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.