Introduced Species in the Galápagos

Feral goats on Isabela Island. Photo by Galápagos National Park Service.

Yesterday I wrote about the case of the North American beaver being purposefully introduced to Argentinian Patagonia for a business venture and having severe unintended consequences on the environment in both Chile and Argentina. Most of us think of Patagonia as a pretty faraway and isolated place, and its location so far down the southern hemisphere merits that. The Galápagos Islands are another place geographically apart from most of us–that distance accounts for the specialized evolution that took place in the archipelago over millennia.

The isolation of the Galápagos from the rest of the world for so long, and the relatively small size of the islands, means that it is especially vulnerable to opportunistic species that can become invasive. In the same way that the Canadian beavers had no natural predators in Patagonia, common domesticated goats, when introduced to different islands in the Galápagos by sailors centuries ago, were able to roam and multiply, which was the travelers’ goal(to create a quick source of meat in the middle of the ocean).

Photo by Fernando Diez for Galápagos Expeditions.

Like the beavers, feral goats in these now Ecuadorian islands multiplied out of control very quickly. Biologists have run the numbers (citation below) and it seems that the goat population can double every two and seven years, since goats can start breeding before they are even a year old and can have two litters per year (and goats normally give birth to a pair of kids). So the population of goats throughout the Galápagos exploded in a very short amount of time, and, although not to the same extent as beavers, goats can do some real damage to local plant life.

And in a small, protected island ecosystem like this one, food webs can be quite fragile. Deforestation and overgrazing of a certain species of plant can lead to both habitat and key food loss for animals like tortoises, which definitely don’t have the same capability as goats to clamber up mountainsides to look for things to eat as supplies dwindle. In the last several decades, goat eradication efforts through aerial and ground-based hunting have been relatively successful, and at least six of the islands are now goat-free.

Volunteers clear a field of blackberry brambles. Photo by WordPress blogger Julie.

Removing the hill blackberry from the archipelago, however, is going to be a lot harder than shooting large mammals from helicopters. The plant not only competes with native flora very effectively, but it also resists eradication through its defenses and its reproduction. As any of you who learned to pick blackberries at a young age know, the brambles are covered with thorns. From direct experience, I can tell you that the thorns on wild Galápagos blackberry bushes, at least on Santa Cruz island, are particularly vicious. They are longer and stronger than those I’ve encountered in Central America and the US, and to make the plant even less desirable, the berries are small and bitter.

You might have guessed that the blackberry plant was introduced to the Galápagos, and you’d be right. It was introduced in the seventies as an agricultural endeavor, and is now completely out of control. If we very conservatively estimate that each blackberry has about twenty-five seeds (which have a high germination rate) in it, and assume that the dense population of finches that the archipelago is known for will enjoy eating the little fruit and disperse those seeds (which can also stay viable in the soil for years), then it is easy to see how the hill blackberry can expand its range with such devastating efficiency. Combine that with the thorns and the fact that thickets can grow up to over ten feet high, and the problem becomes a nightmare.

Photo by Micaela Ayala for Andes.info.ec

For the bulk of the scientific data used in this post, I am indebted to Aida Castillo-Flores’s monograph on goats and blackberries in the Galápagos, accessible via the University of Arizona here.

3 thoughts on “Introduced Species in the Galápagos

  1. Pingback: There Is No Effective Resistance To This Charm | Raxa Collective

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