1.1 Introduction
Humankind is facing three major challenges viz. human overpopulation, urbanisation, and climate change with the onset of the twenty‐first century (Steiner 2014). Presently, about seven billion (expected to reach 8.2 and 9 billion by 2025 and 2050, respectively) people are inhabiting the Earth which is more than any previous time. Urban areas and people living in the cities are increasing rapidly in size, globally (Mitchell et al. 2018). Over half (~54%) of the world's population is residing in the urban areas which is expected to grow to 60 and 80% by the year 2030 and 2050, respectively (Lee 2011; Vasishth 2015). Urbanisation phenomenon can be seen occurring on all the continents (except Antarctica); however, rapid urbanisation is happening, particularly in the Asia and Sub‐Saharan Africa (Yu et al. 2017). Rapid urbanisation is putting severe stress on the planet Earth resulting in changes in the ecosystems from the landscape to the global scales (Steiner 2014; Colding and Barthel 2017). Urbanisation leads to the rapid conversion of natural pervious land surfaces to various impervious surfaces in the built forms like buildings and roads which resulted in changes in many ecosystem functions such as water infiltration and availability, species composition, soil properties, and thermal properties of the surfaces (Gaston et al. 2010; Seto et al. 2012; Yu et al. 2017). Urbanisation has not only affected the tangible features of the natural ecosystems but also resulted in the modifications of intangible aspects such as biogeochemical cycling and climate change (Kattel et al. 2013; Mitchell et al. 2018). Therefore, need for the proper planning and designing of the urban ecosystems has been arisen for reducing the ecological footprints (on per capita basis) of these ecosystems for managing the trio of challenges mentioned in the opening line of this chapter (Steiner 2014; Vasishth 2015).
Urban ecosystems are not only the rich nodes of civilisation but also have become the engines of development and divers of various environmental changes even at fine spatial scales (Kattel et al. 2013; Verma et al. 2020a). Urban ecosystems are characterised by intensive human population and its supportive infrastructures such as built‐up areas in the form of cities, towns, and megacities, developed by profound changes in the landscape structures and energy processes at the cost of natural ecosystems (Forman 2014; Jaganmohan et al. 2016; Yu et al. 2017; Mitchell et al. 2018; Verma et al. 2020a). The urban ecosystems are highly heterogeneous and fragile systems, which are facing many challenges due to massive human interferences (Ma et al. 2020). Since humans represent the core of urbanisation's structural processes, their activities related to aesthetic values, goods and services (food and water), energy and waste generation, and recycling are the driving factors of all the changes occurring in these ecosystems (Carpenter and Folke 2006; Chapin et al. 2011; Kattel et al. 2013). Thus, urban ecosystems are the complex ecosystems characterised by the interplays of socio‐economic dimensions and biophysical (natural processes) interactions occurring at various spatio‐temporal scales (Kattel et al. 2013; Ma et al. 2020). Most notably, the land‐use change and the infrastructural development by the human decide the future of the urban ecosystems which are already facing various unprecedented social, demographic, technological, and environmental challenges (Niemelä 2014; Steiner 2014). Thus, there comes the need to understand the urban ecosystems as an ecological system which can play and sustain even under the human dominance (Grimm et al. 2008). In the next sub‐section, the concept of urban ecology and its need for the sustainable urban development has been highlighted.
1.1.1 Urban Ecology
Nowadays most of the people live in the urban areas; however, they have limited understanding of the benefits derived from the interaction with the natural systems within the cities. Urban ecology is a scientific discipline which integrates a number of concepts from the natural and social sciences along with the landscape approach and ecosystems services at its core (Alberti 2008; Niemelä et al. 2011; Niemelä 2014) and represents the ‘holistic ecology of urban areas’ (McDonnell et al. 2009; Jim 2011). Since this field is still emerging, its hypotheses, models, and theories are still in the process of testing and validation (Niemelä 2014). Since human activities are the dominant factors in shaping the urban ecosystems, the urban ecology unintentionally revolves around the processes and interactions attributed by the human actions (Verma et al. 2020a). For example, urban ecology helps in recognising the restorative behaviour of humans for the natural ecosystems and elucidating the mechanisms responsible for the structuring of urban communities during the process of urbanisation (Steiner 2014; Duffy and Chown 2016). However, poor understanding of the complex interactions between the socio‐ecological and infrastructural developments in the urban ecosystems resulted in the social disharmony in the long term (Kattel et al. 2013). Therefore, Kattel et al. (2013) suggested a concept of complementary framework for urban ecology which represents the development of infrastructure and green spaces in an integrated manner to derive utmost ecosystem services. For instance, integrated development of flora and fauna along with the urban buildings, roads, and railway tracks for providing the utmost scope for the interactions between human inhabitants and wider communities through inter‐habitat‐community systems (Halpern et al. 2008).
Recent studies on urban ecology revealed that the field is now viewed as the ‘ecology of the city’ in addition to the ‘ecology in the city’ (Grimm et al. 2000; Pickett et al. 2001; Kattel et al. 2013; Childers et al. 2014). The ‘ecology in the city’ refers to the study of the natural ecological systems (fragments) within an urban ecosystem, i.e. different urban fragments as the analogues of their non‐urban counterparts, whereas ‘ecology of the city’ represents a much wider context where urban ecosystem itself is studied as an ecological system, i.e. the study of the interactions of various biological, built, and social components of the ecosystems within a city (Vasishth 2015; Pickett et al. 2016; Verma et al. 2020b). Based on these diverse concepts of the urban ecology, following sections shall provide a brief understanding of the urban ecosystems. In the next sections, climate change as an emerging challenge to the urban ecosystems has been discussed, followed by the possible urban ecological approaches for mitigating the ill effects of climate change in these highly heterogeneous and fragile ecosystems.
1.2 Components of Urban Ecology
An urban ecosystem is composed of several tangible and intangible components. Tangible components include the physical structures which can be natural (such as flora and fauna, water bodies, mountains, urban agriculture, etc.) or human‐made (such as built structures like buildings and building materials, roads, railways, health, and related infrastructural developments; energy sources like coal, liquified petroleum gas (LPG), wood; food supplies and waste generation, etc.). In addition, the intangible components include the ecosystems services derived from various natural systems, biogeochemical cycling, solar energy, and material flow in the urban areas (Verma et al. 2020a). These major components can be mainly divided into three sectors: (i) urban infrastructures associated with the urban heat island (UHI) effect; (ii) urban vegetation representing the green spaces and related ecosystem services; and (iii) urban metabolism which represents the flow of energy and materials within the urban ecosystems.
1.2.1 Urban (Built) Infrastructures
Urban infrastructures are built to provide the services and benefits to the urban inhabitants (Ma et al. 2020). Urban habitats are almost similar over large areas or human‐managed cities and microclimatic regions (Savard et al. 2000) having a central square with paved areas, residential areas, urban parks, urban agriculture, and some disturbed/unmanaged plots (Lososová et al. 2018). Considerable impervious surfaces such as roofs, roads, and paving are the most common features of urban infrastructure which sets cities apart from adjacent rural areas (Vasishth 2015). Both the components (viz. natural and artificial) of urban infrastructure help in the maintenance of the ecosystem functioning and health (Kattel et al. 2013). For example, vegetation cover and water bodies in the urban and peri‐urban areas provide habitats for biological diversity and help in maintaining the integrity of the natural/ecological and physical environments (Hofmann et al. 2012). In addition to the natural and artificial classification, urban infrastructures can also be classified into two major components viz. aboveground and belowground (Ma et al. 2020). Aboveground component