DIETARY SIMILARITY IN SYMPATRIC IDAHO AND COLUMBIAN GROUND-SQUIRRELS (SPERMOPHILUS-BRUNNEUS AND S-COLUMBIANUS)

We studied diets of Spermophilus brunneus, a rare endemic species, and S. columbianus, a larger, widespread potential competitor, in two are as where they were sympatric to learn if competition for food might be a factor in S. brunneus' rarity. Because the diet of S. brunneus was unknown, we also wished to determine if dietary specializations might explain the species' limited distribution. Vegetation at the study sit es was sampled using a point frame method. Fecal pellets were collecte d from live-trapped squirrels and analyzed using a microhistological t echnique; plant parts (leaves/stems, seeds/flowers, roots/bulbs) were tallied as separate categories for each species. At the two sites, S. brunneus consumed 45-50 species of plants in 55-75 categories, althoug h only 18-20 categories comprised > 1% of the diet and only 5-7 compri sed > 5%. Similarly, S. columbianus consumed 53-58 plant species in 74 -75 categories; only 10-17 categories comprised > 1% of the diet and o nly 4-6 comprised > 5%. Idaho and Columbian ground squirrel diets were 30-52% similar at a site, whereas each species' diet was only 25-37% similar between sires. Diets were not correlated with plant abundance in the habitat, indicating that diet selection was taking place. The d iet of S. brunneus appears typical of ground squirrels of the subgenus Spermophilus. Both species utilized similar resources, and habitat se gregation probably accounted for most of the observed dietary differen ces. The results suggest that dietary overlap with S. columbianus rele gates S. brunneus to less productive habitats and is a factor in S. br unneus' rarity.