Habitat Loss

One of the major factors of native bee declines has been habitat loss or degradation. Like many other species, they have been directly impacted by anthropological expansion and resource exploitation. However, for bumble bees, this can be expressed in a couple of unique ways.

Nesting Site Availability

Unlike honey bees, most bumble bees just need a hole in the ground to build their nest. Unfortunately, bumble bees lack the strength to excavate large amounts of soil. Consequently, they rely mostly on abandoned rodent holes for nesting sites, but can utilize a large variety of crevasses, such as those formed by the roots of a tree or a pile of grass (Kearns and Thomson, 2001). The most obvious implication of this is that we've simply been reducing the amount of surface that could be used for a nest, by turning natural habitats into impervious surfaces such as buildings or roads. Doing this also results in less opportunities for rodents, their primarily nesting hole providers, to dig their own nests. 

Another, less considered factor, is how humanity has impacted disturbance regimes. When a new queen selects a site for a nest, she prefers to pick a location that is relatively undisturbed and has some cover. The few natural areas that we still keep tend to be grass heavy, and we mow that grass intensely. This does not allow the chance for small grass crevasses to form, and thus no habitat. In addition, we have dramatically altered the fire regime in many ecosystems. Historically, there were relatively frequent, low-intensity fires in areas like prairies and shrub-steppe, which created new habitat and a variety of plants. Currently, we suppress fires, as they typically threaten landowner holdings. As a result, when fires do occur, they are large and intense, destroying all in their path, including bees. 

Example of a Bumble Bee Entering its Nest
Photo Credit: dragonfly-gems.blogspot.com

Forage Availability

Bumble bees, like most other animals, have to find food in addition to having a shelter. Cities and expanding urbanization frequently reduce the amount of native plants, and thus the amount of resources available to forage from. In addition, agriculture is typicially not good for bumble bees, even though we are planting crops. Many of our crops, such as wheat and barley, are wind pollinated, and do not provide resources for bumblebees. Furthermore, the giant monoculture of plants we use generally flower all at once, meaning there are little to no resources available to the bees before or afterwards. These activities can generally make it hard for bumble bees to find food. On the other hand, however, cities do sometimes provide prime foraging opportunities, in the form of urban gardens and along roadsides. These areas are typically planted for a variety of plants that results in a community that offers a high density of resources, different resource types (nectar vs. pollen), and resource availability throughout the year.

An Example of Forage in a Urban Garden
Photo Credit: bringingbackthenatives.net