Seven years of rewilding with giant tortoises

Rosemary Moorhouse-Gann speaks on their latest research which investigates how rewilding Giant Tortoise can affect an Island ecosystem.

The dodo, perhaps the most famous animal originating from Mauritius, is tragically extinct. Less well known are the two extinct species of giant tortoise that were found only in Mauritius. You can see shadows of the lost tortoises in Mauritian plant communities today, in the form of evolutionary adaptations to deter herbivory. Mauritian ecosystems have suffered in the absence of tortoises, by, for example, plants with fleshy fruits losing a key seed disperser. As part of a campaign to restore Mauritian ecosystems, the Mauritian Wildlife Foundation and partners began introducing non-native Aldabra giant tortoises to Mauritian Islands in the 1990’s.

Non-native Aldabra giant tortoises have been introduced to Round Island. These tortoises are analogue species for extinct giant tortoises endemic to Mauritius. Photo Pouvalen Seeneevassin.

Our Round Island Study

Our study took place in 2015 on Round Island: a 219 ha island nature reserve unique for its reptile, seabird and plant communities. Here, introduced Aldabra giant tortoises have been roaming free since 2008.

We analysed plant community data and dietary information (from dietary DNA metabarcoding, feeding observations, and searching for plant remains in faeces) to understand how tortoises modified plant communities and the consequences for endemic Telfair’s Skinks.

To maximise the taxonomic resolution of the DNA metabarcoding data (a method that allows you to identify multiple taxa in a pooled sample from DNA), we also comprehensively DNA barcoded the island’s plant community. We now have a DNA reference library that can be used to identify almost all plant species from a short section of DNA.

Tortoises are unlikely to be detrimental for endemic skinks. A Telfair’s skink is basking on the head of an introduced Aldabra giant tortoise. Photo Nik Cole.

Management Implications

We are often asked if rewilding with tortoises works. The answer of course depends on the goals and vision of the restoration programme. The long-term vision for Round Island is to restore a self-sustainable, functional and resilient Mascarene ecosystem. To what extent we think tortoise introductions have brought us closer to realising this vision is nuanced and evolving as we learn more about the ecosystem. Nevertheless, our study produced the following management headlines:

Tortoises disperse palm seeds

Clusters of young Latanier Bleu palms are only found away from parent trees in areas where tortoises are present. Tortoises are likely to play a role in increasing the extent of the lowland palm-rich community: a community that has disappeared from the rest of Mauritius.

Mauritius’ last fragments of lowland palm-rich habitat are found on Round Island. Tortoises disperse the seeds of Latanier Bleu, the most abundant palm on Round Island. Photo Rosemary Moorhouse-Gann.

Tortoises are unlikely to be detrimental for endemic skinks

We compared the dietary preferences of tortoises and Telfair’s skinks and found that they were negatively correlated. This indicates that tortoises are not typically targeting food resources that are preferred by skinks. Telfair’s skinks, alongside 6 other endemic reptile species found on the island, have increased substantially with restoration efforts. Thus, we are confident that tortoises are not having a negative impact on Telfair’s skinks, but this subject should be revisited as the tortoise population increases and matures.

Further conservation interventions required to restore a native Mauritian tortoise grazed plant community

We hypothesise that a tortoise grazed plant community would have existed in patches on Round Island, composed of grasses, sedges, herbaceous plants and succulents that thrive in open and grazed habitats. We found that tortoise presence is associated with an increase in bare ground, which may provide opportunities for this plant community to regenerate. However, components of this plant community have not increased in the presence of tortoises. We suspect that the Round Island soil seed bank is depleted, and populations are too small and isolated to recolonise. We recommend that restoration ecologists intervene to support the restoration of this plant community type.

Since 2015, planting efforts have accelerated. Pouvalen Seeneevassin, Round Island Warden, waters a planted endemic Aloe tormentorii. Photo Rosemary Moorhouse-Gann.

Tortoise introductions are not a panacea for the problem of non-native plants

We found that tortoises reduced the cover of native herbs and creepers and increased the cover of a non-native herb, Desmodium incanum, which is known to withstand heavy grazing. The examination of plant remains in faeces confirmed that tortoises also disperse the seeds of this plant. Whether these seeds germinate and how their success rate compares to dispersal by other means is unknown.

Beyond 2015

Since the end of our study, the planting restoration work on Round Island has accelerated. The restoration team have extended the irrigation network and a team of wardens work tirelessly to plant out, water and monitor native plants. This includes some of the components of a Mauritian tortoise grazed community. The role of tortoises will continue to be evaluated as the ecosystem undergoes restoration.

Read the full paper Impacts of herbivory by ecological replacements on an island ecosystem in Journal of Applied Ecology.