The feeding habits of the Mazama pocket gopher in the pine region of South-Central Oregon

Abstract

The objective of this study was to determine the feeding habits of the Mazama pocket gopher (Thomomys mazama Merriam) in the pine region of south-central Oregon. Damage caused by pocket gophers to conifer regeneration was a major incentive for this research. A permanent reference collection, consisting of epithelial tissue from leaves, stems, and roots of local plants mounted on microscope slides, was prepared. Pocket gophers were collected on alternate months from July 1973 through September l974. Species of plants eaten were identified by comparing fragments of epithelial tissue in the stomach contents to the reference collection. Shrubs, common mullein (Verbascum thapsus L.), and ponderosa pine seedlings (Pinus ponderosa Dougl.) also were examined in the field monthly from December 1973 through September 1974 for evidence of above-ground feeding by gophers. The relative abundance of plants on the study area was estimated by frequency of occurrence and percentage cover. Preference indexes for frequently eaten foods were obtained by dividing the precentage composition in the diet by the plant cover on the study area. Stomach content analysis revealed that 31 of the 45 plant species identified on the study area were used as food by gophers. Plant materials eaten included foliage, stems, and roots. Forbs comprised 55 percent of the vegetative cover in July 1973, and 61 percent in July 1974. They constituted 40 percent of the annual diet, and were eaten most heavily during the growing season. The most frequently eaten forbs were spreading groundsmoke (Gayophytum diffusum Mutt.), matted nama (Nama densum Lemmon), knotweed (Polygonum douglasii Greene), common thistle (Cirsium vulgare(Savi) Tenore), goosefoot violet (Viola purpurea Kell.), and velvet lupine (Lupinus leucophyllus Dougl.). Grasses comprised 41 percent of the vegetative cover in July 1973, and 30 percent in July 1974. They constituted 32 percent of the annual diet, and were eaten extensively during the dormant season. Cheatgrass (Bromus tectorum L.) was eaten only in May. Western needlegrass (Stipa occidentalis Thurb.) was eaten most heavily in summer and autumn, and mountain brome (Bromus carinatus H. & A.) was consumed most heavily during the dormant season. Stomach content analysis indicated that woody plants were eaten during the dormant season, but they constituted only a small part of the annual diet. Ponderosa pine was the most heavily eaten woody plant. Field observations supported these results. Evidence of gophers feeding above ground on woody plants was found only from December through April. In general, the diets of gophers collected during the growing season, July and September, were not significantly different. The diets of gophers collected during other sampling periods were significantly different (p<O.O5). Availability and relative abundance of foods were identified as important dietary determinants. In July, when all foods were abundant, vegetative components ranked by preference were perennial forbs, grasses, annual forbs, and woody plants. Perennial forbs were the preferred food, but grasses were the most important food in winter, Results indicated that vegetative a1teration as an indirect method of controlling pocket gopher damage to conifers, should be directed at controlling grasses without stimulating the establishment of perennial herbs.