ground nesting bees

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Vacant Lots as Habitat for Insects

Urban areas are increasingly being studied for their potential to help conserve biodiversity and provide habitat for numerous plants and animals. Despite the harsh conditions of the built environment, organisms of all kinds are able to survive in our cities, and as we find ways to make these spaces more hospitable for them, cities actually have great potential for species conservation, even for species that are rare, threatened, or specialized. One obvious way to accomplish this is to manage our yards, parks, and gardens as habitat, such as planting flower strips for pollinators. Another way, perhaps overlooked at times, is to manage and maintain vacant lots as habitat. Every urban area has some degree of vacant land that for one reason or another has not been developed, or that once was developed but has since been bulldozed or abandoned. Spontaneous vegetation quickly moves in to occupy these sites, and while some may see them as eyesores, their potential for providing habitat for an untold number of plants and animals is substantial.

In cities that are growing – like Boise, Idaho – vacant and abandoned lots are disappearing quickly as development strives to keep up with population growth. My first Weeds of Boise post took place at an abandoned Pizza Hut, which has since been demolished and is now the future site of a large building (see photo below). This is happening all over the city – the City of Trees is looking more like the City of Cranes these days. On the other hand, cities that are shrinking due to economic downturn, loss of industry, and other factors, have an increasing number of vacant lots, which offers the opportunity not only to maintain these lots as habitat, but also to carry out research that will help us understand how these locations can be best managed for species conservation.

Abandoned Pizza Hut Lot Now Under Construction

Cleveland, Ohio is one example of a “shrinking city.” Due to significant population decline, Cleveland has a growing number of vacant lots, many of which are maintained by the City of Cleveland Land Bank. For researchers at The Ohio State University, all of this vacant land presents an opportunity to study, among other things, urban biodiversity. Hence, the Cleveland Pocket Prairie Project was born. By assigning different management treatments to groups of vacant lots and observing the differences between each treatment, researchers can help determine the best strategies for managing vacant lots, particularly when it comes to biological conservation. One of the major focuses of the Cleveland Pocket Prairie Project is to determine how vacant land can provide habitat for insects and other arthropods.

In a study published in Sustainability (2018), researchers in Cleveland compared the species richness and abundance of bees found on vacant lots to those found on urban farms. Bee collections were made three times a year over a three year period. Of the more 2733 bees collected, researchers identified 98 total species representing 5 different families. The vast majority of the species were native to the area. Significantly more bees were found in vacant lots compared to urban farms. In both vacant lots and urban farms, the total number of ground nesting bees decreased as the proportion of impervious surfaces near the study sites increased. Plants that received the most bee visits on the urban farms during the study period were common milkweed (Asclepias syriaca), chives (Allium schoenoprasum), and squash (Cucurbita pepo); while the top three plants with the most bee visits on vacant lots were red clover (Trifolium pratense), white clover (Trifolium repens), and Queen Anne’s lace (Daucus carota).

ground nesting bee (photo credit: Sierra Laverty)

Bee communities differed between vacant lots and urban farms: 29 of the 98 total species were seen only in vacant lots, while 14 species were seen only at urban farms. Most of the bees collected in this study were ground nesting species, and researchers suspect the reason more bees were found on vacant lots compared to urban farms is that farms experience frequent soil disturbance in the form of tillage, weeding, mulching, and irrigation, while vacant lots generally do not. The researchers conclude that their study “adds to the growing body of literature advocating for the maintenance of minimally-managed vacant lot habitats as a conservation resource.” Vacant land that is “surrounded locally by high concentrations of impervious surface,” however, may not be the most suitable location to carry out conservation efforts.

In a study published in Urban Ecosystems (2020), researchers in Cleveland looked at the species richness and abundance of lady beetles in vacant lots. They were particularly interested in the potential that vacant lots may have in providing habitat for lady beetles that are native to the region. The study consisted of 32 vacant lots, each assigned one of four habitat treatments: control (seeded with turfgrass and mowed monthly), meadow (seeded with turfgrass and mowed annually), low-diversity prairie (seeded with three species of prairie grasses and four species of native prairie forbs), and high-diversity prairie (seeded with three species of prairie grasses and sixteen species of native prairie forbs). The two prairie treatments were mown annually. The majority of the nearly 3000 lady beetles captured across all of the plots over a two-year study period were exotic (introduced) species. Sixteen species total were collected: four exotic and twelve native.

The researchers predicted that the lots seeded with prairie plants native to the region would support a higher abundance of native lady beetles than those composed of turfgrass, especially those that are frequently mown. Surprisingly, a similar abundance and species richness of both native and exotic lady beetles were found across all treatments. What appeared to be important for native lady beetle abundance were vegetation features like bloom abundance, height, and biomass. The surrounding environment also matters. As the researchers put it, “vacant lots embedded in landscapes dominated by impervious surface and with a high degree of habitat isolation were less suitable habitats” – a similar conclusion to that made in the bee study.

Brachiacantha ursina (photo credit: Wikimedia Commons / NY State IPM Program at Cornell University)

The most abundant native lady beetle collected in the study was the ursine spurleg lady beetle (Brachiacantha ursina). The larvae of this beetle “infiltrate the nests of Lasius ants,” which is “one of the most common genera of ants found in urban environments.” Researchers posit that the abundance of B. ursina reflects the habitat preferences of ants in the Lasius genus. Several species of lady beetles native to the region are experiencing significant population declines, and the researchers were disappointed to find that none of the most rare species were collected during their study period. However, it was promising to find that “pollen and nectar provided by both seeded native and naturally occurring weedy plants” appeared to be important food sources for native lady beetles.

Both studies indicate that vacant lots can be important locations for habitat conservation in urban areas, particularly when they are part of a larger collection of greenspaces. In combination with managing our yards, parks, and urban farms as quality habitat for plants and animals, conserving vacant lots that consist of diverse vegetation (both planted and spontaneous) can help support insect populations within our cities.

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Flowers Strips Bring All the Pollinators to the Yard

The longer I garden the more I gravitate towards creating habitats for creatures that rely on plants for survival. I’ve always been more interested in functional gardens rather than gardens that are simply “plants as furniture” (as Sierra likes to say) – a manicured, weed-free lawn, a few shrubs shaped into gumdrops, sterile flowers for color – and that interest has grown into a way of life. A garden should be more than just something nice to look at, and for those of us who’ve embraced the “messy ecosystems” approach, what’s considered “nice to look at” has shifted dramatically.

Thankfully, I’m not alone in this thinking. Gardens focused on pollinators, birds, habitats, native plants, etc. seem to be gaining in popularity. The question is, is it making a difference? At least one study, referred to below, seems to suggest that it is. And as more gardens like these are planted and more studies like this are done, perhaps we will get a clearer picture of their impact.

In 2017, eight 1000 square meter flower strips were planted in Munich, Germany. The sites had previously been lawn or “roadside greenery,” according to the report published in the Journal of Hymenoptera Research (2020). An additional flower strip, planted in 2015, was included in the study. Over the next year, an inventory of the number of bee species found in these nine flower strips was taken and compared both to the number of bee species that had been recorded in Munich since 1795 (324 species) and the number of bee species recorded in the 20 years prior to the planting of the flower strips (232 species).

In just a year’s time, these newly planted flower strips quickly attracted a surprising number of bees. The researchers identified 68 different species (which is 21% and 29% of the two categories of previously recorded species). As they had expected, most of the bees they identified were common, non-threatened, generalist species; however, they were surprised to also find several species that specialize on pollen from specific groups of plants. Future studies are needed to determine whether or not such flower strips help increase the populations of pollinators in the city, but it seems clear that they are a simple way to increase the amount of food for pollinators, if nothing else.

But perhaps these results shouldn’t be that surprising. Urban areas are not necessarily the biodiversity wastelands that the term “concrete jungle” seems to imply. Though fragmented and not always ideal, plenty of wildlife habitat can be found within a city. In his book, Pollinators and Pollination, Jeff Ollerton lists a number of studies that have been carried out in cities across the world documenting an impressive number of pollinating insects living within their borders [see this report in Conservation Biology (2017), for example]. As Ollerton writes, these studies “show that urbanization does not mean the total loss of pollinator diversity, and may in fact enhance it.” After all, “many of us city dwellers see every day, nature finds a home, a habitat, a place to thrive, wherever it will.”

In a chapter entitled, “The Significance of Gardens,” Ollerton continues to explore the ways in which cities can host a wide variety of flower visiting insects and birds. “Planted patches” don’t necessarily need to be managed as pollinator gardens in order to provide resources for these creatures, nor do all of the plants need to be native to the region to be effective. Rather, diversity in flower structure and timing seems to be key; “floral diversity always correlates with pollinator diversity regardless of the origin of the plants,” Ollerton writes in reference to pollination studies performed in British cities. The more “planted patches,” the better, as “a large and continuous floral display in gardens is the only way to maximize pollinator abundance and diversity.” Add to that, “if you allow some areas to become unmanaged, provide other suitable nesting sites or areas for food plants, and other resources that they need, a thriving oasis for pollinators can be created in any plot.”

ground nesting bee emerging from burrow

Bees and other pollinating insects are finding ways to live within our cities. There is no need to go to the lengths that I like to go in order to help them out. Simply adding a few more flowering plants to your yard, balcony, or patio can do the trick. Eliminating or limiting the use of pesticides and creating spaces for nesting sites are among other things you can do. Learning about specific pollinators and their needs doesn’t hurt either. The continued existence of these creatures is critical to life on earth, and this is one important issue where even simple actions can make a real difference.

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