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March 29, 2005
A new twist in eco-systems: introducing a preditor kills the grass...
This is from the NYTimes Science section today. It is new to me, a case where the ecosystem is so fragile that introduction of a bird eating carnivor is able to destroy the grass on an island.
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Foxes may not graze, but a new scientific study describes how their arrival on Aleutian islands destroyed rich grasslands and left only sparse tundra. The authors of the report, which appeared in Science last week, say this transformation shows how an entire ecosystem may go into a tailspin if just one new top carnivore shows up.
The inadvertent experiment began in the late 1700's and continued into the early 20th century as fur traders looking to expand their supply released nonnative arctic foxes and, in some cases, red foxes on more than 400 Alaskan islands. Some died out, but many populations survived.
The new habitats included much of the Aleutian archipelago that curves west toward Asia. Except for the occasional polar bear rafting in on winter ice, the windswept islands had few predators before.
The botanical impoverishment that has resulted is the reverse of what usually happens when a new meat-eater comes along.
"Traditionally, the predator eats the grazer; the grazer no longer eats the green stuff; and the habitat gets more green," said Dr. Donald Croll, a professor of biology at the University of California, Santa Cruz, and the lead author of the report.
An example of the more usual routine is in Yellowstone National Park, where returning wolves, preying on sapling-browsing elk and confining the wary survivors to areas where they can see wolves coming, have touched off a resurgence of willow, aspen and other vegetation.
The contrary effect in the Aleutians, once sorted out, has a simple explanation.
The grazers on these islands were grass- and seed-eating Aleutian geese, which are smaller cousins of Canada geese. The foxes drove the geese near extinction, which would have been a boon for grasses except that the foxes also feasted on the eggs and hatchlings of puffins, auklets and other ocean-feeding seabirds they found brooding in vast numbers almost everywhere.
Some islands lost almost all birds except for cliff-nesting species. And as ground-nesting birds faded, so did their nutrient-rich excrement, or guano, which had been a natural fertilizer.
The research team concluded that islands with no foxes received an average 361.9 grams per square meter yearly. Fox-infested islands get just 5.7 grams per square meter of guano per year.
"You ever smell one of those rookeries?" Dr. Croll asked. "That is the odor of ammonia, like in fertilizer. Even the wind scatters it around." Without the regular subsidy of nitrogen and potassium-rich nutrients winged in from the sea, grasses lost their competitive edge over tundra shrubs and herbaceous plants...
Posted by rakhier at March 29, 2005 04:50 PM