Catastrophes and Charr
Alaska, some scientists claim, is catastrophe country. Glaciers grind hills into valleys, and volcanoes spew enough ash to fill them up again. Earthquakes sink whole beaches in some places and uplift others into cliffs. Between extreme climate and unsettled terrain, Alaska makes for a high-risk homeland for living things.
Halfway around the earth, another bit of the north could challenge us for catastrophes. The island nation of Iceland sits astraddle the Mid-Atlantic Ridge, a great seam in the earth's structure. The sides of this seam are lurching apart, letting new molten rock well up from within to fill the gap. Most of this activity takes place far beneath the Atlantic Ocean, but at Iceland the ridge rides the surface. So here too, volcanoes spew and glaciers grind over quake-raddled ground.
All this activity helped form Iceland's largest lake, Thingvallavatn. "Thing" here doesn't mean Whatchamacallit; in Icelandic, the word is used in its very old sense of "assembly" and Thingvallavatn is the lake near the site of Iceland's first parliamentary assembly, which was held in the year A.D. 930. Iceland's parliament still governs the country. In 1974, on the 1100th anniversary of the founding of Iceland, the president of the parliament commissioned an ecological investigation of Thingvallavatn.
With that much history behind them, Icelanders logically are more patient than Alaskans. The researchers have been working steadily on Thingvallavatn, and during 1992 finally published summaries of their work.
For specialists, the long, intensive study turned up many interesting points, but most amateurs would be caught by a fascinating question: Has the Thingvallavatn research captured a species-creating process under way? For, in the Icelandic lake, Arctic charr (otherwise called char, or even Dolly Varden) have diverged so much from a common type that, to a casual observer they might seem to be four different kinds of fish--and the lake is less than 10,000 years old. That, in evolutionary time, is an eyeblink.
Thingvallavatn's charr are all of the same species, but they represent four different combinations of body type and hunting behavior. One small sort lives off bottom-dwelling animals, as does one large kind. Another small-size charr hunts the littlest animals found swimming free in the lake, the zooplankton. The big form of the open-water hunter is a cannibal, eating young charr as well as the brown trout and sticklebacks that share the lake.
Technically speaking, the charr representing different body types and occupying different places in the ecosystem are considered to be different morphs. The scientists studying the fishy portions of Thingvallavatn's system had never seen this kind of diversification in charr, and they immediately fell into enthusiastic debate about why the different morphs should have appeared. One likely explanation was that the fishes came from different ancestral stocks that already showed some of the diverse features exaggerated in their many-generations-removed offspring. However, examination of the fishes' DNA showed that they had a common ancestry.
The possible explanation now most favored among the scientists seems ironic: they think the different charr morphs arose because the lake offered an unusually stable habitat. Odd though it sounds for catastrophe country, it seems to be true, thanks to a fluke of local geology. About 90 percent of the water entering Thingvallavatn has percolated through the surrounding lava fields, and the water flow, quality, and temperature end up being remarkably constant year around---features virtually unheard-of in a subarctic lake. Living in this unchallenging environment, the charr could specialize; they didn't need to preserve a safe, good-for-all-purposes set of features and behavior.
Perhaps by Iceland's 2200nd birthday, scientists will know if the charr morphs have become different species---if a local catastrophe doesn't wipe out their stable home in the meantime. It may be Iceland's largest lake, but it covers only 32 square miles.
