In this era of the omnivore’s dilemma, we present you with a choice devoid of complications: oven-roasted branzino with olive oil and lemon, or an albino alli­gator basking on a rock. Both are offered under the living roof of the California Academy of Sciences in San Francisco, but only one pairs well with a biody­namic pinot grigio.

Since consuming the white gator would violate the spirit (not to mention the substance) of the academy, which houses the reptile as part of its tribute to the wonder and fragility of Mother Nature, we recommend the flaky, mild branzino. It’s served at the Moss Room, a restaurant that reminds us that our planet, though depleted, remains home to creatures we can cull without guilt.

Getting to the restaurant, which serves lunch and dinner (the latter after the museum is closed), feels less like the start of an evening out than like the first step in an act of eco-tourism. From the parking lot (shame on you for driving), you make your way along unmarked paths, through a side door, into a gloomy room aglow with a tropical aquarium, and down a staircase beside a wall covered with moss. Water trickles down the wall, much as it might on a spelunking journey, and spills into a pool stocked with Southeast Asian river fish. Like the albino alligator, those fish are not intended for your dining pleasure. Loretta Keller has less endangered items in mind.

The well-established chef, who showed her versatility a few years back by transforming rustic Bizou into cocktail-happy Coco500, has managed to adapt her approach once more, this time to a warming climate. Keller’s menu, carried out by executive chef Justin Simoneaux, reads like a culinary clause in the Kyoto Protocol: The seafood is sustainable, the produce local, the beef grass-fed, and the poultry free-range. There’s no Chilean sea bass, but you will find Spanish mackerel, a PC option whose population in the wild appears to far outstrip the number of diners who claim to like it. The kitchen has the courage to braise it in tomatoes (with mint, preserved lemon, and olive oil) in a manner that retains the mackerel’s oily richness while tempering the intensity of the fish. It’s a bold play, and the Moss Room pulls it off.

Not that dinner here is a feat of derring-do. The flavors are familiar (turnip soup tastes just as you’d imagine, its earthy origins lightly enhanced with cream), and the seasonings are subtle. Grilled Monterey squid, entangled with chickpeas and Swiss chard, rises to the gentle heat of jalapeño pesto. Grilled bavette steak, smoky from cumin-hinted charmoula vinaigrette, lies unfussily in a tender harvest of watercress. If Darwinism applies to dining rooms, this dish should be around for a while.

By now, all but San Francisco’s most benighted Big Mac addicts have become acquainted with the Slow Food ethos—and the Moss Room, to its credit, doesn’t browbeat its clientele. Its conservationist bent is apparent (tap water instead of bottled still water, a wine list divided under headings like sustainable, organic, and biodynamic), but not overbearing. You get the message of eating well and leaving leftovers for the grandkids, but it comes across as sensible instead of shrill.

If anything, the restaurant could be brasher, at least in what it puts on the table. The food is sometimes exceedingly timid, as if by treading lightly on the planet, the chefs are treading too lightly across the plate. A subtle touch works with certain dishes, like that whole-roasted branzino, whose delicate meat is best when unadorned, or a grilled-persimmon salad (with chopped endive, radicchio, dates, and pomegranate) that depends more on freshness than on technique. But even the best ingredients can benefit from intervention: On my visits, thin duck sugo on ricotta cavatelli failed to dive to any depths of flavor, while salt-cod fritters suffered from a stubborn bout of blandness—too much fried coating, very little fish.

The service, however, never stumbles. The quiet expertise of the staff perfectly matches the tranquil setting, with its soft acoustics, golden lighting, and the soothing sound of water rushing down that wall of moss. A friend of mine complained about the res­taurant’s isolation—too hard to find in a darkened park, too displaced from the bustle of the city—but that remove adds to the beautiful sense of place. The Moss Room feels like a kind of underground Eden.

The Academy of Sciences, of course, also provides context. The ultragreen building hosts exhibits that point out the costs of our consumption, complementing a restaurant that asks patrons to be conscientious without demanding that you count your carbon credits—or, for that matter, your calories. Desserts include luscious buttermilk ice cream, followed by pear-ginger custard with a fancy name. The ice caps are melting, just like your ice cream, and you’re partly to blame. But you can beat yourself up about that later. On this night in the Moss Room, you can enjoy your clafouti without sticking future generations with the bill.

The Moss Room: California Academy of Sciences, 55 Music Concourse Dr., 415-876-6121, S.F., $$$, reservations recommended, wheelchair accessible, 2.5 stars