Bee visitation differs in urban, agricultural and natural environments

Although I (and all the bee biologists I know) am fascinated with the vast and surprising diversity of bees, pollination is often the main reason most people care about these invertebrates. We need them for our crops, in both agricultural fields and home gardens, yet they are also important for maintaining the fields of wildflowers that grace natural habitats throughout the world. Yet little is known about the differences in the efficacy of pollination in these three different environments. Misha Leong, a fellow graduate student here in UC Berkeley’s Department of Environmental Science, Policy and Management, has been assessing visitation and seed set in urban, agricultural and natural areas in California’s Bay Area. Leong used yellow starthistle, a common grassland weed found in all her environments of interest, as a sort of probe to test rates of pollination.

It turns out that in natural habitats, bees visit the thistle in far fewer numbers, yet those same visits results in higher seed set than in urban and agricultural areas. This may be because there are more plant species for bees to choose from in the non-natural areas. That’s right, you heard me, in late summer, when yellow starthistle blooms, human-made environments actually provide more resources than natural areas that are usually starved for water. Spring is prime-time for California’s dry grasslands, so in late-summer, when the native plants have senesced, bees may be more reliant on starthistle, thus carry more pollen during each visit despite having fewer overall visits. Nature is full of little paradoxes like this.

Now as I mentioned, yellow starthistle is a terrible invasive weed. It is prickly and can take over areas rapidly. I am not very encouraged that it might do better in natural areas, the places we most need to conserve. This research doesn’t really point to what we can do to prevent its spread. And it’s harder still to imagine taking away resources from bees at the end of their flight season. Nevertheless, it is interesting that a plant can have high rates of visitation in such non-natural environments as those studied. Hopefully the same proves true for wildflowers or crops we add to our urban and suburban gardens.

This is an open access article, which means you can read the real thing by clicking this link:

Pollinator Interactions with Yellow Starthistle (Centaurea solstitialis) across Urban, Agricultural, and Natural Landscapes

Leong M, Kremen C, Roderick GK (2014) Pollinator Interactions with Yellow Starthistle (Centaurea solstitialis) across Urban, Agricultural, and Natural Landscapes. PLoS ONE 9(1): e86357. doi:10.1371/journal.pone.0086357


Drought and bee behavior

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).

What conservation measures actually work?

Sometimes we hear about neat methods for helping species thrive or rebound, but it can be hard to know whether or not they actually work. A new website is helping to answer this question by providing evidence: Conservation Evidence. They provide case studies supported by scientific papers that show efficacy for a variety of important groups, including bees, amphibians and birds. Check them out for innovative ideas or to see if a project you’ve done is proven to work.