Other traits
Beyond the four most commonly assessed functional traits, several others warrant methodological consideration. Parasitism status, i.e., whether or not a given species is a brood parasite or social parasite, was commonly but inconsistently classified, and often excluded from analyses. Studies varied considerably in whether they classified brood parasitism (a reproductive strategy) as a trait state under sociality, nesting biology, diet breadth, multiple of these traits, or as its own trait (e.g., parasite: yes or no). Indeed, parasites may be considered functionally distinct from non-parasitic bees with respect to sociality, nesting, and diet; the appropriate classification scheme will depend in part on the research question. For example, a comparative study assessing impacts of nest microclimate on bee thermal ecology might be better served by assigning parasitic species to the nest type they occupy (e.g., stems), rather than to their own category. Importantly, however, divergence in classification methods across studies will present obstacles to meta-analysis.
Studies also varied in their approach to measuring tongue (proboscis) length, a functional trait strongly implicated in pollination ecology because it mediates access to diverse floral host species. Tongue length presents measurement challenges because it can require dissection of fresh specimens, a tedious process which can compromise subsequent identification (Cariveau et al., 2016). Likely for this reason, only seven of the 28 studies measuring tongue length used actual specimen measurements (Bartomeus et al., 2018; Beyer et al., 2021; Ibanez, 2012; Laha et al., 2020; Persson et al., 2015; Ramírez et al., 2015; Roquer‐Beni et al., 2021). More commonly, species were categorized as “short” vs. “long” tongued according to the literature, sometimes including an intermediate category (e.g., “medium”). However, this approach may obscure important variation within short- and long-tongued groups, and so may not always be appropriate for testing functional hypotheses in pollination ecology. These classifications typically rely only on family information, and so do not capture within-family variation related to body size. To overcome these limitations, Cariveau et al. (2016) described an allometric equation that explains 91% of the variance in bee tongue length, and produced an R package that allows users to predict tongue length from bee family and ITD (“BeeIT” package). Allometric scaling relationships have been instrumental for developing predictive models to estimate biological measurements, especially in plant ecology (McHale et al., 2009; Montagu et al., 2005; Roxburgh et al., 2015). Since its publication, the BeeIT package was used to estimate tongue length in nine of the functional trait studies we surveyed, suggesting that this approach has reduced quantification barriers (Bartomeus et al., 2018; Beyer et al., 2020; Evans et al., 2018; Hass et al., 2018; Hung et al., 2019; Kratschmer et al., 2021; Lane et al., 2022; Staton et al., 2022; Steinert et al., 2020). This approach will enable analysis of older specimens that cannot be dissected, which will be particularly useful for rare and endangered species. However, caution should be taken when applying this approach broadly, as relationships between body size and tongue length vary across regions and taxa. For example, tongue length varies dramatically in neotropical orchid bees (Apidae: Euglossini), with species of similar body size exhibiting both short and very long tongues, sometimes more than twice their body length. Although over-reliance on proxy calculations introduces error that could potentially obscure functional relationships, allometric approaches such as these can represent improvements upon categorical assignments based on bee family alone. However, where possible, actual tongue measurements are preferable for advancing our documentation and understanding of the functional consequences of tongue length variation.