Effects of spotted knapweed on a cervid winter-spring range in Idaho

Abstract

Spotted knapweed (Centaurea maculosa Lam.), an exotic member of the Compositae, infests large areas of rangeland in the northwestern United States. We assessed the impacts of infestation on a wilderness winter-spring range for elk (Cervus elaphus nelsoni Bailey), mule deer (Odocoileus hemionus Raf.), and white-tailed deer (Odocoileus virginanus Raf.) along the Selway River in Idaho and found no evidence of a large reduction in carrying capacity. We estimated cervid densities in open areas by scan sampling known area blocks. Densities in knapweed vegetation were greater than or equal to densities in areas of native bunchgrasses and sedges. Direct observation of animals and laboratory analyses of fecal and rumen samples showed spotted knapweed seedheads and rosette leaves were being eaten by all cervid species. Deer ate large amounts of rosette leaves at times in contrast to elk, which consumed them frequently, but in small amounts. Seedhead consumption was greatest during periods of snow cover. We collected composite samples of knapweed times and determined energy and protein content wtth standard laboratory techniques. Energy and protein content of rosettes was near that of preferred native food plants. Seedheads, while less nutritious than rosettes, remained easily obtainable above the snow. The amount of energy and protein available on sample plots decreased modestly at most after infestation. In composite samples of spotted knapweed the content of cnicin, a sesquiterpene lactone in aerial tissues, was determined by high performance liquid chromotography. Changes in cnicin levels did not appear to be responsible for seasonal changes in the amount of knapweed in cervid diets. When estimating or predicting carrying capacity of a cervid range, spotted knapweed should be considered a potential food.