Habitat Tracking

The tendency for species to change their geographic ranges in response to climate and other forms of physical change on the earth's surface is known as habitat tracking. Habitat tracking in effect confers stability in times of change, for as long as a species can locate suitable habitat—which means habitat to which it is already adapted—it will tend to persist.

At the opening of the nineteenth century, when the natural sciences such as geology and biology were still very much in their infancy, most scholars saw a world of stability— one that, they thought, had been created only a scant 10,000 years earlier. Not only was stability the norm, but there simply had been no time for much change in the earth or its living species. But the science of geology began to change those traditional assumptions; the great physician/farmer James Hutton, who essentially founded geology, said that he saw "no vestige of a beginning, no prospect of an end," but rather an endless cycle of moun tain uplift, followed by slow but inexorable erosion, then uplift again. He saw vast vistas of time when change, far from being possible, was instead inevitable. His intellectual successor, Charles Lyell, took Hutton's lessons and applied them systematically to the earth. Lyell established the Principle of Uniformity ("Uni-formitarianism"), which said that the same processes we see operating around us today were in operation throughout geologic history. Storms, volcanoes, earthquakes—events producing large-scale effects rather suddenly— could be added to the list along with the slower actions of rivers wearing down hillsides and bringing their sediment to the sea, there to build up into thick piles that would, as Hutton saw, one day be uplifted anew into fresh mountain chains.

Lyell was able to show that a series of hills in France were actually the remnants of extinct volcanoes. The Swiss naturalist Louis Agassiz was instrumental in showing that many of the topographic features of his homeland must have been shaped by sheets of ice vastly more extensive and thicker than the glaciers that still cling to the sides of the higher Alpine peaks.

Darwin was fascinated with all the mounting evidence that the earth had undergone great changes since its inception and believed with many of his fellow naturalists that considerable spans of geologic time must have elapsed for all such changes to take place. Darwin, of course, had a deeper motive for wanting to see deep geological time established as a fact: his theory of evolution by natural selection, in which he saw changes in species slowly and gradually accumulating as the world itself changes with passing time, actually demanded a very old age for the earth and its living species. His chapter on geological time in his epochal book On the Origin of Species (1859) was perhaps the boldest and most creative attempt to show that the earth is not mere thousands, nor even a few millions of years old but is in fact hundreds of millions of years old. As we now know, Darwin and Lyell were quite right: the earth is 4.65 billion years old, and life goes back as far as at least 3.5 billion years—in the form of tiny bacteria that are the oldest fossils so far discovered.

Thus everything was in place for Darwin: he saw a very old earth that had a long history of complex transformation—just the backdrop required for natural selection to produce an equally long and complex history of change in the species of earth. Natural selection, Darwin thought, would simply track the climatic and geologic changes the earth was undergoing, slowly changing the adaptations of organisms to keep them matched up perfectly with their environments. If, during the great ice ages, it grew colder, then certain mammals that we associate nowadays with the tropics—elephants and rhinoceroses, for example—would adapt by evolving dense, furry coats. And, of course, there were wooly rhinos and mammoths during the ice ages. Everything seemed to fit: the earth and life were far from stable, but rather were subject to constant change as geological time went on.

But nineteenth-century naturalists, lacking the detailed paleontological and even long-

term neontological (that is, modern species) studies, didn't realize one simple fact: that, when faced with events that happen relatively slowly, such as global cooling events and the growth of continental glaciers, it is not so much natural selection that tracks the change by modifying organisms to suit the changed conditions; rather, it is species themselves that move to, for example, warmer climes via habitat tracking. True, some species remain around the glacial ice fields, adapted to the new colder conditions; but these new mammals of the tundra evolved rather quickly, just under a million years ago at the dawn of the second of the four major glacial advances of the Pleistocene Epoch (Ice Age) that started 1.65 million years ago.

But most of the rest of the world's species retreated southward during the glacial advances. Even plants can "habitat track," for though a rooted tree or bush of course dies when it gets too cold (or when the ice covers it!), nonetheless plants are adapted to disperse their seeds. Thus entire plant communities move south— with the tundra ringing the margins of the advancing ice sheets, and the northern forest advancing southward ahead of it, the mixed hardwoods even further south than the northern forest. This is not to say that these large-scale plant communities simply pick up and move smoothly southward toward the equator when things get colder, and just as smoothly beat a retreat back north when the temperature warms up: nature isn't quite that smooth or simple. But as individual species track favorable habitat (each plant species needing a special combination of soil types and chemistry, water, nutrients, and temperatures, for example, not to mention the right pollinators if they depend on insects rather than the wind for reproduction), the same basic community types keep assembling as species keep on the move in the face of environmental change.

Paleobotanist Margaret Davis has become famous for the work she has performed with colleagues and students plotting the movements of plant species as the Pleistocene ice sheets waxed and waned. And paleoento-mologist G. R. Coope has done the same for beetles in Europe—in one instance finding an Ice Age beetle that was living during one of the warm periods along with tropical species such as lions and hippos in what is now London's Trafalgar Square, alive and well and living in the hot climes of the southern tip of Italy.

Thus habitat tracking is real, and it helps keep species alive in the face of extinction; and species tend to survive unchanged by evolution, as they simply move to habitats to which they are already well adapted. Most of the marine invertebrate species on both coasts of North America simply moved up and down the coasts as climatic conditions (including changes in sea level) kept changing during the Pleistocene. Not only did most mollusks and other marine invertebrate species avoid extinction then, but they also remained pretty much unchanged from their condition when they first evolved, in most cases well before the Pleistocene began.

Nowadays, global warming is causing many species to track to the north. Cardinals and tufted titmice, for example, are species of birds that had been restricted to the Southern states before 1800 but are now year-round denizens in New York City and indeed even much farther north. When the study of natural history was in its infancy, the changeability of species distributions was not documented—nor was it expected in the exciting days when old notions of stability were being replaced by ideas of evolutionary change of both the earth and of life. But patient study has revealed the reality of habitat tracking—and its importance in understanding how species can remain so puz-zlingly stable even in the face of massive envi ronmental change—the phenomenon known as "stasis."

—Niles Eldredge

See also: Darwin, Charles; Evolution; Extinction, Direct Causes of; Geological Time Scale; Hutton, James; Ice Caps and Glaciers; Lyell, Charles; Natural Selection

Bibliography

Darwin, Charles R. 1859. On the Origin of Species. London: John Murray; Eldredge, Niles. 1989. Macroevo-lutionary Dynamics. New York: McGraw Hill; Eldredge, Niles. 1999. The Pattern of Evolution. New York: W. H. Freeman; Futuyma, Douglas J. 1997. Evolutionary Biology. Sunderland, MA: Sinauer; May-nard Smith, John. 1993. The Theory of Evolution. Cambridge: Cambridge University Press.

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