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Last week, my dad and I had yet another conversation about privacy. It makes him nervous to consider what gets broadcast where and stored away by whom on the World Wide Web. Chances are, your parents feel similarly about the explosion of tell-all networking sites and one-click shopping pages that save your credit card info and life history details. Meanwhile, many of us don’t bat an eye when asked to supply birth dates and cell phone numbers, while a string of relationship dramas play out across our Facebook walls.

There’s a term for this inter-generational switch in sensibilities: shifting baselines. Because you and I grew up in a virtually privacy-free reality, we’re much more relaxed about our personal information than previous generations, who came of age when “normal” meant sharing less.

However, both you and your parents probably agree that lobster makes for a classy meal, but eel isn’t fit for the holiday table — even though neither of those beliefs were in vogue a few generations ago. That’s because shifting baselines apply to our environmental perspectives, too.

Indeed, the phrase was coined by a fisheries scientist: Daniel Pauly first wrote about the phenomenon in 1995 to describe how our forgetfulness drives overfishing. Here’s the troubling, but typical, timeline. Fish X swims its native waters in high numbers. Life is good. Then, someone discovers that it is delicious and — because it is plentiful — cheap. X’s popularity skyrockets: it graces every menu and dinner table. But then X becomes overfished, and populations fall. Prices rise, and the new premium imparts an air of exclusivity, driving fishermen to scoop up the last few individuals for exorbitant profit. Meanwhile, for those who can no longer afford it, the fish simply fades off the map: it is no longer part of daily existence on the dinner plate, or present in local streams and rivers. Fishermen become acclimated to smaller catches, then no catch at all. Eventually, the once-popular food is forgotten altogether; even conservation plans don’t consider X. It is a species lost in time.

On the East Coast, that’s the story of shad (fishing boats used to haul four million pounds out of the Hudson River each year; now we just pull out the occasional airplaneful of people), salmon (ever eaten an Atlantic salmon that wasn’t farm raised?), and even eel (yes, it was once part of the Christmas spread). We’ve since turned to new targets. Animals that used to be considered “trash fish” fit for only the poor or prisoners (think lobster in the 1880s and abalone around 1900) have become expensive delicacies.

The phenomenon isn’t exclusive to the ocean. Picture Hawaii. Chances are, images of palm trees and pineapple spring to mind. But these species aren’t native to the islands: they’re just two of the thousands introduced over the course of human history. Today, “native” Hawaiian ecosystems survive in a few isolated places at high elevations. Most of us never see them, so our baselines consist of coconut milk and guava juice.

For an example that hits closer to home, think of our own treasured Northeast forests. Today’s leafy patchwork may be beautiful, but it’s nothing like the dense woodland that was first clearcut to make way for colonial cropland. Or consider the Great Plains to our West, which once teemed with bison, and today hold corn, wheat, and highways with 70mph speed limits.

If we’re lucky, some of these historic baselines are immortalized in history textbooks, sketches, and photographs. Still, they have taken on the air of myth and legend, of tall tales and exaggerations told by the old to impress the young. We have a hard time believing such things once really existed because it means denying our own baseline, the conditions we’ve experienced ourselves.

This is a huge problem in conservation because present-day improvements lack historical perspective. In the Northwest’s Columbia river, where wild salmon do still run, the fish population has doubled since 1930. But this seemingly monumental improvement is considerably less profound when we remember than the 1930 baseline represents only 10 percent of the historical population. Still, locals describe the salmon conditions as “better than ever,” which is admittedly true in their personal experience.

Unfortunately, it’s in our nature to forget history in favor of the evidence of our own experience. How many times have your parents rolled their eyes as you made the same mistake they warned you about because, well, you just had to “see for yourself”? How often do you dismiss old-timer stories about fish of monstrous proportions because you’ve never seen a fish half that size yourself? In China, the fishermen of our generation have already forgotten about the Yangtze River dolphin, the charismatic species that generated international protest against dams and pollution. The dolphin probably went extinct within only the past ten years.

Putting “beautiful”, “natural”, and “normal” into context is extraordinarily difficult in the face of changes that develop over decades and centuries. Our expectations are reduced as our collective memories fade: history becomes myth becomes forgotten. Meanwhile, our children spend less and less time outdoors, so they place diminished value on the natural world. Our wild places shrink and disappear, leaving science and society with fewer and fewer opportunities to experience at least somewhat authentic baselines.

Of course, even if our memories were perfect, we couldn’t turn back time and restore ecosystems to pristine states. We have too many mouths to feed to hand the bison back their entire range, too many dams for power and water to let spawning fish run free. And the inexorable forces of evolution and, lately, climate change, ensure that nothing can remain frozen in time.

Still, calibrating our baselines is a good idea. Go see as much “wilderness” as you can. Marvel at its beauty, but also learn some of its history. Imagine how the place must have looked before humanity came to play. Listen, really listen, to the stories of the past: are they so unbelievable if you remember how much the world has changed? Tell your own stories, and keep telling them, to your children and their children after them. Make sure we remember the wealth of nature around us, but also understand what it was once worth.

Here’s one for table talk at your next $50 surf-and-turf dinner. It begins, “Did you know that lobster used to be prison food because no one else would eat it?”

Holly Moeller is a graduate student in the MIT/WHOI Joint Program in Biological Oceanography. She welcomes reader feedback at hollyvm@mit.edu. “Seeing Green” runs on alternate Tuesdays.