This is a re-post of one that Max Lambert and I wrote for the blog “Life in the City: Evolution in an Urbanizing World.”
A variety of species – animals, plants, fungi, microbes – share our cities with us. Some we love, some we hate, and some we hardly notice. Some of these species existed naturally before we built cities. Some were obliterated during urban construction but have since re-entered the city. And others have been purposefully or accidentally introduced by people from all other corners of the Earth. While many – or most – urban species are relatively common, others can be quite rare or surprising to find in cities as they are declining elsewhere. Together, all these species represent urban biodiversity. Recent years have seen two interesting ideas about urban biodiversity emerge and take hold.
Greening our cities
First, society is increasingly interested in “greening” our cities. We are tearing up pavement and concrete to build more parks. Trees are planted where there were none before. Bare dirt in the medians between roads are planted with wildflowers. Community gardens are becoming common to make food more local and accessible. Ponds, wetlands, and streams are being built or dredged – for functional, recreational, or aesthetic reasons. Whether intentionally or incidentally, these activities increase biodiversity in urban places. This cultural push to not only increase the availability of green places in cities but to fill those places with species has happened because we now realize that biodiversity is good for human health and well-being and that managing biodiversity in cities is ethical.
The second idea emerging lately is that urban species are evolving. What does this mean? It means that species aren’t a single, static entity. Rather, individual populations change over time. Some of these changes are neither good nor bad for urban life. Other changes can be adaptive and help populations live better city lives. And some changes can end up being maladaptive, making some aspects of life a bit harder. We first discussed the idea that species might be adaptively evolving to live in cities a few years back. Since then, there’s been an explosion of researchers studying how species are evolving in and because of cities.
Evolving urban biodiversity management
These two ideas – valuing efforts to bolster urban biodiversity and the fact that some urban species are evolving – have an important consequence. As stewards of the urban environment, our goals are often to conserve species we care about and to manage the harmful effects of species we consider nuisances. But if some of these urban species are evolving, biodiversity conservation and management goals are moving targets.
This is the focus of a framework we recently published in Nature Ecology & Evolution. Urban species are living in new and very weird environments, built by and primarily for people. Cities are fractured by buildings and roads and are full of stressful conditions like urban heat islands or chemical, noise, and light pollution. By shaping the flow of genes or causing populations to adapt to stressful circumstances, urban landscapes fundamentally change how species evolve.
Why does evolution matter?
Society spends a lot of money and time trying to build greener and more biodiverse cities while also managing harmful pests. But, if we don’t account of the fact that species are dynamic and evolve over time, we may not be getting the most out of our greener cities. The evolution of some species may also have consequences for some conservation efforts (see frogs below) or pest management (e.g., pest like rats evolving tolerance to pesticides).
What does it mean to account for evolution when we do urban biodiversity conservation and management? Well, there are two ways we typically do conservation and management. We can manipulate habitat by making it less suitable for species we don’t want or we can enhance it for species care about. Or we can directly manipulate populations of species themselves. This can mean bolstering existing populations by adding individuals from elsewhere, creating whole new populations where ones currently don’t exist, or removing species we would prefer to not have. But the ways in which species have evolved to city life can change the outcomes of any one of these approaches.
Urban frog conservation
This is the crux of current work by one of us (Max) in collaboration with colleagues at the City of Portland, City of Gresham, the USGS, and elsewhere. We are studying red-legged frogs (Rana aurora), a species of conservation concern in the Pacific Northwest. Red-legged frogs have declined across their entire range. They’re not common in cities but they’re also not absent. But we’d certainly like to see more urban red-legged frogs. There are a couple of ways we might go about doing this.
We could put frogs in places where they currently don’t exist, thus creating dozens or hundreds of new frog populations. Or we can increase the amount of forested spaces in the city, making it easier for frogs to hop around from urban pond to urban pond. This would also better connect the ponds in cities to ponds in the forested hinterlands beyond the urban boundary. In many ways we are lucky. Urban managers have built tons of new urban ponds. They didn’t do this for the frogs. They did it to redirect flood water out of our homes and off our roads. But frogs use these ponds nonetheless. But how might evolution reshape our thinking on urban red-legged frog conservation?
We have built enclosures in a diversity of city ponds as well as ponds in forests. In these enclosures, we placed red-legged frog embryos which will develop into tadpoles and eventually metamorphose into little froglets. In each pond, some enclosures contain frog embryos from forests and others contain embryos from existing city frog populations. With our experiment, we are asking a number of questions. Have urban red-legged frogs adaptively evolved to live in polluted, warm city water? If so, can we take tadpoles from existing urban populations to start new city populations where frogs currently don’t exist? If we connect our city populations to the outside forests, will we dilute the urban gene pool and do more harm than good to our city frogs? The data are starting to trickle in and we will hopefully have answers to these questions soon!
Evolutionary thinking for all urban species
But of course, thinking about evolution doesn’t only matter for managing species of conservation interest like red-legged frogs. Urban evolutionary insight matters for making sure native, common species stay common in our cities and for mitigating the harm done by a variety of pest species like brown rats. Cities are functioning ecosystems, composed of species – both native and introduced – with diverse histories from all over the globe. That means that many species living in cities are interacting with each other, and with humans, for the first time in their evolutionary stories. If we want to keep making our cities greener and more biodiverse, we have to know how species are evolving to life in the city.