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Body size constrains the generality of generalist bee species.
Roulston, T'ai*,1, Brewster, Amanda2, Smith, Stephen3, 1 Blandy Experimental Farm, Boyce, VA2 Yale University, New Haven, Ct, USA3 Sarah Lawrence College, Bronxville, NY, USA
ABSTRACT- In animal groups with extreme body size variation among taxa, dietary differences commonly reduce competition for food. Bee species within a habitat, however, commonly differ more than 100x in body weight but consume the same basic food items: pollen and nectar. We envision three possible outcomes of this potentially competitive interaction: 1) Segregation: bees of different size utilize flowers of different morphology, each maximizing foraging profitability on a different set of hosts; 2) Convergence: all bee species converge on the most productive hosts, or 3) Body-size dependent host breadth: host breadth is inversely proportional to body size because foraging requirements, and thus the potential profitability of hosts, scales with body size (i.e., all bees use the best hosts but small bees utilize more hosts because more hosts meet their minimum foraging requirements). To examine this question we established 3 1-hectare plots and collected floral visitors to each of the main flowering plants for 1 whole day. We found that, as predicted by size-dependent host breadth theory, small bee species utilized significantly more host plants within a plot than large bees. Larger bees were significantly associated with the most productive hosts (as measured by total nectar sugar per flower) but small bees often used less productive hosts, which were, presumably still profitable for their relatively more modest foraging requirements. We hypothesize that two body-size dependent factors combine to yield this ecological pattern: large bees, for which flight is relatively more cost efficient over longer distances, find it more profitable to forage further while visiting only the most productive hosts, while small bees find it more profitable to minimize travel by visiting more species of hosts near their nest.
Key words: body size, bees, pollination, apidae