Tristan Wang ’16, THURJ Staff
Urban environments are one of the most influential yet ignored ecosystems in our everyday lives. Especially today – with over half the world’s population living in cities and the natural plant and animal life around us increasingly shrinking – urban ecology research is particularly relevant because of our need to understand how we interact with the natural world, particularly with plants (Pickett, 2001). Approaching flora in the urban setting allows us to better understand the ecological and social world and appreciate the impact of flora on urban areas.
There is however, a lack of consensus about what constitutes an “urban area.” Urban areas are sometimes defined by the anthropogenic impact on land but other times by ecological trends. For example, some studies use the geographer’s definition of urban land, stating that lands with more than 6.2 people per hectare (ha) can be considered urban, but others focus more on type of land-use and properties of urban settings (McDonnell et al. 1997, McIntyre et al. 2008). Thus, disagreement makes it difficult for research to produce meaningful data about urban environments if different definitions are used.
All of these definitions of urbanization lie on the fundamental premise of anthropogenic influence. Simply said, any underdeveloped area outside of human influence is assumed to be “natural” and the opposite of urban (McIntyre et al. 2008). Oftentimes, urban areas are associated with dense human populations and infrastructure, leading to a quickly evolving landscape and influencing the biota within and around the area (McDonnell et al, 1990).
Consequently, urban environments allow us to distinguish specific traits of plants as a result of human influence. For example, anthropogenic disturbances such as mining and weeding often mimic disturbances in the wild like storms or fires allowing successful species adapted to disturbances to flourish (McIntyre et al. 2008). Urban plants are only one clear example of the consequences of human-nature interaction.
What Urban Plants Can Tell Us
How plants are situated and where they grow in urban settings often reflect human intervention (Pickett et al. 2007). While plants are omnipresent in many cities, research in urban plant ecology tends to focus on urban green spaces and other “plant sanctuaries” in cities, ignoring the more casual vegetation that may not be intended for floral growth (Pickett et al. 2007). Exploring all areas of floral growth in cities provides us with insight into current ecological and social trends (Pickett et al. 2007).
Urban plants reflect gradients of the natural and human world, due to their impacts on biodiversity and economic factors. The gradient model for ecology has been used to study several anthropogenic causes and ecological effects throughout urban areas. For example, when looking at an urban-to-rural gradient in a forest, researchers have found factors such as air quality, water supply and contamination of heavy metals to impact soil microbial processes (McDonnell et al. 1997). Fungal and microbial mass decreased but certain natural processes such as leaf litter decomposition along with nitrification processes showed higher rates in urban forests than in rural forests. This is partly due to the higher temperatures found in urban settings, which reflect some of the complex effects of urbanization (McDonnell et al. 1997). Thus, physical and chemical environments impact the flora near urban locations indirectly through ecosystem processes (McDonnell et al. 1997).
Even more interestingly, plant heterogeneity may also be a strong indicator of resource availability in urban ecosystems, reflecting economic and cultural gradients in cities (Hope et al. 2003). The diversity of anthropogenic vegetation proves to be a suitable model for describing human intervention, independent of natural processes. Human maintenance helps to remove resource limits, allowing planted species to compete with other native species (Hope et al. 2003). This can help explain the relationship between wealth and plant diversity. In fact, one study has even pointed out that residential neighborhoods have three times the landscape vegetation diversity than surrounding neighborhoods (Martin et al. 2004). It is also possible that wealthier people are more drawn to newer housing and to urban landscapes that have more plant diversity and thus, the influence between plant diversity and social gradients may be mutual (Hope et al. 2003).
Significance of Urban Plants
Moreover, urbanization influences ecological processes through biota, disturbance and landscape structure, which in turn directly impact city dwellers (McDonnell et al. 1990). For example, urban plants function in cities to improve city microclimates by moderating extreme temperatures, collecting pollution through tree canopies, and contributing to the collection of storm water through absorptive urban surfaces (McDonnell et al. 1990). Taking away urban plants effectively rids cities of their metaphorical skin, lungs and kidneys. The significance of plants’ ecological roles only gives more importance to studying this type of flora.
However, aside from the ecological roles of urban flora, plants also directly impact people on a social level. Interestingly enough, plants may not only help us relax, but recent studies have also indicated that an increase in vegetation is associated with a decrease in crime (Donovan et al. 2012; Kuo et al. 1998). While this phenomenon may not necessarily be a cause-and-effect scenario, it has been pointed out that tree and grass maintenance may lead to an increased sense of safety (Donovan et al. 2012; Kuo et al. 1998). And although it is true that trees and shrubs might provide additional cover where crimes can occur, trees may also attract people into public spaces, providing more eyes that criminals have to worry about (Donovan et al. 2012). Even more telling is the broken window theory conceptualized by James Q. Wilson and George L. Kelling. The theory attests that criminals are pulled to rundown neighborhoods, an indication of neglect, and that the opposite is true in maintained neighborhoods, in which visual clues generally indicate working law enforcement (Donovan et al. 2012; Sridhar 2006).
Often ignored as pedestrian street décor, urban plants serve as significant indicators of ecological and social gradients imposed by urbanization. Their impact on cities’ health is only part of the incentive for further research on these unappreciated ecosystems. This is especially important as the continuing growth of urban areas could ultimately lead to even more of an increase in the influence of urban plants.
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McDonnell, Mark J., Steward T. Pickett, Peter Groffman, Patrick Bohlen, Richard V. Pouyat, Wayne C. Zipperer, Robert W. Parmelee, Margaret M. Carreiro, and Kimberly Medley. “Ecosystem Processes along an Urban-to-rural Gradient.” Urban Ecosystems 1 (1997): 21-36. Print.
McIntyre, Nancy E., K. Knowles-Yánez, and D. Hope. “Urban Ecology as an Interdisciplinary Field: Differences in the Use of “Urban” Between the Social and Natural Sciences.” Urban Ecosystems 4.5 (2000): 49-65.
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Pickett, S. T. A., and M. L. Cadenasso. “Linking Ecological and Built Components of Urban Mosaics: An Open Cycle of Ecological Design.” Journal of Ecology 96.1 (2007): 8-12.
Hope, Diane, Corinna Gries, Weixing Zhu, William F. Fagan, Charles L. Redman, Nancy B. Grimm, Amy L. Nelson, Chris Martin, and Ann Kinzig. “Socioeconomics Drive Urban Plant Diversity.” Proceedings of the National Academy of Sciences 100.15 (2003): 8788-792.
Martin, Chris, Paige S. Warren, and Ann P. Kinzig. “Neighborhood Socioeconomic Status Is a Useful Predictor of Perennial Landscape Vegetation in Residential Neighborhoods and Embedded Small Parks of Phoenix, AZ.” Landscape and Urban Planning 69.4 (2004): 355-68.
Donovan, Geoffrey H., and Jeffery P. Prestemon. “The Effect of Trees on Crime in Portland, Oregon.” Environment and Behavior 44.3 (2012): 3-30.
Kuo, F. E., M. Bacaicoa, and W. C. Sullivan. “Transforming Inner-City Landscapes: Trees, Sense of Safety, and Preference.” Environment and Behavior 30.1 (1998): 28-59.
Sridhar, C. R. “Broken Windows and Zero Tolerance: Policing Urban Crimes.” Economic and Political Weekly 41.19 (2006): 1841-843.