Using Exotics as Temporary Habitat: An Accidental Experiment on Rodrigues Island
Warren Cornwall Sarah DeWeerdt
By Douglas Fox Illustration by Ken Orvidas It’s a typical November morning on Rodrigues Island, a tiny speck in the Indian Ocean 560 kilometers northeast of Mauritius and just south of the equator. Sweat oozes from a chorus of pores across my back, soaking my shirt, turning my backpack into a sponge. Mary Jane Raboude,
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Anthropocene
By Douglas Fox

Illustration by Ken Orvidas

It’s a typical November morning on Rodrigues Island, a tiny speck in the Indian Ocean 560 kilometers northeast of Mauritius and just south of the equator. Sweat oozes from a chorus of pores across my back, soaking my shirt, turning my backpack into a sponge.

Mary Jane Raboude, an educator with the Mauritian Wildlife Foundation (MWF), a dark-skinned Rodrigan seemingly impervious to the heat despite her long pants, is guiding me through Grande Montagne forest preserve, where remnants of the island’s decimated native forests still persist.

We climb a stair-step path up a steep hill, through rank after rank of ferns, palms, and bushes with red berries. Every time I point to an especially attractive plant, Raboude’s response is always the same: the confirmatory glance, the slight frown, “No, that plant is introduced, too.”

Many of these plants were imported during the 1980s by the Mauritius Department of Forestry. The goal was simple: halt the island’s rapid erosion by replacing forests that were mowed down two decades before. But the methods were brutally clumsy.

Rodrigues’ native flora was ignored, and exotic plants were used instead. Only during the last six years have heroic efforts been underway to restore the native flora one hectare at a time.

Perhaps it’s ironic, then, that our entire walk is serenaded by the chic-chic-chic of Rodrigues fodies (Foudia flavicans), unseen but hardly unheard. These birds exist nowhere else on Earth, and when I ask Raboude if we can find one, she commences her own amazingly realistic chic-chic-chic, and soon a pair of them are twittering about above our head: a green-breasted male, a brown female. Even though its native forest is mostly absent, here in the foreign foliage the fody itself seems a dime a dozen.

But it wasn’t always that way. Just 30 years ago, before the exotic forest was planted, the Rodrigues fody perched at the lip of extinction, with fewer than ten birds surviving. The same is true of Rodrigues’ only other two surviving endemic vertebrates, the Rodrigues warbler (Acrocephalus rodricanus) and the Rodrigues fruit bat (Pteropus rodricensis): 30 years ago they teetered on the brink of oblivion, but today they’re doing well. These recoveries — especially those of the fody and fruit bat — are nothing short of “spectacular,” according to MWF Scientific Director Carl Jones.

And therein lies the paradox: normally, the restoration of native fauna is tied directly to restoration of a native flora habitat, or so goes conventional thinking. But Rodrigues is a glaring case of just the opposite. What began as an ecological blunder on a grand scale — the planting of an entire forest of rapidly growing exotic plants, some of them highly invasive pests — may have actually snatched Rodrigues’ three remaining endemic vertebrates from the maw of extinction just in time. And now, equipped with lessons learned the first time around, the caretakers of this accidental experiment are planning to try it elsewhere.

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Rodrigues’ forests were already degraded by the close of World War II, but their final demise came between 1955 and 1968, when most of the island’s remaining forests were converted into terraced farmland. The effects of habitat destruction were exacerbated by a series of severe droughts during the 1960s and 1970s, which further reduced the supply of food (fruit, nectar, and insects). Fody, fruit bat, and warbler populations crashed.

A survey in 1968 found fewer than ten fodies. The fruit bat population, according to a survey conducted in 1974, dropped to 70 to 100 individuals. And the warbler was thought to be extinct — several surveys conducted during the early 1970s failed to find even a single bird.

It was in the early 1970s that luck began to turn. The government began replanting forests in the watershed. Their goals were hardly conservational; commercially useful timber and fruit trees were the top priority, and native trees, which no one had bothered learning how to propagate, were forgotten. But the enterprise wasn’t entirely haphazard, either. At the very least, the MWF attempted to advise the Mauritian Forestry Department on which combinations of exotics might also benefit the endemic fauna. The goal of that advice was to provide three critical components (see Table ): rapidly growing fruit and nectar trees to feed the fody and fruit bat, leafy trees to provide nesting and roosting sites, and canopy-forming trees to support bustling insect populations for the warbler. “It wasn’t done as well as we would have liked,” observed Jones when I visited him at the MWF office in Black River, Mauritius, “but they certainly did their bit to try and help.”

And that imperfect solution paid off. From fewer than ten individuals in 1968, the fody mushroomed to about 500 by 1990 and currently stands at 1,500 to 2,000 and growing. The bat population has also grown, from about 100 in 1974 to about 1,000 in 1990, and 5,000 and growing at present. The recovery of the warbler has been less dramatic. After increasing to about 100 birds by 1990, the population has leveled off at 200 to 250 over the last five years or so because of rat predation.

These recoveries are made all the more impressive by the fact that they’ve been achieved without costly investment in efforts such as captive breeding or predator control.

Of course, purists will ask whether using exotic plants was absolutely necessary, or whether these recoveries could have just as easily been achieved with native plants. It’s a question that can never be answered for sure. But MWF plant conservation manager John Mauremootoo points out that with only a handful of individuals remaining, the fody and warbler in particular were ripe to be plunged overnight into extinction by any chance event, be it a cyclone, a disease outbreak, or yet another year of drought. As such, it was important to put up a tree canopy as quickly as possible. “I think the faster-growing nature of some of these exotic plants may have helped,” concludes Mauremootoo, “because you get the endangered fauna out of the bottleneck a bit quicker. At the time it may have saved them, which is deeply ironic.” Using exotic plants may have also sped the process for another reason: methods for propagating them were already known, eliminating several years of trial and error.

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MWF headquarters on Rodrigues Island is a no-frills white brick building shaded beneath a dense stand of tecoma trees, at a place called Solitude — an ironic name because you’re never alone: the air is teeming with mosquitoes queuing up to slurp away every last drop of your blood.

Inside the building, I speak with Richard Payendee, a thick muscular man (immune to mosquitoes) who is directing the next phase of the Rodrigues campaign. Now that the fody, fruit bat, and warbler are recovering, the exotic forest is nearing the end of its usefulness. Payendee is hell-bent on ripping it out and replacing it with newly planted native forest.

It’s a monumental effort. In the com-pound’s fenced-in nursery, row upon row of freshly potted native seedlings — over 40 species in all — stand ready to be dispatched into the forest, like Greek soldiers arrayed on a battlefield.

When large-scale restoration started in 1996, Payendee tells me, the only remaining native forest consisted of a few clumps of trees, but mostly isolated plants scattered among the exotics. The goal is to convert this to native forest bit by bit, without decreasing habitat availability. The formula for achieving this has been empirical. One-hectare plots are progressively thinned of exotic plants once every 6-12 months and each time, more native seedlings are planted. This gradual approach of maintaining levels of shading and moisture that resemble the canopy environments to which the seedlings are adapted allows the natives to grow more rapidly than does simply clear-cutting of the exotics.

So far, 18 hectares of native forest have been restored in two preserves: Anse Quitor, a wedge of arid coastal forest, and Grande Montagne, the uplands spot where Raboude showed me around. The impact of restoration is especially obvious at Grande Montagne. It’s true that exotic plants still dominate the lower slopes, but on the mountaintop, natural-looking plots of restored forest bristle with native trees and shrubs.

As the conversation winds down, I ask Payendee whether the more useful exotic trees should remain indefinitely. His reply is that any exotic plant can be replaced by a native that does the same job at least as well. Take the exotic jamrosa tree (Syzygium jambos), for example. For three decades, the Rodrigues fody happily nested among its leafy branches, and yet the fody clearly prefers the native carrot wood trees that are now being planted — choosing to build nests in newly planted specimens that are barely six feet tall, even when there are plenty of 30-foot jamrosas still to be had.

“If it’s doing some good,” Payendee concludes, “of course leave it for now. But one day it will be useless. When it’s useless we will cut it down.” He pauses and I reflect on his words. He is well spoken and gentle — but his hands are the sturdy hands of an ax-wielding executioner, and I take his words seriously. “It’s gonna take time,” he says again, this time almost to himself, “but I’m gonna cut it down.”

With that, our interview is over, and he hurries out the door on his way to a chainsaw training session.

***

Payendee’s stalwart stance on removing every shred of exotic plants is understandable, for whereas the recovery of the fody, fruit bat, and warbler has been hands-off, the restoration of native forest has not. Every square meter of newly minted native forest has been hard-won. This is because some of the same exotics that aided recovery of birds and bats turned out to be highly invasive pests in their own right, metastasizing throughout the island. And these days, the perpetual chore of weeding them from restored plots devours most of the workers’ time. They’re resorting to brush cutters, herbicides, and chainsaws to get the job done. But if Rodrigues’ entire watershed is to be restored, as MWF hopes, then less labor-intensive ways must be found. One possibility is carefully managed grazing — perhaps by giant tortoises.

Of course, Mauremootoo has better advice for others who might wish to try their own Rodrigues experiment. “The first thing you’ve got to ask,” he says, “is can a native plant do the same job? Second thing is, if you do choose an exotic, send emails out around the world and ask whether there’s reason to think this plant will be invasive.”

Another consideration is choosing exotic plants to match the geography and climate. Rodrigues hasn’t traditionally lacked running water, yet the reforestation of the 1970s brought water-hungry exotics that transpired vast quantities of the island’s runoff back into the atmosphere. Although rainfall has not declined appreciably, many of the island’s creeks — water holes where Payendee swam as a child — have since dried up.

A final bit of wisdom to be gleaned from Rodrigues is that reforestation should be practiced as a whole-ecosystem approach, rather than as an exercise in providing one plant to help one animal and another plant for another animal. In this respect, the Rodrigues experiment was well conceived. “A lot of the trees that were planted have reasonably sized fruits,” says Mauremootoo, “food for bats. And fruits have fruit flies and fruit flies have fruit fly predators, and that will help feed the warblers. You’ve started to create an exotic ecosystem.”

Armed with these lessons, the MWF is preparing to try the exotic flora approach, on a more limited scale, in Mauritius. The benefactor will be the Mauritius fody (Foudia rubra), which currently numbers just 100 pairs or so. A fair number of native hardwood trees (where the Mauritius fody traditionally nested) remain on the island. But surveys show the Mauritius fody is eight times more likely to reproduce successfully if it nests instead in the Japanese cedar (Cryptomeria japonica) — an exotic tree. The reason is that fody eggs are frequently eaten these days by introduced rats and monkeys, and the Japanese cedar provides a superior hiding place from these predators.

What’s more, experience has shown Japanese cedar not to be invasive, making it an ideal tool. MWF hopes that planting forest corridors containing Japanese cedar will provide the Mauritius fody with secure nesting and coax it to spread to other habitat sites. Another apparently noninvasive exotic, the bottlebrush tree (Callistemon citrinus), may be planted as a food source.

To purists, this business of planting exotic trees may seem like selling out to the devil, but this island paradise — like so many others — has already been to hell and back. Gone from Mauritius are the dodo, the giant tortoises, and 30 other animal species. In their place roam droves of hungry macaques, mongooses, and rats — beasts that never before trod the island since it poked its volcanic nose out of the Indian Ocean nine million years ago.

Table: Rodrigues Reforestation Key Exotic Trees

Fruit & nectar trees

Indian almond (Terminalia catappa)

Tecoma (Tabebuia pallida)

Jamrosa (Syzygium jambos)

Tamarind (Tamarindus indica)

Papaya (Carica papaya)

Fig (Ficus benghalensis)

Mango (Mangifera indica)

Nesting & roosting trees

Norfolk Island pine (Araucaria cunninghamii)

Jamrosa (Syzygium jambos)

Mango (Mangifera indica)

Insect & canopy trees

Jamrosa (Syzygium jambos)

Mahogany (Swietenia mahogani)

FILLING A VACANT NICHE

Wander the forests of Rodrigues or Mauritius, and you’ll notice that juvenile plants often have narrower, darker, waxier leaves than adult plants. And even on adult plants, the leaves closest to the ground have the “juvenile” form. These curious observations could help solve Rodrigues’ invasive plants problem.

Biologists suspect this leaf adaptation, called “heterophylly,” resulted in resistance to grazing by the herds of giant tortoises that once roamed Rodrigues and Mauritius. The MWF is thinking about introducing giant Seychelles tortoises (Dipsochelys hololissa) (the closest-surviving relative of the extinct Mauritius and Rodrigues tortoises) into Rodrigues in hopes their grazing might control exotic pests. Unlike native plants, the exotics aren’t heterophyllic, and preliminary experiments suggest the tortoises prefer dining on better-tasting, nonheterophyllic leaves. Tortoises may also help native plants by disseminating the seeds in their dung.

Giant tortoises are especially appealing because they’d fill a niche left vacant by extinctions — and they’re unlikely to become invasive pests. But even if further experiments with them pan out, it could take 20 years to breed enough of them to graze Rodrigues. In the meantime, other grazers such as goats are also being considered.

For More Information:

The Mauritian Wildlife Foundation website:

<a “http://www.mauritian-wildlife.org” href=”#”>www.mauritian-wildlife.org

Impey, A.J., I.M. C?t?, and C.G. Jones. 2002. Population recovery of the threatened endemic Rodrigues fody (Foudia flavicans) (Aves, Ploceidae) following reforestation. Biological Conservation 107(3):299-305.

Showler, D.A., I.M. C?t?, and C.G. Jones. 2002. Population census and habitat use of Rodrigues Warbler Acrocephalus rodericanus. Bird Conservation International 12(3):211-230.