Field Notes From a Catastrophe: Man, Nature, and Climate Change (9 page)

Wyeomyia smithii
completes virtually its entire life cycle—from egg to larva to pupa to adult—inside a single plant,
Sarracenia purpureaor
, as it is more commonly known, the purple pitcher plant. The purple pitcher plant, which grows in swamps and peat bogs from Florida to northern Canada, has frilly, cornucopia-shaped leaves that sprout directly out of the ground and then fill with water. In the spring, female
Wyeomyia smithii
lay their eggs one at a time, carefully depositing each in a different pitcher plant. When flies and ants and occasionally small frogs drown in the leaves of the pitcher plant—
Sarracenia purpurea
is carnivorous—their remains also provide nutrients for developing mosquito larvae. (
Sarracenia purpurea
does not digest its own food; it leaves this task to bacteria, which don’t attack the mosquitoes.) When the young mature into adults, they repeat the whole process, and if conditions are favorable, the cycle can be completed four or five times in a single summer. Come fall, the adult mosquitoes die off, but the larvae live on through the winter in a state known as diapause—the insect version of hibernation.

The exact timing of diapause is critical to the survival of
Wyeomyia smithii
and also to Bradshaw and Holzapfel’s research. In contrast to most insects, which rely on a variety of signals, including temperature and food availability, to regulate the onset of dormancy,
Wyeomyia smithii
depends exclusively on light cues. When the larvae perceive that day length has dropped below a certain threshold, they stop growing and molting; when they perceive that it has lengthened sufficiently, they take up again where they left off.

This light threshold, which is known as the critical photoperiod, varies from bog to bog. At the southern end of the mosquitoes’ range, near the Gulf of Mexico, conditions remain favorable for breeding well into fall. A typical
Wyeomyia smithii
from Florida or Alabama will, consequently, not go dormant until day length has shrunk to about twelve and a half hours, which at that latitude corresponds to early November. At the far northern edge of the range, meanwhile, winter arrives much earlier, and an average mosquito from Manitoba will go into dormancy in late July, as soon as day length drops below sixteen and a half hours. Interpreting light cues is a genetically controlled and highly heritable trait:
Wyeomyia smithii
are programmed to respond to day length the same way their parents did, even if they find themselves living under very different conditions. (One of the walk-in-freezer-like rooms in the Bradshaw-Holzapfel Lab contains locker-size storage units, each equipped with a timer and a fluorescent bulb, where mosquito larvae can be raised under any imaginable schedule of lightness and dark.) In the mid-1970s, Bradshaw and Holzapfel demonstrated that
Wyeomyia smithii
living at different elevations also obey different light cues—high-altitude mosquitoes behave as if they were born farther north—a discovery that today might seem relatively unremarkable but at the time was sufficiently noteworthy to make the cover of
Nature
.

About five years ago, Bradshaw and Holzapfel began to wonder about how
Wyeomyia smithii
might be affected by global warming. They knew that the species had expanded northward after the end of the last glaciation, and that at some point in the intervening millennia, the critical photoperiods of northern and southern populations had diverged. If climatic conditions were changing once again, then perhaps this would show up in the timing of diapause. The first thing the couple did was go back to look at their old data, to see if it contained any information that they hadn’t previously noticed.

“There it was,” Holzapfel told me. “Just hitting you right in the eye.”

When an animal changes its routine by, say, laying its eggs earlier or going into hibernation later, there are a number of possible explanations. One is that the change reflects an innate flexibility; as conditions vary, the animal disable to adjust its behavior in response. Biologists call such flexibility “phenotypic plasticity,” and it is key to the survival of most species. Another possibility is that the shift represents something deeper and more permanent—an actual rearrangement of the organism’s genetic code.

In the years since they established their lab, Bradshaw and Holzapfel have collected mosquito larvae from all over the eastern United States and much of Canada. The couple used to do the collecting themselves, driving across the country in a van equipped with a makeshift bed for their daughter and a miniature lab for sorting, labeling, and storing the thousands of specimens they would gather. Nowadays, they more often send out their graduate students, who, instead of driving, are likely to fly. (Getting through airport security with a backpack full of mosquito larvae is a process that, the students have learned, can take half a day.)

Every subpopulation exhibits a range of light responses; Bradshaw and Holzapfel define critical photoperiod as the point at which 50 percent of the mosquitoes in a sample have switched from active development to diapause. Each time they collect a new batch of insects, they put the larvae in petri dishes and place the dishes in the controlled-environment light boxes, which they call Mosquito Hiltons. Then they test the larvae for their critical photoperiod, and record the results.

When Bradshaw and Holzapfel went back to their files, they looked for populations that they had tested at least twice. One of these was from a wetland called Horse Cove, in Macon County, North Carolina. In 1972, when the couple had collected mosquitoes for the first time from Horse Cove, their files showed, the larvae’s critical photoperiod was fourteen hours and twenty-one minutes. They collected a second batch of mosquitoes from the same spot in 1996. By that point, the insects’ critical photoperiod had dropped to thirteen hours and fifty-three minutes. All told, Bradshaw and Holzapfel found that in their files they had comparative data on ten different subpopulations—two in Florida, three in North Carolina, two in New Jersey, and one each in Alabama, Maine, and Ontario. In every single case, the critical photoperiod had declined over time. Also, their data showed that the farther north you went, the stronger the effect; a regression analysis revealed that the critical photoperiod of mosquitoes living at fifty degrees north latitude had declined by more than thirty-five minutes, corresponding to a delay in diapause of nearly nine days.

In a different mosquito, this shift could be an instance of the kind of plasticity that allows organisms to cope with varying conditions. But in
Wyeomyia smithii
, there is no flexibility when it comes to timing the onset of diapause. Warm or cold, all the insect can do is read light. Bradshaw and Holzapfel knew therefore that the change they were seeing must be genetic. As the climate had warmed, those mosquitoes that had remained active until later in the fall had enjoyed a selective advantage, presumably because they had been able to store a few more days’ worth of resources for the winter, and they had passed this advantage on to their offspring, and so on. In December 2001, Bradshaw and Holzapfel published their findings in the
Proceedings of the National Academy of Sciences
. By doing so, they became the first researchers to demonstrate that global warming had begun to drive evolution.

The critical photoperiod for
Wyeomyia smithii
has declined markedly over time. Changes are most dramatic at higher latitudes. Credit: After W. Bradshaw and C. Holzapfel
, PNAS,
vol. 98 (2001).

The Monteverde Cloud Forest sits astride the Cordillera de Tilarán, or Tilarán Mountains, in north-central Costa Rica. The rugged terrain in combination with the trade winds that blow off the Caribbean Sea make the region unusually diverse; in an area of less than two hundred and fifty square miles, there are seven “life zones,” each with its own distinctive type of vegetation. The cloud forest is surrounded on all sides by land, yet, ecologically speaking, it is an island and, as is often the case with islands, it displays a high degree of endemism, or biological specificity. Fully 10 percent of Monteverdean flora, for example, are believed to be unique to the area.

The most famous of Monteverde’s endemic species is—or at least was—a small toad. Known colloquially as the golden toad, it was officially discovered by a biologist from the University of Southern California named Jay Savage. Savage had heard tell of the toad from a group of Quakers who had settled at the edge of the forest; still, when he came across it for the first time, on May 14,1964, at the top of a high mountain ridge, his reaction, he would later recall, was “one of disbelief.” Most toads are dull brown, grayish green, or olive; this one was a flaming shade of tangerine. Savage named the new species
Bufo periglenes
, from the Greek word meaning bright, and titled his paper on the discovery “An Extraordinary New Toad (Bufo) from Costa Rica.”

Since the golden toad spent its life underground, emerging only in order to reproduce, most of what was subsequently learned about it had to do with sex. The toad was, it was determined, an “explosive breeder”; instead of staking out and defending territory, males simply rushed the first available female and fought for the chance to mount her. (“Amplexus” is the term of art for an amphibian embrace.) Males outnumbered females, in some years by as much as ten to one, a situation that often led bachelors to attack amplectant pairs and form what Savage once described as “writhing masses of toad balls.” The eggs of the golden toad, black and tan spheres, were deposited in small pools—puddles, really—often no more than one inch deep. Tadpoles emerged in a matter of days, but required another four or five weeks for metamorphosis. During this period, they were highly dependent on the weather; too much rain and they would be washed down the steep hillsides, too little and their puddles would dry up. Golden toads were never found more than a few miles from the site where Savage originally spotted them, always at the top of a mountain ridge, and always at an altitude of between forty-nine hundred and fifty-six hundred feet.

In the spring of 1987, an American biologist who had come to the cloud forest specifically to study the toads counted fifteen hundred of them in temporary breeding pools. That spring was unusually warm and dry, and most of the pools evaporated before the tadpoles in them had had time to mature. The following year, only one male was seen at what previously had been the major breeding site. Seven males and two females were seen at a second site a few miles away. The year after that, a search of all spots where the toad had earlier been sighted yielded a solitary male. No golden toad has been seen since, and it is widely assumed that after living its colorful, if secretive, existence for hundreds of thousands of years,
Bufo periglenes
is now extinct.

In April 1999, J. Alan Pounds, who heads the Golden Toad Laboratory for Conservation in the Monteverde Preserve, published a paper in
Nature
on the toad’s demise. In it, he linked the toad’s extinction, as well as the decline of several other amphibian species, to a shift in precipitation patterns in the cloud forest. In recent years, there has been a significant increase in the number of days with no measurable precipitation, a change that, in turn, is consonant with an increase in the elevation of the cloud cover. In a separate article in the same issue of
Nature
, a group of scientists from Stanford University reported on their efforts to model the future of cloud forests. They predicted that as global CO
₂
levels continued to rise, the height of the cloud cover in the Monteverde Preserve and other tropical cloud forests would continue to climb. This, they speculated, would force a growing number of high-altitude species “out of existence.”

Climate change – even violent climate change – is itself, of course, part of the natural order. For the earth’s flora, the last two million years have been particularly turbulent; in addition to the glacial cycles, there have also been dozens of abrupt climate shifts, like the Younger Dryas.

Thompson Webb III is a paleoecologist who teaches at Brown University. He studies pollen grains and fern spores, in an effort to reconstruct the plant life of previous eras. In the mid-seventies, Webb began to assemble a database of pollen records from lakes all across North America. (When a grain of pollen falls on the ground, it usually oxidizes and disappears; if it is blown onto a body of water, however, it can sink to the bottom and be preserved in the sediment for millennia.) The project took nearly twenty years to complete, and, when it was finally done, it showed how, as the climate of the continent had changed, life had rearranged itself.

A few months after I visited Bill Bradshaw and Chris Holzapfel in Eugene, I went to talk to Webb in Providence. He has an office in the university’s geochemistry building, and also a lab, where, on this particular day, one of his research assistants was examining charcoal particles from an ancient forest fire. Webb took some slides from a cabinet and slipped one under the lens of a microscope. Most pollen grains are between twenty and seventy microns in diameter; to be identified, they must be magnified four hundred times. Peering through the eyepiece, I saw a tiny sphere, pocked like a golf ball. Webb told me that what I was looking at was a grain of birch pollen. He replaced the slide, and a second tiny golf ball swam into focus. It was beech pollen, Webb explained, and could be distinguished by a set of three minute grooves. “You see, they’re really very different,” he said of the two grains.