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? 

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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 notices the same suite of species in many cities ... biological invasions creating a kind of global Cuisinart where the urban biota becomes homogeneous."

"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........

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