Precursors to success and establishment
In general, looking at urban gradients, a trend of increasing proportion of non-native species toward the urban core has been found in birds, mammals, and insects as well (McKinney 2006). The most common ‘urban exploiters’ (synanthropes) include cats, rodents such as mice and rats, or scavenging birds, such as the house crow. Synanthropes are usually omnivorous, they occur worldwide, and they are rarely native to a region. That said, introduced animals are also found in moderately developed (sub)urban areas. These ‘urban adapters’ include medium-sized omnivorous or carnivorous mammals, such as the raccoon (Procyon lotor), but also amphibians, reptiles, and insects. Globally, native suburban species of carabid beetles, among the commonest urban insects, tend to be smaller, and trophically more generalized than species in fully natural habitats (Ishitani et al. 2003).
Some species also colonized urban systems following their introduction in neighboring areas. For example, the American mink (Neovison vison) now occurs in urban areas of Copenhagen (Denmark) following escape or intentional release from European fur farms. Actually, independent of the arrival pathway, urban areas can provide disproportionate benefits to introduced
Figure 34.1 Number of alien urban species per taxonomic group (Padayachee et al. 2017), compared to the total number of species included in the GISD
Source: After Padayachee et al. (2017)
species. They offer overabundant resources and shelter from natural enemies (e.g. large predators which occur in natural areas but are almost absent in towns and cities), and more favorable climatic conditions, so providing a different ‘niche opportunity’ (Shea and Chesson 2002).
Furthermore, urban invasive mammals are mostly represented by carnivores or rodents. Together, they include almost 50 percent of the mammal species introduced around the world (Figure 34.2) and several of them are medium or small-sized adaptable species. Indeed, in order to persist in towns and cities, species need to be flexible or pre-adapted to the unique combination of environmental conditions that characterize (sub)urban habitats (Cadotte et al. 2017).
Introduced animals occurring in cities throughout the world are the result of a two-stage process, depending on (1) the reasons for, and pathways of, introduction and/or invasion and (2) their ability' to adapt and become established populations along gradients of urbanization, from urban centers to the rural hinterlands of cities. Introductions have led to the emergence of a particular subset of introduced animals with biological and behavioral traits (e.g. large reproductive rates, relatively' long life expectancy, behavioral and ecological flexibility) that have allowed them to fully exploit the new opportunities offered by cities and, in some cases, to become invasive.
