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Research Questions in Urban Ecological Sustainability

By Amy Karpati

Credit: Flickr
Photo: Flickr

Despite having been regarded as a series of “externalities” by conventional systems of economic thinking, our natural environment is not merely a backdrop to human activities, but is the very base upon which all human systems are built. This reality is certainly not new to sustainability science, but it is easy to lose sight of our close dependence on natural ecosystems when it comes to living in urban environments. Amidst the concrete, buildings, and tightly managed green spaces, it can be hard to see the natural environment — the nature – that exists in cities when we’re so used to “nature” being something you find in national parks and preserves, fenced off and isolated from the places we live and work.

Rockström et al. (2009) presented initial estimates of “planetary boundaries” – the limits of global ecosystem properties which, if transgressed, could trigger major environmental changes that threaten human sustainability. By the authors’ estimates, three of these boundaries – climate change, changes to the global nitrogen cycle, and rate of biodiversity loss – have already been crossed. These aspects of the global ecosystem are most dramatically altered in urban environments. Given that cities play a major role in altering natural ecosystem properties at a global scale, it makes sense that we focus sustainability efforts in cities in an attempt to slow, stop or even reverse deleterious effects. Not surprisingly, multiple research questions surround urban ecosystems, especially within the context of sustainability management.

Defining Urban Ecosystem Sustainability and Associated Indicators. If we cannot realistically use pristine forests and wetlands as model ecosystems for our urban landscapes, what then do we use as a reference point for sustainability? What do we use as the gold standard for urban ecosystem function and biodiversity? Do we define urban ecological sustainability by the existence of the ecosystem processes we value most strongly in the urban environment, while ignoring others? To what degree must these ecosystem services – water infiltration, carbon sequestration, biodiversity – function in order to impart true urban sustainability? And what indicators will provide meaningful measures of urban ecological sustainability? We have no long-term reference for this, and in this area much research is needed.

Ecological Value of Urban Spaces and Species. Beyond the cultural and ethical values inherent in protecting biodiversity and minimizing species extinctions, biodiversity holds major importance to ecosystem function. A recent study by Pasari et al. (2013) found that local species diversity is a strong contributor to landscape-level ecosystem functionality, particularly when multiple ecosystem functions are considered together. Another by Hooper et al. (2012) cites evidence that species extinctions are altering ecosystem processes critical to sustainability with a magnitude comparable to effects of climate change. How then do we approach biodiversity conservation in urban ecosystems, which have changed so dramatically from their historic state?

Perhaps one part of the urban sustainability solution is to integrate true ecological restoration of native species communities and ecosystem functions in moderately altered sites along the urban fringe (in which restoration is more likely to succeed) with enhancement of novel species communities and functions in drastically altered urban ecosystems. The spontaneous vegetation seemingly growing out of the concrete at the bases of our buildings and in the cracks in our sidewalks are volunteering to colonize our most derelict urban spaces with no intervention or maintenance required of us. Recent studies have shown that such “weeds” have ecosystem values like carbon sequestration, temperature regulation, and insect pollinator habitat (i.e., Robinson and Lundholm 2012). Extensive research is needed to further evaluate how spontaneous urban species can contribute to urban ecosystem sustainability. Even more research is needed to find out how we can reconcile human land use with the needs of other species, given the enormous role of biodiversity in promoting ecosystem services.

The Need for an Ecological Sustainability Index. More than 15 years ago, Mathis Wackernagel and William Rees proposed the idea of using an “ecological footprint” analysis to measure a nation’s level of ecological sustainability in their book Our Ecological Footprint: Reducing Human Impact on the Earth. The ecological footprint essentially measures how much land area is needed per capita to provide all food, water, material goods and waste assimilation services at a country’s current standard of living. Naturally, the lower the resource use, the smaller the ecological footprint, and the lower the risk of developing an ecologically unsustainable lifestyle.

A country’s prosperity is traditionally measured by a series of economic indicators: GDP, currency rates and stock prices, for example. However, these measures say nothing about the ecological sustainability of a nation. Several sustainability scientists have proposed that we develop an ecological sustainability index, which takes into consideration real environmental properties such as energy usage, waste production, water quality, and biodiversity conservation, just to name a few. An ecological sustainability index would recognize the inextricable feedbacks between our lifestyles and the natural environment and provide a measure of prosperity based on true environmental limits to sustainability.

Beyond application to nations as a whole, an ecological sustainability index would be particularly useful if applied to individual cities, as this would allow for smaller scale city-by-city sustainability plans for global improvement. Though considerable research has already been devoted to the development of ecological sustainability indices, many ambiguities remain regarding what variables and thresholds should go into calculating such an index. This all brings us back to the biggest underlying research question of urban ecological sustainability: What does ecological sustainability actually require of our cities?

Amy Karpati holds a Ph.D. in ecology and evolution from Rutgers University. Her research has focused on urban ecology, ecological restoration and conservation biology. She currently works as the director for conservation science with Pinelands Preservation Alliance, a non-profit environmental organization dedicated to the preservation of the New Jersey Pinelands. Amy teaches The Science of Urban Ecology in the Masters in Sustainability Management program at Columbia University.



Hooper, D.U., E.C. Adair, B.J. Cardinale, J.E.K. Byrnes, B.A. Hungate, K.L. Matulich, A.

Gonzalez, J.E. Duffy, L. Gamfeldt, and M.I. O’Connor.  2012.  A global synthesis reveals biodiversity loss as a major driver of ecosystem change.  Nature 486: 105-108.

Pasari, J.R., T. Levi, E.S. Zavaleta, and D. Tilman.  2013.  Several scales of biodiversity affect ecosystem multifunctionality. PNAS 110: 10219-10222.

Robinson, S.L and J.T. Lundholm.  2012.  Ecosystem services provided by urban spontaneous vegetation.  Urban Ecosystems 15: 545-557.

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Wackernagel, M. and W. Rees.  1996.  Our ecological footprint: Reducing human impact on the Earth.  New Society Publishers, B.C., Canada.

Science for the Planet: In these short video explainers, discover how scientists and scholars across the Columbia Climate School are working to understand the effects of climate change and help solve the crisis.
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