New research shows that loss of a group of pollinators doesn't always have a predictable decline in pollination success.
Certainly, it would be bad news for plants like orchids that have very specific relationships with specific species of bee. But would if be bad for all plants? Someone’s done an experiment and like most real-world science it turns out the answer is a bit more complicated than that.
The experiment is written up in “Pollination success following loss of a frequent pollinator: the role of compensatory visitation by other effective pollinators” by Hallet et al. They’ve looked at milkweed, Asclepias verticillata, and they stopped bumble bees from pollinating it to see what happens. The method they used was a mix of careful planning and bloody-mindedness.
The planning was in how the plots were prepared. There were six populations and at each site, four plots. The plots were tended to make sure only the milkweed was flowering. They were also weeded to make sure that there was the same density of plants in each spot.
The plots were caged to prevent pollinators getting to the flowers. They were then opened up, so insects could visit between 0930 and 1530 local time. Bumblebees don’t actually work in shifts, but they need warm muscles to fly, so they’re most active during the warmest part of the day. They could be expected to be around as usual because they didn’t know what was going to hit them.
What hit them was a 1.2m white fiberglass rod, which the experimenters used to gently tap bees. The bees were then put off eating at the milkweed, much like anyone would be put off eating at a fast food place if the management starting gently tapping them with a rod when they approached a table. It sounds like a quite a task.
They then looked to see if the milkweeds got any other pollinators. A. verticillata is a good plant to experiment on for this, as it’s self-incompatible. So if pollination happened, it had to be through a non-bumblebee pollinator.a is a good plant to experiment on for this, as it’s self-incompatible. So if pollination happened, it had to be through a non-bumblebee pollinator.
What they found was that other pollinators visited the flowers, in particular Polistes wasps. Not only that, but the wasps were as good at pollinating the plants in the bee-free plots as the bumblebees were in the control plots.
The results don’t mean we should all go mad with 1.2m white fiberglass rods of our own. There are some things to investigate. For example, the wasps visited more in plots with a large milkweed population. In the longer term would that be a hint at problems for smaller populations? It would also mean a shift in pollinators and that might have more implications elsewhere in the food chain. It might also mean that loss of a pollinator could affect flora in strikingly different ways, with some plants being much better at recruiting replacement pollinators than others.
You can read the research free at AoB PLANTS.
Hallett, A. C., Mitchell, R. J., Chamberlain, E. R., & Karron, J. D. (2017). Pollination success following loss of a frequent pollinator: the role of compensatory visitation by other effective pollinators. AoB PLANTS, 9(3). https://doi.org/10.1093/aobpla/plx020