Birds and Mammals
Readings in Nature
Collections Care and Conservation
Binational Multidisciplinary Expeditions
Symposia and Workshops
BRCC Staff Directory
The southern right whale, Eubalaena australis, visits Patagonia's Valdez Peninsula
each spring while the calves nurse.
Winds of Change
By Lynett Gillette Photographs by Fran?ois Gohier
The California coast has been a good place for whale watching for a long, long time. Three and a half million years ago, tall camels, wild horses, and primitive mastodons witnessed a cornucopia of cetaceans and other marine mammals visiting these rich coastal waters.
It would have been quite a sight. The long-legged camel (Titanotylopus) stops browsing on the hillside, startled at a young and enthusiastic breaching right whale. A gray whale yearling engages a sea cow baby in rollicking play near the kelp beds as indulgent mothers watch. Walruses nose the clam beds for dinner. A solitary albatross soars above it all, riding the trade winds, following schooling squid.
Fossils of marine mammals—birds, fish, and invertebrates from the sandstones of the Pliocene epoch’s San Diego Formation—reveal that these animals lived in a broad embayment that stretched from La Jolla, California, to Rosarito, Mexico, several miles inland of today’s shoreline. Toothed and baleen whales claimed the top of the ocean food chain; miniscule phytoplankton and zooplankton supported everything else.
Pliocene Climate Change
A long reign of very warm climate, lasting for millions of years, came to an end in the early Pliocene. Global sea levels had remained high as world temperatures averaged between five and fourteen degrees Fahrenheit warmer than today. Some paleoclimatologists describe that period as being much like El Ni?o events of modern times, when weak southeast trade winds fail to foster upwelling currents along the North and South American coast. Today, primary ocean productivity drops significantly when iron, calcium, manganese, and other minerals aren’t raised from the ocean bottom. In the early Pliocene these conditions seem to have been permanent features of the tropical Pacific. But in Earth’s history, nothing is permanently permanent.
While sea levels were still high and temperatures were warm, ice sheets began to expand inexorably at high latitudes in the Northern Hemisphere. About 3.7 million years ago, the Isthmus of Panama emerged above sea level, as sea levels dropped worldwide. Prior to that, the Central American Seaway separated North and South America and provided a direct passage between the tropical eastern Pacific and the Caribbean.
Trade winds and ocean circulation shifted. Valuable nutrients began to cycle upwards on the southern California coast. In turn, oceanic phytoplankton bloomed. New studies document a sudden increase 3.5 million years ago in fossil opal production from microscopic diatoms with their glassy exoskeletons. A new regime was well underway.
On the Move
As late Pliocene ice sheets continued to grow in the Northern Hemisphere, the climate cooled further. New species of cooler-water mollusks and foraminifera begin to show up in the rock record. Along with the walruses and sea cows, some whales coped with the cooling climate by beginning to shift their ranges into higher latitudes. Today beluga (Delphinapterus) and right whales (Eubalaena), and very closely related bowheads (Balaena), never enter into warm, low-latitude waters. Perhaps their considerably thick layers of blubber now make such migrations dangerous.
Old Habits Remain?
Gray whales now make the longest migration of any whale, leaving their northern feeding grounds each winter when ice pushes them south. They rear their calves 4000 miles away in the quiet lagoons of Baja California.
In spite of occupying new ranges since the Pliocene, modern right whales (Eubalaena) share the gray’s habit of seeking protected bays to give birth and nurse their calves. Did both these whales’ migratory habits begin with climate change in the Pliocene? Cooling temperatures forced major changes in another marine mammal’s habits. Sea cows (Hydrodamalis), distantly related to today’s tropical dugongs (Dugong), had once chewed gritty tropical sea grasses in Baja California and the tropical Caribbean. They evolved into giant, toothless kelp-eating sirenians with the capacity for surviving in cooler water. As the Pliocene ended and the polar ice sheets grew, the range of Hydrodamalis shifted from southern California further and further north. The last of their lineage, the Steller’s sea cows, suffered extinction at the hands of humans in the 1700s in Alaskan nearshore waters.
As climatologists strive to come up with more and more exact models of past climates, we begin to understand that humans have a direct effect, not just on the survival of other species, but on the pace of climate change itself, which now outpaces many animals’ adaptive abilities.
Questions are raised by today’s scientists—are we headed again into a climate regime like that of the warm early Pliocene? As ice sheets retreat, will upwelling stop again along the California coast? Which animals can adapt, which ones will become extinct? Will there still be any whales to watch a million years hence?
SAN DIEGO NATURAL HISTORY: FIELD NOTES, March 2007