While the rest of the country braces for another polar vortex, California is officially in a drought. The state is seeing record highs for this time of year, and no forecast of much-needed rain anytime soon. We know what this means for our water use, but what does it mean for the pollinators of our crops and wildflowers?
This question sent me into the literature, where I found a study published just last year that broached this very topic. The researchers studied a desert system during a drought event. They found that in drier years, the bee community shifted towards generalists– bees that forage on a variety of different species of flowers. In contrast, specialists– bees that specialize in pollination of a single species or genera of flower– were absent or in much lower abundance than during wetter years. The wildflower community was also depauperate, with patches of wildflowers instead of lush fields. Further, the plants that did bloom had fewer flowers. Thus, it appears that bees are able to track conditions that are favorable to their species of choice.
There were differences, however, between annual and perennial plants. Annuals are shallow-rooted, whereas perennials are deep rooted and able to draw on resources deeper into the soil, as well as store resources. The authors found that specialist pollinators of perennial plants such as mesquite, still emerged in drought years to visit the flowers produced by the tree despite the low levels of precipitation.
How are specialists able to determine what years are good ones for them to emerge versus which might be bad? Bees are able to go into diapause, where their system becomes dormant (it’s akin to what we think of as hibernation, where the metabolism slows and development can be delayed). Diapause can be induced by certain environmental conditions that basically cue a creature into whether or not they should emerge into the world to collect food, or remain protected and in a state that requires little nourishment when resources they require may be in short supply. Similarly, environmental stimuli, such as moisture or temperature, can bring creatures in diapause out of that state so that they can then take advantage of pulses of resources if they are available.
Over short periods of drought, diapause is important adaptation, allowing species to ensure that they will have their appropriate food sources available. However, the authors caution that if there are prolonged periods of drought, such as those predicted by climate change, species in diapause may be forced to emerge or die if conditions do not improve. Additionally, species differ in their ability to undergo diapause, thus some may fare better than others. If drought periods are long-lasting, resources that perennial plants rely on may dry up, affecting the specialists that visit them as well. Generalist bees may not be as clued into environmental cues of droughts, and therefore may suffer reduced reproduction during drought years harming their populations over the long term. In short, the droughts predicted may have severe and lasting negative affects on the community of native bees. The authors predict that some species would be extirpated (go locally extinct) from areas strongly affected by drought.
Not a rosy picture at all. For now, we should all hope for the rains to come again, if not this year, then next year, so the bees in diapause get their much needed pollen and nectar.
Minckley, Robert L., H. Roulston T’ai, and Neal M. Williams. “Resource assurance predicts specialist and generalist bee activity in drought.”Proceedings of the Royal Society B: Biological Sciences 280.1759 (2013).