The ecological future of cities

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Science  20 May 2016:
Vol. 352, Issue 6288, pp. 936-938
DOI: 10.1126/science.aaf3630


The discipline of urban ecology arose in the 1990s, primarily motivated by a widespread interest in documenting the distribution and abundance of animals and plants in cities. Today, urban ecologists have greatly expanded their scope of study to include ecological and socioeconomic processes, urban management, planning, and design, with the goal of addressing issues of sustainability, environmental quality, and human well-being within cities and towns. As the global pace of urbanization continues to intensify, urban ecology provides the ecological and social data, as well as the principles, concepts and tools, to create livable cities.

For the past half century, the prevailing global economies have created employment opportunities in urban areas, driving massive migrations and overcrowding cities. In 2014, the global urban population reached nearly 4 billion people; it is predicted to gain an additional 2.5 billion people by 2050, mostly in African and Asian cities (1). The 2011 United Nations Global Report on Human Settlements warned that future synergistic interactions between urbanization and global climate change will threaten the stability of human societies. Urban ecology has developed the ecological and social data, principles, concepts, and tools required to create livable cities in this increasingly urbanized world.

The discipline of urban ecology arose in the 1990s out of a need to increase our knowledge and understanding of the ecological and human dimensions of urban ecosystems (2, 3). Urban ecology studies have mostly focused on biodiversity, ecosystem processes, and ecosystem service attributes of cities. Increasingly, urban ecologists are integrating these components with socioeconomic processes, urban management, planning, and design (2). Furthermore, the direct and indirect socioecological effects of urban environments extend far beyond the physical borders of cities. Some of the most alarming global environmental problems are the result of urbanization, including land-use and land-cover change, alteration of biogeochemical cycles, climate change, biodiversity loss, and biological invasions (4). Thus, the scope of urban ecology research extends well beyond city limits.

An evolving discipline

Scientists use a number of different approaches to study urban systems. Some studies focus on describing small-scale physical and ecological elements and patterns within cities. In this vein, early urban ecology studies commonly described the distribution and abundance of organisms residing in remnant woodlands and wetlands within cities. Other studies aim to elucidate the interacting physical, biological, and social components at city- or ecosystem-wide scales (5). For example, Hope et al. (6) discovered a positive relationship between plant diversity and human wealth in metropolitan Phoenix. They found higher plant diversity in wealthier neighborhoods, which adds to growing evidence that the quality of the urban social environments is strongly related to socioeconomic status in developed regions. More recent studies explicitly address issues of sustainability, environmental quality, and human well-being in urban ecosystems. For instance, researchers involved in the Baltimore Ecosystem Study, one of the first in this emerging research direction, have been working to better connect neighborhood urban design with fundamental ecological theory and sound environmental practices.

Ecological understanding of cities

Our ecological understanding of cities has lagged far behind that of nonurban areas, mainly because of the widely held notion that humans disrupt the natural ecological conditions and processes that scientists are attempting to understand (3). Thus, nature within cities was long considered unworthy of scientific study, except when it involved solving environmental problems that threatened human well-being.

To address the critical environmental issues facing our cities, ecologists have expanded their traditional knowledge base. This has been necessary because urban systems are highly complex and heterogeneous. Cities often contain large human populations, as well as high proportions of exotic species, distinctive disturbance regimes, few top predators, and elevated levels of nutrients and pollutants (2).

Not only has the growth of ecological knowledge about the structure and function of cities improved our understanding of basic ecological principles, it has also provided us with valuable information for creating livable, healthy, and resilient urban environments. Today, cities serve as important living laboratories that present opportunities to test fundamental social and ecological questions, such as the effect of landscape fragmentation on the distribution of organisms, the effect of night lighting on the circadian rhythms of humans and other organisms, and the role of urban green spaces in providing ecosystem services to urban dwellers. Cites also provide opportunities for learning about people’s knowledge, attitudes, and values in regard to nature. Increasingly, ecologists and landscape architects work together to develop urban design experiments that deliver both cultural amenities and important ecological information (7). For example, in design experiments initiated to increase urban tree cover, ecologists are active participants in the design process as well as the follow-up assessments.

Due to the many physical, biological, and social variations between different cities, urban areas also provide unprecedented opportunities to study the evolutionary and adaptive processes affecting urban biota. For example, in a recent global comparative study of more than 800 species of urban birds, Sol et al. found that species loss was primarily due to the birds’ inability to adapt to urban conditions, especially with regard to exploiting novel resources and avoiding new risks (8). However, other studies have shown that some species currently thriving in cities have adapted to urban environments via behavioral, physiological, morphological, and evolutionary responses (9). Examples include great tits (Parus major) modifying their song to communicate at noisy locations (10) and plants living in highly fragmented urban environments shifting to localized seed dispersal strategies (11).

Global ecological assessments on the distribution of plants and animals in urban environments have documented the biological richness of cities and offered insights into maintaining this richness. For example, Aronson et al. (12) found that the presence of remnant and restored natural vegetation is important for maintaining bird and plant populations in urban areas. Hahs et al. (13) reported that fewer local plant extinctions occurred in cities that maintained at least 30% native vegetation cover. Unfortunately, few studies have addressed the ecological effects of urban growth in developing countries, where levels of biodiversity are highest and urbanization is progressing rapidly (14).

Urban ecology in action

Urban ecology research commonly has an applied outlook, with a focus on identifying mitigation and adaptation strategies to reduce the negative effects of human activities (3). Urban ecologists in Melbourne, Australia, have begun to develop action plans to improve the health, livability, resilience, and sustainability of their city (Fig. 1). For example, an urban forest strategy aims to ensure that the city will have a resilient, healthy, and diverse urban forest in the future. Other cities around the world—including London (United Kingdom), Chicago (United States), Singapore, Durban (South Africa), and Portland (United States)—are incorporating similar strategies and plans.

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Fig. 1 View of the central business district and Kings Domain gardens in Melbourne, Australia.

Like other cities around the world, Melbourne is developing urban ecology action plans to improve quality of life for both humans and other species in the region.


Local governments have been participating in research partnerships focused on diverse themes such as understanding, protecting, and enhancing biodiversity; creating and managing ecological linkages; creating biodiversity-friendly environments by minimizing negative effects on animals, plants, soils, and ecosystems; and assisting the public in experiencing and valuing biodiversity and urban ecology in general. The Chicago Wilderness regional alliance is one such partnership. This group aims to improve the quality of life of both humans and other species in the region. Initially, researchers, government and nongovernment agencies, and the public came together to restore and enhance the region’s biodiversity. Their mission has expanded to include issues related to climate change, green infrastructure, and connecting people with nature.

Increasingly, urban ecologists, local communities, and governments are also working together in developing countries. For instance, Chilean researchers conducted detailed studies of urban wetlands in Valdivia (southern Chile). Their acquired environmental and social knowledge encouraged the local government, together with the interested community, to develop a land management and planning tool for the protection and future sustainable use of the region’s wetlands.

Another example is that of one of the remaining perennial springs (La Carbonera) surrounding the city of Querétaro, located in a semi-arid region of central Mexico. Because the area’s wetlands have been studied for several years, it was possible to rescue the spring, together with strong local stakeholder support and action. This is clearly a successful case in which locals worked closely with academics to achieve a common goal. This periurban spring has been managed, and the area is currently used to conduct recreational and educational activities while also contributing to the restoration of several critical ecological processes.

Toward livable cities

The discipline of urban ecology has made great strides over the past three decades. To further our knowledge of urbanization’s effects on people, biodiversity, and ecosystem processes, urban ecologists must shift from studying patterns to untangling the emerging mechanistic processes behind the reported patterns (15). However, it is also crucial to create the tools and procedures for transforming scientific knowledge into action. The need for these advances is pressing, as there is a growing discontent among urban dwellers worldwide, related to the erosion of their quality of life. Many urban dwellers are now calling for the creation of green, sustainable cities that are also healthy and resilient (16). Incorporating urban ecology principles into the design, construction, and management of cities will require the cooperation, alliance, and synergy of all stakeholders, thus reforming the way we conceive and prepare land to fulfill the needs of modern human agglomerations.


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