Tree islands for tropical forest restoration: the outlook is rosy after 10 years

Planting tree islands has many of the benefits of larger plantations, but entails significantly less cost. Karen Holl (University of California, Santa Cruz), Leighton Reid (Missouri Botanical Garden), and Zak Zahawi (American University of Beirut) describe recent findings on tree seedling recruitment in a long-term experiment in southern Costa Rica.

Over the past few years there have been a growing number of commitments at the global, national and regional scale to restore forests because of their importance to conserve biodiversity, sequester carbon, reduce erosion, and provide goods and services to people. For example, Initiative 20×20, led by the International Union for the Conservation of Nature, aims to restore 20 million hectares of tropical forest by 2020, an area roughly equivalent to the size of Uruguay or Nebraska.

A common strategy to restore forests is to plant trees. But, the big question is: where will the money come from to plant billions of trees when there are so many pressing needs? As restoration ecologists, we started thinking about how we could most efficiently allocate resources to get the best bang for the buck and restore the largest area of forest.


Trade-offs in forest restoration strategies. Planting fewer trees leaves more to chance and can require more time, but tree plantations are more expensive and leave a bigger ecological footprint. Our study tests an intermediate option, and after 10 years it appears to provide a good balance. Figure modified from Corbin & Holl (2012).

Starting over 10 years ago, we set up a large-scale tropical forest restoration experiment in southern Costa Rica to test two ideas.

First, we tried planting tree “islands”. The idea is to plant groups of trees that attract birds and bats, which disperse most tropical forest tree seeds. The tree canopy also shades out light-demanding grasses that can outcompete tree seedlings. In one experimental treatment, we planted tree islands that covered about 20% of 50 × 50 m plot of former cattle pasture. We compared that to plots where no trees were planted (natural recovery) and to the more intensive (and more typical) restoration strategy of planting trees in rows throughout the plot (plantation).

Second, we asked: is it only possible to restore forest near remnant forests or can you restore forest anywhere in the landscape? This is important information to help guide forest restoration efforts. To do this we set up our entire experiment at 13 sites, some of which were mostly surrounded by agricultural land and some of which were adjacent to the largest remaining forests in the region.

Then we monitored establishment of new tree seedlings in our research plots over a decade. We compared the number of seedlings, number of species, and types of species in the restoration plots with those found in the nearby forest to evaluate how well the forest is recovering.

The tree island planting method not only saves money on buying, planting, and maintaining seedlings, but it also results in a more heterogeneous distribution of trees, so it looks more like a natural forest.


Profuse tree seedling and sapling recruitment in the understory between two tree islands in southern Costa Rica.

We counted over 6000 tree seedlings, 88% of which have seeds that are dispersed by animals. On average there were many more tree seedlings in the tree island and plantation treatments than in the natural recovery plots. These results suggests that some tree planting helps the forest to recover faster, but that it is not necessary to plant the whole area with trees. The tree island planting method not only saves money on buying, planting, and maintaining seedlings, but it also results in a more heterogeneous distribution of trees, so it looks more like a natural forest.

Even though there were many tree seedlings in the island and plantation plots, on average there were less seedlings of tree species that have big seeds (>0.5 cm/0.2 inches across) compared to mature, reference forests. It seems that the larger-seeded species that are common in mature forests are much slower to colonize restored sites, likely because they are eaten and dispersed by a small number of larger animals, such trogons and agoutis. Many of those dispersers are less likely to visit early successional forest.


Small frugivores, small seeds. Most of the birds we see in these experimental plots are small-gaped omnivores (e.g., Yellow-bellied Elaenia, Elaenia flavogaster, left), but it usually takes large-gaped species to disperse larger seeds1. The figure at right shows the maximum fruit size that a bird species with a given gape size was able to consume in a cloud forest in central Costa Rica (modified from Wheelwright (1985)). In our experiment, small seeds were ubiquitous, but large seeds were mostly absent.

We were surprised that the amount of forest cover around the experimental plots had a weak effect on the number of seedlings establishing. In other words, isolated plots had just as many tree seedlings as plots right next to old-growth forests. We think that this is likely due to the fact that there are many trees in the agricultural landscape surrounding our plots; these trees include remnant trees, living fence rows, and riparian corridors. Trees in the landscape can serve an important role in both providing sources of seeds and stepping stones for the movement of seed-dispersing fauna. We anticipate that having forest nearby will be more important in future years as these forests build up greater diversity of rare, large-seeded species. Nonetheless, our results suggest that there are good prospects for restoring forests in many locations in this landscape.

Our key finding is that planting tree islands can be a cost-effective way to restore tropical forests at our study site in Costa Rica, but we hasten to note that the strategy should be tested in other locations, particularly areas with fewer forest elements in the surrounding countryside. Our study also demonstrates that tropical forests can recover some species quickly but it will take many decades, if ever, for forests to fully recover. So, preserving existing rain forests is critical to conserve biodiversity and the services they provide to people.


Diverse tree cover in an agricultural landscape in southern Costa Rica. Remnant trees in pastures, trees along fence rows, and riparian forests provide important sources of flora and fauna to speed up forest recovery.

1See Melo et al. (2009) for an example to the contrary: small-gaped animals dispersing fairly large fruits and seeds.

Ecological Restoration in a Changing Biosphere

If you were at the MBG Fall Symposium, we want to hear from you! How did the symposium change your perception of restoration? Send us an email at

On October 8th, Missouri Botanical Garden hosted its 63rd annual Fall Symposium. This year’s theme was Ecological Restoration in a Changing Biosphere. Author and journalist Paddy Woodworth moderated the day, and seven speakers presented contemporary perspectives on a core challenge in modern restoration ecology. Namely: in the post-COP21 world, when all three UN conventions call for scaling up and mainstreaming of restoration, it is clear that restoration will affect hundreds of millions of hectares – and as many people – over the coming decade. At the same time, we find ourselves in an era of unprecedented change where climate, ecological baselines, and future land-use changes are highly uncertain. This raises the question: What should large-scale restoration look like in the remainder of the 21st century?


2016 Fall Symposium speakers. From left to right: Peter Wyse Jackson, Curt Meine, Robin Chazdon, James Aronson, Leighton Reid, Pedro Brancalion, Karen Holl, Don Falk, Paddy Woodworth, and Jim Miller. Photo by Andrea Androuais.

Talks during the morning focused on tropical forests, where much of the international restoration dialogue is focused.

  • Leighton Reid (Missouri Botanical Garden) opened with a presentation on restoration longevity – the idea that some restoration projects create ecosystems that persist for more than a century (e.g., Floresta da Tijuca), while other projects fail quickly. Dr. Reid argued that how long restored ecosystems persist is quantifiable, predictable, and manipulable, opening the possibility for more ambitious restoration planning.
  • Robin Chazdon (University of Connecticut and beyond) then spoke about forest landscape restoration, an approach that aims to regain ecological integrity and enhance human well-being in deforested, human-impacted, or degraded forest landscapes. Drawing on a wealth of large-scale studies, Dr. Chazdon made the case that natural forest regeneration is the most ecologically effective and economically feasible approach to forest restoration globally.
  • Karen Holl (University of California Santa Cruz) presented her take on research priorities for forest restoration in the Neotropics. She highlighted that researchers could make an impact by studying forest restoration at larger spatial scales, at longer temporal scales, and in collaboration with stakeholders. Improving information exchange and standardizing monitoring protocols were also among her top priorities. (Graduate students, take note!)
  • Dr. Pedro Brancalion (University of São Paulo) completed the morning session with a TED talk-style discussion of the linkages between science, technology, policy, and best practice in Brazilian Atlantic Forest restoration. Using Thomas Kuhn’s structure of scientific revolutions, Dr. Brancalion argued that restoration ecology is in a crisis period, in part because disciplinary research has predominantly created solutions at smaller spatial scales than the (growing) problems the discipline seeks to address. Perhaps restoration is ripe for a paradigm shift?

Dr. Pedro Brancalion (right) asks whether restoration ecology is ready for a new paradigm shift, as Paddy Woodworth (left) moderates. Photo by Robin Chazdon.

After lunch, the conversation turned towards a major academic debate in restoration ecology. Has global change outpaced the restoration approach? And is a new approach needed?

  • Curt Meine (The Aldo Leopold Foundation) drew on his long experience in the upper Midwest, and, in particular, his studies of author and environmentalist Aldo Leopold (1887-1948). He argued that Leopold avoided the simple polarities through which some contemporary restoration debates are framed. He viewed nature in a relative way, neither entirely wild, nor entirely domesticated in any given landscape. Although he practiced ecological restoration in some contexts, he also advocated soil conservation and sustainable agriculture – activities motivated by his core values, as expressed in The Land Ethic (1949).
  • James Aronson (Missouri Botanical Garden) followed with an elucidation of the reference ecosystem concept. Reference ecosystems, he noted, help determine the social and ecological vision for a restoration project or program – a critical issue for restoring historic continuity in degraded landscapes. Dr. Aronson described a family of restorative actions for achieving progress towards the reference system, drawing on examples from Jordan and South Africa. He argued we need to look deeper into the past and ponder our choices from many angles as we decide how to do more effective restoration at the landscape and larger scales.
  • Donald Falk (University of Arizona) delivered the keynote address. He painted a disturbing portrait: rapid climate change is driving a massive forest-to-non-forest transition in the southwestern United States. In particular, many ponderosa pine forests will not be able to persist in the future where they have been in the recent past and present. Perhaps restoration ecologists should transition too. Rather than “chasing the ambulance”, maybe we could get out ahead of disasters and ease transitions between stable ecosystem states. Anticipating ecosystem transitions could mitigate the loss of ecosystem functioning that accompanies major climate-driven forest fires, but it would require a shift in restoration thinking. Importantly, Dr. Falk noted that ecosystems do not care what words we use – ecosystems respond to actions.

With moderator Paddy Woodworth’s help, we finished the day with a panel discussion, inviting questions from the audience. Among the thoughts and questions that we were left with:

  • Is ecological restoration more difficult in places with greater population density?
  • Should restoration focus on policy, economic, or cultural motivations for engaging people?
  • Are values a better guide for land management than ecological history? Are the two complementary?
  • How can the reference ecosystem concept accommodate rapid biome changes, as we are seeing in the Southwestern USA?
  • What is the way forward to mainstream serious, multisectorial monitoring and evaluation with all these new factors to consider? Who will fund it?
  • To what extent can we move from restoring degraded ecosystems to avoiding degradation in the first place?
  • Can forest landscape restoration and natural forest regeneration bridge the gap between small-scale, past restoration experience and present, large-scale restoration needs?

PhD candidates Ricardo Cesar (University of São Paulo) and Leland Werdan (University of Minnesota) compare notes on seedling functional traits in dry tropical forest restoration. Leland was the recipient of the annual Delzie Demaree award. Photo by Robin Chazdon.


More than 150 people registered for the symposium. They came from three continents, five countries, and seven US states.

Epiphyte restoration in Brazil’s Atlantic Forest

CCSD restoration ecologist and PARTNERS member Leighton Reid spent 10 days collaborating with scientists and students in the Tropical Silviculture Lab (LASTROP) at the University of São Paulo. Epiphytes were a central theme of the visit.

Vascular epiphytes are plants that live non-parasitically on other plants. Readers from the tropics will be quite familiar with some epiphytes, like the ubiquitous Tillandsia of Neotropical powerlines, but temperate zoners will have seen many epiphytes as well, at the florist, the botanical garden, and the mall. These plants are incredibly diverse; by one estimate, epiphytes make up 9% of all vascular plants worldwide. But epiphytes also face serious challenges in today’s world. Habitat loss and overharvesting threaten some epiphyte species with extinction. Many epiphytes also have a hard time recolonizing new habitat in regenerating forests, but new studies on epiphyte restoration could help.

I spent the past 10 days in the State of São Paulo learning about epiphyte ecology, conservation, and restoration from students and scientists at the University of São Paulo’s College of Agriculture (Escola Superior de Agricultura Luiz de Queiroz). This part of Brazil was once covered in semideciduous tropical and subtropical forests, which hosted about 150 vascular epiphyte species. Today, only ~15% of the forest remains, but there is a large effort underway to restore 15 million hectares (nearly 58,000 square miles) of it by 2050.


ESALQ maintains shade house with more than 3,000 orchids, including (A) Cattleya loddigesii, (B) C. forbesii, and (C) Arpophyllum giganteum.

Frederico Domene is a doctoral student studying epiphyte reintroduction in restored Atlantic Forest. Like his advisor, Pedro Brancalion, Fred’s interest in epiphyte restoration stems from a passion for orchids. He grows a variety of them at his house in Piracicaba, preferring true species over horticultural varieties.

Fred picked me up in his black pickup, “mamangava”, and took me on a tour of several tree plantations where he has been developing methods for reestablishing populations of epiphytic orchids, bromeliads, cacti, and aroids. Fred’s basic procedure involves collecting epiphyte seeds (or purchasing small plants, in the case of orchids), growing them out in a nursery, and then attaching them to trees using twine or plastic. He started his work in 2010 and has been monitoring his plants, and reintroducing new plants, every year since. He uses a ladder to put the orchids up high, out of easy reach for would-be poachers.


Atlantic Forest restoration plantations. Left: 60-year old plantation along the Rio Piracicaba near Rio Claro. Right: 12-year old plantation at the Anhembi Forest Science Experimental Station. The older restoration site had considerably more naturally recolonizing epiphytes than the younger site.

Late August is mid-winter in São Paulo, and while it doesn’t get particularly cold, it is quite dry. The restoration plantations were crunchy with desiccated leaves and twigs. These are harsh conditions for epiphytes, which do not have the luxury of soil to buffer to their roots from the sunlight and dry air. Some of Fred’s epiphytes have withered and died, especially during a 100-year drought in 2012. But others are thriving, thanks to special adaptations, such as the velamen of orchid roots, which wicks up rainwater when it drips down the tree trunk during storms. Many individuals have started fruiting and flowering, a good sign for the future viability of these reintroduced populations.


Epiphyte reintroductions in restoration plantations. (A) A reintroduced festoon of bromeliads, orchids, and cacti. (B) A fruit-bearing orchid (Cattleya forbesii), six years after reintroduction. (C) This reintroduced cactus (Epiphyllum phyllanthus) seemed to grow better in tree forks than on vertical stems, as did an aroid, (D) Philodendron bipinnatifidum. (E) Two tiny cacti have germinated in this direct seeding experiment, using seeds enrobed in paper discs. (F) Even where epiphytes have dessicated and died, experimental infrastructure continues to enhance epiphyte development; here a small bromeliad (Tillandsia recurvata) uses a piece of natural twine as a foothold.

To identify the key challenges for epiphyte restoration, it is also important to study epiphyte recolonization in naturally regenerating forests. Alex Mendes, an undergraduate researcher at ESALQ, is doing just that. On an unseasonably rainy morning, Alex, Fred, and I visited three regenerating forests near the sugar town of Rio Claro. We ducked under barbed wire fences and wandered through low, dense vegetation where Alex is systematically searching for vascular epiphytes. Two forests had rather few epiphytes – mostly generalist bromeliads – but one forest had a high density of orchids, which happened to be flowering spectacularly on the day we visited. Based on historical aerial photos, Alex knows that these three forests are at least 20 years old. They are part of a network of 75 sites that he will ultimately search for epiphytes. By the end of his undergraduate program, Alex hopes to be able to predict where epiphyte communities will regenerate on their own, and where they will need more assistance.


This secondary forest near Rio Claro might have felt like your average overgrown Psidium guajava patch had it not been  decorated with dozens of Ionopsis sp. orchids.

These are early days for learning about epiphyte restoration, and there is still a lot of work to be done. The projects that I visited in Brazil are making headway, complementing our research in Costa Rica. It remains to be seen under what circumstances epiphyte reintroductions will be most successful. Perhaps an even more important issue will be convincing funding agencies and land managers to think beyond trees.


Fred Domene and Alex Mendes are making strides in the ecology of epiphyte reintroductions and community assembly. Here, they pose with a reintroduced bromeliad (Billbergia zebrina) at Anhembi experimental station.

Fig Stakes: Shoreline Restoration for a Costa más Rica

Andres Santana is the graduate program coordinator at the Organization for Tropical Studies. During a recent fieldtrip in southern Costa Rica, he and CCSD restoration ecologist Leighton Reid compared notes on using fig stakes for ecological restoration.

Tropical beaches are many things to many people. To plants, beaches are hot, sandy, and salty – complicating their restoration.

Costa Rica has 1228 km (763 mi) of coast line – including 1016 km on the Pacific side and 212 km on the Caribbean. Along Costa Rica’s northern Pacific coast, the beach forms the natural edge of the dry forest. Farther south the adjacent forest is more humid. Giant trees, 40 m or more in height, grow right up to the high tide mark, particularly along the Caribbean.

But as with so many tropical ecosystems, Costa Rica’s coastal forests have been subject to human impacts. Many shoreline forests were cleared for cattle ranching, and exotic grasses were introduced as forage. Some of these grasses are fierce competitors and prevent tree seedlings from establishing, even long after the pastures have been abandoned.

Playa Hermosa Antes y Despues

Playa Hermosa, before (left) and after (right) planting 2-m long cuttings of a coastal fig species (Ficus goldmannii).

In 2009, a small non-profit organization, Costas Verdes, was formed to restore coastal forests along degraded shorelines, particularly wildlife refuges. The restoration work was initially challenging; tree seedlings were hard to establish along the coast because of the harsh environment – high temperatures and salinity and lack of freshwater were among the most significant obstacles. Not to mention the invasive cattle forage grasses.


Coastal restoration at Playa Hermosa

Playa Hermosa, a surfing destination on the Central Pacific coast, was among the most heavily deforested project sites. This area, part of a wetland and river estuary, was declared a national wildlife refuge in 1998. By 2009, very little forest had naturally regenerated. This led Costas Verdes to implement a restoration project at this beach. Planting plots were established where invasive grass was removed. In other areas, grasses left intact, as a comparison. It quickly became evident that tree seedlings were outcompeted by the grass. Those in the cleared plots grew better, but they still faced the other coastal habitat challenges.

Some native trees are resistant to hot substrates and high salinity, but these species were not available in tree nurseries, most of which focused on ornamental species. This meant that seedlings needed to come from locally collected and germinated seeds. We realized that this would take time to get going. Tree seedlings under 50 cm rarely survive, even if they have the proper coastal adaptations.

To accelerate the restoration, we decided to use tree cuttings rather than growing seedlings from seed. A colleague suggested Ficus goldmannii as a candidate species, so in 2011 we conducted a planting trial. We planted 225 2-m long cuttings. Of these, 195 (87%) survived their first year. By the second year all 195 survivors had become established and were quickly providing canopy cover and lowering the temperature of the sand.


An established fig stake with a dense canopy. Note the weak, patchy grass below it.

Once fig stakes created some canopy cover, we brought in other tree species – mostly from the coastal tree nursery that we created. Shade from the fig canopy also began to inhibit the invasive grasses, which require high sunlight to photosynthesize efficiently. Reduced competition with these grasses allowed other tree seedling species to survive.

In this instance Ficus cuttings turned out to be useful in promoting restoration. We have since used cuttings for other plots with similar success.


Coastal trees and shrubs growing below established fig cuttings at Playa Hermosa.

Transplanted bromeliads improve microclimate and facilitate arthropods in restored forests

Estefania Fernandez is a masters student at the University of Montpellier, France. She spent the past six months working with scientists in the Center for Conservation and Sustainable Development on a tropical forest restoration experiment in southern Costa Rica.

Costa Rica is one of the world’s most biodiverse countries, hosting 4% of flowering plant species in an area representing only 0.03% of the Earth’s terrestrial surface. With a large diversity of ecosystems, ranging from mangroves to cloud forests, Costa Rica hosts a unique family of (almost exclusively) Neotropical plants: the Bromeliaceae, commonly called bromeliads. With their colorful inflorescences and strikingly patterned leaves, numerous bromeliads are cultivated around the world for their ornamental value. Less is known, however, about their ecology in tropical ecosystems, particularly in regenerating forests.

Werauhia gladioliflora rosette, showing its overlapping leaves.

Werauhia gladioliflora rosette, showing its overlapping leaves.

Many of the so-called “tank bromeliads” are epiphytes, meaning that they grow non-parasitically on other plants. These bromeliads have ample rosettes of overlapping leaves, capable of holding considerable amounts of water. These water tanks keep them hydrated, and plant detritus that accumulates in these structures also provides bromeliads with nutrients. Arthropods take refuge in bromeliad rosettes, and consequently these plants attract mammals and birds seeking prey. Mutualistic ants build their nests in bromeliad rhizospheres, or root zones, and frogs lay eggs in the tanks. When sufficiently numerous in tree canopies, bromeliads can stabilize local temperature and humidity.

Water stored inside a W. gladioliflora tank.

Water stored inside a W. gladioliflora tank. (Photo courtesy of Dave Janas)

Despite these important ecological roles, vascular epiphytes like bromeliads are often scarce in regenerating tropical forests. Their recovery could be slowed by limited seed dispersal or by a lack of suitable recruitment sites. One way to overcome dispersal limitation is to transplant individuals. In our study area in southern Costa Rica, transplanting bromeliads is relatively simple because they are easily found on fallen tree branches in the old growth forest reserve at Las Cruces Biological Station. We hypothesized that transplanting bromeliads from the old growth forest into 10-year old forest restoration sites would buffer local temperatures and increase arthropod abundance and diversity compared to bare, control branches.

Measuring local temperature in a transplanted Aechmea dactylina.

Measuring local temperature in a transplanted Aechmea dactylina.

To test our hypothesis, we transplanted 120 bromeliads into three restoration sites in southern Costa Rica. The restoration sites are part of the Islas Project, an NSF-funded restoration experiment led by Drs. Karen Holl and Rakan Zahawi. Bromeliads were sterilized and attached to tree branches in the restoration sites with twine. Each day, we measured the microsite temperature on branches with and without transplanted bromeliads, as well as ambient temperature in the nearby air. To characterize arthropod colonization, we extracted and identified arthropods (to order) from transplanted bromeliads after two and three weeks.

We found that transplanted bromeliads decreased local temperatures on tree branches, creating a less stressful microclimate for other organisms. Bromeliads also facilitated arthropods; transplanted bromeliads were quickly colonized, especially by ants. We also observed small frogs inside of some bromeliad tanks, but none on the bare branches where we did not transplant bromeliads.

We found this frog (Craugastor stejnegerianus) in a small  Catopsis sessiliflora tank. (Photo courtesy of Dave Janas)

We found this frog (Craugastor stejnegerianus) in a small Catopsis sessiliflora tank. (Photo courtesy of Dave Janas)

Our observations suggest that bromeliad transplantation can buffer microclimates and create useful structures for invertebrates. If so, this method could improve restoration outcomes for canopy flora and fauna. Given that this experiment was conducted over a single field season, it is still an open question whether transplanted bromeliads will survive over longer time periods. It will also be important to learn whether transplanted bromeliads will facilitate colonization by other epiphytic plants. We did find some evidence of this as ferns were already growing in several bromeliads’ rhizospheres after two months.

Galápagos: A Restoration Reference for Arid Archipelagos?

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.

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.

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.

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.

Land iguana and tree cacti (Opuntia echios var. echios) on Plaza Sur Island.