Leighton Reid, a postdoctoral fellow in the Center for Conservation and Sustainable Development, reflects on tortoises, tree cacti, and ecological isolation.
The Galápagos is the world’s most pristine tropical archipelago, and it is utterly unique. Nearly the entire island group is a national park, and 200,000 visitors per year come to witness its ecological singularities ‒ things like penguins and iguanas swimming side-by-side through a mangrove lagoon. The archipelago consists of fourteen large, volcanic islands and over a hundred smaller rocks and islets. Most of the land surface is low and dry. The easternmost island is about 900 km from mainland Ecuador, which is a probable source for the organisms that first began to colonize Galápagos when its volcanic peaks surfaced above the Pacific five million years ago. Indeed, the islands’ ecology is characterized by their isolation. Each island contains a relatively low diversity of organisms, many of which are unafraid of large primates. The biotas’ ecological simplicity and naiveté have facilitated major scientific discoveries, such as that small, heritable variations can have life or death consequences for individuals and ultimately change populations.
One of the more bizarre life forms on Galápagos is the tree cactus. Prickly pear cacti (Opuntia species) are not particularly rare in the western hemisphere. In the United States, for instance, they occur in every state except Alaska. But over millions of years in Galápagos they have become quite varied. Some grow low to the ground, like the familiar continental forms, whereas others grow as trees, towering up to 15 m above the ground. The first botanist to speculate on this phenomenon was Alban Stewart (1911), a scientist-sailor with the California Academy of Science. He noted that erect, tree cacti tended to grow on islands that also housed another over-sized organism – the Galápagos tortoise (Chelonoidis nigra). Galápagos tortoises eat the fleshy cactus pads, which contain water – a limiting resource in arid environments. Stewart posited that the pressure from tortoises craning their long necks upward to munch cactus pads may have favored taller cacti.
Opuntia echios var. barringtonensis is one of the taller tree cacti, presumably made that way by pad depredation by giant tortoises over many generations.
A low-growing cactus (Opuntia echios var. zacona) growing on Seymour Norte, an island that historically had no tortoises or iguanas. Herbivore pressure is visible here; an introduced land iguana (Conolophus subcristatus) has been taking bites from the lowest pads.
The relationship between tortoises and cacti was thrown into disarray after the Galápagos were discovered (accidentally) by Panamanian Bishop Tomás de Berlanga in 1535. By the late 19th Century, pirates and whalers removed thousands of tortoises from the islands, stowing the living animals in their holds as fresh meat for their long Pacific voyages. Eventually, overharvesting extirpated tortoises from several of the islands, with rippling effects on the rest of the ecosystem. Even where tortoises survived, they were often unable to reproduce because their offspring were eaten by introduced, European rats. Tree cacti were among the hardest hit; tortoise decimation stripped these plants of their main seed disperser.
Reintroduced giant tortoise in the littoral zone on Isabela Island.
In response to tortoise declines, the Charles Darwin Foundation and the Galápagos National Park Service began a captive breeding program on Santa Cruz Island. Since 1965 they have raised and repatriated thousands of tortoises to several islands, waiting to release them until the tortoises have gotten big enough to be “rat proof”. By and large the reintroductions have been successful. On Española Island, for example, tortoise populations had crashed to fifteen individuals in 1960, but by 2007 more than 1500 individuals had been repatriated, and the population appeared stable. Moreover, these reintroduced tortoises reinitiated seed dispersal for an endangered tree cactus (Opuntia megasperma var. megasperma), increasing the number of juvenile plants.
In addition to species reintroductions, ecological restoration in Galápagos has often entailed species eradications. Isolation historically shaped Galápagos ecology; nine hundred miles is a long way for a snake or a lizard to float on a vegetation raft. But Galápagos’s isolation was compromised by seafaring humans, who facilitated island colonization by domesticated animals and hundreds of plant species. Goats have been among the worst invaders. Until recently, goats overgrazed the islands’ vegetation, converting it into habitat unsuitable for native species. One of the most ambitious restoration projects in Galápagos has been eradicating goats from the archipelago. On the largest island, Isabela, more than 140,000 goats were killed in 2004-2005 using unconventional restoration tools, including helicopters, AR15 rifles, and Mata Hari goats – sterilized female goats induced into long-term estrus and fitted with radio telemetry collars to root out the last hold-outs. Goat eradication has resulted in spontaneous vegetation recovery. In addition to goats, the Charles Darwin Foundation and the Galápagos National Park Service have also eradicated eight exotic plant species. Other species will be harder to get rid of, like rats, guava, blackberry, and domestic cats.
Despite its one-of-a-kind nature, can the world’s most pristine tropical archipelago serve as a reference for other arid, tropical islands? That is, can we evaluate the success of other island restorations by comparing them to the relatively intact Galápagos’s ecosystem structure, function, and composition? Perhaps to some extent we can. Historical contingency leads to unique island assemblages (for example: giant tortoises in Galápagos, giant skinks in Cabo Verde, giant lizards in Komodo), but many islands may be characterized by their lack of functional redundancy. In other words, if you remove a species from an island, the ecosystem consequences may be greater than if you had removed a species from a more diverse mainland ecosystem. Additionally, plant restoration in the arid Galápagos suggests that when disturbances are removed, vegetation can recover rapidly. This may also be true of other oceanic archipelagos, whose plants and animals have already colonized difficult terrain from a long way away.
Land iguana and tree cacti (Opuntia echios var. echios) on Plaza Sur Island.
Natural History of Ecological Restoration 