Bees in Oak Savannas

ABSTRACT: In western North America one of the major communities most threatened by habitat conversion is the deciduous oak savanna. Originally occurring from southern California to southern British Columbia. The oak savanna has become almost entirely extirpated north of California. In western Oregon and Washington oak savanna occurred on valley-bottoms to low foothills. More than 80% of the human population in the Pacific Northwest now is located within this region, which also comprises the most valuable farmland in the area as well. Consequently, the oak savanna has been largely lost to anthropogenic uses. The scattered isolated remnants have been further transformed by introduced herbaceous species, increases in shrub density and canopy closure due to fire suppression in this fire-adapted community.

            There is considerable interest in reestablishing the native oak community. Current management methods are employing canopy thinning, elimination of conifer re-growth and removal of alien annual grass species, in addition to native plant re-introduction. However, mutualistic pollination interactions are central to insure the success of re-introduced or re-invigorated native plant species. Evidence from pollination mutualisms in intact northern California oak savannas indicate that the rich bee fauna is of primary importance to the reproductive success of understory species. Many of the bees are morphologically and behaviorly specialized to visit only a single plant genus. Prior to widespread agriculture and habitat conversion the oak savanna probably supported about 375 species of bees, approximately 25% of which were specialized feeders.

            Attempts during 2001 to locate relictual areas with both significant bee richness and native herbaceous understory revealed two possible isolated regions. Mitigating the oak savanna ecosystem will require successful reintroduction of both plant and pollinator species.