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Who's the real invasive species: us or them? Ecologists are rethinking urban biodiversity
Climate change is already having a profound impact on cities, as global urbanization pushes more and more people to live in them. The animals who cohabitate with human, whether we appreciate their presence or not, are changing too. Specifically, there's been an increase in invasive species — a term used to describe introduced organisms that bring dramatic and often destructive changes, and sometimes can drive other species to extinction.
But here's the thing: Invasive species don't stop evolving themselves. Consider the infamous brown rats of New York City, which have evolved longer noses and shorter upper molar tooth rows, the better to enjoy the Big Apple's colder weather and higher-quality food. Other invasive species are adapting behaviorally, physically and genetically to life in cities as well. While invasive species pose major public health implications and can certainly affect humans' quality of life, their adaptive abilities can rival those of human migrants and pose a puzzling question: Who's the real invader here?
Brian Verrelli, a professor in the Center of Biological Data Science at Virginia Commonwealth University, sees many of these questions as philosophical.
"What does it mean to invade something, right?" he asked in a video interview. "What does it mean for humans to invade one area or another? Are there areas that we invade, or are humans welcome freely to move around the planet? We know that's the case in some places, and certainly not the case in others.
"It's a bit of an interesting, hypocritical conversation," Verrelli added, given that "we are facilitating the movement of these organisms around the planet."
Rats have invaded new continents because ships carrying them in the hold. Spiders move across continents inside our cars. And we have all literally "translocated" organisms from one place to another, Verrelli said, because we wanted to — flowers and household pets, for example.
"Traveling globally, one can notice the same suite of species in many cities around the world — biological invasions creating a kind of global Cuisinart where the urban biota becomes homogeneous," Laura Meyerson, a professor at the University of Rhode Island who studies invasive species and ecological restoration, told Salon. "
She cited the nearly ubiquitous tree of heaven (Ailanthus altissima), which is found in cities around the world. "This introduced tree is threatening important ancient ruins such as the Roman Colosseum. Interestingly, it is the host plant in its native range for the spotted lanternfly," an invasive pest that has recently gotten a lot of attention in North America. "Other common species in urban areas globally are Norway rats and English pigeons, both of which can spread disease," Meyerson said, along with "European starlings and the common reed (Phragmites australis), both of those being aggressive introduced species that displace native species."
But Verrelli believes that if we can't exactly determine who counts as an invader, we also can't really say what's being invaded. That might sound like an abstruse philosophical argument, but he says it's a hot debate within the scientific community.
"We have what are called 'stud books' for our primates at zoos all over the world," Verrelli said. "We don't count that as invasion, but we are directly facilitating that. I'm hitting this note because it very much underlies our idea of invasion and gene flow, how genetics moves around and what it means to point to an organism and say it belongs to a certain species."
Verrelli has studied the spread of black widow spiders, an increasing medical health concern in cities along the West Coast. People now encounter these potentially deadly spiders, which used to be found in isolated desert areas, in urban or suburban garages and yards. "As we're altering the landscape within cities, these areas are becoming attractive to black widows," Verrelli said. "They're all around us now, so we need to be able to understand what's happening as they're moving into cities."
There are important health and environmental questions about whether black widows are becoming more deadly or displacing other arachnid species, but Verrelli raises a different question: "They were here first. Are they really invading the urban area? Yes, because we plopped the urban area down in the middle of the desert." Verrelli spent nine years in Arizona, an arid environment where many organisms struggle to survive, but that has now been altered and made more hospitable by human habitation.
Indeed, the American Southwest is particularly vulnerable to invasive species, including highly adaptive species that can affect human health, largely thanks to the rapid spread of urbanism — which can bring water to desert environments while also creating artificial heat islands in colder climates, and allowing tropical species to expand their ranges.
Meyerson defines invasive species as "non-native species introduced either intentionally or accidentally by humans outside of their native range," while noting a crucial distinction between non-native but non-invasive species — tulips were imported to North America from Europe, for example, but do no ecological harm — and invasive ones.
Most invasion scientists aim "to prevent introductions of harmful invasive species in the first place through public education, better screening tools and risk assessments," she said. But once such species are introduced, detecting and eradicating them quickly becomes the goal. If that doesn't work, the next goal is about managing and containing them, preventing further spread, and supporting native species by managing green spaces and natural habitats within the urban environment. "Urban areas can also be important stopover sites for migrating species," she added. "We need to manage these areas to help support these seasonal migrations."
It's no longer reasonable, Verrelli believes, to draw a neat line between urban and wild spaces. "For a very long time, even most scientists studying urban areas as ecologists didn't really treat urban areas as 'natural' environments, because humans lived in them," he said. "That's something that I smile at, because if we don't believe humans are part of the natural world, we're in trouble. Most of our natural world is going to be consumed by humans, in which case we're ignoring a major influence on how organismal biodiversity is successful."
Patterns of evolution play out in urban environments. A classic example often taught in high school biology classes is that of industrial melanism in the peppered moth, a British species tha looks, well, "peppered," with black dots on a mostly white background. There had always been occasional all-black moths, but from the mid-19th century, observers began to see more and more black moths in the industrial cities of England and Scotland. The mostly white moths stood out in the sooty environment and became easy prey for predators. Ultimately, the ones able to blend in were more likely to survive and reproduce.
This is an adaptation — in evolutionary biology, a trait that arose due to natural selection. But when we casually talk about species adapting to life in the big city, that's not always what we're describing. A bear that overturns your trash can and figures out how to get your leftovers isn't "adapting"; it's just repurposing a behavior that evolved in its natural or ancestral environment. This kind of transfer of an evolutionary adaptation to a new context is called "exaptation."
"It's really important that we know the difference between these two," Verrelli said. "We're not just trying to classify for the purpose of classification. If we can tell the exaptations versus the adaptations, then we are going to really understand the selective agents." Verrelli cites the work of Kristen Winchell at NYU, who has studied anoles lizards moving from forests into urban areas in Puerto Rico. In cities, she found, these lizards evolved longer legs relative to their body size, allowing them to crawl up metal pipes. That's not an exaptation, where the same structures that let them climb trees are applied to the new setting, but actual evolutionary adaptations. In Verrelli's view, understanding this difference isn't about discouraging these animals' presence, but making it easier for us and them to live in harmony and reasonable comfort. If an urban lizard can't climb up a pipe, an urban cat may select it right out of existence.
"How do we design the urban environment to make them more readily livable to these organisms?" Verrelli asks. "What kind of surfaces should we be developing? What kind of areas? Where do we put light, where do we not put light?" Such questions will be "very important to understanding how organisms can move through urban environments."
Sometimes our attempts to address invasive species can be counterproductive: Meyerson cites the example of the American elm, a formerly popular street tree appreciated not only for their beauty but also their hardiness under harsh urban conditions. But Dutch elm disease, an invasive fungus spread by bark beetles to which the native trees had no defense, has decimated the elm by tens of millions across the continent. "Ironically, many of these street trees were replaced by other introduced invasive species such as Norway maple or callery pear," she said, "which in turn have their own impacts." Introduced species are wreaking havoc due to similar lack of evolved defenses in the American beech, many amphibian species and North American bats of different kinds. Generalist creatures, those that can thrive in a variety of environments, will most easily adapt to city life and may go on to decimate native species.
There is general agreement that we are no longer dealing with either a purely urban, human environment or a purely wild and untouched one.
But while Verrelli belongs to one side in the ongoing debate among conservation biologists, the other includes those who believe cities are an incurable blight and that their spread must be reduced or reversed. His side, roughly speaking, sees urbanization as a permanent reality and harmonious biodiversity — sharing these spaces with our plant and animal relatives — as the most important goal. They see the distinction between wild spaces and urban spaces collapsing, which is happening anyway, not always harmoniously.
"In understanding invasion, we need to learn more about cities," Verrelli said. "We need to learn more about invaders. We need to learn more about how they're doing, what they're doing. That's why we study things like bedbugs and black widows, because these are good models to learn about how this is happening, and because it's going to keep happening.
"Instead of the idea of, let's go study things outside of cities, because this are the important areas we need to conserve, I'm on the other side. We need to study invasions in cities, because this is largely the future, whether we like it or not. As we move into the future, we're preparing a new landscape that's evolving all the time. Let's prepare biodiversity for it as well."
