It's no secret that our success as a species has been pretty bad for a lot of other species on Earth.

In fact, some scientists believe we're currently in the midst of Earth's sixth mass extinction event, caused entirely by us. 

Us, taking up more land that used to be wild habitat, using up more fresh water, and belching out greenhouse gasses that are changing the climate. 

So, for many species we're just terrible news. But there are exceptions. Some species even seem to thrive in the biggest most populated cities. Those animals that we see in our downtowns and suburbs and exurbs --the ones that are best at putting up with us -- may well be the survivors of the sixth extinction. So what's their secret?

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Okay, I'm not passing judgement here, but there are a lot of animals out there that, when it comes to getting along in an increasingly human-dominated world, aren't really survivor material.

These are animals that are most sensitive to disturbances in their habitat. They're just not flexible about things like what they need to eat or where they need to sleep or when and where they mate.

Animals like these, like large predators or ground-nesting birds, are "urban avoiders" and they're in the most danger from encroaching human development.

But lots of other species are so thoroughly accustomed to living with humans that they've become regular city-dwellers they're practically right in front of you in line at Starbucks.

These are "urban exploiters" and the most obvious example, for anyone who's ever been in any city, is the feral pigeon. Also known as the Rock Dove, it used to make its home on the cliffs and rock faces of Europe.

But the most successful survivors are probably creatures that, though they still maintain populations in the wild, are extremely flexible around people.

These so-called "urban adapters" have been able to take advantage of the way humans alter their natural habitat by altering themselves.

Many of these adapters are mammals, specifically carnivores and they tend to have some features in common. One of these is body size, they're neither too big or too small.

Small animals can't traverse the vast fragmented habitat of cities to find enough food, but big animals are often considered dangerous by humans and are eliminated. Plus they need more space to roam and hunt so you don't see many mountain lions waiting at your corner bus stop.

Another thing that successful adapters have in common is the ability to change their circadian rhythms, or daily biological cycles, to become more active at night than they were in the wild.

Both bears and coyotes are good examples of this. In the wild their counterparts are usually at home laying low at night but in the city night time is when you're free to look for food without running into a noisy human, which takes us to our third trait.

The best urban adapters also tend to be diet generalists, which just means they can adapt to use whatever food is available. Like it's kind of adorable that all giant pandas wanna eat is bamboo but they wouldn't last very long in London, or New York, or Mumbai because those cities don't have that one thing that they have a taste for.

But humans provide a great deal of food for animals that are willing to experiment. Urban raccoons, for example, have managed to make a living eating almost nothing but our trash.

Bears aren't above grazing through our garbage either and they love the fruit trees in our yards. And scavengers, like crows, are totally happy to eat roadkill; a source of calories that wouldn't be available if it weren't for us.

So when it comes to surviving the sixth mass extinction, biological flexibility seems to be the name of the game. And maybe it's not all bad news. The urban niche offers a pretty specific combination of both resources and threats so like any ecological niche, it'll benefit a very specific type of creature.

Some ecologists thing that the urban environment might even help provide a way for some species that are in trouble. Take the peregrine falcon. In the 1960s it was all but extinct in North America. But then it was introduced into cities like New York and Montreal where it found plenty of pigeons to eat and cliff-like building to nest in. And today, falcons are found nearly all over the continent including in our biggest cities.

As we reshape the world we change habitats. So as we keep building cities and replacing old habitats with new ones we might want to keep in mind that we're basically picking winners and losers in the next great extinction.

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