Splendid Isolation in Thingvallavatn
Campbell, David G., Natural History
Our planet has been graced by life for at least three and a half billion years. Today, most places have a long and convoluted history of interactions with living things. Even the frozen interior of Antarctica hides a medley of fossils. But the earth is not entirely without newborn places. One is in Iceland, where the Mid-Atlantic Ridge rises above the surface of the sea and traverses the country from south to north, splitting the land asunder. Here, where two tectonic plates pull away from each another, magma is extruded through the earth's crust, pushed up through an uneasy suture between the plates. Iceland's rifting activity produce some of the youngest rocks on earth, a few millennia--in some places, just a few years--old. (Millions of years from now, these flows will be swallowed by subsidence, as one tectonic plate slides beneath another, in a world that will have oceans and continents diferent in shape from those of today and that, undoubtedly, will have long forgotten humans.)
The centerpiece of this youthful land is Thingvallavatn in southwestern Iceland. This clear and deep lake was born ten thousand years ago in a valley gouged from volcanic rock and ash by Langjoekull Glacier, which today has retreated twenty-four miles to the north. Thingvallavatn--Iceland's largest lake--is one of the most thoroughly studied bodies of water on earth, thanks to a monumental effort by a team of more than a dozen scientists of various disciplines, led by Petur Jonasson, of the Freshwater Biology Laboratory, University of Copenhagen. These researchers have found that as a result of its youth, its isolation on an island away from the continents, and its high latitude, Thingvallavatn is impoverished in animal species. It supports only three species of fish: three-spined stickleback, brown trout, and arctic char. Cut off from the ocean, it was never colonized by Atlantic salmon or by European and American eels (whose ranges overlap elsewhere in Iceland). Other vertebrates that in varying degrees depend on the lake include the common loon, the red-breasted merganser, the arctic tern, and an introduced species, the mink.
Young ecosystems like Thingvallavatn are the delight of ecologists because their simplicity helps elucidate fundamental principles of ecology and evolution. The chars of Thingvallavatn have been of special interest to evolutionary biologists because in only a few millennia, they have diversified from a common ancestor into a splendid array of four varieties, or morphs; all, however, are still members of the same species. (Icelanders have long recognized at least two morphs. Fish-eating chars were sought by the Vikings, who used baited hooks on hand lines dropped through holes cut in the ice. By the eighteenth century, Icelanders were using woolen nets to capture the murta, a plankton-eating variety of char whose pink, firm flesh tastes to me of fresh-ground walnuts.)
Arctic char (Salvelinus alpinus) is a notoriously plastic species. Northern lakes in Canada and Europe commonly have two or three varieties of this relative of trout and salmon, but Thingvallavatn is the only lake to support four. To understand why the chars of Thingvallavatn have diversified to such a degree, we must first understand this rift lake's short, but dynamic, history, which distinguishes it from most ofits counterparts in the Arctic.
Thingvallavatn is a flooded graben, a piece of the earth's crust stuck in the middle of a fault zone. As the rift widens, the graben sinks. Today the bottom of the lake subsides an average of four or five millimeters a year. Since its formation, four separate volcanic eruptions have flowed into its basin, resulting in a variety of terrains: a shallow, tapering shoreline, tiered shelves of lava on its flanks, and a relatively flat bottom strewn with boulders. Much of the lake's shoreline consists of lava, with stones and boulders strewn on top, although there are a few sandy beaches and pockets of wetlands.
Turbid from waterborne sediments, subarctic lakes of glacial origin are often robin's-egg blue or chalky white--strangely beautiful, but nearly devoid of life. Thingvallavatn, by contrast, is extraordinarily transparent. Soon after Langjoekull Glacier retreated, fresh lava flows blocked the direct passage of meltwater from the glacier, forcing it to soak into the earth. Today up to 90 percent of Thingvallavatn's source is spring water, percolated through basaltic dunes and lava fields and therefore filtered of its sediment. The water takes fifty to sixty years to make the journey, and by the time it enters the striated rifts at the north end of the lake, it is air-clear. The lake is more than 370 feet deep at its deepest point. Light may penetrate 230 to 260 feet into the lake, depending on the time of year, and overall primary productivity--the biomass produced by photosynthetic phytoplankton and the beds of algae that coat the shallow rocks--is generally high. The bounty of the lake is especially apparent during the ever-bright arctic summer, when the air erupts in clouds of uncountable midges and stoneflies and the water dances with small crustaceans, such as cladocerans and copepods.
Their sides flecked with gold or shot through with silver, Thingvallavatn's four variously colored morphs of arctic char move through the clear waters of the lake like living rainbows. Adult morphs vary considerably in size: from three inches (the smallest mature males of the small bottom-feeding char) to twenty inches (the largest piscivorous char caught in the lake). Two of the morphs, one large and one small, are benthic, keeping to the bottom most of the time. These benthic morphs have subterminal mouths (positioned slightly beneath the tip of the fish's blunt "snout") adapted to extract food from the lake floor. Both are dark, camouflaged for living in the somewhat dusky depths, and feed principally on snails along the stony lake bottom near shore. The benthic morphs are partly separated by habitat: the small one stays mainly among the stones, plucking snails from crevices between and under the rocks; the large one seeks out snails on the rocks' upper surfaces.
Two other morphs are largely limnetic (living and feeding in open water). These two have pointed "snouts" and terminal mouths ideal for snatching prey from the open water. Both are silvery for most of the year, blending in with the sunlit surface of the lake. One is a fish eater that inhabits both open water and shallow areas near shore, where it hunts among the algae for sticklebacks and, for that matter, any other fish smaller than it is; the second is the murta, a pelagic char that feeds on small crustaceans and aquatic insects, especially the pupae of the abundant midges.
If each morph of char had originated in isolation from the others, and each had in turn independently colonized Thingvallavatn from a neighboring river or lake, their coexistence in the lake would be unremarkable. This process is known as allopatric differentiation, the divergence of sibling groups that have been geographically isolated from one another and have come to share a habitat only after differentiation has occurred. It was long assumed that speciation almost always took place in this manner. The origins of this dogma are no surprise: some of the most evocative early clues to species evolution came from archipelagos--Charles Darwin's insight in the Galapagos, for example, or Alfred Russel Wallace's in Malaysia--or from isolated populations, such as the varieties of domestic animals and plants that have been fostered by selective breeding. Yet all the evidence so far indicates that the four char morphs in Thingvallavatn evolved within the lake's watershed, perhaps within the lake itself. Even more intriguing, their genesis must have been rapid, within the 10,000 years since the lake was formed.
The most likely explanation for Thingvallavatn's char morphs is that they are an example of resource polymorphism. As morphs radiate from an ancestral form to exploit different components of a diverse and open environment, such as Thingvallavatn, each becomes proficient at exploiting the resources of a particular habitat. Eventually, the morphs evolve their own pattern of development, growth, morphology, and behavior. Together, these characteristics determine a distinct ecological niche for each of the morphs and minimize competition between them.
Resource polymorphisms occur in a variety of vertebrate species. The African seed-eating finch (Pyrenestes ostrinus), for example, has diverged into two morphs, one with a broad bill for crushing hard seeds and one with a narrow bill for opening soft seeds. The two morphs of the oystercatcher (Haematopus ostralegus), a mud-waddling denizen of many of the world's shorelines, are distinguished by behavior: those that use their bill as a wedge to open mussel shells and those that bash shells open. The spadefoot toad (Scaphiosus multiplieatus) of the American Southwest produces two kinds of tadpoles: cannibals, adapted to living in rain puddles where rapid growth is an advantage, and omnivores, adapted to more permanent ponds where there is less pressure to mature quickly.
Morphs that develop this way may or may not become reproductively isolated, that is, they may or may not diverge enough to stop interbreeding and then, perhaps, evolve into separate species. Sometimes differences may be the result of nonheritable variations in development that are induced by the local environment. If this were the case in Thingvallavatn, one would expect to find intermediate forms among the char morphs. Jonasson, Odd Terje Sandlund, Sigurdur Snorrason, and their colleagues examined such variables as larval and adult morphology, spawning behavior, internal parasites, and habitat and food preferences of the four chars. Their analysis, however, turned up no such intermediate forms. In the case of the Thingvallavatn chars, the morphs, while genetically very similar, do indeed appear to be reproductively isolated.
But what is it that prevents the morphs from interbreeding and ultimately disappearing? One possible explanation is that hybrids are less likely to survive and produce offspring than are the more efficient, specialized morphs. Selection, therefore, favors the specialists. Unfortunately, this idea is hard to demonstrate in the wild: the mere existence of distinct morphs does not prove that hybrids have been selected against. Another possibility is that characteristics that differentiate resource use--for example, habitat or diet--contribute to reproductive isolation. With their subterminal mouths, benthic chars are well equipped to feed on the lake bottom but not to snatch animal plankton and fish from the bright lake surface, where their limnetic relatives dwell. Natural selection favors benthic chars that linger on the lake bottom and tend to breed with other bottom-dwellers. Fish-eating chars also venture into the lake's shallows. However, their diet of sticklebacks may keep them among the algae and thus limit their opportunities to interact and interbreed with the snail-eating benthic chars.
Whatever the mechanism of differentiation, fish in new, relatively empty lakes such as Thingvallavatn appear to evolve in a predictable sequence: first into limnetic and benthic morphs--a pattern observed, for example, in whitefish and sticklebacks--and then, as we see in Thingvallavatn chars, into more specialized forms. This pattern has been observed in pumpkinseed sunfish, whitefish, and several other populations of arctic chars. Over time, ecosystems change in complexity and structure. But do they evolve according to rules? The resource polymorphisms in freshwater fish suggest that ecosystem evolution is, to a degree, deterministic--a controversial and, to some, disturbing idea in evolutionary biology, in which so much is attributed to contingency and chance. Thingvallavatn teaches us that there may be certain limited ways in which pieces of an aquatic community may bejoined.
But the lesson may go unfinished. The land and lakes of Iceland are transitory. The seasons, the plants, animals, even the rocks themselves, will change. Thingvallavatn exists momentarily by the measure of geological time, during this respite between ice ages. The char morphs may never have the opportunity to develop into full species. The glaciers will return; the lake is doomed by the same forces that created it. In the near future, Thingvallavatn may lose its clarity and become opaque with sediment. It will become a lake like all the others nourished by silty glacial meltwater, beautiful to behold but denied life's bouquet.…
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Publication information: Article title: Splendid Isolation in Thingvallavatn. Contributors: Campbell, David G. - Author. Magazine title: Natural History. Volume: 105. Issue: 6 Publication date: June 1996. Page number: 48+. © American Museum of Natural History Dec 2008/Jan 2009. Provided by ProQuest LLC. All Rights Reserved.
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