By Peter Carr, ZSL

New research published this week in Restoration Ecology, led by scientists at ZSL’s Institute of Zoology, shows the potential benefits to breeding seabirds of converting coconut plantations to native habitats after invasive predators have been eradicated.

On many Pacific and Indian Ocean islands, colonisation by humans brought invasive species, the destruction of native habitats, and widespread growth of coconut plantations, leading to the decimation of seabird populations. The coconut industry on oceanic islands has since crashed, leaving the legacy of abandoned coconut plantations that, by themselves, create species-poor biomes. When an island’s flora is dominated by abandoned coconut plantations and it has invasive rats, it becomes an avian desert.

Experts from the Institute of Zoology, Zoological Society of London, along with colleagues from the Royal Botanic Gardens, Kew, the University of Exeter, and Heriot-Watt University, have been researching the possible outcomes for breeding seabirds (e.g., Red-footed Booby and Lesser Noddy) of eradicating rats from oceanic islands, with and without conversion of surrounding abandoned coconut plantations.

Working for over a decade, collecting data in the Chagos Archipelago, central Indian Ocean, scientists counted every breeding seabird on all 55 islands of the archipelago, and mapped and recorded the habitat they were breeding in. By comparing the number of seabirds breeding in a specific habitat on rat-free islands, they were able to predict the number of seabirds that could potentially colonise an island if invasive rats were eradicated, and abandoned coconut plantations were converted to native habitats. This is especially relevant in the Chagos Archipelago, as some 94% of the terrestrial landmass is rat-infested, where the vegetation is dominated by abandoned plantations.

One island, Ile du Coin in Peros Banhos atoll which, at 1.26km squared, is the fourth largest in the Chagos Archipelago, was hypothetically ecologically restored. This island has rats, and 92% of its vegetation is made up of former coconut plantation. At present, there are 51 pairs of breeding seabirds on this island, made up of three generalist species (Brown and Lesser Noddy and Common White Tern). Scientists predict that, following rat eradication, without any habitat management, the number of breeding pairs could rise to 4,306 pairs of 14 species. However if 1 km squared of abandoned plantation is converted to equal measures of native savannah and forest (example shown below), the number of breeding pairs could potentially increase to 319,762 of 16 species – more than the entire archipelago at present.

This research has practical applications throughout the Tropics. It shows that in order to restore tropical oceanic seabird islands that have been ecologically degraded, due to introduced predators and the destruction of native habitat, eradicating the predator as a single intervention method is unlikely to result in fully functional, seabird-driven ecosystems. On degraded islands where invasive rats and abandoned coconut plantations exist together, to restore seabird-driven ecosystems, rats must be eradicated, but also the plantations must be converted to native habitat.

In the Chagos Archipelago, as elsewhere in the Tropics, restoring seabird islands is no longer a ‘green dream’, it is a matter of funding and political goodwill. This research shows the requirement for an ecosystem-wide approach in order to fulfil these green dreams, and demonstrates the potential colossal gains to the family of birds that are suffering the greatest decline in number – seabirds.

You can access the paper here, and find out more about our work in the Chagos Archipelago here.

Recent work published by our researchers has shown that differences in movement strategies between two closely-related reef shark species can influence how vulnerable they are to poaching risk in the Chagos Archipelago and that by studying these movements, we can help to inform enforcement decisions in the MPA. Network analysis led by Dr David Jacoby, investigated the movement and space use of silvertip sharks (Carcharhinus albimarginatus) and grey reef sharks (Carcharhinus amblyrhynchos) around Peros Banhos and Salomon atolls in the Chagos Archipelago and showed that despite living side-by-side, the two species move through the MPA in very different ways. The study used acoustic tracking to study how individuals from these two shark species moved within the MPA. This method involves catching sharks, surgically implanting acoustic tags and setting up an array of underwater receivers which record the tag ID as a tagged shark swims past. From this, the researchers were able to highlight key movement corridors that MPA enforcement patrols can focus on to help protect the sharks from illegal, unregulated and unreported (IUU) fishing activity. This study also showed that silvertip sharks were more wide-ranging and therefore more vulnerable to IUU activity than grey reef sharks.

Another way to monitor species in the MPA is to use DNA. Genetic analysis can be used to identify the diversity of species in the territory and even explore whether there are distinct populations within species. One recent monitoring technique that is gaining attention in ecosystems all over the world is species detection using environmental DNA, also known as eDNA. As animals pass through their environment, they leave traces of DNA behind them. In the MPA surrounding the Chagos Archipelago, we can collect eDNA from the water and use it to determine what species are present and this is a great way to detect shark species that we otherwise might not see.

To achieve genetic monitoring and detection through eDNA, researchers must first know the genetic sequence of the species they want to study. To this end, researchers Nick Dunn (from ZSL and Imperial College London) and Shaili Johri (from Stanford University) have sequenced and published the mitochondrial genome sequences of the grey reef shark, silvertip shark, whitetip reef shark (Triaenodon obesus) and silky shark (Carcharhinus falciformis) from the Chagos Archipelago. Mitochondrial DNA is commonly used in genetic studies because it is easier to target than nuclear DNA and can be used to identify populations within species. To do this, small fin clips were taken from each of the species during tagging expeditions in the territory and their DNA was sequenced using two contrasting DNA sequencing techniques. Shaili used a handheld device called the Oxford Nanopore MinION to sequence whole mitochondrial genomes and Nick  produced his sequences using a more conventional approach on an Illumina sequencer that involves breaking DNA into small fragments and reproducing the complete sequence by using overlapping fragments, a bit like a puzzle. The mitochondrial genome sequences produced were the first ever published for the grey reef shark and the silvertip shark, and a first from populations in the Chagos Archipelago for the whitetip reef shark and the silky shark.

The sequences produced from the two techniques were over 99% similar, showing that both methods can be used in future research as the few differences were most likely to be due to differences between the individuals sequenced rather than errors. These mitochondrial genome sequences will be used to produce eDNA detection protocols for the species, which will help researchers understand how eDNA can be used to monitor shark species in the MPA. The sequences and methods used can also be used to investigate the connectivity of sharks in the Indian Ocean and to determine how significantly Chagos Archipelago reef sharks contribute to the shark fin trade.

References

Dunn, N., Johri, S., Curnick, D., Carbone, C., Elizabeth, A., Chapple, T. K., et al. (2020) Complete mitochondrial genome of the gray reef shark , Carcharhinus amblyrhynchos ( Carcharhiniformes : Carcharhinidae ). Mitochondrial DNA Part B 5, 2080–2082. doi:10.1080/23802359.2020.1765208.

Jacoby, D. M. P., Ferretti, F., Freeman, R., Carlisle, A. B., Chapple, T. K., Curnick, D. J., et al. (2020) Shark movement strategies influence poaching risk and can guide enforcement decisions in a large, remote Marine Protected Area. J. Appl. Ecol. doi:10.1111/1365‐2664.13654.

Johri, S., Chapple, T. K., Dinsdale, E. A., Schallert, R., and Block, B. A. (2020a) Mitochondrial genome of the silky shark Carcharhinus falciformis from the British Indian Ocean Territory Marine Protected Area . Mitochondrial DNA Part B. doi:10.1080/23802359.2020.1775147.

Johri, S., Chapple, T. K., Schallert, R., Dinsdale, E. A., and Block, B. A. (2020b) Complete mitochondrial genome of the whitetip reef shark Triaenodon obesus from the British Indian Ocean Territory Marine Protected Area . Mitochondrial DNA Part B. doi:10.1080/23802359.2020.1775148.

Johri, S., Dunn, N., Chapple, T. K., Curnick, D., Savolainen, V., Dinsdale, E. A., et al. (2020c) Mitochondrial genome of the Silvertip shark, Carcharhinus albimarginatus, from the British Indian Ocean Territory. Mitochondrial DNA Part B Resour. doi:10.1080/23802359.2020.1765210.

News piece by Nick Dunn