with a further 290,000 km2
expected to be lost between
2000 and 2030 (The Nature Conservancy 2018; McDonald et al. 2020). Species composition in urban areas may also change through the introduction of non-native or invasive species (Müller et al. 2013).
The active selection of organisms by humans through activities such as landscaping, gardening and pet breeding alters the composition of species in city landscapes (Williams et al. 2009; Kendal, Williams and Williams 2012; Aronson et al. 2016; Jenerette et al. 2016; Pearse et al. 2018). These changes may cause losses and gains in species and result in urban assemblages of species that are both native and non-native to the surrounding area (Ives et al. 2016; Lepczyk et al. 2017a). Despite the potential for losses, it is nonetheless possible to maintain a significant portion of native biodiversity in the city. For example, in a sample of 110 cities, a majority of native bird and plant species are present in urban areas but their density is significantly lower than non-urban habitats (Aronson et al. 2014). In this same sample, another study found that a median value of 52 per cent of plant species were native (La Sorte et al. 2014), although not all species have the same adaptability to the urban environment (Lin et al. 2012).
Some domesticated areas, such as gardens and parks, may have a greater number and variety of plant species than natural areas in cities (Pearse et al. 2018) or the landscape outside the city (Kühn, Brandl and Klotz 2004). Other urban biodiversity trends include biological homogenization, where species composition becomes similar across different urban areas (McKinney 2006; La Sorte, McKinney and Pyšek 2007; McKinney 2008), a phenomenon that may be explained by global plant exchange and the nursery trade, shared aesthetic ideals and trends (Ignatieva and Stewart 2009) and the introduction of potentially invasive alien species. The movement and exchange of organisms can also introduce microorganisms through waste disposal, tourism, food and global transport (Zhu et al. 2017), which can favour the spread of some diseases.
There are few studies of urban biodiversity at the global scale (McDonnell, Hahs and Breuste 2009). Most have focused on a single type of organism across multiple locations or patterns of multiple types across a single city (Aronson et al. 2014). Most urban biodiversity studies have a regional bias focused on the Global North and temperate areas (Aronson et al. 2014; McDonald et al. 2020).
Cities are usually a heterogeneous mosaic of habitat patches, offering valuable opportunities for the conservation of certain species and ecosystems (Elmqvist et al. 2013; Aronson et al. 2014; Ives et al. 2016) and to improve their functioning and connections with the surrounding landscapes (section 4.2). The extent of biodiversity within a city depends largely on how much green space is kept intact both inside and outside the city, as well as its connectivity and size (Goddard, Dougill and Benton 2010; Beninde, Veith and Hochkirch 2015). Green spaces include parks, conservation areas, abandoned lots, green roofs, private residential gardens, rivers and reservoirs. Most of
these areas also contribute to the well-being of citizens. For example, the Complete Streets approach to transport design promotes street space not only as a transport corridor but also as a social space that enhances the urban environment through leisure, culture and recreation activities, and greenery (Achuthan et al. 2019). This improvement contributes to people’s health by reducing noise and air pollution and providing opportunities for urban biodiversity. Examples of this initiative can be found globally in cities as diverse as New York, Paris, Bangalore and Buenos Aires.
Urban planning also plays a critical role in improving levels of biodiversity, through ecosystem restoration, the implementation of green and blue infrastructure, biodiversity corridors and nature-based solutions through which species can move (Connop et al. 2016; Raymond et al. 2017; see also chapter 2). The characteristics of urban green infrastructure determine the environmental quality and ecosystem services provided in the urban landscape (Andersson et al. 2020). To be functional, urban ecosystems should be linked to other ecosystems in rural areas through corridors or other restoration efforts (Cohen-Shacham et al. 2016; see also chapter 4 and section 5.4.1). Careful consideration of the benefits and trade-offs (for example, social, environmental, and economic) of different ecological configurations is required when deciding the kinds of biodiversity and corresponding functions to be supported in the “novel ecosystems” of cities (Kowarik 2011; Lepczyk et al. 2017b; Backstrom et al. 2018; see also chapters 4 and 5).
3.3.3 Freshwater
Urbanization increases soil sealing (Ferreira, Walsh and Ferreira 2018), a term used to describe the covering of the ground by impermeable materials that interferes with natural drainage patterns, increasing stormwater run-off and flood risks (Oudin et al. 2018; Ren et al. 2020). It also prevents groundwater recharge and increases pollution of urban and downstream water bodies. Surface treatments, such as metalled roads, are a major source of ions in groundwater, which can impact the drinking water supply and infrastructure and cause coastal alkalization (Kaushal et al. 2017). The effects are worse in sprawling cities (Lee et al. 2006; Chen et al. 2020). Cities encroach on springs, wetlands and coastal ecosystems, contributing to direct habitat loss, modifying hydrological and sedimentation regimes and altering the dynamics of nutrients and chemical pollutants far beyond urban boundaries (Lee et al. 2006). Unplanned urbanization, especially in the cities of the Global South, creates further challenges through the occupation of hillsides and floodplains and the persistence of water infrastructure deficits (Mguni, Herslund and Jensen 2016; Williams et al. 2019; see also section 4.2).
Domestic and industrial wastewater and other contaminants are still frequently discharged untreated into water bodies and their instream habitats due to inadequate or absent wastewater infrastructure (McGrane 2016). This has a major impact on the water quality of lakes, wetlands, rivers, aquifers and aquatic life, both within and outside cities (section 4.2). It is also responsible for waterborne
The State of the Environment in Cities 51
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80 |
Page 81 |
Page 82 |
Page 83 |
Page 84 |
Page 85 |
Page 86 |
Page 87 |
Page 88 |
Page 89 |
Page 90 |
Page 91 |
Page 92 |
Page 93 |
Page 94 |
Page 95 |
Page 96 |
Page 97 |
Page 98 |
Page 99 |
Page 100 |
Page 101 |
Page 102 |
Page 103 |
Page 104 |
Page 105 |
Page 106 |
Page 107 |
Page 108 |
Page 109 |
Page 110 |
Page 111 |
Page 112 |
Page 113 |
Page 114 |
Page 115 |
Page 116 |
Page 117 |
Page 118 |
Page 119 |
Page 120 |
Page 121 |
Page 122 |
Page 123 |
Page 124 |
Page 125 |
Page 126 |
Page 127 |
Page 128 |
Page 129 |
Page 130 |
Page 131 |
Page 132 |
Page 133 |
Page 134 |
Page 135 |
Page 136 |
Page 137 |
Page 138 |
Page 139 |
Page 140 |
Page 141 |
Page 142 |
Page 143 |
Page 144 |
Page 145 |
Page 146
