Jeremy Hemberger, Neal Williams

A rapidly warming climate is a primary force driving changes in biodiversity worldwide. The impact of warming temperatures on insect communities is of particular interest given their importance for ecosystem function and service provision and the uncertainty around whether insect communities can keep pace with the rate of increasing temperatures. We use a long-term dataset on bumble bee species occurrence along with summer maximum temperature trends across North America to characterize community-level responses to recent climate warming. We examined responses using the community temperature index (CTI) – a measure of the balance of cool- and warm-adapted species within local communities. Starting in 2010, bumble bee average CTI across North America has rapidly increased after a period of slight increase from 1989 to the late 2000s. This increase is strongly associated with recent increases in maximum summer temperatures. The increase in CTI is spatially extensive, but the areas exhibiting the largest increase include mid to high latitudes as well as low and high elevations - areas relatively shielded from other intensive global changes (e.g., land-use). On average, bumble bee CTI has increased 0.99°C from 1989 to 2018, a change of similar magnitude to the increase in maximum summer temperatures. This shift has been driven by the rapid loss of cold-adapted species and an increase in warm-adapted species within bumble bee communities across North American ecosystems. Despite evidence that the spatial velocity of community change is keeping pace with temperatures, this is a direct result of the decline of cool-adapted species. Our results provide strong evidence of the pervasive impacts posed to insect communities by temperature increases in the last 30 years.