IntroductionInferring generalizable patterns in species dynamics, distributions, and functional variation are central aims of ecology and evolutionary biology (MacArthur, 1972). Trait-based approaches, which quantify phenotypic characteristics that impact organisms’ fitness and/or functional role, provide a tractable comparative framework for understanding communities, ecosystems, and evolutionary processes (Mcgill et al., 2006; Violle et al., 2007). Functional trait studies have proliferated over the past two decades, addressing foundational questions in community ecology (Cadotte et al., 2015; Mcgill et al., 2006; Violle and Jiang, 2009), biogeography (Violle et al., 2014), and conservation biology (Cadotte et al., 2011; Wellnitz and Poff, 2001) across taxonomic groups. These works emphasize the promise of trait-based research for generating novel insights into central ecological concepts and theories.Increasingly, bee researchers are recognizing the utility of trait-based approaches for a wide variety of applications in ecological research. Bees (Hymenoptera: Apoidea: Anthophila) represent more than 20,000 species worldwide and display dramatic interspecific variation in morphology and behavior (Figure 1), including traits that mediate pollination services and responses to global environmental change (Supplementary Table 1). Exploration of functional traits has long been a cornerstone of bee research, yet only recently have these traits been systematically applied in bee ecological studies as a comparative framework for understanding community-level processes. Given their major functional role as the primary animal pollinators of terrestrial ecosystems (Ollerton et al., 2011), the bees represent a group ripe for exploration through a functional ecological lens.Here, we review an emerging body of literature that quantifies functional traits across bee communities to address questions in bee ecology. In doing so, we address the following questions: How have functional traits been used to study bee ecology? What have been the major outcomes and limitations in bee functional trait research? How might this framework be leveraged to address urgent questions in the study of global bee declines? We review the variety of methods used to quantify bee trait variation, highlight common methodological problems and inconsistencies, and recommend best practices. Additionally, we describe geographic, taxonomic, and trait biases across the body of bee functional trait work, and highlight research areas that merit particular attention in future studies. Finally, we emphasize the value of open trait data sharing, and propose a roadmap toward a global bee functional trait database, including an initial aggregated dataset of 3369 morphological measurements from 1209 bee species.

1. Bumble bees are key pollinators with some species reared in captivity at a commercial scale, but with evidence of population declines and with alarming predictions under climate change scenarios. While studies on the thermal biology of temperate species are still limited, they are entirely absent from the tropics where the effects of climate change are expected to be greater. 2. Herein we test if tropical bumble bees’ lower (CTMin) and upper (CTMax) critical thermal limits decrease with elevation and if the stable optimal conditions used in laboratory-reared colonies reduces their thermal tolerance. 3. We assessed changes in CTMin and CTMax of four species at two elevations (2600 and 3600 m) in the Colombian Andes and of laboratory-reared individuals of B. pauloensis. In addition, we examined the effect of body size and compiled information on bumble bees’ thermal limits from the literature to assess potential predictors for broad-scale patterns of variation. 4. CTMin decreased with elevation while CTMax did not. CTMax was slightly higher (0.84 °C) in laboratory-reared than in wild-caught bees while CTMin was similar. CTMin decreased with increasing body size while CTMax did not. Latitude is a good predictor for variations in CTMin while annual mean temperature and extreme monthly temperatures are good predictors for both CTMin and CTMax. 5. The stronger response in CTMin with increasing elevation supports Brett’s heat-invariant hypothesis. Tropical bumble bees appear to be about as heat tolerant as those from temperate areas, suggesting that other aspects of climate besides temperature (e.g., water balance) might be more determinant environmental factors for these species under global warming. Laboratory-reared colonies are adequate surrogates for addressing questions on thermal tolerance and global warming impacts.