Lindberg and Pyenson initiated the study several years ago in the face of conflicting and contentious estimates for past gray whale populations. They thought that an understanding of how gray whales adapted to climate change over the past 3 million years, the period called the Pleistocene, might provide insight into how they will adapt to climate change today.
Since gray whales arose—the oldest fossils date from 2.5 million years ago—Earth has gone through more than 40 major cycles of warming and cooling, each of which significantly affected the world's flora and fauna. During the last glacial cold spell, between 50,000 and 10,000 years ago, most of the large terrestrial mammals disappeared through a combination of climate change and human depredation, Lindberg noted. The marine realm, however, experienced almost no extinctions and very few new originations during that same period.
The California, or eastern, gray whale, one of two surviving populations of gray whale, can be traced back about 150,000-200,000 years. Pyenson and Lindberg looked closely at only the past 120,000 years, during which Earth transitioned from a warm period to a glacial period and then to today's warmer climate.
During the glaciated period, ocean water became locked up in land-based glaciers, drawing down the sea level by about 120 meters, or nearly 400 feet. That drop eliminated nearly 60 percent of the Bering Sea Platform, a shallow area that is part of the continental shelf and the major summer feeding area for today's gray whales. Gray whales can engage in benthic feeding no deeper than about 75 meters (250 feet), Pyenson said, and during the glacial period, waters offshore of the Bering platform would have been much deeper than that.
"If gray whales were primarily feeding on the Bering Platform, it's hard to see how they could have avoided a population crash," Lindberg said.
By calculating the amount of food lost because of dropping sea levels, and combining this with estimates of the food needed to keep a whale alive, the two researchers calculated the impact of global cooling on gray whale populations and the populations that would have had to exist in order for the whales to survive.
They concluded that populations would have had to have alternative feeding modes sufficient to support a population of around 70,000 during warm periods so that population drops during glacial periods wouldn't be below 5,000-10,000 whales. Much lower numbers would have produced a genetic bottleneck obvious in the DNA of the whales, and such a signature has not yet been seen.
"We don't yet have the ability to look deep enough into the whale genome to see this type of bottleneck," Pyenson added, though genetic analysis that has been done shows no evidence of a bottleneck much shallower in time, just before humans targeted the mammals for whaling.
The carrying capacity of the North Pacific could have been as high as 170,000, "assuming modern day values for benthic productivity, food density, and gray whale energetics," the authors concluded. If gray whales also exploited non-benthic organisms, such as krill, the populations could have been even higher.
If gray whales do respond well to the rising temperatures and sea levels predicted for the future, that may not be true for the birds and other marine mammals that feed in the Bering Sea, one of the most productive marine ecosystems during the summer.
"If this environment disappears in glacial maxima, we really need to rethink what we know about the ecological history of all the other organisms that make a living in the Bering Sea," Pyenson said. He and Lindberg urge other scientists to focus on the historical ecology of species to fully understand their complex interactions with a changing environment.
"We really make a lot of conservation decisions without a lot of data," Lindberg said. "Integrating paleontological and geological data in the context of known ecological traits can help us address impending biological changes in marine ecosystems."