arthropods

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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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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.

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Book Review: Bringing Nature Home

Since Bringing Nature Home by Douglas Tallamy was first published in 2007, it has quickly become somewhat of a “classic” to proponents of native plant gardening. As such a proponent, I figured I ought to put in my two cents. Full disclosure: this is less of a review and more of an outright endorsement. I’m fawning, really, and I’m not ashamed to admit it.

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The subtitle pretty much sums it up: “How You Can Sustain Wildlife with Native Plants.” Ninety three pages into the book, Tallamy elaborates further: “By favoring native plants over aliens in the suburban landscape, gardeners can do much to sustain the biodiversity that has been one of this country’s richest assets.” And one of every country’s richest assets, as far as I’m concerned. “But isn’t that why we have nature preserves?” one might ask. “We can no longer rely on natural areas alone to provide food and shelter for biodiversity,” Tallamy asserts in the Q & A portion of his book. Humans have altered every landscape – urban, suburban, rural, and beyond – leaving species of all kinds threatened everywhere. This means that efforts to protect biodiversity must occur everywhere. This is where the You comes in. Each one of us can play a part, no matter how small. In closing, Tallamy claims, “We can each make a difference almost immediately by planting a native nearby.”

A plant is considered native to an area if it shares a historical evolutionary relationship with the other organisms in that area. This evolutionary relationship is important because the interactions among organisms that developed over thousands, even millions, of years are what define a natural community. Thus, as Tallamy argues, “a plant can only function as a true ‘native’ while it is interacting with the community that historically helped shape it.” A garden designed to benefit wildlife and preserve biodiversity is most effective when it includes a high percentage of native plants because other species native to the area are already adapted to using them.

Plants (and algae) are at the base of every food chain – the first trophic level – because they produce their own food using the sun’s energy. Organisms that are primarily herbivores are at the second trophic level, organisms that primarily consume herbivores are at the third trophic level, and so on. As plants have evolved they have developed numerous defenses to keep from being eaten. Herbivores that evolved along with those plants have evolved the ability to overcome those defenses. This is important because if herbivores can’t eat the plants then they can’t survive, and if they don’t survive then there will be little food for organisms at higher trophic levels.

The most important herbivores are insects simply because they are so abundant and diverse and, thus, are a major food source for species at higher trophic levels. The problem is that, as Tallamy learned, “most insect herbivores can only eat plants with which they share an evolutionary history.” Insects are specialized as to which plants they can eat because they have adapted ways to overcome the defenses that said plants have developed to keep things from eating them. Healthy, abundant, and diverse insect populations support biodiversity at higher trophic levels, but such insect populations won’t exist without a diverse community of native plants with which the insects share an evolutionary history.

That is essentially the thesis of Tallamy’s book. In a chapter entitled “Why Can’t Insects Eat Alien Plants?” Tallamy expounds on the specialized relationships between plants and insects that have developed over millennia. Plants introduced from other areas have not formed such relationships and are thus used to a much lesser degree than their native counterparts. Research concerning this topic was scarce at the time this book was published, but among other studies, Tallamy cites preliminary data from a study he carried out on his property. The study compared the insect herbivore biomass and diversity found on four common native plants vs. five common invasive plants. The native plants produced 4 times more herbivore biomass and supported 3.2 times as many herbivore species compared to the invasive plants. He also determined that the insects using the alien plants were generalists, and when he eliminated specialists from the study he still found that natives supported twice as much generalist biomass.

Apart from native plants and insects, Tallamy frames much of his argument around birds. Birds have been greatly impacted by humans. Their populations are shrinking at an alarming rate, and many species are threatened with extinction. Tallamy asserts, “We know most about the effects of habitat loss from studies of birds.” We have destroyed their homes and taken away their food and “filled their world with dangerous obstacles.” Efforts to improve habitat for birds will simultaneously improve habitat for other organisms. Most bird species rely on insects during reproduction in order to feed themselves and their young. Reducing insect populations by filling our landscapes largely with alien plant species threatens the survival of many bird species.

In the chapters “What Should I Plant?” and “What Does Bird Food Look Like?,” Tallamy first profiles 20 groups of native trees and shrubs that excel at supporting populations of native arthropods and then describes a whole host of arthropods and arthropod predators that birds love to eat. Tallamy’s fascinating descriptions of the insects, their life cycles, and their behaviors alone make this book worth reading. Other chapters that are well worth a look are “Who Cares about Biodiversity?” in which Tallamy explains why biodiversity is so essential for life on Earth, and “The Cost of Using Alien Ornamentals” in which Tallamy outlines a number of ways that our obsession with exotic plants has caused problems for us and for natural areas.

Pupa of ladybird beetle on white oak leaf (photo credit: wikimedia commons)

Pupa of a ladybird beetle on a white oak leaf. “The value of oaks for supporting both vertebrate and invertebrate wildlife cannot be overstated.” – Doug Tallamy (photo credit: wikimedia commons)

Convincing people to switch to using native plants isn’t always easy, especially if your argument involves providing habitat for larger and more diverse populations of insects. For those who are not fans of insects, Tallamy explains that “a mere 1%” of the 4 million insect species on Earth “interact with humans in negative ways.” The majority are not pests. It is also important to understand that even humans “need healthy insect populations to ensure our own survival.” Tallamy also offers some suggestions on how to design and manage an appealing garden using native plants. A more recent book Tallamy co-authored with fellow native plant gardening advocate Rick Darke called The Living Landscape expands on this theme, although neither book claims to be a how to guide.

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