Abstract
AbstractRoughly a third of described insect species visit flowers, making the flower–insect interface one of the chief pillars of global biodiversity. Studying flower–insect relationships at the scale of communities and landscapes has been hindered, however, by the methodological challenges of quantifying landscape‐scale floral resources. This challenge is especially acute in urban landscapes, where traditional floral surveying techniques are ill‐suited to the unique constraints of built environments. To surmount these challenges, we devised a “honey bee foraging assay” approach to floral resource surveying, wherein continuous colony weight tracking and DNA metabarcoding of pollen samples are used to capture both the overall availability and taxonomic composition of floral resources. We deploy this methodology in the complex urban ecosystem of Philadelphia, Pennsylvania, USA. Our results reveal distinct seasonality of floral resource availability, with pulses of high availability in May, June, and September, and a period of prolonged scarcity in August. Pollen genus richness mirrored this pattern, with peak richness in May and June. The taxonomic composition of pollen samples varied seasonally, reflecting underlying floral phenology, with especially strong turnover between May and June samples and between August and September samples delineating well‐defined spring, summer, and fall floral resource communities. Trait analysis also revealed seasonal structure, with spring samples characterized by trees and shrubs, summer samples including a stronger presence of herbaceous “weeds”, and fall samples dominated by woody vines. Native flora predominated in spring, giving way to a preponderance of exotic flora in summer and fall. At a basic level, this yields insight into the assembly of novel urban floral resource communities, showcasing, for example, the emergence of a woody vine‐dominated fall flora. At an applied level, our data can inform urban land management, such as the design of ecologically functional ornamental plantings, while also providing practical guidance to beekeepers seeking to adapt their management activities to floral resource seasonality. Methodologically, our study demonstrates the potential of the honey bee foraging assay as a powerful technique for landscape‐scale floral resource surveying, provided the inherent biases of honey bee foraging are accounted for in the interpretation of the results.
Highlights
It has been estimated that roughly a third of all described insect species are either directly or indirectly dependent on flowers for food (Wardhaugh 2015). This ecological centrality of flowers-as-food extends to systems in which historic floral communities have been dramatically altered, such as urban landscapes characterized by novel assemblies of native and exotic flora (Aronson et al 2014)
Weight dynamics were characterized by distinct gain and loss motifs that were largely consistent across years, with periods of strong weight gain in May and June, weak gain or weak loss in July, strong loss in August, weak to strong gain in early September, and strong loss from the second week of September to the end of data collection in October (Figure 3)
Continuous colony weight monitoring revealed strong gain and loss motifs. We interpret these dynamics as a measure of the overall abundance of floral resources within the colonies’ foraging range, with the caveat that colony weight is affected by overall health and honey bee life history in addition to resource availability
Summary
It has been estimated that roughly a third of all described insect species are either directly or indirectly dependent on flowers for food (Wardhaugh 2015). This ecological centrality of flowers-as-food extends to systems in which historic floral communities have been dramatically altered, such as urban landscapes characterized by novel assemblies of native and exotic flora (Aronson et al 2014). While not an unbiased representation of local flora, such an approach would manifestly be relevant to the question of trophic function at the flower-insect interface
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