Urban Ecology

Its Nature and Challenges

Pedro Barbosa, Pedro Barbosa

• Éditeur: CAB International
• Année: 2020
• ISBN:
  9781789242621
• Sujet: Biological Sciences
• Sous-sujet: Ecology

Today, 55% of the world's human population lives in urban areas. By 2030, up to 90% of the global human population will live in cities and the global population is expected to increase by 68% by 2050. Although land cover categorized as "urban" is a relatively small fraction of the total surface of the Earth, urban areas are major driving forces in global environmental change, habitat loss, threats to biodiversity, and the loss of terrestrial carbon stored in vegetation biomass. These and many other factors highlight the need to understand the broad-scale impacts of urban expansion as it effects the ecological interactions between humans, wildlife and plant communities. In a series of essays by leading experts this book defines urban ecology, and provides much-needed focus on the main issues of this increasingly important subdiscipline such as the impacts of invasive species, protecting pollinators in urban environments, the green cities movement and ecological corridors. The book stresses the importance of understanding ecological forces and ecosystem services in urban areas and the integration of ecological concepts in urban planning and design. The creation of urban green spaces is critical to the future of urban areas, enhancing human social organization, human health and quality of life. Urban ecology is becoming a foundational component of many degree programs in universities worldwide and this book will be of great interest to students and researchers in ecology and conservation science, and those involved in urban planning and urban environmental management.

1 Urban Ecology: What Is It and Why Do We Need It?

Nancy B. Grimm*

Arizona State University, Tempe, Arizona

*[email protected]

The Growth and Rationale for Urban Ecology

Urban ecology has blossomed within a discipline that once shunned cities as unworthy of its attention (Collins et al., 2000), especially in the USA. Hundreds of papers on the topic are now published each year compared to 25 in a five-year period in the 1990s (Collins et al., 2000), and urban ecology sessions at the Ecological Society of America’s (ESA) annual meeting have been ‘standing room only’ in recent years. In the ESA’s family of journals, urban ecology papers have increased from just four in the first half of the 1990s to almost 100 between 2015 and 2019 (Fig. 1.1). Accompanying the increased attention to cities has been an expansion of conceptual frameworks guiding urban research (see McPhearson et al., 2016b for a summary). Most of these frameworks build upon the idea of cities as novel ecosystems, rather than seeing cities as disturbances of existing ecosystems. They, by necessity, incorporate social dimensions (Alberti, 2008; Grimm et al., 2000, 2008; Pickett et al., 2001, 2008; Groffman et al., 2017; see also Chapter 7).

The growth in interest in urban ecology is well founded given patterns of human migration in the past century, migration that continues to accelerate along with other drivers of change in the Anthropocene. In the USA, the 2012 census reported that more than 80% of the US population lives in urban areas, the major transition to urban and suburban areas having occurred in the post-World War II era (Grimm et al., 2008). Moreover, the percentage of total surface area in the USA that is developed or built up is projected to increase from 5.2% in 1997 to 9.2% by 2025 (Alig et al., 2004).

The pattern of urbanization in the USA and in Europe is being repeated today in developing countries. Rapid urbanization is occurring in the global south, with the fastest growth in African and Asian cities of less than one million inhabitants (United Nations, 2015). North America, the Caribbean and Europe already are more than 75% urban, and most increases in the urban population are expected to occur in low- to middle-income countries. As in the USA, the rate of urban expansion exceeds the rate of urban population growth in many world regions (Seto et al., 2012). By mid-century, 80–90% of the global population is projected to live in cities (Grimm et al., 2008; Seto et al., 2012). In 1950, 24% of the world’s 233 countries were urbanized (i.e. had an urban population greater than the rural population); by 2014, that proportion had increased to 63% and by 2050, over 80% of countries are projected to have more than half of their population living in cities with about half of these countries being more than 75% urbanized (United Nations, 2015). Sometime in the next 20–30 years, developing countries in Asia and Africa are likely to cross a historic threshold, joining Latin America in having majority-urban populations. The world’s population as a whole is expected to undergo substantial further growth over the period, almost all of which is expected to take place in the cities and towns of poor countries.

Fig. 1.1. Increase in the number of urban ecology papers published in the Ecological Society of America family of journals, 1990–2019, by half-decade. Search was conducted on the ESA journal website with the following search terms: urban, urbaniz*, city, cities. Journals include Ecology, Ecological Applications, Ecological Monographs, Ecosphere, Ecosystem Health and Sustainability, Frontiers in Ecology & the Environment, and Bulletin of the Ecological Society of America.

Today’s cities exhibit a wide range of population sizes and densities. The median urban population density is 5800 people/km², equivalent to the population density of Shanghai, China, but the range of densities is huge (Grimm and Schindler, 2018). If the global population rises to 11 billion by the end of this century, an evenly distributed population density would be ~725–1550 people/km² – less than today’s median (Grimm and Schindler, 2018). But that is an unlikely outcome: in the fast-growing, poor cities of the global south, much of the population growth is occurring in slums and informal settlements, which present huge challenges for meeting infrastructure needs, providing clean water, sanitation and housing, and protecting populations from extreme events.

People live virtually everywhere on earth and significantly transform natural habitats where they settle (Berry, 1990; Meyer and Turner, 1992) and in distant lands they rely on to supply resources. Near the end of the last century, human dwellings occupied 1–6% of the earth’s surface; human agriculture covered another 12% (Meyer and Turner, 1992). Virtually all lands have experienced human settlement or agriculture, or have been used to provide the natural resources or recreational opportunities needed to sustain the burgeoning human population. One estimate holds that only 17% of the earth’s surface is untouched by human activity (Kareiva et al., 2007). Models suggest that over the last three centuries forests have declined by 19%, grasslands by 8%, and cropland has increased over 400% (Meyer and Turner, 1992; Marzluff and Hamel, 2001). Human domination of planet Earth is evidenced by our use of 40% of all terrestrial net primary productivity (Vitousek et al., 1986) and lights that are visible from space at night (see Fig. 10.1; Elvidge et al., 1997).

We are thus living in an urban century – a part of the epoch of the Anthropocene, which is characterized by the indelible imprint of human impact on the earth’s system (Steffen et al., 2018). In this century, we will see the movement of the vast majority of the global human population to cities, accompanied by other accelerating changes in the environment. Changes in human activities, as recorded by exponentially increasing trends in, for example, urban population, foreign investments, vehicle miles and carbon dioxide in the atmosphere, match in scale and acceleration troublesome environmental trends. The earth is getting hotter, extreme events are increasing in frequency and magnitude, water security is increasingly threatened, and species are being lost at astonishing rates. Perhaps most urgent among these are climate change and increases in the frequency and severity of extreme events. The resulting collision course is one that presents opportunities for building better cities or rebuilding existing ones, and in which an ecologist’s perspective, along with the perspectives of social scientists, planners, designers, engineers and builders, has potential to move cities along a trajectory toward greater liveability, resilience to extreme events, and sustainability (Childers et al., 2014; McPhearson et al., 2016b).

Social-ecological systems (SES) models enable urban ecologists to describe emergent dynamics among ecosystems, people and institutions, such as how existing social norms influence choices made about landscape vegetation, and thus its appropriateness as habitat for birds (e.g. Cook et al., 2012; Chapter 3). Existing conceptual models, such as the Human Ecosystem Framework (Machlis et al., 1995), the Integrated Social-Ecological System Model (Redman et al., 2004), the Press-Pulse Dynamics Model (Collins et al., 2011) the Long-Term Ecological Research Program, and, most recently, the SES Framework (McGinnis and Ostrom, 2014) have advanced social-ecological systems theory. But to understand cities, we must integrate social, ecological and built infrastructure (including roads, buildings, power, transportation systems, and water delivery and removal systems). This built infrastructure and its associated governance, which we refer to as the technological dimension, is often left out of traditional SES research (Ramaswami et al., 2012a; Grimm et al., 2013, 2015; McPhearson et al., 2016b; Advisory Committee for Environmental Research and Education (AC-ERE), 2018; Markolf et al., 2018; Partelow, 2018; Fig. 1.2). Together, the social, ecological, and technological dimensions form the foundation of a truly new urban ecology, an urban systems science. This expanded view is reflected in the conceptual frameworks adopted by the two urban long-term ecological research projects in the USA; the Central Arizona–Phoenix LTER and the Baltimore Ecosystem Study.

The foundations of this new urban ecology are actually old; they can be found in the early writings of Sir Arthur Tansley, who argued that ‘The “natural” entities and the anthropogenic derivates alike must be analyzed in terms of the most appropriate concepts we can find’ (emphasis added). Tansley (1935) made this argument in the same paper in which he defined one of the most enduring concepts in the whole field of ecology, that of the ecosystem. While there are disparities between ecologists and non-specialists on exactly what constitutes an ecosystem, its utility to scientists, managers and the public’s understanding is well established. I write this chapter from the perspective of an ecosystem scientist, asserting that the ecosystem concept is highly appropriate to understanding the structure, dynamics and interactions of ecological, social and technological components in cities, for learning how cities interact with surrounding local and global ecosystems. In addition, it is highly appropriate for predicting how expected changes in landscapes and regions resulting from increased urbanization coupled with other environmental changes will affect the future of the earth system. But as we see from the proliferation of conceptual frameworks to guide ecosystem study of urban areas, ecosystem study, as traditionally applied, is necessary but not sufficient to understand urban ecosystems. Rather, the new urban ecology is an ecology of complex, urban, SETS; it is an interdisciplinary science of the Anthropocene (i.e. the epoch [as yet unofficial] during which human activity has been the dominant influence on climate and the environment). The primary objective of this chapter is to provide an overview of ecosystem study of cities that illustrates the need for integration of SETS, showing how an integrated urban systems science can address the challenges we face in the urban century and into the future.

Fig. 1.2. Whereas in the press-pulse dynamics framework for social-ecological systems (Collins et al., 2011) the interaction of ecosystem structure and function within a biophysical template is seen as delivering ecosystem goods and services (and disservices), a SETS (social-ecological-technological systems) framing also identifies the interaction of built structure and technological function as delivering services and disservices to the human population. Note that this diagram depicts only the ecological and technological components of the SETS, which also includes the social dimension as part of the system (see, for example, Grimm et al., 2013).

The Physical Environment of Cities

From the earliest times of established urban centres, beginning some 7500 years ago in the Fertile Crescent (Redman, 1999), urban populations have benefitted from aggregation to solve challenges of living on earth. In many cases, these urban centres have arisen and succeeded where transportation is facilitated, such as along coasts and rivers, and this is true today, with 42% of the US population living in coastal counties (Fleming et al., 2018). Other cities have grown up in proximity to railroads (Cronon, 1991) or in inland, arid regions (e.g. Phoenix (Gober, 2011), Albuquerque and Denver) where life outside a concentrated urban centre would be difficult.

The most obvious feature of a city is its built or engineered elements. Indeed, when one thinks of a city, it is likely that a skyline of tall buildings, bridges, or rows of brownstones or apartment buildings come to mind. Infrastructure that supports human well-being and livelihoods includes road networks, water and power delivery systems, stormwater and wastewater systems, and buildings for home and work activities. Built infrastructure, thus, is a basic component of the structure of a city (Pickett and Grove, 2009) and its physical environment that has a strong influence on climate and hydrology. The built environment also presents habitat, stresses such as noise and light pollution, or barriers to movement (and direct mortality) for organisms (see also discussion in Chapter 3).

Urban climate and the urban heat island, a phenomenon wherein temperature in the city exceeds temperature outside the city (Oke, 1973), provides an example of modulation of local climate by built environment and human activity (see also Chapter 3). Contributing factors include the high heat absorption by building materials, waste heat from urban activities (air conditioning, manufacturing etc. (Chow et al., 2014)), reduction in vegetative cover, and changes in the wind flow owing to urban geometry (Oke, 1973). Younstead et al. (Chapter 8) draw an important contrast between the urban heat island as a primarily surface phenomenon and global warming as an atmospheric phenomenon, but outline ways in which similarities among the two drivers of urban heat can be exploited for a better understanding of evolutionary and adaptive responses to heat. Urban heat island and extreme heat in cities often disproportionately aff...