Historical changes in northeastern US bee pollinators related to shared ecological traits

Abstract

Pollinators such as bees are essential to the functioning of terrestrial ecosystems. However, despite concerns about a global pollinator crisis, long-term data on the status of bee species are limited. We present a long-term study of relative rates of change for an entire regional bee fauna in the northeastern United States, based on >30,000 museum records representing 438 species. Over a 140-y period, aggregate native species richness weakly decreased, but richness declines were significant only for the genus Bombus. Of 187 native species analyzed individually, only three declined steeply, all of these in the genus Bombus. However, there were large shifts in community composition, as indicated by 56% of species showing significant changes in relative abundance over time. Traits associated with a declining relative abundance include small dietary and phenological breadth and large body size. In addition, species with lower latitudinal range boundaries are increasing in relative abundance, a finding that may represent a response to climate change. We show that despite marked increases in human population density and large changes in anthropogenic land use, aggregate native species richness declines were modest outside of the genus Bombus. At the same time, we find that certain ecological traits are associated with declines in relative abundance. These results should help target conservation efforts focused on maintaining native bee abundance and diversity and therefore the important ecosystems services that they provide.

Fig. 1.

Trends in species richness over time. Number of bee species (± SE) in a standard number of independent specimen records per time period. Dashed line indicates a nonsignificant trend and solid line a significant trend. (A) All native bee species excluding Bombus (rarefied to 1,000 specimens). (B) Genus Bombus (rarefied to 400 specimens). (C) Exotic bee species (rarefied to 1,000 specimens).

Fig. 2.

Estimates of relative rate of change over the 140-y interval for different bee genera. Estimates are derived from logistic regression models. Negative values indicate declining trends. Boxplot width is proportional to the number of species sampled for each genus.

Fig. 3.

Relationships among species traits, phylogeny, and rates of change in relative abundance over time. The three most speciose genera are colored to demonstrate how traits are often shared among species within a genus: Bombus (blue), Andrena (red), and Lasioglossum (green). (A) Dietary breadth (floral specialization); boxplot width is proportional to number of species and individual species values are indicated along each side. (B) Body size measured as the intertegular distance (mm). (C) Phenological breadth (days of adult activity per year). (D) Northern distributional limit (degrees latitude).