use of Habitat

Leafy spurge (Euphorbia esula), smooth brome (Bromus inermis), Japanese brome (B. japonicus), and downy brome (B. tectorum) are exotic plant species that dominate and displace native forage species throughout much of central North America. However, information on how exotic plant infestations affect native ungulate use of habitat is limited. We used pellet-group densities to estimate use of habitat by bison (Bos bison), elk (Cervus elaphus), and deer (Odocoileus spp.) during 1992-93 growing seasons within 4 exotic plant-infested and 4 comparable noninfested grassland habitats in Theodore Roosevelt National Park, North Dakota. We used twig count and twig measurement methods to estimate use of browse during summer (1992) and winter (1992-93), respectively, for both leafy spurge-infested and noninfested woodland habitats. Bison use of 2 leafy spurge-infested grassland habitats averaged 83% less than that for noninfested sites (P < 0.001). Deer pellet-group densities, normally highest within creeping juniper (Juniperus horizontalis)-little bluestem (Schizachyrium scoparium) habitat, were reduced ≤70% by infestations of leafy spurge (1992, P = 0.035; 1993, P = 0.002). Use of bromegrass-infested grassland by bison, elk, and deer was similar to that for noninfested sites for 1992 and 1993 (P > 0.05). Use of browse in green ash (Fraxinus pennsylvanica)-chokecherry (Prunus virginiana) habitat during summer and winter was reduced an average of 32% by infestations of leafy spurge (P < 0.05). The reduction in native ungulate use of leafy spurge-infested sites may be attributed to lower forage production in infested sites as well as simple avoidance.