Two regrettable recent stories, each with mammals in the villain’s role: The humble mouse terrorizes seabird populations on a remote island; and Dutch shorebirds abandon a marine reserve because of shellfish dredging.
In far and away the more macabre story, on tiny Gough Island in the South Atlantic, thousands of endemic albatrosses and hundreds of thousands of petrels are being gnawed to death by house mice run out of control. Sorry if it sounds like TV news hyperbole – it isn’t.
After escaping onto the island from fishing boats a century ago, the mice have bulked up to twice their normal body size and surged to an estimated 700,000 strong on the island (that’s about half again as dense as people in San Francisco). Through constant, aimless nibbling, one imagines, they learned that the docile chicks around them were full of protein and fat.
The knee-high, 20-pound chicks, for their part, remain flummoxed, never in their evolutionary past having dealt with mammalian predators attacking from below. Scientists estimate the carnage at more than 700,000 chicks each year. New Scientist has the story, which hit the South African newspapers as well. (thanks Charles)
Some 6,500 miles to the north, in Holland’s Wadden Sea, even good intentions lead to bad news. Resource managers tried to allow shellfish dredging on the same mud flats where half of Europe’s red knots (see picture) feed for – you guessed it – shellfish.
The science of the study is ferociously good. The authors sampled shellfish availability in 2,800 different locations in each of five years. They traipsed across those same mud flats scraping up red-knot droppings to analyze what they’d been eating. They captured the birds and put bands on their legs to track their yearly survival – and while they were at it they even measured the birds’ gizzards with an ultrasound, for crying out loud. (Knots swallow mussels whole, then crunch up the shells in their gizzards to get to the flesh.)
Dredging didn’t reduce shellfish numbers appreciably, but it did hurt their quality as knot food – more shell and less flesh. Analysis showed that unless a knot could expand its gizzard capacity during the season, it likely could not eat enough food to meet its energy requirements. The result, the scientists calculated, was 58,000 knot deaths over five years, and the departure for parts unknown of four out of every five knots that used to frequent the Wadden Sea.
The good news is that the Dutch government halted dredging in 2004. But the bad news is not just bad for shorebirds. It’s bad news for conservation, because one of the encouraging selling points of marine protected areas is the idea that we can balance resource extraction (and jobs) with habitat protection. This study adds to the evidence that it’s simply harder than that.
(from PLoS Biology: news story and research article; image: Varnamo bird club)
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My thoughts exactly.