aesthetic values by the urban inhabitants hold a crucial significance in current scenario. Exotic plants constitute a major portion of the urban vegetation; however, their contribution to the functioning and diversity at the socio‐ecological scales have been less explored (Cook‐Patton and Agrawal 2014; Pearse et al. 2018; Cubino et al. 2021). Most of the exotic species present in the urban areas are ornamental plants introduced intentionally by the humans (Čeplová et al. 2017; Lososová et al. 2018). Due to suitable environmental (heterogeneous) conditions or positive anthropogenic interferences (fertilisation and irrigation at regular intervals) in the urban areas, exotic plants have substantial scope to establish and colonise (Lososová et al. 2018). Human‐induced dispersal of the plant species in the urban areas contributes to the plant distribution and community composition (Møller et al. 2012; Lososová et al. 2018). Moreover, local fauna and their preferences for the fruits/flowers also contribute to the biodiversity of the area. For example, the preference of frugivorous birds affects the dispersal of fleshy‐fruited plant species in the urban areas (Møller et al. 2012). Moreover, birds help in seed dispersal of ornamental (exotic) trees from the gardens to the nearby natural ecosystems, thus, may facilitate plant invasion (Milton et al. 2007). Thus, exotic plants can be a major cause of ecosystem imbalances when the sufficient source of seeds/propagules and its dispersal agents are present in the nearby areas (Rai and Kim 2019).
Vegetation, particularly trees, plays a crucial role in maintaining the harmony in the urban ecosystems (Tigges et al. 2013). For example, trees store sufficient amount of carbon (C) and help in maintaining the overall C‐pool of the urban ecosystems (Davies et al. 2011). Urban ecosystems have sufficient potential to store C in their above‐ and belowground components (Hutyra et al. 2011; Nowak et al. 2013), even in dense urban areas (Mitchell et al. 2018). Urban areas have abundant shade trees (recreational purpose), trees grown for hazard removal, or exotic trees, all have potential to store substantial amount of C in their vertical structures. However, C‐density of the urban areas varies at spatio‐temporal scales (Mitchell et al. 2018; Upadhyay et al. 2021). Detailed view on the urban C‐stocks and their ecosystem services have been highlighted in the latter part of the chapter.
1.2.3 Urban Metabolism
The urban areas can be considered as an organism where consumption of materials, flow of energy and information, and waste generation (as end‐products) are the common processes occurring at various spatio‐temporal scales (Liu et al. 2013; Vasishth 2015; Verma et al. 2020a). These processes not only occur within a city but also affect the environment beyond the borders of the city, as like the natural organisms where different cells and tissues interact and involve in the metabolic processes and excrete the wastes outside the cell/body (Liu et al. 2013; Verma et al. 2020a). To understand the concept of material and energy supply for the functioning of the cities and the resultant waste (pollutants) generation in the urban ecosystems, the concept of urban metabolism has emerged (Restrepo and Morales‐Pinzon 2018). The concept was first proposed by Wolman (1965), who believed that processes occurring in the urban systems are analogous to that occurring in the metabolic processes of the living organisms. This approach helps in quantifying and identifying the movement of energy and materials as well as management of the environmental problems in an urban ecosystem (Wang et al. 2021). Thus, the research focus has now been shifted from quantifying resource consumption and environmental impacts to identifying and analysing the internal processes and the mechanisms involved in the outcomes of end‐products. Urban metabolism approach is getting wider attention of the urban ecology researchers as it helps in simulating the material flows and managing the environmental problems at different spatio‐temporal scales (Wang et al. 2021). In a CiteSpace analysis, Wang et al. (2021) identified the research trends in the urban metabolism. They found that now research communities are focussing on different micro‐ and macro‐scales in the urban areas such as by differentiating the central urban areas from the suburbs and rural areas to refine the results from the urban systems. Moreover, developing nations and the developing or less explored cities are being recognised as the new objects for the research, as several case studies are already available from the cities from the developed nations. In addition, future research should integrate the role of developing economies and the climate change phenomenon for exploring the urban metabolism at different scales (Wang et al. 2021). In the next sections, an insight has been given on the climate change and its impacts on the urban ecosystems. Moreover, the adaptation mechanisms of the urban ecology to the climate change has also been highlighted in the later sections.
1.3 Climate Change as Emerging Challenge for Urban Ecology
Climate change is the most challenging environmental change the whole humanity is facing nowadays (Niemelä 2014). It affects both the biotic and abiotic components of the urban ecosystems. The impact of climate change may become more severe with the UHI effects, particularly for the ageing and sensitive urban populations (UN 2011). Thus, climate change and rapid urbanisation are considered as the two major challenges the world is going through recently (Yu et al. 2017). Intensive land‐use change and high consumption of fossil fuels for different purposes have resulted in the substantial GHGs emissions (~78%) from the urban ecosystems which further contribute to the global climate change (Kattel et al. 2013; Weissert et al. 2014; Mitchell et al. 2018). Sustainability of the urban development, and C and energy metabolism are emerging topics in the light of climate change scenario (Wang et al. 2021). Climate change is affecting the urban ecosystems in different ways such as by increasing the UHI effect, reducing the ecosystem services provided by the natural systems, occurrence of extreme events such as floods and droughts, wildfires, diseases, and health problems (Niemelä et al. 2010; Ma et al. 2020; Verma et al. 2020b). Thus, there is a need for proper land‐use planning and improved infrastructural resilience for reducing the urban vulnerability to the extreme environmental events occurring (and expected to intensify) because of climate change‐urbanisation nexus in the near future (Green et al. 2016; Ma et al. 2020). Moreover, there is a need of transdisciplinary research including both the natural and social sciences along with the major stakeholders for the climate change mitigation (Niemelä 2014). A brief insight on the impact of climate change on urban ecosystems has been given in the following sub‐sections.
1.3.1 Urban Ecosystems as Indicators of Future Ecosystems
Urban ecosystems having comparatively higher temperature as compared to their surrounding (rural) areas are viewed as projections of the future ecosystems in the context of climate change (Grimm et al. 2008). These ecosystems represent the locations where human activities utilise higher proportion of primary productivity and produce comparatively higher amount of GHGs (CO2 emission); thus, influencing the global C cycle (Gaston et al. 2013; Mitchell et al. 2018). Land‐use change‐related urban expansion has been reported as the cause of ~5% (1.38 PgC) emissions from the deforestation in the pan‐tropics during the first three decades of the twenty‐first century (Seto et al. 2012). Recent estimates on C‐estimation revealed high potential of C‐storage in the urban ecosystems, even at higher magnitude as proposed earlier (Davies et al. 2011). Therefore, researches on measuring the citywide C‐stock and C‐sequestration potential of different cities are the hot topics of research which will further help in developing strategies for climate change mitigation (Pedersen Zari 2019). Further, the development of novel plant communities and their formation processes under the combined impact of urbanisation and climate change may impact the health and livelihood of urban inhabitants (Knapp et al. 2017; Lososová et al. 2018). Healthy ecosystems have self‐regulating capacity by regenerating the ecological and social health enabling humans to better adapt to the climate change, therefore, enabling the cities to evolve this ability should be the priority agenda of the future urban planning (Pedersen Zari 2019). A detailed insight on the impact of climate change on the urban flora has been given in the next sub‐section.
1.3.2 Impact on Urban Flora
As mentioned earlier, urban plant communities are composed of both native and exotic origins with different
