Diversity, composition and seasonality of wild bees (Hymenoptera: Apoidea) in a northern mixed-grass prairie preserve

Information deficiencies exist regarding patterns of wild bee (Hymenoptera: Apoidea) diversity and community composition in representative habitats worldwide. The main objective of this study was to characterize the wild bee fauna of mixed-grass prairie in southwestern Manitoba. Weekly sampling of wild bees using two methods, sweep-netting and bee bowls, was conducted over two years (2005-2006) at three sites within the Yellow Quill Mixed-grass Prairie Preserve. Spatial and seasonal patterns in diversity indices, taxonomic composition and ecological composition of the bee fauna were identified and investigated in relation to sampling method, environmental conditions, and floral resource availability.;Overall, bee catch and species richness were greater in the second year, but diversity was lower. Observed spatial and temporal patterns in the abundance, diversity and composition of the bee community were strongly modified by sampling method. Sweep-net sampling detected more diverse, even and species-rich assemblages, while catch frequencies for each method were reciprocally related to resource availability. That the second year was characterized by high temperatures and deficient rainfall allowed some of the observed patterns to be interpreted within the context of floral resource limitation. While high densities of the exotic invasive plant Euphorbia esula altered the diversity and seasonality of the floral resource base at one site, other factors were deemed responsible for supporting a qualitatively species-rich assemblage at that site relative to the other two sites. In both years, seasonal bee diversity values, measured as Hill's effective species richness (N1), peaked in the early part of the season and subsequently declined. Redundancy analysis was used to relate this pattern to high early-season species richness of miners and to an increase in the relative abundance of social nesters over the season.;A total of 7014 individual bees representing five families, 21 genera and 100 species were collected during the course of this study; however, this assessment is incomplete as taxon accumulation curves did not achieve asymptotes. While the fauna was biogeographically biased toward eastern-distributed taxa, there was nonetheless solid representation of species with central and western affinities. Fifteen species are newly recorded for Manitoba. A total of 23.8% of the taxa were pollen specialists. Miners were the most species-rich nesting guild (50 species); while the bulk of the individuals were social nesters (75.3%) of the genera Lasioglossum and Bombus. Representing 11% of the sample, the proportion of parasitic taxa was lower than expected compared to other studies, likely owing to sampling effects or taxonomic difficulties.