Conserving an extinct species
Tracking changes in dugong populations in the Andaman and Nicobar archipelago
From what is known of dugong numbers in the Andaman and Nicobar archipelago, populations here may already be biologically extinct. But dugongs can be remarkably stubborn in the face of these odds. Understanding what enables their persistence will be critical to evolving a strategy to conserve this species.
Tracking an elusive species
As a long-lived, slow moving and slow-breeding marine mammal, dugongs are particularly vulnerable to extinction. They are globally endangered and, in Indian waters, are restricted to a few tiny pockets like the Andaman and Nicobar Islands. They are primarily herbivores and are highly dependent on seagrass meadows in which they graze. In the A&N Islands, where they were once relatively abundant, our studies have shown a clear historical decline. This decline appear to be caused by accidental trapping in gill nets, boat strikes and stranding, poaching by local communities and legally mandated hunting by the indigenous island communities .
Estimating current populations across this island chain is not easy. The species is elusive and generally shy, and the aerial techniques normally used to estimate densities are generally impracticable in these remote island groups. Fortunately, dugongs leave signs of their passing. They graze extensively in seagrass meadows and leave behind distinctive feeding trails. Our studies have shown that a feeding trail is a clear sign that a dugong has visited a meadow within the last 8 to 10 days. By regularly surveying meadows across the archipelago we can get a good sense of where dugongs are active, even without seeing the animal. Together with direct sightings and an informer network geared to inform us whenever an individual is seen, we have been able to build up a clearer picture of the occupancy of dugongs in the islands.
Conserving dugongs and their habitats
Our occupancy studies are showing that dugongs continue to persist across the island group, albeit at very low densities. Their occurrence is strongly linked to the presence of large contiguous seagrass meadows dominated by fast-growing species like Halophila and Halodule. As these meadows get fragmented, dugongs appear to move away from the area. We have been studying how dugongs use these habitats and our results show that, even at low densities, they can exert a strong pressure of herbivory on the meadow and we are now exploring if they can additionally influence the community composition of seagrass ecosystems in the island group.
Conserving dugongs in the light of their current densities is not an easy task. We are working together with the Department of Environment and Forests in the A&N Islands to evolve a suite of strategies to protect what remains of this population. So far, we have developed a strategy with specific protection and management recommendations for 56 seagrass meadows across the Andaman and Nicobar island chain, based on the level of suitability of a meadow for dugong use and the threat present. Our next step will be to help the management execute some of these recommendations.
- Journal Article2015Seagrass Herbivory Levels Sustain Site-Fidelity in a Remnant Dugong PopulationSeagrass Herbivory Levels Sustain Site- Fidelity in a Remnant Dugong PopulationPLoS ONE 10(10): e0141224. doi:10.1371/journal.pone.0141224Download
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Seagrass Herbivory Levels Sustain Site-Fidelity in a Remnant Dugong Population
Herds of dugong, a largely tropical marine megaherbivore, are known to undertake long-dis- tance movements, sequentially overgrazing seagrass meadows in their path. Given their drastic declines in many regions, it is unclear whether at lower densities, their grazing is less intense, reducing their need to travel between meadows. We studied the effect of the feeding behaviour of a small dugong population in the Andaman and Nicobar archipelago, India to understand how small isolated populations graze seagrasses. In the seven years of our observation, all recorded dugongs travelled either solitarily or in pairs, and their use of seagrasses was limited to 8 meadows, some of which were persistently grazed. These meadows were relatively large, contiguous and dominated by short-lived seagrasses spe- cies. Dugongs consumed approximately 15% of meadow primary production, but there was a large variation (3–40% of total meadow production) in consumption patterns between meadows. The impact of herbivory was relatively high, with shoot densities c. 50% higher inside herbivore exclosures than in areas exposed to repeated grazing. Our results indicate that dugongs in the study area repeatedly graze the same meadows probably because the proportion of primary production consumed reduces shoot density to levels that are still above values that can trigger meadow abandonment. This ability of seagrasses to cope per- haps explains the long-term site fidelity shown by individual dugongs in these meadows. The fact that seagrass meadows in the archipelago are able to support dugong foraging requirements allows us to clearly identify locations where this remnant population persists, and where urgent management efforts can be directed.
- Popular Article2015A day to celebrate the dugong-a story of their conservationAndaman Chronicle, 04, October
- Popular Article2015Marine Meadows – Following The Feeding Trail Of The DugongSanctuary Asia, Vol. XXXV No. 1, February 2015.
- Popular Article2014Mermaid's second lifeDown to Earth, September issue, http://www.downtoearth.org.in/content/mermaids-second-life
- Popular Article2014The Constant GardnerCurrent Conservation, Issue 8.2, http://www.currentconservation.org/?q=articles/feature&n=297
- Journal Article2013Long-Term occupancy trends in a data-poor dugong population in the Andaman and Nicobar ArchipelagoPLoS One. 8(10): e76181
Prioritizing efforts for conserving rare and threatened species with limited past data and lacking population estimates is predicated on robust assessments of their occupancy rates. This is particularly challenging for elusive, long-lived and wide- ranging marine mammals. In this paper we estimate trends in long-term (over 50 years) occupancy, persistence and extinction of a vulnerable and data-poor dugong (Dugong dugon) population across multiple seagrass meadows in the Andaman and Nicobar archipelago (India). For this we use hierarchical Bayesian dynamic occupancy models accounting for false negatives (detection probability,1), persistence and extinction, to two datasets: a) fragmentary long-term occurrence records from multiple sources (1959–2004, n = 40 locations), and b) systematic detection/non-detection data from current surveys (2010–2012, n = 57). Dugong occupancy across the archipelago declined by 60% (from 0.45 to 0.18) over the last 20 years and present distribution was largely restricted to sheltered bays and channels with seagrass meadows dominated by Halophila and Halodule sp. Dugongs were not found in patchy meadows with low seagrass cover. In general, seagrass habitat availability was not limiting for dugong occupancy, suggesting that anthropogenic factors such as entanglement in gillnets and direct hunting may have led to local extinction of dugongs from locations where extensive seagrass meadows still thrive. Effective management of these remnant dugong populations will require a multi-pronged approach, involving 1) protection of areas where dugongs still persist, 2) monitoring of seagrass habitats that dugongs could recolonize, 3) reducing gillnet use in areas used by dugongs, and 4) engaging with indigenous/settler communities to reduce impacts of hunting.
- Newsletter2013Where have all the dugongs gone? A study on long-term occupancy trends in the Andaman and Nicobar archipelago, India,Sirenews, Newsletter of the IUCN Sirenia Specialist Group, Vol. 60.
- Newsletter2013Protecting the dugong: Better late than neverSpecial bulletin of the 59th Wildlife Week Booklet Department of Environment and Forests, Andaman and Nicobar Administration, September issue.
- Book Chapter2013Dugongs in Asia. In Sirenian conservation: Issues and strategies in developing countries.Florida: University of Florida Press.
Ellen Hines. John Reynolds, Lemnuel Aragones, Antonio A. Mignucci- Giannoni, Miriam Marmontel. (Ed.)
- Popular Article2012Dugongs, mermaids of the seaThe Hindu in School, 26 September