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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 traits and niches (Lososová et al. 2018). They hold crucial importance in the urban ecosystems by supporting massive biodiversity and providing ecosystems services such as temperature regulation, primary productivity, nutrient cycling, pollution reduction, carbon storage, and recreational opportunities to the human inhabitants (Dallimer et al. 2016; Lososová et al. 2018; Cubino et al. 2021). Studies revealed that climate change acts as a major driver for changes in the plant diversity, community structure, and composition (Lososová et al. 2018; Cubino et al. 2021). Plant species with different traits and origins have been supposed to respond differently to the future climate change. Based on the global climate model predictions for two representative concentration pathways (2.6 and 8.5), Lososová et al. (2018) reported that the perennial herbs and woody trees will respond more slowly to the climate change as compared to the fast‐spreading annual herbs. However, studies revealed that the responses of both native and alien plant species will be similar under changing climate conditions (Lososová et al. 2018).

      Different resource conditions such as soil water availability, precipitation, nitrogen deposition, photoperiod length, and CO₂ conditions also regulate the plant phenological events (Jeong et al. 2011; Cong et al. 2013). In temperate latitudes, climate change has played a major role in advancing and extending the growing seasons of plants (Menzel and Fabian 1999). Therefore, the projected climate change will alter the future species composition of the urban flora by modulating the temperature and precipitation conditions having direct impact on the plant phenological events (Neil et al. 2014; Lososová et al. 2018). Shifts in vegetation phenological events will have considerable impacts on different ecological functions which lead to the alteration in water, carbon and energy balances, and thus, primary productivity and interspecific interactions (Dallimer et al. 2016). Impact of urbanisation and climate change on the phenological events of birds (migration), amphibians (reproduction), plants (leafing and flowering), and arthropods (appearance and development) has been studied considerably (Grimm et al. 2008; Neil et al. 2010). Similarly, extensive studies have been done to measure the abundance and richness of bee species under changing climate and urbanisation conditions (Neil et al. 2014). A general decrease in bee species abundance and richness has been reported in the urban areas with respect to the climate change conditions. However, mechanistic understanding of these changes on flowering phenology and the pollinator communities is still needed to be explored (Neil et al. 2014). Therefore, for effective management of the urban green spaces, it is critically important to understand how the future plant and related pollinator communities will respond to the combined impacts of urbanisation and climate change conditions (Dallimer et al. 2016).

      1.3.2.1 Invasive Species and Climate Change

As mentioned earlier, even under severe climate change conditions, the distribution and responses of both native and exotic plants will be similar. However, there will be greater chances of invasion of the alien species to the new sites which have become naturalised in the urban gardens/areas (Richardson et al. 2000; Lososová et al. 2018). Moreover, the risk of invasion will further intensify under the combination of warmer climate and urbanisation (with UHI effects), particularly in the parts of Europe (Central and Western) where native

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