Erle C. Ellis
University of Maryland, Baltimore County, USA
Conserving biodiversity is a grand challenge of the Anthropocene (Kareiva et al., 2011). As this challenge is often equated with the need to conserve native species, an important question is raised: what does it mean to conserve native species in the city — especially when species are on the move?
Cities might be considered the opposite of wilderness. Yet as human societies increasingly transform the biosphere, urban landscapes may now be as critical to conserving native wildlife as wilderness areas have been in the past (Goddard et al., 2010; Seto et al., 2012; Ellis, 2015). Depending on how their areas are estimated, cities currently cover as much as 3% of Earth’s land, and this area will likely double by 2030 (Schneider et al., 2010; Seto et al., 2012). Further, human settlements sprawl across Earth’s most productive landscapes and are interconnected across continents by the networks of roads and other built infrastructures needed to sustain their populations- enhancing their significance both as habitats and as barriers to wildlife movement (Seto et al., 2012; Ellis, 2015). As climate change, land use, pollution, and other anthropogenic forces displace wildlife from their historical habitats, urbanizing landscapes are increasingly where the conservation action is — and needs to be.
The first calls to sustain native species in urban landscapes originated many decades ago in Europe and the USA (Kendle and Rose, 2000; Gröning & Wolschke-Bulmahn, 2003). While the earliest calls for native plantings were more political than scientific, native wildlife, from songbirds to foxes, beavers, coyotes, wolves, and bears, are indeed returning to and thriving within urbanized regions they were driven from long ago (Bateman & Fleming, 2012). There are multiple explanations for this good news — a reversal of earlier trends towards native declines and extinction, and native plantings are at least partly responsible, as native plant communities can often sustain larger populations of native wildlife than nonnative plantings (Goddard et al., 2010). Nevertheless, a focus on native plantings as the key to biodiversity conservation in urban areas can create more problems than it can solve. To begin with, the concept of “native species” itself can be the problem.
Species are generally considered native based on a long history of inhabiting a specific area. But how long does it take to become a native? In parts of Europe, for example, “naturalized” species that have established a reproductive population in a region before 1492 are known as “archaeophytes” and distinguished as “more native” than more recent arrivals (“neophytes”), even when they are known to be introduced by humans (e.g., the Romans). In North America, earthworm species introduced from Europe predominate over now rare indigenous species, yet few consider these nonnative species to be unwelcome invaders despite their transformative effects on soils and entire ecosystems. Even more broadly, all species outside the Tropics have always had to move up and down the continents ahead of glacial ice as Earth warmed and cooled over its many glacial/interglacial cycles. The native habitats of the temperate zones have always been dynamic. Now, with climate changing faster than ever, species are on the move again- and will need to move faster than ever to keep up. To define a spatial limit where a species is “native” — and where it is not — when climate is changing so rapidly may already be more of a problem than a solution.
What does “native” even mean for a species inhabiting an anthropogenic landscape composed of built structures, engineered soils, excess nutrients and heat, pollutants, and other human-altered conditions? Urban ecosystems are inherently novel ecosystems — permanently transformed in biota and environmental conditions from those that existed before human habitation (Kowarik, 2011; Ellis, 2015). It should surprise no one that urban ecosystems are full of species from other places- introduced both intentionally and unintentionally (Ellis et al., 2012).
Urban communities are the biodiversity mixing pots of the Anthropocene — bringing natives together with introductions and immigrants from all over the world. Moreover, these novel communities often produce high levels of ecosystem services and serve as valuable habitats for native wildlife (Davis et al., 2011; Schlaepfer et al., 2011). The fact that seemingly inhospitable environments like brownfields and vacant lots can support diverse and thriving novel communities rife with ecosystem services is truly remarkable — and these would generally not be possible if only native species were present. Some of the most common species in urban landscapes, such as Tree of Heaven (Ailanthus altissima) and the Rock Pigeon (Columba livia) are found across the cities of the world. These are the true natives of the urban landscape (Del Tredici, 2010; Kowarik, 2011).
To sustain biodiversity across the Anthropocene it will be necessary to avoid extinctions while sustaining the evolutionary processes that produce biodiversity. A focus on native plantings – inherently the natives of the past — has the potential to make cities into biodiversity museums – or worse. The “native plantings” of parks and yards are often domesticates produced by large scale nurseries and seed corporations, and many are reproduced by cloning. These “natives” might as well be crops — and when they breed with remnant populations of wild natives, their narrow genetic base can reduce the genetic diversity of those native populations that are still regenerating without human management. By planting native cultivars, it is possible that we may be ending the evolution by natural selection of our most favored native species.
Is the city really the best place to focus on native species? If this is nothing more than the planting of native domesticates, the answer is likely no. Yet urban areas and their infrastructures still play a critical role in efforts to sustain Earth’s remaining wild species, far beyond their relatively small global extents. As Earth moves deeper into the Anthropocene, species are moving and coming together in ever more novel and dynamic communities, ecosystems and landscapes (Ramalho & Hobbs, 2012). Urban landscapes are the biodiversity melting pots of the Anthropocene. It is time to assist native species in moving and embrace a dynamic vision of what it means to be native — to belong in a place — whether it is urban, agricultural, semi-natural or wild. By rethinking, redesigning and rebuilding our urban spaces, transport networks and other urban infrastructures, it may yet be possible to assist the migration of species responding to climate change.
To sustain biodiversity in the city, the time has come to evolve new “cultures of nature” that welcome new immigrants together with natives in a “more-than-human democracy” of Anthropocene ecology (Ellis, 2015; Heise, 2016).