By guest author Dr. James Gibbs of the State University of New York’s College of Environmental Science and Forestry (SUNY-ESF). All photos © James Gibbs.
Wacho and I are gearing up for a seven-day-long field trip into the middle of Española Island. However, seas are extremely rough (nine-ten foot waves reported) and the port captain is not letting open boats leave the harbor. When the wave size decreases, we will be on our way.
What will we do on Española? First we will measure what is happening inside the fenced tortoise "exclosures" we built surrounding 25 cacti two years ago. These are strong, six-by-six-meter fences well anchored to keep tortoises out. Why build fences to keep tortoises out, you ask? During our work over the years, we have learned that the island is getting progressively more covered by dense stands of woody plants. We don’t know why but it’s likely that the introduced goats, now eradicated, removed all grasses, triggering a major change in which plant species thrive and which do not.
We also need to determine what role tortoises play in all this. Will the introduced and expanding tortoise population help push the woody plants back by flattening and eating them? Or will they actually hasten the expansion of woody plant growth through seed dispersal? The magnificent tree cacti on Española, which were hammered by the goats, are also important. Now the few remaining cactus get much attention from the tortoises, maybe too much as the tortoises eat every bit they can find.
Exclosures are a fabulous way to get a clear understanding about the effects any herbivore has on its ecosystem. In a classic experiment, we initially measured every single plant on the 25 six-by-six-meter-square plots, which involved lots of crawling around on hands and knees, then installed the exclosures on 13 of those plots. We will continue to measure all 25 plots (with and without fences) for at least five years. We’ll have news on this year’s findings when we return from this trip.
Our second objective is to measure the response of nesting waved albatross (globally unique — they only nest on this island) to large brush clearings (pistas or "airstrips") we created in the interior of the south side of the island. Large numbers of albatross still nest in the interior. They must scramble in and out of small open areas for take-off and landing and then walk great distances through the brush to their nest site, although they don’t have developed nests and they roll their eggs around when so inclined.
We believe that these ungainly birds have also suffered from the extensive growth of woody plants due to increased difficulty when landing and taking off. But we don’t really know. In May/June, these birds are incubating eggs. Two years ago we made counts of nesting albatross on ten 50-meter-radius plots scattered through the interior of the island. Then we cut down every single woody plant by hand on five of these plots (hot work!).
Last year we counted nesting albatross. The cleared areas gained five nesting albatross on average, the controls (uncleared plots) none. We’ll repeat the counts on this trip. These are long-lived, site-faithful birds, so we don’t expect them to respond immediately if in fact they do at all. Only time and continued monitoring will tell.
And if we have time, we hope to tag 200 cactus of all sizes to track them long into the future. How well do they grow and survive? Just what is the cactus population doing — declining or expanding? Cactus is critical to so many species on the island — finches that eat the seeds, mockingbirds that peck the pads, insects that take the pollen and nectar, and, of course tortoises that devour every bit of cactus they can find. By tracking what happens to cactus we can get a handle on what the future might bring for this keystone species in this extremely arid, once goat-devastated, now recovering environment.
Together these studies will help us understand if and how much woody plants are a problem for Española Island, how tortoises are shaping the vegetation, and if the critically endangered albatross will benefit from brush clearing. This is part of a larger effort by Galapagos Conservancy to aid the Park in decision-making concerning whether to let the island continue to recover without intervention from the ravages of 100 years plus of goat infestation, or if large-scale habitat management, which could include cactus seeding and brush clearing, is needed to get island restoration back on track.
Dr. James Gibbs is Professor of Vertebrate Conservation Biology and Associate Chair of the Department of Environmental and Forest Biology at the State University of New York (SUNY-ESF). He has partnered with Galapagos Conservancy for many years in efforts to restore giant tortoise populations in Galapagos through the Giant Tortoise Restoration Initiative, and is a frequent guest contributor to the GC blog.