Many endangered species like the Giant Tortoises of the Galápagos islands share a similar fate with the introduction of human beings to their natural landscape. With humans so too come the pets and the plants that invade their space. And more often than not it means that these endangered species either die off or adapt to avoid competition or costly meals. But for some, in very rare conditions, it means that the species can thrive even better than it could on its sparse local sources for food. And in a new study published this week in the Biotropica, researchers say that after a four year study, tracking the movement and foraging behavior of giant tortoises, it appears that they are seeking out sweeter treats of invasive flora and fruits instead of their local fare.
Weighing in excess of 500 pounds, the Galápagos' Giant Tortoises are peculiar creatures that, to date, have revealed that not only are they not active foragers, but that they can even survive for up to a year without food or drink. It's an impressive feat, and an adaptation that has allowed them to survive on the archipelago for quite a long time. But when researching a pair of the tortoises on the island of Santa Cruz, researchers were stunned to find that the tortoises there would trek up and down an extinct volcano now populated with humans in search of vegetation. And the question that arose was: why? If the species could live without food for such a long time, then why would they be seeking out food year-round.
"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?" lead author of the study, Stephen Blake says. "Couldn't it just wait out the dry season until better times came with the rains?"
Together with Blake, a researcher with Washington University, the pair of tortoises were followed and studied by Fredy Cabrera of the Charles Darwin Foundation in the Galápagos. The researchers outfitted the giant tortoises with GPS tracking devices to track their movements throughout the year, and after four years in search of an answer, they finally came to the conclusion that the slow-moving tortoises spent more time foraging non-native plant species which forced them to change their travel plans quite a bit. With their diets now being comprised of more than half non-native flora, the tortoises are making an advantageous choice in seeking out fruits and greens that are higher in sugar content in that they will give the tortoises a richer diet, packed with more energy.
"Consider it from a tortoise's point of view" Blake says. "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."
Fortunately for the tortoises, of the 750 invasive plant species now introduced onto the island, some are advantageously sweet with the taste and sugars that these slow-moving creatures need in the dry months. Now, instead of waiting out the bad season, which comes at a cost to their health, the tortoises can dine year round on the invasive fruits instead. And while it may not be ideal from an ecologist's standpoint, it appears that failed flora conservation techniques may just mean the survival of another large species.
