Nesting biology
Nesting biology is highly variable in bees, with consequences for habitat preferences and exposure to environmental stressors. For example, ground-nesting bees may be better insulated from extreme temperatures and fire than are twig- or wood-nesting bees, yet may be more vulnerable to habitat loss under certain environmental pressures (e.g., urbanization). As such, nesting traits have figured prominently in bee functional trait studies (Figure 2d). Most studies reviewed here categorized nesting biology at the species level according to nest location (e.g., “ground,” “cavity,” “stem,” etc.), though others additionally or alternatively considered nesting ability, i.e., whether a given species excavates its own nest or occupies pre-existing cavities. More so than other categorical traits like sociality and diet breadth, nesting trait analysis prompted the use of a large number of unique terms, reflecting the sheer diversity of bee nesting strategies, especially when considering data on tropical bees (e.g., nesting in termite mounds or in exposed nests, Borges et al., 2020; Giannini et al., 2020; Munyuli, 2014); (Supplementary Table 3). By far the most common method of assigning bee species to nesting categories was by distinguishing broadly between above-ground and below-ground nesters. Importantly, several authors create categories (e.g., “variable” or “mixed”) that account for within-species variation in nesting location, for example, bumble bees that can nest either in below-ground soil cavities or above-ground tree cavities. Nesting information was predominantly sourced from the literature and only about half of the studies provided source information for their nesting trait data. Importantly, nesting categories were often inferred from literature observations at the generic or family level; these inferences may obscure important species-level variation. Finally, definitions for nesting categories were rare. Of the 62 studies that categorized bee species by nesting location, only 13 (21.0%) provided some definition (whether written or by referencing a previous paper’s definition) for at least one of their nesting states. We recommend that authors specify the sources of their nesting data, rigorously define the boundaries of their categorizations, and clarify whether data is inferred from the species or generic level.