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.


When Alien Plants Invade – The Four Stages of Invasion, part two

In a review published in New Phytologist (2007), Kathleen Theoharides and Jeffrey Dukes examine four stages of invasion as they relate to alien (i.e. introduced or non-indigenous) plant species. In part one we discussed transport and colonization, in which species must survive being transported long distances and then take root and reach maturity in an unfamiliar location. Alien plant species don’t become invasive until they have reached the last two stages: establishment and landscape spread. Removal of the species upon reaching these stages is no easy task. Luckily, introduced species have a few barriers to overcome before this point.

An established population is one that is self-sustaining and expanding. Environmental conditions may be a limiting factor, as they were during colonization, but the main constraints at this stage are “biotic filters.” Theoharides and Dukes define these as “barriers to invasion created by the actions or presence of living organisms.” Through competition for various resources, as well as herbivory and disease, neighboring organisms affect the survival, growth, and reproduction of introduced plant species. Thus, “traits that enhance competitive performance, reduce niche overlap between [introduced species] and natives, or increase enemy resistance may be most important during establishment… Other advantageous traits include secondary chemical compounds that deter herbivores, ‘novel weapons’ such as root exudates that negatively impact other plants, fast growth, and high fecundity.”

Plants compete for light, moisture, and soil nutrients, as well as for pollinators and seed dispersers. Competition inhibits the establishment of invaders when neighboring plants consume available resources more efficiently. Introduced plants risk being outcompeted by plants that are of the same functional type (plants that are “morphologically, phenologically, and physiologically similar”). They also risk competition by a single dominant species (or group of similar species) or by “an assemblage of species with different traits.” As a general rule, plant communities with greater diversity are more resistant to invasion.

“In forests of the northeastern USA, Alliaria petiolata, an herbaceous mustard species, contains a type of phytotoxic glucosinolate that appears to disrupt the mutualism between arbuscular mycorrhizal fungi and hardwood canopy trees.” – – Theoharides and Dukes (2007) [photo credit: eol.org]

Two hypotheses postulate the success of some plant invaders in establishing themselves: the enemy release hypothesis and the evolution of increased competitive ability hypothesis. In the first hypothesis, plant species – having been removed from their native habitat – are freed from their natural enemies and are thus able to allocate more resources to growth and reproduction. The second hypothesis states that, in light of “reduced enemy pressure,” introduced species quickly evolve to allocate resources “from enemy defense to faster growth.” Escape from herbivory and diseases, however, is likely not the only factor in the success of invaders, and much still depends on the competitiveness of the plant and the availability of key resources.

After introduced plants become established, a lag phase generally occurs before landscape spread. This can be a result of a lack of genetic variation, a dearth of suitable habitat, unfavorable environmental conditions, or some combination of the three. New introductions may occur, and the population may continue to adapt and expand. Suitable habitat may be made available, and environmental conditions may shift. In time, landscape spread becomes a possibility.

Landscape spread occurs when multiple populations of a species are connected via long-distance dispersal. At this “metacommunity scale,” populations of an introduced plant species interact across a large area, with each population in a different stage of colonization and establishment. This means that transport, colonization, and establishment are all at play during the landscape spread stage.

Abutilon theophrasti (velvetleaf) was originally introduced before 1700 in the USA. This species has only recently become an aggressive invader as a result of the evolution of different life-history strategies based on the nature of competition in its new environment.” — Theoharides and Dukes (2007) [photo credit: wikimedia commons]

Dispersal ability and habitat connectivity are key factors in determining the success of an introduced plant species during landscape spread. Long-distance dispersal can occur via wind, water, or animals. Species that depend on animals to spread their seeds rely on specific animals to be present. The seeds of Prunus serotina (black cherry), for example, are dispersed by birds. So, landscape spread is reliant on birds and “roosting trees” where the birds can perch and defacate the seeds. In many cases, “humans also play a large role in intraregional dispersal.”

Habitats vary across the landscape due to a combination of numerous geological and biological processes. The disturbance regime – “the frequency, spatial extent, severity, and intensity of killing events over time” – also helps determine landscape patterns. Natural disturbances, such as fire, weather, and natural disasters, are differentiated from disturbances caused by human activity. Large scale development and disturbance of natural areas by humans disrupts the natural disturbance regime and alters historical landscape patterns. As the authors write, “alterations of the disturbance regime that increase resource availability or change landscape patterns can promote non-indigenous plant species spread by creating favorable patches for colonization and establishment.”

Fragmented landscapes consisting of small patches of natural areas dispersed among large areas of human development are particularly prone to invasion by introduced plant species for many reasons, including increased influx of propagules and a high degree of edge effects (habitat edges have environmental conditions that are generally more prone to invasion than habitat interiors).

Habitat patches can be connected via corridors. It is through these corridors that dispersal can occur between populations in a metacommunity. Corridors connect populations of both introduced and native plant species. However, “native plants often require wide undisturbed corridors of intact habitat, while [introduced plant species] may disperse best through strips of human-disturbed habitat or ‘disturbance corridors.'” The environmental conditions in disturbance corridors and the presence of dispersal agents (including humans and domesticated animals) help facilitate the connectivity of populations of introduced plant species and promote the colonization and establishment of new populations.

In their abstract, Theoharides and Dukes write, “both research and management programs may benefit from employing multiscale and stage approaches to studying and controlling invasion.” With their conclusion they provide a list of potential management strategies for each stage, and they advise employing “natural filters in order to prevent invasion succees.” Examples include reducing habitat fragmentation and edge effects, promoting intact native communities, reducing human disturbances, promoting natural disturbance regimes, and minimizing disturbance corridors.

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