Bees are the giant pandas of the insect world. They are charismatic and almost universally admired by the lumbering humans around them. We recognize that they pollinate not only our food plants but also a significant percentage of the plants that form the structure and substance of the ecosystems around us. I like to think of them as “gateway insects” that can be used to draw people into appreciating the millions of other insects that collectively make life on Earth possible.
Our perspective on bees, though, needs a closer look. What are we thinking about when we say “bee?”
We probably have in mind the larger members of the seven families and 20,000 species that are classified as “bees.” Honey bees and bumblebees are the usual species that people recognize as “bees,” but they represent a very small number of the “bees” that are around us and upon which our natural ecosystems depend.
There are four thousand native species of bees in North America. Most of these bees are tiny and most go about their ecological and reproductive tasks with very little human notice. In fact, the honey bees that so many us are familiar with are actually alien species brought to North America by European settlers. Fortunately, we don’t append the second adjective “invasive” to the honey bee because they seem to carry out their life functions without greatly impinging upon the ecological niches of the native bee species. As important as the honey bees are especially for pollinating the large number of plant species that European settlers brought with them to North America (i.e. a great deal of the plant community of our agricultural ecosystems!), they are not able to pollinate many of the plants that are native to North America. The often small, solitary, native bee species are needed for that.
The NPR show “Science Friday” had an interview on November 10, 2017 with a University of Texas researcher who studies many of these tiny, native bee species. She talked about the almost unbelievably small size of several of the species (the size of grain of rice!) and the vigor with which these very small bees pollinate their plants. One rice-sized species that she studied flew over a mile to find their proper plants. She related this “bee distance” to a human equivalent of traveling from Chicago to Los Angeles! These tiny bees include the “sweat bees” that drink the perspiration and tears of many vertebrates and tiny carpenter bees that tunnel their way into stems and twigs to make their brood chambers. Most of these tiny bees are solitary, but they all make a substance called “bee bread” (a semi-solid mix of honey, pollen, bee saliva and associated fungi and bacteria) upon which they lay their eggs and on which their developing larvae feed.
These tiny bees do not have the ability to overwinter in our cold climates. Unlike honey bees who are able to generate enough collective metabolic heat in their hives to maintain warm (90 degrees F!) inter-hive conditions through even the coldest winter (as long as their food supply lasts!), these solitary bees only live for part of the warm, plant flowering season and then leave their eggs and larvae resting on their stored reserves of bee bread to over-winter and continue the species in the next growing season.
(Great thanks, by the way, to Jennifer Wood who passed along the link to the recording of the Science Friday show on tiny bees! Click here and check it out!)
Paul Glaum is a PhD student in ecology at the University of Michigan. Glaum and a group of fellow students became interested in how urbanization affects populations of bees. They examined research published in the scientific literature and found that some studies indicated that the more urbanized a site is the fewer bees it has, but that other studies showed that urbanization did not affect bee populations at all. Further, there were even some studies that indicated that intermediate levels of urbanization actually increased numbers and species diversity of bees. Glaum wondered if the lack of agreement in these studies might be arising from different ways previous research teams defined and described the “bees” they were studying. Could the great diversity of species referred to as “bees” (remember, there are 4000 native species of bees in North America alone!) along with their wide ranges of ecological and life history adaptations be obscuring real trends in these vital pollinators’ adaptations to urbanized environments? To try to correct for this potential source of error, Glaum decided to look at just one type of wild bee, the bumblebee (Bombus) and explore how it responds to varying degrees of urban development.
Five urban sites in southeastern Michigan were selected. These cities ranged from small, very recently incorporated cities to the largest city in Michigan, Detroit. The team found that four of these five cities had very clear relationships of their bumblebee populations to the area of impervious surfaces (concrete and asphalt roads, sidewalks, parking lots etc.) per square kilometer: the higher the percentage of concrete, the lower the numbers of bumblebees. This was very logical considering that bumblebees dig their resting and brood burrows into the ground and, thus, require penetrable soil in order to survive. The one urban site that did not display this relationship, though, was Detroit. Detroit had a high percentage of impenetrable surfaces per unit area but still had high numbers of bumblebees.
Glaum and his team explained this divergence in their observation by describing the unique landscape configuration of Detroit. The impact of decades of financial decline and human depopulation has left great areas of Detroit vacant. These unoccupied areas generate a patchwork of greenspaces in the cityscape that can be utilized by bumblebees for their ground burrows. Sustaining a population of bumblebee pollinators in an urban environment, then, can be accomplished by designing patterns of open spaces in which burrows can dug and reproduction carried out.
One last observation made by Glaum and his team had to do with the impact of urbanization on sex ratios in bumblebee populations. It turns out that male bumblebees are almost completely unaffected by degrees of urbanization, but that females are significantly diminished in numbers in sites that are more urbanized. This alteration of gender ratios could significantly affected rates of growth and the sustainability of a population of bumblebees. The reason for this differential gender impact is very obvious: only the females dig burrows (in which they subsequently rest at night and into which they lay their eggs). The males remain above ground and actually sleep at night sheltered in the petals the very flowers they gather nectar from during the day. Questions about which types of flowers are preferentially utilized by male bumblebees for shelter should be answered in subsequent studies.