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Evolution in Galapagos

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Fast evolution

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The Galapagos marine iguana is a diving reptile that eats algae. This guy is about 1.2 meters long, from schnozz to tail. Notice the white "wig," formed by expulsion of excess salt. Image (c) M. Horning, 1991.










































New evidence indicates danger of trace oil pollution.


Ever since the Santa Barbara oil spill of 1969, oil spills are linked in the public eye to the gruesome death of petroleum-slathered sea birds. But does petroleum have more subtle effects? If an oil spill seems to spare a shoreline, can it still interrupt the web of life on that shore?

Iguana with tropical sea in background.

The questions arise from a new study of the aftermath of a spill in the Galapagos Islands in the Pacific Ocean. Since most of the oil blew out to sea, the January of 2001 spill was originally considered a close call for the islands where Charles Darwin made key insights into evolution by natural selection.

The Galapagos archipelago has been a nexus of conservation biology and evolutionary studies for many years. Good records on wildlife abundance indicate -- if not yet prove -- that trace levels of oil harmed marine iguanas (Amblyrhynchus cristatus).

The oil apparently did not kill the iguanas directly. Instead, it seems to have snuffed bacteria living in their gut that degrade the algae -- seaweed -- that is the lizard's main course.

On Santa Fe island, the population of marine iguanas plunged from 25,000 to 10,000 in the year after a tanker ran aground and spilled about 3 million liters of petroleum products. On a nearby island not affected by the spill, the iguana population was stable.

map of the south pacific, showing where the Galapagos Islands are, off the coast of Ecuador, South America. Inset shows a closeup of the islands, with Isla Santa Fe labelled as well...a smaller island in the southern 1/2 of the islands.

The five-foot iguana in question may never win a beauty contest. Libeled by Darwin as "imps of darkness," they have a blunt nose and can dive 50 feet to harvest algae from the ocean. To balance electrolytes in their systems, they expel salt from their noses, forming a white "wig."

But for symbiotic bacteria that live in the iguanas gut, the algae would be more indigestible than yesterday's grease-coated burger dumpster-dived from the Ptomaine City drive-in. These bacteria break down cellulose in plant cell walls, making the nutrients available to their hosts. The same process lets cows and other ruminants digest cellulose-rich plants.

Cud be true
We might know little of the marine iguana's fate but for a long-term research project by Martin Wikelski, an evolutionary biologist at Princeton University. Since 1987, Wikelski has been marking and tracking marine iguanas in the Galapagos, studying how animal size evolves over time.

about a dozen black, mottled marine iguanas on the beach
Marine Iguanas (Amblyrhyncus cristatus), Gal┬Ěpagos Islands, Ecuador. Courtesy Charlie Urbanowicz, California State University, Chico.

When Wikelski went looking for his research subjects after the spill, however, he found that 62 percent were dead on Santa Fe island. On an island not affected by the spill, the population had been stable.

The spill in 2001 deposited about one liter of oil - a mix of heavy bunker oil and lighter diesel fuel - on each meter of windward beach on Santa Fe island. Levels of oil in the water reached 44 parts per million.

The iguanas periodically die off when el Nino-induced changes harm the algae in their lunchbucket. Otherwise, they are more likely to die from old age - they have no significant predators.

El Nino was quiet during the die-off. So what killed the iguanas? Wikelski and his co-authors raised these possibilities, all related to the oil spill:

"It's poison": direct toxicity to the iguanas or algae.

"It tastes yukky": oil-tainted algae simply quit tasting like food.

"Honey, we've got to stop meeting in the hindgut of a marine iguana": the oil harmed the symbiotic relationship between the iguana and the endosymbionts (the bacteria living in their gut).

In ruling out the first two explanations, the researchers noted that iguanas and algae seemed healthy in the first two weeks after the spill.

That left a soured relationship as the most likely cause for the die-off. Although the conclusion is not conclusive, the researchers wrote, "We infer that the fermenting endosymbionts in the iguanas' hindgut must be very sensitive to low-level environmental disturbance or contamination. This sensitivity probably compromises the digestive efficiency..."

Short, stocky and none-too-eye-pleasing is this iguana.
©2000 Robert George Daniel (www.rgdaniel.com). All rights reserved.

One stressed-out lizard
A further indication of trouble was a high level of stress hormones called corticosteroids in the iguanas. This, the researchers wrote, was "a reliable indicator of the induction of life-threatening environmental stress."

To nail down the conclusion, the scientists intend to experimentally feed captive iguanas normal or oil-tainted seaweed ("Honey, I'd prefer virgin-pressed Italian diesel oil"), and then check the health of the symbiotic bacteria.

If that study confirms the present one, the findings will be a further indication that "low-level contamination" does not equal "no contamination."

That could cast a shadow on the safety of petroleum exploitation and transportation in other pristine habitats, in places like the Arctic National Wildlife Refuge, for example.

-- David Tenenbaumlittle iguana



Marine Iguanas Die from Trace Oil Pollution, Martin Wikelski et al, Nature, June 6, 2002.


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