“Wow, you get river otters here?” exclaimed Abby as she scanned a sign near the ruins of the Sutro Baths at the Western edge of San Francisco.

I glanced at the temporary placard. “Don’t feed the wild animals,” it read, alongside a picture of a playful-looking river otter.

“There’s no way,” I said. I’ve only seen a river otter once in my life – a lucky glimpse from a kayak along a particularly isolated stretch of Southeast Alaskan coastline. I couldn’t imagine such a shy animal tolerating all the commotion of city life and tourist foot traffic. “They probably ran out of signs with sea lions.”

Then, two days later, I stumbled across a news article about Sutro Sam, a genuine river otter (I clicked through the pictures to be absolutely sure) recently seen frolicking around the concrete bath pools. I ate crow, forwarded the article to Abby, and made a mental note to spend a few hours next weekend staking out the Baths in hopes of catching a glimpse of my own.

We have known for a long time that some animals tolerate humans remarkably well. One has only to witness the daily standoffs between dockworkers and sea lions around the corner at Pier 39 to know that at least one marine mammal has no problem claiming human structures for itself. Some species – pigeons, cockroaches, rats, and even the deer of suburbia – seem almost pre-adapted for existence in the built environment, where they are so successful that they have become pests. Others eke out an uneasy coexistence at the expanding edges of human development, like the Los Angeles coyotes that occasionally devour a purse dog out of sheer desperation.

Still, we get a pleasant thrill of surprise whenever we glimpse a bit of wildness in our urban enclaves, like Sutro Sam, or the red-tailed hawks of New York’s Central Park. In part, we’re recognizing that rusty but eternal link between ourselves and our evolutionary past. And in part, we’re responding to our social history – our visits to zoos and viewings of nature documentaries that taught us that such experiences are a rare privilege.

For me – and possibly Abby, nerdy biologists that we are – much of the excitement stems from awe at the adaptability of life. Perhaps I’ve grown too anthropocentric, come to feel too unique as I stumble about on my two legs, but I’m continually struck by the variety of forms and behaviors that life can adapt to tolerate environmental stress. From the trees sprouting in the gutters of houses to the cattle egrets that follow industrial lawnmowers instead of herds of cows, as Dr. Ian Malcolm told us in Jurassic Park, “Life finds a way.”

Ecologists, of course, have an elaborate framework of technical terms to describe and categorize such phenomena. At the leading edges of human activity – which seem to become more remote with each passing day – we concern ourselves with “resilience:” the ability of an ecosystem (and its organisms) to tolerate human disturbance. And at the trailing edges – places like the Sutro Baths, where our structures are crumbling into the sea – we wonder about “recovery,” asking who will return, and when.

After a few centuries of increasingly industrialized civilization, and a few decades of increasingly meticulous science, the closest we’ve come to a conclusion is that the story is complicated and heavily context-dependent. We do know that every ecosystem has its breaking point. And long before that breaking point is reached, some irreversible transitions may be made: the local extinction of a particular species, a hydrological transformation, or the introduction of a new community member. And, while life will persist long after humans depart (I have infinite confidence in our silent microbial overlords), it may no longer take on the forms that we associate with “nature” and “beauty.”

The point, then, is that while Sutro Sam may already be cruising the outskirts of San Francisco, we should exercise what restraint we can in our future environmental modifications. Although we are inarguably born from and a part of nature, we have outstripped the planet’s ability to support us. If we value the preservation of our evolutionary past, rather than playing guessing games about environmental tolerances, we should eschew development on pristine landscapes wherever possible. Where we must build, let us tread lightly, to maximize the chances of rapid recovery.

Meanwhile, I’ll be watching for river otters along my running routes. It’s a New Year: a time for new starts, and new meetings in novel environments.

Holly welcomes reader comments, feedback and reports of weasel family sightings at [email protected].