Over at the excellent The Nature of Cities blog, editor David Maddox is hosting the site’s 8th Global Roundtable, this time focusing on the challenge of invasive and exotic species in cities. David recently invited me (as one of the regular contributors to the blog) to take part in this roundtable with a brief essay stating my perspective. My essay was posted to the roundtable earlier this week, along with about a dozen other perspectives from urban ecology practitioners around the world. Please visit the forum site to read all the different views, and share your own by commenting in the discussion – I really hope you will do so. All of the authors are reading comments and participating in the discussion actively over the next few weeks. So I hope to see you there. Meanwhile, as promised, here is a somewhat expanded (or more rambling) version of my essay:
Aliens! Invasives! Species that don’t belong here and are taking over the ecological roles of native species. Those weeds choking the understory of native forests. The Starlings and House Sparrows pulled out of Shakespeare’s England that now so tragically dispossess native birds of their nest holes!
How can we tolerate these aliens among us? How fast can we get rid of them? Exterminate the aliens!
Pretty exotic flowers that make our suburban gardens so lovely. Grasses from halfway around the world that knit the dense mats of our lush lawns. Lovely succulent plants and delicately spined cactuses that dot our newly water-wise xeric landscapes and window ledges. Plants that flower and fruit out of their time and place, maybe year round.
Where can we get more of them so our gardens in Arizona look just like the ones in England or Singapore or Australia? So the corporate landscapes in Bangalore make the techs returning home from Silicon Valley feel like they never left California? Like the peacocks whose plaintive calls and stunning displays might make California feel more like Gujarat!
We humans sure do have a passionately ambivalent relationship with nature. Never more so than when it comes to the plants and animals we move around among our global habitats. The very words we use to describe them are so heavily laden with value judgment: invasive, alien, exotic. We move them around with us so we can evoke the landscapes of our childhoods halfway around the world, or recreate imagined ones from somewhere else entirely. We wage war on them when they refuse to stay in the gardens and pens where we put them, and start exploring new worlds on their own.
Humans are the most invasive species on Earth. Our cities, ecosystems we build and replicate around the world, are also focal points from which other species have invaded native habitats. Just as wanderlust defines our species, so does the biophilia which makes us take living elements of our habitats with us wherever we go. Carrying a suite of species as sources of food, comfort, companionship, and beauty, has always been part of our cultural and evolutionary baggage. Invasiveness is something evolution tends to reward, and our own evolutionary success springs from a certain restless invasiveness.
Ever since our ancestors realized they could control the lives of other plants and animals, so they could rely on them to feed the body and nurture the mind during lean times, humans have been moving species around the world. In scattering across the planet, we deliberately carried many such species to help fill our bellies, beautify our homes, and stave off homesickness in strange new lands. Meanwhile, other species latched on to our coattails, hitching rides around the world with this walking ape, this most efficient global form of transport and dispersal they had ever encountered! Wittingly and unwittingly, we set off evolutionary changes in species we want, species we tolerate, and species we hate, as they all became part of a human-ecosystem-complex we have set about replicating across the world. These places we call cities often serve as outposts from which many other species launch their invasions into surrounding native habitats. And so we end up with the dilemma of wanting to move some species with us to our new homes, but struggling to keep some of them from spreading out of our gardens and farms and taking over entire native ecosystems.
It is in the very nature of life to try to take control of its immediate environment, to transform resources to nurture itself, and to expand outwards to occupy and transform new places. Invasiveness, the ability to “invade” new habitats, is always favored by evolution. Natural selection rewards traits that allow species to grow rapidly, to outcompete other species vying for the same resources, or simply eat them as food. Moving to a completely new habitat, while quite risky, can also bring the benefits of escaping the co-evolved competitors and predators and parasites that might hold a species back in their native range.
So most species are always sending out their seeds and young ones into new habitats by whatever means available. Most often, these dispersals end in tragedy, but every once in a while, they open up vast unimagined new territory for the species, and new evolutionary opportunities to express and expand the range of their genetic potential. Humans, increasingly, provide some of the most perilous, exciting, and potentially high-risk/high-reward challenges to other species: if you can attach yourself to a human, and figure out how to survive in the strange new urban world, you stand a good chance of surviving in this Anthropocene, sometimes in places far from your evolutionary home.
We have spent millennia figuring out how to make some species grow where and when we want them. Meanwhile, other species have latched on to our coattails making the most of this new mode of hyper-efficient long-range dispersal: the hairless ape that travels the world, with baggage. Only recently have we realized the often devastating consequences of bringing exotic species into native habitats. Invasive species fuel some of the most intense debates among conservationists, often laden with hysterical rhetoric about alien, exotic, invaders who must be exterminated.
Invasiveness is in the nature of the most successful species. To know the truth of this, one need but look in the mirror. We are, after all, the most successful invasive species now occupying the planet! But our success has been the downfall of many other species, and we are only just now grappling with the consequences of our invasiveness. Yet the most passionate debates about what to do with invasive species, heavy with the metaphor of war and genocide and extermination, tends to somehow tiptoe around this elephant in the room: the fact that we humans are the ultimate invasive species on Earth, and are responsible for most other invasions that are destabilizing native ecosystems worldwide. Indeed, displaced species often form a big part of our own preferred global habitats.
Cities are where most humans now live, where we often first introduce new species, and whence some of these species launch invasions into new habitats. Indeed, cities themselves seem like invasive habitats proliferating in and destabilizing ecosystems around the world. Cities must therefore be central to our efforts to address the challenge of invasive species. Cities embody the contradiction between our desire to control nature, shaping entire ecosystems to suit our purposes, and our growing desire to conserve nature and biodiversity.
How do we reconcile our innate desire to build habitats for our own biological and cultural needs with a growing awareness that perhaps we should leave nature alone? It must start with owning our central role in this ecological conundrum. It is time we accepted our responsibility, as the ultimate invasive species that has moved entire ecosystems around and built new ones. It requires us to transform our role beyond the dichotomy of active perpetrator / passive bystander in the drama of invasive species. We must embrace the role of more deliberate stewards of the lands we now dominate.
As more people recognize the problems of invasive species, many now seek ways to build native species friendly urban landscapes. Ecologists are good at understanding the effects of non-native species in native habitats, and in raising the alarm about invasive species. We haven’t done enough to actually transform the practices that contribute to the invasive species problem. Urban ecologists have been lax in engaging with one group who arguably wield the greatest influence on this challenge: gardeners, nurseries, and landscapers. The growing desire to make urban gardens native-friendly is constrained by lack of available species options in local nurseries, and of expertise in nurturing native species. Ecologists must fill this knowledge gap by developing better ways to support native species in urban habitats in partnership with the people who actively transform the landscape.
Forget “leave nature alone”; in cities we must become better ecosystem engineers, designing habitats more consciously to enhance native biodiversity while limiting opportunities for non-native species. We must also recognize that some non-native species have become naturalized to play important roles in their adoptive ecosystems, so simply eradicating them is not the ideal solution. People move and grow plants and animals to fulfill complex social, cultural, aesthetic, and emotional needs. We must develop a broader vision of biodiversity that includes both the ecological roles of species and their cultural resonance for people. Balancing these will be key to managing invasive species in and around urban landscapes.
Humans will continue to move species around, despite conservationists’ (and agriculturalists’) best efforts to limit the movement of exotic species into regions outside their native range. Some of these species can and do become invasive in their new habitats. As good gardeners in the city, we will therefore also have to keep a close eye on all the species in urban ecosystems to make sure they don’t escape and start threatening native habitats nearby. At the same time, it is also worth remembering the plight of the House Sparrow, that ultimate city slicker introduced around the world as part and parcel of the global urban template. Alarmingly, it has recently disappeared quite mysteriously from many of its native cities in Europe and Asia. If the species were to somehow go extinct in its native range, at least its emigre populations, like the House Sparrow diaspora in the Americas, may remain the only living populations of a threatened species! We therefore have to be careful even in our efforts to eradicate non-native species lest we destabilize ecosystems, or drive species to extinction in unexpected ways.
Leaving nature alone is not really a viable option in the social-ecological systems we call cities. Instead, our respect for nature, and our growing enthusiasm for enhancing native species diversity, must be tempered by Constant Vigilance (if I may borrow the wise Mad-Eye Moody’s words) when it comes to the interlopers from elsewhere.