Deep Time Lags: Lessons from Pleistocene Ecology

Scientists involved in Gaian research—also known as geophysiology, Earth systems science, or whole-Earth science—as a matter of course provision their global climate and chemical cycling models with their best understandings of time lags inherent in Earth’s thermal and chemical reservoirs. For example, how long will it take the carbonic acid content of the world’s oceans to equilibrate with today’s (and tomorrow’s) elevated concentrations of carbon dioxide in the atmosphere? Time lags are just as important to understand for biodiversity preser-vation. New forms of population modeling help conservation biologists estimate the probabilities that a particular population (of any given size) of plant or animal will “wink out ” owing to fl uctuations in natural conditions—even if the population seems to be self-maintaining in the present. Such models have served as wake-up calls to conservationists that even stabilized populations of threatened species may be doomed to extirpation unless their numbers can be increased or corridors established to facilitate cross migration with neighboring populations. Another kind of time lag also impinges on biodiversity preservation. This time lag has come to the attention of conservation biologists, thanks to the work of those who specialize in Pleistocene ecology. In the late 1970s ecologist Dan Janzen, working in Costa Rica, began to suspect that his studies of seed dispersal in the large-seeded, fruit-bearing plants had gone awry. The studies were fl awed by the then-unexamined (and universal) assumption that dispersal candidates could include only those fruit- or seed-eating mammals that currently were native to the plant’s home range—or that had likely been there just prior to the arrival of Europeans in the Western Hemisphere. Janzen had previously concluded that several large-seeded tropical plants were dispersed by rodents who extracted and buried the seeds for later consumption. But when he Crist_10_Ch10.indd 165 4/22/2009 3:52:17 P