TATOOSH ISLAND, Wash. — From a stretch of rocky shoreline on this tiny island, one can, on any given morning, watch otters floating on their backs, elephant seals hauling out of the water and a bald eagle flying past murres huddled along a cliff face. The startled birds perform a synchronized dive into the sea, their ovoid black-and-white bodies resembling miniature penguins.


It appears as if the island’s wildlife is thriving at this remote outpost, which is also a former Coast Guard station crowned by a decommissioned lighthouse. It was also once a whaling base for the Makah tribe, who maintain treaty rights to the land.


But for over four decades, with the blessing of Makah leaders, Tatoosh has been the object of intense biological scrutiny, and scientists say they are seeing disturbing declines across species — changes that could prove a bellwether for oceanic change globally.


Cathy Pfister and Timothy Wootton, both biology professors at the University of Chicago, have been trekking to the island since the 1980s, often accompanying their former graduate adviser, Robert T. Paine, a nominally retired zoology professor from the University of Washington. At 79, Dr. Paine still returns to Tatoosh several times a year to continue the ecological research he began in the 1960s.


Dr. Pfister and Dr. Wootton met and fell in love while studying the island’s species with Dr. Paine. Now married, they often bring their two children on research trips, where the family sleeps in bunk beds in a one-room cabin, a former Coast Guard facility affectionately dubbed the “Winter Palace.”


On their frequent visits to the island, usually lasting several days each, the researchers haul duffel bags of clothing and equipment up a steep path cut into the rock until the landscape plateaus into a field of salmonberry bramble. It is no quiet retreat. The dull roar of the surf, the screeching gulls, the groaning seals and a distant foghorn all layer into a cacophony on the island. Even the mussel beds creak and crackle.


Among the declines the researchers are noticing: historically hardy populations of gulls and murres are only half what they were 10 years ago, and only a few chicks hatched this spring. Mussel shells are notably thinner, and recently the mussels seem to be detaching from rocks more easily and with greater frequency.


Goose barnacles are also suffering, and so are the hard, splotchy, wine-colored coralline algae, which appear like graffiti along rocky shorelines.


While not entirely understood, the declines are not entirely mysterious. Biologists suspect that the shifts are related to huge declines in the water’s pH, a shift attributed to the absorption of excess carbon dioxide being released into the atmosphere in ever-greater amounts by the burning of fossil fuels for energy.


As the carbon dioxide is absorbed, it alters the oceanic water chemistry, turning it increasingly acidic. Barnacles, oysters and mussels find it more difficult to survive, which can cause chain reactions among the animals that eat those species, like birds and people.


During a research trip in 2000, Dr. Pfister and Dr. Wootton first began testing the pH of water samples. They found the water around Tatoosh and along nearby coastlines to be 10 times as acidic as what accepted climate change models were predicting. Even after collecting seven years of data, when they published their findings in the Proceedings of the National Academy of Sciences in 2008, their data were met with skepticism.


“People think we just don’t know how to use the instrument — I still hear that,” Dr. Pfister said. “Luckily for our reputations, I guess, this has been corroborated by a lot of other people.”


It was on this island and a nearby mainland shore that Dr. Paine developed his keystone species hypothesis, which describes how top predators dominate an ecosystem, often to the benefit of species diversity.


Now, the island species are once again helping to solve important biological questions. Mussels appear to be succumbing more easily to crashing waves, with barren patches on the rocks growing larger and appearing with greater frequency, said Dr. Wootton, who carefully documents patch sizes at various island sites.


“We all agree, it just looks different,” Dr. Pfister said, pointing to a weedy-looking barren area near where Dr. Paine conducted his studies. More research needs to be done to definitively explain what is happening, she added.


While their parents are out counting barnacles or collecting water samples, the children, Anna, 9, and Ben, 12, lie in their bunks and read. Or they head out to try to catch a fish for dinner, hopping and skipping their way across slippery rocks and past several dark caves to a perch on the island’s north end.


What the researchers call “happy time” comes in the afternoon when the tides return and they gather — along with a rotating cadre of graduate students — outside the Winter Palace on an old dock laid out on the lawn to compare notes, gossip and have a snack.


Everyone gets a brownie, courtesy of Dr. Pfister, but only one — and everyone keeps close watch.


“Being a graduate adviser is like doing a second round of parenting,” Dr. Paine said.


He speaks of the calcareous sponges that live in the caves of Tatoosh and, like hard-shell species, use dissolved calcium carbonate, in this case to form their skeletons or spicules, thus making them vulnerable in more acidic waters.


“Almost nothing is known about this species,” Dr. Paine said.


“No one in their right mind has the time to sample calcareous sponges, let alone recognize them,” he added. “They’re likely to disappear.”


While some species may be able to adapt to new oceanic conditions, many will not.


“You can predict change,” Dr. Paine said, “and most of the changes are going to be in a direction we don’t want.”