Robertson's bees were mostly solitary bees, small, nondescript bees that lay a few eggs in cells and leave their young to develop on their own.
"Some of these bees have broad distributions, encompassing, for example, the entire eastern United States, so even though they're locally extirpated, most are not extinct," said Knight.
The re-collection also revealed timing mismatches between the bees and the plants. Plants were flowering earlier than they had in Robertson's time: on average. 9.5 days earlier. Bees were active earlier too: on average 11 days earlier.
But despite similar average shifts, timing mismatches occurred, because the early-season bees advanced a lot more than the late-season bees, said Knight, and no such pattern occurred among the plants.
Moreover, everything had speeded up. The flowers were in bloom eight fewer days on average and the insects flew for 22.5 fewer days. Because everything was more compressed, there was less overlap and less time for successful pollination.
Of the 532 pairings between the plants and bees that linked the subset of Robertson's network Knight and Burkle studied, 406 had been lost (but 120 new pairings had been gained). Forty five percent of the links were broken because bee species were missing, and the rest had broken for other reasons, including timing mismatches or habitat fragmentation.
A 40-year-old re-collection of Robertson's network
But Burkle and Knight were aware that counting network links was a crude measure of pollination services. "All the network diagrams say is the bee is present, the plant is present, and we saw them interacting at least once," Knight points out.
"Robertson didn't keep track of how much time he spent in the field watching each flower, so we couldn't get visitation rates from his data. But of course we searched the literature to see whether anybody h
|Contact: Diana Lutz|
Washington University in St. Louis