Are invasive species good for giant tortoises? Ask the dung.

By Ben Gruber

Invasive species usually spell trouble for isolated ecosystems but in the Galapagos, an archipelago of islands off the coast of Ecuador that Charles Darwin credited with inspiring his theory of evolution, giant tortoises are in love with non-native fruit and grass species that appear to be keeping them happy and healthy.

« While introduced species in general are generally a bad thing for Galapagos ecosystems and any ecosystem, there are bits to that story that make it a little bit more complicated, » said Dr. Stephen Blake, a scientist who has dedicated his career crisscrossing the world to protect endangered animals.

For the tortoises, non-native grass and fruit species like guava and passion fruit appear to be putting a spring in their step.

Blake is a co-ordinator for the Galapagos Tortoise Movement Ecology Program. He and his colleagues observed that tortoises, some weighing more than 500 pounds, travel out of their way to where invasive species thrive for some lunch.

Apart from watching that massive animals eat, Blake needed quantify exactly how much and which invasive species the tortoises were munching on.

« So we complimented direct observations with analyzing tortoises dung piles, » Blake said.

« I think we have been through 300 to 400 hundred dung piles by this point counting every single seed in a pile of dung and sometimes you get 7 or 8 thousand seeds in a single dung pile, so it’s been a lot of work, » he added.

According to Blake, it’s important work that couldn’t be done without the political and logistical support of the Galapagos National Park.

While the tortoises appear to be thriving on the invasive species, the long term effects are still unknown.

« One could imagine tortoises modifying potentially their migration and movement behavior through special memory and things like that. Suddenly there is a nice new patch of nutritious stuff here that didn’t exist ten years ago, so I will come back here next year, » said Blake.

That change could affect migratory routines that could, in turn, affect reproductive patterns. But to answer those questions, only time and a lot more dung piles will tell.