The presence of suitable nesting material was at least as important in determining how many types of bees might use a site as was diversity of plants, which provide nectar and pollen to the bees. The composition of an entire bee community was linked to higher plant variety, less canopy cover and soil characteristics that may be best-suited for nesting.
The study found that specialist bees—those picky native bees that gather pollen from only a few kinds of plants—were more likely to live in open areas than areas with a higher density of trees.
"Specialist bees, not surprisingly, were also more associated with the presence of native plants in the areas, but a lot of these native plants were more likely to occur in disturbed areas, including areas that had recently been burned and, somewhat to our surprise, residential areas where soil disturbance is commonplace," Grundel said.
However, specialist bees were often rarer and mainly used open habitats, such as grasslands and savannas. According to a 2005 study, said Grundel, such open Midwest habitats are today perhaps the most poorly conserved habitats on the planet, causing concern about long-term conservation of such bee species.
"At several locations around the world, specialist bumblebees living in plant rich areas, such as these open habitats, have declined significantly," Grundel said. "Similar bumblebee declines have been documented in the Midwest U.S. Documenting how diet breadth, rarity and habitat use are related is important for understanding such patterns of decline and was one of the main objectives of our study. We collected six bumblebee species in our study while a 1930s study in this area collected twelve species. Four of the species we did not find in our study have been identified as bumblebees of special concern due to their disappearance from sites across the Midwest."
USGS researchers in Indiana and Maryland are following up on this research with a recently initiated study examining how native bee populations across the national park system might be affected by climatic variation.
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