Tue, Aug 03, 2021:On Demand
Background/Question/Methods
The general pattern reported for cities is that generalist species are dominant and pervasive in these environments. The negative effects, i.e., filtering of urbanization on specialist bee species presence and diversity, are well investigated. Typically, diet specialists (oligolectic) bees are rare in cities where the unpredictable availability of resources favors generalists. However, these broad trends mask nuanced interactions between specialists and the urban environment. Consequently, little attention has been given to the role of pollen specialists in cities. Ptilothrix bombiformis (Apidae: Emphorini) is a habitat and diet specialist on Hibiscus pollen and is commonly found throughout St. Louis, MO, USA, and the surrounding suburban and rural areas. Using Pt. bombiformis populations as a study model, we assess the variation in pollen diet and foraging behavior in 12 study areas across an urbanization gradient. We combine field observations, measures of habitat preference, scopal pollen load analysis, and floral choice experiments in the St. Louis metropolitan area to answer if these specialists are changing their diet or foraging strategies to adjust to life in the city.
Results/Conclusions Scopal pollen load analysis indicates that all 12 populations of Pt. bombiformis are carrying pure or mostly pure loads of Hibiscus pollen, independent of location along the gradient. In areas where the bees emerge before the native Hibiscus flowers bloom, the bees rely on the non-native H. syriacus to provision their nests. This has conservation implications because the non-native species quickly outcompetes native species, and it may not be a reliable food source if removed by landowners in favor of a native landscape. Floral choice experiments revealed sex differences in foraging strategy, with males being quicker to find and guard new Hibiscus resources in their sites. Females in urban areas more quickly discovered and used new floral resources presented to them as compared to females from rural areas. Urban bees are more resourceful in their water acquisition strategies – using urban analogs like dog bowls or alley puddles instead of natural ponds. Similarly, instead of switching nesting site type, bees used urban analogs of baseball fields and dirt paths with similar compaction level, but not soil type as rural bees. These results indicate that specialists can thrive in urban areas if the correct urban analogs for habitat and resources exist.
Results/Conclusions Scopal pollen load analysis indicates that all 12 populations of Pt. bombiformis are carrying pure or mostly pure loads of Hibiscus pollen, independent of location along the gradient. In areas where the bees emerge before the native Hibiscus flowers bloom, the bees rely on the non-native H. syriacus to provision their nests. This has conservation implications because the non-native species quickly outcompetes native species, and it may not be a reliable food source if removed by landowners in favor of a native landscape. Floral choice experiments revealed sex differences in foraging strategy, with males being quicker to find and guard new Hibiscus resources in their sites. Females in urban areas more quickly discovered and used new floral resources presented to them as compared to females from rural areas. Urban bees are more resourceful in their water acquisition strategies – using urban analogs like dog bowls or alley puddles instead of natural ponds. Similarly, instead of switching nesting site type, bees used urban analogs of baseball fields and dirt paths with similar compaction level, but not soil type as rural bees. These results indicate that specialists can thrive in urban areas if the correct urban analogs for habitat and resources exist.