As conservation biologists, we are deeply concerned with preventing species from going extinct, and rightly so. There is a frightening number of species teetering on the brink, and we have demonstrated that it is possible to bring some of them back. We are in the midst of an era of defaunation – ecosystems worldwide are being drained of their wildlife, at heavy ecological and human costs. Although it would take the most dewy-eyed of optimists to hope that we could save all species, we are still compelled to do our utmost to reverse the extinction trend.
What happens when a species disappears from an ecosystem? In many cases, ecological processes begin to unravel. Aldo Leopold, well ahead of his time, beautifully described the devastation of losing a top predator, as he learnt to ‘think like a mountain’. In Mauritius, evidence of ecosystem dysfunction is present all around, to those with the eyes to see it. For example, a number of endemic trees are failing to regenerate, for lack of an effective seed disperser. Although the extinction of the Dodo was famously put forward as the cause for one species’ demise, the loss of endemic tortoises has been much more devastating. The effects of tortoise herbivory on the local flora is also visible in the plants’ very morphology: many lowland species demonstrate heterophylly, whereby their juvenile leaves, often long, thin and brightly coloured, differ greatly from adult leaves. It has been suggested that heterophylly arose as a defence against browsing from tortoises. The persistence of these adaptations in plant populations, centuries after they have become effectively futile, nicely illustrates the words of John Lawton, that “the ecology of the afterlife is alive and well.”
While it is not possible, just yet, to bring back extinct species, using extant species to re-ignite lost ecological processes is an exciting addition to the conservationist’s tool belt. On two offshore islands in Mauritius, tortoises from the Seychelles and Madagascar have been introduced for the ecological benefits they can provide. I asked Dr. Christine Griffiths, world expert in the use of ecological replacements and one of the masterminds behind the Mauritian tortoise translocations, to help clarify some aspects of the project:
MK: How would you define rewilding and why is the concept relevant to Mauritius?
CG: Rewilding is when animals or plants are introduced to restore natural processes or ecosystem functions. It is an ecosystem-focused translocation approach rather than one focusing on the status of the animal or plant being introduced. It can include species being reintroduced into their indigenous range (termed reintroduction) or those being introduced outside of their indigenous range, in which case it is an ecological replacement.
Mauritius is an oceanic island which has lost the majority of its large-bodied vertebrates, of which some were keystone species, such as the giant tortoises. Because the fauna and flora on Mauritius evolved in isolation and due to its high levels of endemicity, the loss of a species often meant the loss of ecosystem functions which shaped the community. Such losses can have what we call trophic cascades whereby the loss of one species can affect the future survival of other species. For example, on Mauritius, there are many large-seeded fleshy fruit, which today are no longer dispersed because the remaining native frugivores are too small to consume these fruit. Consequently, most of the seeds which were once dispersed by the large native animals rot close to the parent trees. As plants are reliant on the dispersal of their seeds for maintaining healthy populations and to be able to adapt to changes, this has significant implications for the future of Mauritius’ forests.
MK: How do you think science can best inform the use of ecological replacement species?
CG: Conservation work should always be evidence-based, whether it be based on initial observations or from simple experiments. Only by testing things can we really have an understanding of how things work. We must be able to prove and justify all our management decisions.
MK:Given the evidence you have seen so far, how successful would you say the introduction of Aldabra Giant tortoises to the offshore islands has been in assisting the conservation of Mauritian endangered species?
CG: To date, the evidence of using the Aldabra giant tortoises as an ecological replacement to disperse native seeds and control invasive weeds is positive. Native plant species which were previously not dispersed are being carried considerable distances and their germination and survival has been vastly improved. Time will tell if the seedlings of tortoise-dispersed seeds will grow into adult trees and produce viable offspring – the real test as to the success of the tortoises as successful ecological replacements. The importance with all rewilding projects is that monitoring is ongoing and like with any translocation project (reintroduction, reinforcement, assisted colonisation, ecological replacement) success at one time can be failure at another time.
Another important aspect of rewilding projects is looking at the cost-benefits of using, say, an ecological replacement. We should always compare the impacts with and without a replacement species and also consider what impact a native species would have on the ecosystem if it was possible to reintroduce it.
MK: How do you foresee the future of rewilding in Mauritius and the rest of the region?
CG: Mauritius, and the rest of the Indian Ocean Islands, offers a unique opportunity to do many similar rewilding experiments as these oceanic islands have a relatively simple species richness compared to continental systems, and have well-studied ecological histories because they were recently colonized by humans. Hence we can easily conduct hypothesis-driven experiments with a relatively good understanding of how the ecosystem functioned before the arrival of humans, which is invariably taken as the baseline to which we should aim to rehabilitate an ecosystem, and also what were the impacts of species extinctions on the remnant species.
Continuing to test the use of Aldabra giant tortoises on the island of Reunion, Rodrigues, mainland Mauritius and other Western Indian Ocean islands which had a history of tortoises would further our understanding of how ecosystems worked and how we can rebuild them to enable them to persist in a changing world.