What Bugs Can Tell Us About the Value of Vacant Urban Land

Back in October 2017, we discussed some potential benefits of spontaneous urban vegetation (commonly referred to as weeds) and the abandoned or undeveloped urban spaces they inhabit. There is much to learn about the role these plant communities play in the ecology of cities and their contribution to vital ecosystem services. In a review published in the December 2013 issue of Environmental Entomology, researchers from Ohio State University discuss how studying arthropod communities on vacant lands can help “advance our ecological understanding of the functional role” these habitats may have in our cities.

Arthropods were selected as the subject of study because their “populations respond quickly to changes in the urban environment, making them key indicators of how land use change influences biodiversity.” Urban-dwelling arthropods “are diverse and occupy multiple trophic levels” and are “easy to sample.” Additionally, many of the services that vacant, unmanaged land offers are “arthropod-mediated,” including “pollination, decomposition, nutrient cycling, and biological pest control.”

photo credit: wikimedia commons

Vacant land was selected as the study site because “understanding [its] ecological value is important to the advancement of urban ecology and ecosystem management,” and even though such areas are often overlooked in conservation planning, studies have shown that they “have the potential to be valuable reservoirs of biodiversity.” In order to determine just how valuable vacant land might be, more research is needed comparing these spaces to other parts of the city. In addition, vacant lots are generally ephemeral and in due time may be developed. Whether this means that a building or parking lot takes their place or that they are converted into a park, garden, or urban farm, it is important to understand what these land use changes mean for urban biodiversity and ecological functions.

Urbanization is often measured by comparing the amount of built area to the remaining green space. Where there is a high degree of urbanization, there is a low degree of green space comparatively. As urbanization increases, so does habitat fragmentation, pollution, and the urban heat island. In the meantime, biodiversity suffers. The authors cite a number of studies demonstrating that increased urbanization negatively impacted beneficial insect populations. For example, a study in the United Kingdom found that bumblebee diversity in gardens “decreased with increasing urbanization of the surrounding landscapes.” Similar results were found in a study we wrote about.

photo credit: wikimedia commons

Together with remnant natural areas, parks, private and public gardens, greenways, and commercial landscapes, vacant lots are part of a mosaic of urban green space. Each of these areas “experience different levels of disturbance and harbor varying plant species,” which, in turn, “influence arthropods and the services they can supply within and between patches.” Because vacant lots can remain undisturbed and virtually unmanaged for long periods of time, they help provide a contrast to the homogeneous, highly managed green spaces that are common in cities. By their very nature, they “have the potential to aid conservation and enhance green space quality and connectivity within city centers.”

It’s one thing to recognize the value of vacant lots; it’s another thing to change the negative perception of them. Aesthetics are important, and to many people vacant lots are an eyesore and a sign of neglect. Some management may be necessary in order to retain their important ecological value and assuage the feelings of the public. The authors present a number of ways that vacant lots can be and have been managed in order to achieve this goal. They also consider how certain management strategies (mowing, removing and/or introducing plant species) can impact arthropod populations for better or worse. Yet, where vacant lots are left alone and allowed to advance in the stages of ecological succession, changes in arthropod diversity and ecosystem function also occur. For this reason, “the regional species pool of a city requires a mosaic of all successional stages of vacant land patches.”

photo credit: wikimedia commons

Finally, the authors discuss the conversion of vacant land to urban agriculture. Even this land use change can have dramatic effects on the arthropod community. For example, undisturbed or unmanaged areas are a habitat requirement for cavity and soil nesting bees, and regular disturbance associated with farming may interfere with this. Also where pesticides are used or plant diversity is minimized, the arthropod community will be affected.

Thus, “the study of vacant land ecology necessitates a transdisciplinary approach” in order to determine how changes in vacant, urban land “will affect diverse ecosystem functions and services.” A variety of management strategies are required, and land managers must “determine the most appropriate strategies for improving the sustainability of cities from a connected landscape perspective.” It is clear that vacant lots have a role to play. The extent of their role and our approaches to managing them requires careful investigation.

One thing is certain – for biodiversity’s sake – don’t pave over vacant lots to put up parking lots.


Bumblebees and Urbanization

Urban areas bear little resemblance to the natural areas that once stood in their place. Concrete and asphalt stretch out for miles, buildings of all types tower above trees and shrubs, and turfgrass appears to dominate whatever open space there is. Understandably, it may be hard to imagine places like this being havens for biodiversity. In many ways they are not, but for certain forms of life they can be.

An essay published earlier this year in Conservation Biology highlights the ways in which cities “can become a refuge for insect pollinators.” In fact, urban areas may be more inviting than their rural surroundings, which are often dominated by industrial agriculture where pesticides are regularly used, the ground is routinely disturbed, and monocultures reign supreme. Even though suitable habitat can be patchy and unpredictable in the built environment, cities may have more to offer than we once thought.

Yet, studies about bee abundance and diversity in urban areas show mixed results, largely because all bee species are not created equal (they have varying habitat requirements and life histories) and because urban areas differ wildly in the quality and quantity of habitat they provide both spatially and temporally. For this reason, it is important for studies to focus on groups of bees with similar traits and to observe them across various states of urbanization. This is precisely what researchers at University of Michigan set out to do when they sampled bumblebee populations in various cities in southeastern Michigan. Their results were published earlier this year by Royal Society Open Science.

common eastern bumble bee (Bombus impatiens) – photo credit: wikimedia commons

The researchers selected 30 sites located in Dexter, Ann Arbor, Ypsilanti, Dearborn, and Detroit. Most of the sites were gardens or farms in urban centers. They collected bumblebees from May to September using pan traps and nets. The species and sex of each individual bumblebee was identified and recorded for each site. The percentage of impervious surface that surrounded each site was used as a measurement of urban development. Other measurements included the abundance of flowers and average daily temperatures for each location.

Bumblebees were selected as a study organism because the genus, Bombus, “represents a distinct, well-studied set of traits that make it feasible to incorporate natural history into analysis.” Bumblebees live in colonies – eusocial structures that include “a single reproductive queen, variable numbers of non-reproductive female workers, and male reproductive drones.” They are generalist foragers, visiting a wide variety of flowering species for pollen and nectar, and they nest in holes in the ground, inside tree stumps, or at the bases of large clumps of grass. The authors believe that their nesting behavior makes them “a good candidate for testing the effects of urban land development,” and the fact that members of the colony have “distinct roles, [behaviors], and movement patterns” allows researchers to make inferences regarding “the effects of urbanization on specific components of bumblebee dynamics.”

Across all locations, 520 individual bumblebees were collected. Nearly three quarters of them were common eastern bumblebees (Bombus impatiens). Among the remaining nine species collected, brown-belted bumblebees (Bombus griseocollis) and two-spotted bumblebees (Bombus bimaculatus) were the most abundant.

brown-belted bumblebee (Bombus griseocollis) – photo credit: wikimedia commons

Because bumblebees are strong fliers with an extensive foraging range, impervious surface calculations for each site had to cover an area large enough to reflect this. Results indicated that as the percentage of impervious surfaces increased, bumblebee abundance and diversity declined. When male and female bumblebee data was analyzed separately, the decline was only seen in females; males were unaffected.

Female workers do most of their foraging close to home, whereas males venture further out. The researchers found it “reasonable to hypothesize that worker abundance is proportional to bumblebee colony density.” Thus, the decline in female bumblebees observed in this study suggests that as urban development increases (i.e. percent coverage of impervious surface), available nesting sites decline and the number of viable bumblebee colonies shrinks. Because male bumblebees responded differently to this trend, future studies should consider the responses of both sexes in order to get a more complete picture of the effects that urbanization has on this genus.

Interestingly, results obtained from the study locations in Detroit did not conform to the results found elsewhere. Bumblebee abundance and diversity was not decreasing with urbanization. Unlike other cities in the study, “Detroit has experienced decades of economic hardship and declining human populations.” It has a high proportion of impervious surfaces, but it also has an abundance of vacant lots and abandoned yards. These areas are left unmaintained and are less likely to be mowed regularly or treated with pesticides. Reducing disturbance can create more suitable habitat for bumblebees, resulting in healthy populations regardless of the level of urbanization. Thus, future studies should examine the state of insect pollinators in all types of cities – shrinking and non-shrinking – and should consider not just the amount of available habitat but also its suitability.

two-spotted bumblebee (Bombus bimaculatus) – photo credit: wikimedia commons