Galapagos tortoises prefer to munch on invasive plants over those that grow naturally on the island, which could help keep their growth under control.

Introduced plants started to spread across the Galapagos Islands in the 1930s and have been on the rise ever since, Washington University in St. Louis reported. On the other hand, the Galapagos tortoise population is considered to be either vulnerable or endangered. Surprising new research suggests the introduced plants make up about half the diet of the subspecies of the endangered tortoises. The plants are believed to have benefitted the tortoises by providing more nutrition than their previous diets.

"Biodiversity conservation is a huge problem confronting managers on the Galapagos Islands," said Stephen Blake, an honorary research scientist at Washington University in St. Louis and Fredy Cabrera of the Charles Darwin Foundation. "Eradicating the more than 750 species of invasive plants is all but impossible, and even control is difficult. Fortunately, tortoise conservation seems to be compatible with the presence of some introduced species."

Past studies using GPS devices showed Santa Cruz tortoises migrate seasonally between the lowlands  to the highlands during the dry season; this finding puzzled the researchers.

"This struck us as pretty odd since a large Galapagos tortoise can survive for a year without eating and drinking. This is why sailors would collect the tortoises to serve as a source of fresh meat aboard ship," Blake said. "Why would a 500-pound animal that can fast for a year and that carries a heavy shell haul itself up and down a volcano in search of food? Couldn't it just wait out the dry season until better times came with the rains?"

The team then closely recorded what and how much these tortoises were eating. The findings showed the tortoises tended to choose introduced plants over local ones.

"We weren't really that surprised," Blake said. "Consider it from a tortoise's point of view. The native guava, for example, produces small fruits containing large seeds and a small amount of relatively bitter pulp in a thick skin. The introduced guava is large and contains abundant sweet pulp in a thin, pliable skin."

The findings were published in a recent edition of the journal Biotropica.