Tortoise predation

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(Host) America’s desert tortoises are under several threats, but commentator Ruth Page explains that one of the worst is predation from the sky.

(Page) Human activity may be the greatest threat to other life on this planet, but we’re not alone in the destruction. Turtles and tortoises, I’ve always believed, are so well-protected by their tough, horny – but charmingly decorated – shells, they’re pretty much safe from other dangers. A horrifying picture in National Wildlife some time ago disabused me of this naive belief. There stands a scientist, looking down at the ripped-open shells of eight young tortoises. They’re empty as soup-bowls after lunch.

Ravens have been feasting. Our Mojave Desert tortoises are soft-shelled, delicious morsels for their first five to seven years of life, until their shells harden. Huge black crows descend and rip their way through the shells, killing the slow-moving young animals. It’s a free and easy dinner for the sharp-eyed killers from the sky.

In the early nineties, scientists noticed there were a surprising number of shells from tortoise hatchlings and youngsters under the raven nests. They looked into the problem to determine whether it was serious enough to endanger tortoise populations. Setting up a study site for hatchlings, in a few months they saw ravens swarm down and kill 18 of 25 of the youngsters at the site, and eight more nearby.

Since 1960, raven numbers in the Mojave shot up by a thousand percent; during the same time, tortoise populations dropped by ninety percent in some areas. Ravens weren’t entirely to blame, but they were the worst, causing fifty percent of the deaths. Now, scientists fear extinction of the species in the Mojave, because tortoises also face threats from disease and habitat destruction.

Usually when predators manage to wipe out most of a population of prey, the predator numbers drop with the food supply.

Not so with ravens. Nearby towns and cities provide irrigation water and garbage aplenty that keep ravens healthy. Even if they eat the tortoises to extinction, the crows can still thrive.

Our desert tortoises are one of four species native to North America, and their family line goes back as much as eighty million years. The creatures can normally live as much as 100 years. They can spend most of their lives underground to avoid the worst cold of winter and the worst heat of summer. They can go up to a year without a water source except for the moisture in springtime grasses and wildflowers. They store the water in their bladders to carry them through drier seasons.

Many efforts are being made to save these ancient species, but because they can’t reproduce until they’re 15 to 20 years old, only our grandchildren are likely to know for sure whether we’ve succeeded.

This is Ruth Page in Shelburne, Vermont.

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