Goats Be Gone

Getting rid of goats and donkeys benefits Galapagos' giant tortoises

Giant tortoises in the Galapagos Islands have gotten a boost from a program eliminating goats and donkeys, a new study says.

In the 1990s, Alcedo Volcano on the Galapagos’ Isabela Island was over-run by tens of thousands of goats. These non-native animals ate many of the same plants as giant tortoises, destroyed trees, and stripped shrubs that normally collect and provide water to thirsty tortoises. Donkeys also rolled and stomped on tortoise nests.

To protect the tortoises, managers began a program in 1995 to get rid of goats and donkeys in the area. Meanwhile, researchers tracked the effects on Alcedo’s giant tortoises, catching and measuring 8,112 of the animals over the next decade. The team compared the measurements to those of tortoises on Santa Cruz Island, where goats and donkeys still roamed.

From 1995 to 1999, 5 percent of the tortoises caught on Alcedo Volcano were juveniles; by 2000 to 2005, that number went up to 24 percent. The tortoises also grew faster in the later years of the study, the researchers report in Restoration Ecology. The team found no such changes in the Santa Cruz Island tortoises.

The removal of non-native animals wasn’t the only factor improving the tortoises’ quality of life. In the late 1990s, an unusually strong El Nino event dumped a lot of rain on the islands, allowing plants to flourish. But since Isabela and Santa Cruz Islands both benefited from the plant growth, the main reason for the improvement in Alcedo’s giant tortoises is probably their goat- and donkey-free habitat. — Roberta Kwok | 13 July 2012

Source: Marquez, C. et al. 2012. Population response of giant Galapagos tortoises to feral goat removal. Restoration Ecology doi: 10.1111/j.1526-100X.2012.00891.x.

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