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.