Assessing community composition and nutrition of forage plants in prime elk summer habitat in the Absaroka Range, Wyoming.

摘要

Traditionally, the Clarks Fork elk herd (Cervus elaphus) has migrated seasonally from Cody, Wyoming through the Absaroka Range to the high mountains surrounding the Lamar River Valley of eastern Yellowstone National Park. About 50% of elk in the Clarks Fork herd reside the entire year closer to Cody. Whereas the average pregnancy rate for Rocky Mountain elk is about 90%, recent surveys of the herd have detected nonmigratory Clarks Fork population pregnancy rates well within the normal range at 84-94%, while the migratory population ranges only from 59-76%. Because nutritional limitations can lower pregnancy rates within elk herds, the reduction in productivity in the migratory subpopulation may be linked to limitations in the availability of nutritious forage on the migratory summer range. Vegetation sampling was conducted on resident and migratory elk summer ranges to characterize the species composition of both ranges as well as to estimate biomass available of key forage types. Fecal samples were collected on both summer ranges to estimate elk diets and calculate selection ratios based on diet and forage availability. I used Timmer diagrams with vector analysis to make graphical comparisons of the nutritional status of the migratory and resident summer ranges, to look specifically for nutritional deficiency on the migratory range. Summer ranges of migratory and resident Clarks Fork elk contain distinct vegetation types, regardless of which summer range is considered. The summer ranges are also different from one another regardless of vegetation type. Although absolute and relative biomass of plant groups differed between migratory and resident summer ranges, elk selection of forage plants was the same between migratory and resident elk. Elk selected plant groups in the same proportions on each summer range, but ingested different amounts of plant material. Sedges constituted a greater percentage of migratory elk diets, and were more available on the migratory range. Similarly, on the resident range, legumes occurred much more frequently than on the migratory range. Resident elk have more highly desirable forage available on their summer range, including grasses and legumes. The migratory summer range of Clarks Fork elk is more nutritious with respect to both crude protein and digestible energy than the resident summer range. There were differences in the crude protein and digestible energy contents of some analogous vegetation types, often with the plants from the migratory range containing higher nutrient content than the plants sampled from the resident range. I found no nutrient deficiencies in protein or energy on the migratory range. Agricultural areas available on the resident range have much greater amounts of nutritious biomass available per land area than either the resident range in general or the migratory range. These results quantify the benefit of migration to elk through the nutritional quality of forage. However, other studies indicate that the length of time this nutritious forage is available to migratory elk is shrinking, which could lessen the benefit of migration and cause the depression of pregnancy rates seen in the migratory Clarks Fork elk.