Normally when you see "invaders" and "endangered" in the same sentence, it usually means invaders are causing trouble for a species already on its way out. That then would make the iconic tortoises found in the Galapagos Islands a very startling exception to the rule. New research has revealed that these gentle giants are completely pigging out on the islands' otherwise harmful invasive plants.
That's at least according to a study recently published in the journal Biotropica, which details how Galapagos giant tortoises are not only gorging themselves on invasive plants first introduced to the islands in the 1930s, but in-fact prefer them, receiving the nutritional support they need to swell their numbers in the wake of recovery and conservation efforts.
The authors of the study reported how, until the late Pleistocene epoch, giant tortoises could be found on all of today's seven continents, save for Antarctica. Today, however, they can only be found on the Aldabra Atoll in the Indian Ocean, and the Galapagos Archipelago in the eastern Pacific Ocean. Even there, most of these species are threatened or endangered thanks to shrinking ecosystems and threats from humans and their livestock.
Humans also introduced countless new plant species, many of which have handily outcompeted local fauna in the last few decades.
"Biodiversity conservation is a huge problem confronting managers on the Galapagos Islands," researcher Stephen Blake, of Washington University, said in a statement. "Eradicating the more than 750 species of invasive plants is all but impossible, and even control is difficult."
"Fortunately," he added, "tortoise conservation seems to be compatible with the presence of some introduced species." (Scroll to read on...)
In an assessment of biodiversity on the island of Santa Cruz, an extinct volcano that is home to two species of giant tortoise, Blake and his colleagues wanted to determine just how much the mere presence of humans has impacted the Galapagos ecosystem. Santa Cruz is home to the largest human population in the archipelago and this has led to a stunning 86 percent of the highlands to be converted for agriculture.
In past work, Blake and his colleagues had determined that tortoises in this region were regularly migrating between the low- and highlands, but why had remained a mystery.
"This struck us as pretty odd, since a large Galapagos tortoise can survive for a year without eating and drinking," he explained. "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?"
He quickly found out that they were after nutritious invasive species in particular, which were more common in the highlands.
"Consider it from a tortoise's point of view," Blake said. "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."
And with these heartier meals coming their way, the tortoises were able to be more fit and active throughout the region's dry season.
The researcher and his colleagues added that they are encouraged by these findings, as the chances that the Galapagos Islands will ever return to how they once were is slim-to-none. The fact that the radical change it has undergone could be acceptable or even preferable for the endangered tortoises is no-doubt happy news.
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