Are wild ungulates eating your conservation lunch? Aspen and shrub recruitment on a remnant Pacific Northwest bunchgrass prairie

Background/Question/Methods

Western landscapes evolved with a diverse suite of wild ungulates and predators which often constrained their numbers. In the past 150 years these areas have also been used to pasture a variety of domestic ungulates.  Excessive levels of herbivory can prove a significant obstacle to maintaining and restoring native species. We conducted a study aimed at assessing the impact of wild and domestic ungulate herbivory, past and present, on the abundance of deciduous woody vegetation on a conservation area located within a remnant Pacific Northwest bunchgrass prairie in northeastern Oregon. To test the hypothesis that excessive herbivory by elk (Cervus elaphus) and/or mule deer (Odocoileus hemionus) was limiting aspen (Populus tremuloides) and shrub recruitment, we examined contemporary patterns of herbivory across three treatments: fenced to exclude all wild and domestic ungulates; fenced to exclude only cattle (Bos taurus), and unexclosed. To assess the effects of herbivory over past decades, we used dendrochronological techniques to determine the age structure of aspen and several shrub species. These data, along with estimates of the abundance of wild and domestic herbivores, were used to gauge the relative importance of wild vs. domestic herbivores in constraining aspen and shrub recruitment from the 1940s to present.

Results/Conclusions

Our assessment of browse intensity based on shrub architecture revealed that that 96% of individuals across 57 unexclosed sites on the Zumwalt Prairie Preserve had levels of herbivory incompatible with aspen and shrub recruitment. Browse intensity, as determined via measurement of the heights of live vs. dead/browsed stems, was significantly lower in areas excluded from all ungulates than in areas where only cattle were excluded. Exclusion of only cattle had no significant effect on browse pressure. Though current levels of browse may be explained by exponential increases in elk abundance since 2000, aspen age structure suggests that recruitment declined precipitously decades prior when elk were scarce in the area. Declines in recruitment >1960 are best explained by increases in cattle numbers in the region beginning after World War II. Although agents of herbivory have changed over the decades, chronic heavy browsing has limited the abundance of aspen and deciduous shrubs in this landscape impeding efforts to conserve certain species, such as Columbian sharp-tailed grouse (Tympanuchus phasianellus columbianus). While overgrazing by livestock has long been considered by land managers, herbivory by wild ungulates should also be assessed and abated if it undermines conservation efforts.