What to do when a key tortoise goes extinct? Introduce an exotic...

Cylindraspis peltastes or Domed Rodrigues Giant Tortoise went extinct c. 1800 likely from hunting and introduced predators. Illustration from 1770 by Jossigny.

What do you do when a keystone species goes extinct causing major disruption to ecosystem interactions and processes?

Christine Griffiths and fellow researchers offer a controversial solution to this problem for the case of the extinct giant tortoises endemic to the Mascarene islands. Introduce two exotic species of giant tortoises to replace the ones that have gone extinct.

Ecologists believe that the cylandrispis tortoises of the Mascarene islands served as ecosystem engineers, structuring vegetation communities by browsing and grazing on certain plants and dispersing seeds from fruits.

The extinction of the five species of cylandraspis tortoises by the late 19th century has set in motion the deterioration of the ecosystems of the Mascarene islands. An increase in grazing intolerant invasive grasses have encroached upon native grassland communities. Forests have suffered from limited seedling disperal and recruitment which has hampered natural regeneration.

The Mauritius government and a local NGO, the Mauritian Wildlife Foundation introduced small populations of Madegascar radiated and Aldebran giant tortoises to Round Island - one of the Mascarene Islands with a relatively intact ecosystem. The researchers profiled their work in the journal Restoration Ecology.

The exotic tortoises were initially kept in enclosures to more carefully study their effect on plant communities. Initial observation indicated that the tortoises preferentially grazed exotic plants. Furthermore, since their release from the enclosure, the tortoises were seen consuming fruits and dispering the seeds of a dispersal limited, endemic palm.

This approach is likely to generate substantial controversy given the long history of well-intentioned introductions of exotic species wreaking unanticipated ecological havoc. But based on a risk assessment for biological control agents, the researchers argue that the tortoise introduction poses little danger.

It helps that the island is small (255 hectares) and the ecosystem is relatively simple to understand. Furthermore, the limited noxious weed control efforts on the island are expensive and fail to address the fundamental issue driving the problem. The study authors write,

"We argue that blanket condemnation of using exotic species in restoration and conservation contexts to either restore ecological interactions or prevent the extinction of a species, respectively is counterproductive. Each introduction proposal should be rigorously examined on a case-by-case and locality-bylocality basis as proposed by Donlan et al."

--Reviewed by Rob Goldstein    

Griffiths, C., Jones, C., Hansen, D., Puttoo, M., Tatayah, R., Müller, C., & Harris, S. (2009). The Use of Extant Non-Indigenous Tortoises as a Restoration Tool to Replace Extinct Ecosystem Engineers Restoration Ecology DOI: 10.1111/j.1526-100X.2009.00612.x