POPULATION BIOLOGY OF BUMBLEBEES (BOMBUS) IN SUBALPINE MEADOWS IN NORTHEASTERN UTAH (COMMUNITY, COMPETITION, LIFE-HISTORY).

Abstract

The distribution and abundance of six bumblebee species (Bombus) and their associated flowers were studies for 12 weeks in 26 discrete subalpine meadows over the summer of 1981. Principal component analysis of flower densities identified phenological patterns of blooming and differential affinities of plants for moisture as the major contributors to flower variation among the meadows. A positive relationship between bumblebee species diversity and meadow area in the early summer and mark-recapture experiments suggested that queens are attracted to large meadows for nest establishment. After colony initiation meadow floristics govern Bombus species diversity and abundance. Results showed that: (1) mid- to late-summer bumblebee diversity, the temporal occurrence of new queens, and the densities of two species were correlated with the density of nectar producing flowers; (2) flower composition in meadows where local extinctions of at least one Bombus species occurred differed from those where all species persisted; and (3) the number of species reaching reproductive fruition was independent of bumblebee species composition. These results underscore the importance of nectar resources in Bombus colony and guild development. Multiple regression analyses were used to relate the effects of meadow size, elevation, and floristics to observed densities of Bombus flavifrons and B. rufocinctus. In mid- to late summer a significant portion of the variation in densities not explained by these variables was accounted for by densities of congenerics. In meadows where all Bombus were removed except B. flavifrons or B. rufocinctus significant positive density compensation was observed for the latter, but not the former. These results are suggestive of relationships among bumblebee densities, intensity of competition, colony ontogeny, colony reproduction, and local floristics that question both the (1) simplistic views of Bombus guild organization based on the paradigm of tongue length, corolla depth, and character displacement; and (2) the stochastic view that flowering phenologies and developmental time lags thwart resource-based guild organization.