giant tortoise santa cruz galapagos

A tortoise makes its way through the thick foliage in the highlands of Santa Cruz. © Jeff A. Goldberg.

Invasive species are often bad news. This is especially true in a place like the Galapagos Islands, where a single introduced species can wreak havoc on sensitive ecosystems and have a devastating impact on endemic flora and fauna, which haven’t evolved to cope or compete with new arrivals.

That’s why a recently released study on the feeding habits of what is perhaps the islands’ most iconic species, the endangered Galapagos giant tortoise, is making international news and shedding new light on the complex relationships between species, both native and introduced, in the islands.

The study, lead by Stephen Blake, PhD, found that tortoises on the island of Santa Cruz spend more time foraging among non-native plants than native plants and that non-native plants make up about half of their overall diet.

Santa Cruz is the most heavily populated island in the archipelago and much of its land is used, or has been used in the past, for agriculture. So the island is teeming with invasive plants. All told, there are about 750 species of introduced plants in the Galapagos. While some are relatitively benign, others have been disastrous. Invasive wild blackberry, for example, has destroyed huge swaths of endemic Scalesia forests on Santa Cruz. Scalesia forests provide habitat for certain giant tortoise subspecies and other endemic wildlife.

Blake and Fredy Cabrera, of the Charles Darwin Foundation, tracked the tortoises for four years, watching what they ate and examining the tortoises’ dung to find out what they were eating.

The study was inspired by some perplexing tortoise behavior that Blake discovered on a previous project. He noticed that tortoises outfitted with GPS tracking devices were making an arduous journey from the lowlands, which are flush with vegetation in the wet season, to the highlands, which have plenty of vegetation year round. He wondered why a slow-moving, 500-pound animal that can survive without food and water for up to a year would bother hauling itself up and down a volcano in search of food.  It could easily survive the dry season in the low lands until the rains returned. He also wondered where the tortoises were getting the energy to make such a trek.

The results of the new study seem to explain the tortoises behavior. The tortoises, according to Blake, were simply making a wise consumer decision.

“We weren’t really that surprised. 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,” Blake told interviewers.

It’s worth it, it seems, for a tortoise to journey miles and miles up the side of a volcano as long as the metabolic payout is big enough.

The study points to an interesting conclusion: while there is no doubt that some introduced plants have had a devastating effect on tortoises, others, such as domesticated guava, may be helping tortoises survive and even thrive in their altered environments. It’s also a reminder that the relationships between species, both native and non-native, are incredibly complex.