Christian Zeigler / Washington Univrsity in St. Louis

Male tortoise in Santa Cruz eating plants.

We’ve written extensively here at Galápagos Digital about the problem of introduced plants, imported by humans, crowding out the native plant species on the islands.  But now, scientific research reveals some Galápagos tortoises prefer them to native plants.

According to a study published in the journal Biotropica, the introduced plants are a good diet for the tortoises, helping them thrive. The research focused on Santa Cruz Island, home to two species of giant tortoise.

It was conducted by Stephen Blake, PhD, an honorary research scientist at Washington University in St. Louis and Fredy Cabrera of the Charles Darwin Foundation. The team also included a wildlife veterinarian and epidemologist, Sharon Deem, from the St. Louis Zoo. She helped assess the tortoises’ health and nutritional intake, weighing them by suspending the creatures from a spring balance.

Santa Cruz is the most populated of the Galápagos islands  and its highland moist regions have been largely converted to farmland. At least 86 percent of those areas are considered environmentally degraded by agriculture or invasive species.

“Eradicating the more than 750 species of invasive plants is all but impossible, and even control is difficult,” Blake said. “Fortunately, tortoise conservation seems to be compatible with the presence of some introduced species.”

The scientists closely observed the feeding habits of the tortoises, tracking them with GPS devices and, at times, counting every bite they took as they ate.  What the researchers discovered is that the tortoises spent more time browsing on introduced species than on native 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.”

So, the iconic tortoises of Galápagos are munching away on those guavas and other introduced plants, blissfully unaware of the environmental issues that they represent. Another helping, please.