The relationship between reproductive success, vegetation types in the home range, and diet composition and nutritional quality was examined in female Roosevelt elk on the Olympic Peninsula, Washington. Nine elk herds were radio tracked during 2001 to quantify reproductive success (as calf:cow ratio), home range size and location, and vegetation types used. I measured diet composition from fecal samples, and collected plant samples of forages found in the diet. I analyzed plant samples for nutritional quality (protein content, digestibility, and energy).
Herds were grouped into highly productive (December calf:cow ratio > 0.30) and less productive (December calf:cow ratio < 0.30). Highly productive herds had more 2 – 9 year old timber (P = 0.03) and less 11 – 24 year old timber (P = 0.03) available in the home range than less productive herds. Highly productive herds consumed more grasses (P = 0.008) and less shrubs (P = 0.02) and ferns (P = 0.02) than less productive herds. In addition, I found negative correlations between percentage grass or forb in the diet and percentage shrub or fern in the diet. Percentage shrub in the diet was positively correlated to percentage 20 – 24 year old timber, and negatively correlated to percentage 2 – 9 year old timber available in the home range.
Grasses and forbs were high in protein (> 12%), digestibility (> 55%), and digestible energy (> 2.1 Kcal/g). Ferns and shrubs were lower in protein except during spring when Athyrium felix-femina and Equisetum had protein content greater than 17%. Ferns and shrubs were less digestible (< 46%), and had lower digestible energy (< 1.7 Kcal/g) than grasses and forbs.
Regression and correlation analyses were used to investigate relationships between home range size, vegetation types used, vegetation types available, diet, and reproductive success. For highly productive herds, there was no change in calf survival as home range size increased. However, for less productive herds, calf survival decreased and mean calf:cow ratio increased with increasing home range size. Less productive herds could increase home range size to include more forage, but the higher cost of travel and predation risk may have lowered calf survival. Highly productive herds had areas of high quality forage (high protein and digestible energy) within their home range, therefore did not have to increase home range size to provide adequate nutrition.
Available marsh, riparian, and pasture in the home range, as well as used and available 2 – 9 year old timber in the home range, were positively related to reproductive success of female elk (P < 0.05). 20 – 24 year old timber used by herds was negatively related to reproductive success (P < 0.05). The amount of grasses, sedges, and forbs in the diet, all forages that were high in protein and digestible energy, were positively related to reproductive success (P < 0.001). Elk herds with home ranges that included riparian, pasture, and some young timber had access to abundant, good nutritional quality forages, allowing for high reproductive success regardless of home range size. Elk herds with home ranges that consisted of expanses of maturing timber did not have access to as much high quality forage, and therefore had lower reproductive success.
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Diet, Nutrition, And Reproductive Success of Roosevelt Elk in Managed Forests
