Trends recorded in urban biodiversity

All biodiversity is impacted by the urbanization process and ensuing urban form. A review by McKinney (2008) across a diversity of taxa demonstrates variable responses across different taxa. McKinney sought to add to Marzluff et al.’s (2001) review of avian responses to urbanization which showed a decline in avian species richness in two thirds of 51 reported studies.

Similarly, after examining literature on gradient analyses across an array of taxa, McKinney (2008) reached two central conclusions: (1) an almost universal decline in species richness in response to extreme urban cover, such as found in the urban core; and (2) an increase in richness at moderate levels of urbanization. The first, while certainly a function of the pressures associated with urban development such as pollution, noise, loss of habitat, and general disturbance, is also likely an artifact of species—area relationships where a decrease in habitable area will result in an automatic decrease in richness. The second is in keeping with the intermediate disturbance hypothesis, suggesting that diversity is highest when ecological disturbance is neither too frequent nor too rare; although this response varies across taxa. These responses include an increase in plant species richness for well over two thirds of the studies, a lower reporting of an increase in invertebrates found in a third of studies, and limited reporting of an increase in vertebrate species richness (excluding avian species) in just over 10 percent of all studies (McKinney 2008). At a finer scale, attention to some of the less explored areas have shown for example the persistence and dynamic nature of soil biota in cities (see Chapter 32), the relevance of novel niches in urban settings for fostering diversity (see Chapter 19), and the often overlooked value of industrial and brownfield sites for their contributions to urban biodiversity (see Chapter 25). All studies acknowledge the role of the original biogeographic template and associated biodiversity, and the nature of urbanization in informing biodiversity outcomes (Gaston 2010).

Scale effects and sample bias

While the effects on urbanization processes can vary depending on the specific taxonomic group, the scale of consideration can also be relevant (McIntyre 2010; see also Chapter 30). Numerous deviations to the rules exist, for example 1’auw and Louw (2012) demonstrate three different responses to the urban form among four species of bird of the nectar feeding guild in the City of Cape Town. There is also a bias in the focus of urban biodiversity research over the years with greater attention on some taxa than others, resulting in a significant sampling bias (Seto et al. 2012). Birds, plants, and arthropods for example are well researched, while reptiles, amphibians, and mammals have received less attention (McIntyre 2010). These considerations aside, there are some agreed global understandings and trends in relation to biodiversity and urban biodiversity. Urban settlement can lead to increased biodiversity through arrival of new species, particularly leading to high plant diversity at the city edge and in post-industrial environments (see Chapter 25) where native and introduced species occur together in something of an ecotone, or through the exploitation of novel niches, often created by unusual soil chemistry (see Chapters 20, 21, and 30). For the most part these added species tend to be from a pool of urban adapted species which are ubiquitous across the globe (Aronson et al. 2014).

Human social implications

A further significant identified trend relates to urban social structures, socio-economic stratification, and social and institutional resource allocation in relation to urban biodiversity. Higher biodiversity is often associated with the higher income areas of cities (Pickett et al. 2001). Social differentiations in cities tend to be well researched and understood within the social sciences and can be readily applied to biodiversity studies (Pickett et al. 2001). As population pressure grows in cities, and cities themselves grow, these social aspects, and frequent associated injustices relating to access and ecosystem services will become increasingly relevant to human well-being and biodiversity patterns and conservation agendas (see Chapters 74 and 75).

Urbanization patterns in relation to global biodiversity patterns

Increasingly cities are expanding in area faster than their associated human inhabitants are increasing in number, with cities currently growing twice as fast in area than in population (Seto et al. 2012). Modeled projections show dramatic increases in the physical area occupied by cities, with the highest growth in area projected for Asia and Africa as the regions with fastest rate of city expansion (Seto et al. 2012). Seto et al. (2012) go on to overlay their spatial predictions in urban expansion with hotspots of biodiversity (Myers et al. 2000). At the time of their analysis, urban areas intruded on 1 percent of global hotspots. If their projections of areas of high probability of becoming urban are realized in the coming years, there will be a more than 200 percent incursion of urban land cover in biodiversity hotspots. The top six sites of concern are in Guinea, Japan, the Caribbean, the Philippines, the Western Ghats, and Sri Lanka (Seto et al. 2012). People have historically tended to settle in sites of high biodiversity, for example along rivers or on coastlines, and this pattern is unlikely to change (McDonald et al. 2008).

Exploring the encroachment of new urban development in relation to protected areas, McDonald et al. (2008) note an ever increasing proximity of dense human settlement to protected areas. In keeping with a more recent analysis by Seto et al. (2012), urban growth and encroachment on protected areas is occurring primarily in developing regions, and McDonald et al. (2008) highlight the developing status of these regions with fiscal constraints and limited institutional capacity to secure the conservation of these threatened areas. This is certainly true for Africa, which is experiencing urban growth faster than any other region (Anderson et al. 2013a). While Seto et al. (2012) acknowledge the heterogeneity of hotspots and that some areas may be less significant in terms of local biodiversity than others, equally so even small areas of land cover change associated with cities could result in disproportional threat and extinction. Urban development threatens some 8 percent of the IUCN Iked Listed vertebrate species (McDonald et al. 2008).

Although it is not the focus of this chapter, regional impacts should also be considered, where the role of the city in affecting the broader regional ecology and biodiversity has been demonstrated as significant with regional and global impacts (see Chapter 39). Folke et al. (1998) effectively demonstrate this for seafood consumption across Northern Europe, invoking the notion of footprinting. An increasingly common view is that all global ecological considerations must pay explicit attention to the role of cities (Seto et al. 2011, 2012). There is evidently an urgent need for conservation policies to take into consideration urban growth as a significant driver of changes in local and regional biodiversity.

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