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Eats shoots and doesn’t leave: Dugong herbivory and movement patterns in the seagrasses of the Andaman and Nicobar Islands

You probably have to be lost and alone at sea for a very long time to mistake a dugong for a luscious mermaid. At first sight, this large lumbering beast resembles little more than a shapeless gunnysack with whiskers at one end and a roughly-hewn paddle at the other. It is a graceless, grunting and utterly uncharismatic animal. And yet, there is something about its benign guilelessness that gives the dugong an irresistible patina of mystery—we would even go so far as call it mystique.

Once abundant across the shallow coastal waters between east Africa and the islands of Vanuatu in the South Pacific Ocean, dugongs are now facing alarming declines. Their slow breeding rate and extended period of parental care have made them extremely vulnerable to human threats. And with seagrass meadows world-over shrinking at an accelerated pace, things aren’t looking very promising for these mystical creatures of the sea.

Almost everything we know of these vanishing animals comes from a few populations in Australia and the Arabian Gulf.  Here, in small pockets, their populations are still relatively healthy. They form large congregating herds that wander together from meadow to meadow, covering extensive distances as they graze down grasslands in their path. It can often be several months before they return to the same seagrass patch on these long migrations. This, to the best of our knowledge, was how dugongs behaved. But these populations are the exception not the rule. In most places where dugongs continue to exist, they occur in small and fragmented numbers. And we know scarce little about these small populations.

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Aerial view of a seagrass meadow
 Seagrasses — which much like grasses on land, often grow into vast plush meadows — 
are teeming with life. These undersea grasslands can support a diversity of 
species, right from urchins and molluscs to turtles and large marine 
megaherbivores like dugongs.

Photo: Vardhan Patankar

In the Andaman and Nicobar archipelago, researchers from the Nature Conservation Foundation have been studying how a small, isolated dugong population uses seagrass meadows. Quite unlike their counterparts in Australian waters, the dugongs in the Andaman  and Nicobar Islands stayed surprisingly faithful to their meadows, feeding in the same meadows all through the year, without moving much across the archipelago. To better understand this unusual feeding behaviour, perhaps typical of small populations, the team set sail on a seven-year long study.

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Dugongs are one of the main herbivores in seagrass ecosystems across the 
archipelago. But over the last 50 years or so, their numbers have plunged 
drastically — and in some islands, they are now even locally extinct.

Photo: Vardhan Patankar

For a creature so large, long-lived and slow-moving, it is surprisingly difficult to spot. Feeding as it does in the murky depths of shallow seagrass meadows, you may catch an oh-so-fleeting glimpse of it as it surfaces to breathe before descending once again to eat. The best chance you have of knowing a dugong has been around is to put on a snorkel and mask and swim over a seagrass meadow on which it has been feeding.  Because the dugong requires large quantities of seagrass every day, it spends a large part of its day grazing the underwater sandy slopes, mowing down long, snaking paths in the meadow as it goes. Like a lamplighter, you know where it has been by the trail it leaves behind. But this means swimming over large stretches of seagrass meadow. And often. The researchers covered nearly 75% of the archipelago in small wooden dugouts, looking for surfacing dugongs and listening for their tell-tale snorts.  In addition, they put the word out among fishers and divers and other frequent users of these waters to report any dugong sighting—a database that was maintained from 2007-2013. Gathering these various strands of information, researchers of this study identified and repeatedly sampled some 44 seagrass meadows for possible dugong occurrence. Only eight of these meadows were used regularly, and dugongs clearly preferred these meadows over others, returning time and again to them even though there were several other apparently suitable meadows nearby.

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Researchers attempted to validate every sighting with field visits, detailed 
interviews, and surveys of meadows — ferreting around these undersea grasslands 
for signs and sights of dugongs!

Photos: Vardhan Patankar

What differentiated these meadows from all the others around? From what the researchers could gather, dugongs gravitated to meadows that were characteristically large, and had a continuous cover of nutrient-rich seagrass shoots. In these meadows, dugongs consumed up to 40% of seagrass shoots, significantly altering the grasslands through selective feeding, maintaining them with the juicy, fast-growing species they prefer—like gardeners of the sea.

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Studies show that dugong-frequented meadows are home to a rich assortment of marine 
lifeforms. Meadows that aren’t used by dugongs anymore change over time — with 
nutrient-rich seagrass species being replaced by those of lower nutritional 
value — which could affect other animals that depend on them as well. 

Photo: Teresa Alcoverro

What was interesting was that even though dugongs nearly halved the shoot densities with repeated grazing, the meadows were quick to recover from this foraging pressure—rebounding to original shoot densities in a little over a week! The ability of these seagrass meadows to cope with this sustained feeding probably means that dugongs do not have to move the long distances they do when they are found in larger herds. What this means for their conservation is that we can identify clear foraging areas across the archipelago that can be monitored and protected—by curtailing boat traffic, net fishing and other threats in these areas, we will have gone a long way in protecting the last remaining dugongs in the Andaman and Nicobar Islands.

Of course, researchers say, the foraging and movement patterns of dugongs may change drastically as populations plummet further. It was the very rarity of these strange creatures that generated the myths that surround the dugong and its seductress ways. Yet, if we do not work quickly to conserve what remains of their populations, the species itself will join the long list of legendary creatures that once roamed wild in the mystic sea.

 

The study was published recently in the journal PLoS ONE and can be accessed here.

 

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