INTRODUCED AND INVASIVE ANIMALS: Species interactions in towns and cities

Whether accidentally or intentionally, humans have always moved animal species, introducing them outside their natural range, and historically cities have played a key role in this phenomenon. When introductions were intentional, such as plants for parks and gardens, or exotic animals as pets, the species would have been chosen according to their traits and availability and to satisfy specific socio-economic needs (Blackburn et al. 2017). When they were accidental, only some species established viable populations in the introduced range, since successful colonization depends on environmental conditions and on the species’ life-history traits too (Duncan et al. 2014). The introduction patterns could differ, but selective forces were somehow involved in the whole process, or part of it, and they ultimately led to the successful establishment of a subset of introduced species. Among these species, some have had unexpected impacts on the environment and became invasive, in that their introduction and/or spread has started to threaten biological diversity (see: Convention on Biological Diversity, terms.shtml).

Nowadays, the role of cities as entry points for alien species has been exacerbated. Cities house most people and are centers of movement of human beings, their animals, goods and services. Therefore, the ‘propagule pressure’ (Lockwood et al. 2005) in towns and cities tends to be particularly high. Cities have gradually become hotspots of invasions (van Ham et al. 2013), with introduced animals and plants becoming parts of complex socio-ecological systems and establishing multiple interactions with other species, including humans.

In particular, species from warmer climatic regions often survive in mid-latitude urban areas through the urban heat island effect, where susceptibility to winter cold is reduced (see Chapter 11).

The aim of this chapter is to describe the processes related to animal introductions in urban contexts. First, we describe the main patterns and pathways of introduction and colonization and we focus on the precursors to success and establishment, highlighting how these patterns led to the establishment of a particular subset of animal species. Then, we provide examples of the multiplicity of interactions that introduced animals can establish within the urban ecosystem, taking into account the potential cascade effects on animal communities and the social and sanitary interactions with humans and their pets. Emphasis is given to vertebrate introductions, since they are particularly interesting from a socio-ecological perspective. Finally, we highlight the peculiarities of species interactions in cities, and we provide some suggestions on how future research might deal with issues related to introduced animals in urban environments.

Patterns and pathways of introduction and colonization

Human settlements import non-native species for a variety of reasons and through several pathways, from the accidental importation by traffic (trucks, planes, and ships) associated with centers of commerce, more concentrated in cities, to the intentional importation for human uses (McKinney 2006). Several vertebrate species have been imported in cities as pets and then left behind. Examples range from domestic cats (Fells Cains'), squirrels (Sciurus carolinensis), and ferrets (Mustela furo), to monk parakeets (Myipositta monachus) and reptiles such as the redeared slider (Trachemys scripta). Nevertheless, among mammals some species were historically introduced in urban environments as stowaways, e.g. mice (Mas ntusculus), rats (Rattus norvegicus and Rattus rattus), and even shrews (e.g. Suncus murinus). More recently, invertebrates such as the Asian tiger mosquito (Acdes albopictus) (through international trade in vehicle tires, which brings mosquitoes into European ports (Vaux and Medlock 2015) or the zebra-mussel (Dreissena polymorpha) (in ballast water in both ocean and inland waterway shipping) have been spread unintentionally (Global Invasive Species Database (GISD) 2015). Pathways have varied in time and across the taxonomic groups of alien species. Currently, the most prominent pathway for vertebrates to invade is by escaping from containment, while invertebrates escape from traded goods or from containment (Padayachee et al. 2017).

At least 42 invertebrates and 61 vertebrate alien species are present in cities of over one million inhabitants worldwide (Figure 34.1) (Pagad et al. 2015, 2018). Insects are clearly the dominant taxonomic group within invertebrates, with 18 species recorded in urban environments. Among vertebrates, fish are the most represented, with 9 species out of 24 belonging to the order Perciformes (bony, perch-like fish), and they are then followed by mammals (16 species) and birds (14). Invasive birds in cities are mainly Passeriformes (passerines or perching birds), such as the house crow (Corvus splendens), or Anseriformes (ducks, geese, swans, and screamers) such as the Canada geese (Branta canadensis), originally introduced for hunting purposes. Interestingly, while more than 40 percent of bird and fish species included in the GISD are also reported in large cities, for mammals this percentage is smaller (about 24 percent. Figure 34.1).

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