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An Unlikely Farmer - Can Dugong Grow Their Own Grass?

Text by Ivan Lawler and Helene Marsh
James Cook University, Townsville, Australia

 

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Dugong
Dugong or sea cow, Dugong dugon, Indonesia, Indo-Pacific Ocean Image #: 005042

Up to our knees, and elbows, in intertidal mud we realise that it's tough work trying to be a dugong. For this is the realm in which they feed, spending most of their day uprooting and consuming seagrasses, and creating serpentine trails 20cm wide and up to several metres long in seagrass meadows. In contrast to our clumsy and messy efforts, dugongs are superbly equipped to do an efficient job – underwater.

The dugong (Dugong dugon) looks rather like a cross between a rotund dolphin and a walrus. Its body, flippers and fluke resemble those of a dolphin but it has no dorsal fin. Its head looks somewhat like that of a walrus without the long tusks. Like a walrus, the dugong is a bottom feeder. Unlike the walrus, which feeds on fish, the dugong, which can be up to about 3 metres long, is a specialist seagrass feeder.

Why were we pretending to be dugongs? We wanted to recreate the feeding trails of dugongs under controlled conditions. This would enable us to examine more closely the relationship between dugongs and their staple food. Being able to compare exactly the same sites before and after human "grazing" (and to compare them to ungrazed sites at the same time) gives a clearer picture of the effects of dugong grazing on the seagrass species present, their recovery from grazing and the changes in their quality as dugong food. Being a specialist seagrass feeder, like a dugong, is not as simple as just eating what's on your plate.

Dugongs are herbivorous mammals that spend their entire lives in the sea. Their close relatives the manatees also venture into or live in fresh water. Together dugongs and manatees make up the order Sirenia or sea cows, so-named because dugongs and manatees are thought to have given rise to the myth of the mermaids or sirens of the sea. Despite our fondness for dugongs, we can only imagine that a sailor would have to spend a particularly long time at sea to regard one as a temptress!

Seagrasses are the staple diet of dugongs, and the distribution of dugongs very closely follows that of these marine flowering plants. As seagrasses grow rooted in the sediment, they are limited by the availability of light. Consequently they are found predominantly in shallow coastal waters, and so too are dugongs. But, this is not the whole story. Dugongs do not eat all species of seagrass, preferring seagrass of higher nitrogen and lower fibre content. And the presence of dugongs is not always a result of them finding suitable seagrasses. In fact, it appears that the reverse can be true - the occurrence of palatable species of seagrasses can caused by the dugongs' grazing. Can they really grow their own grass?

This phenomenon was first observed by another member of our group, Dr Tony Preen, while he was studying dugongs in Moreton Bay, at the southern limits of their range in eastern Australia. Tony coined the term "cultivation grazing" to describe a process by which dugongs can manipulate seagrass beds to keep them in a state they like to eat.

To explain this process properly we need first to describe how dugongs feed on seagrasses. The mouthparts of dugongs are highly adapted to their feeding style. The mouth opens ventrally to suit bottom feeding (more so than their manatee cousins, which also feed on surface vegetation). In fact, in respect of their feeding habits dugongs might more aptly be named "sea pigs" rather than sea cows. This not a snide reference to the aesthetics of their eating habits, but rather to the fact that when they feed they prefer to dig out the entire plant (roots and all). In contrast, grazing cows remove only the above ground portion of grasses. This has significant consequences for the regeneration of plants after grazing.

As a herbivore, a dugong has horny plates and teeth adapted for manipulating and masticating plants. Its expanded upper lip takes the form of a horseshoe shaped disk, or muzzle, a very complex and mobile structure, well endowed with sensory bristles. This muzzle is used to detect, select and dig up seagrasses and manipulate them into the dugong’s mouth. How do we know this? While a PhD student at the University of Florida, Chris Marshall stuck seagrass through holes in a perspex plate in an aquarium in Toba Japan, mounted an underwater video camera below the plate and filmed a captive dugong feeding.

When we dig up seagrass we are covered with mud. So are our samples. Removing the mud to allow better chemical analysis of the plants is a laborious process. Being more dexterous, dugongs manage to consume seagrass sans mud. However, their table manners are far from impeccable and muddy water, floating seagrass and dugong faeces are characteristic signs of dugong feeding. These are used by traditional hunters (and scientists) to find dugongs.

The result of this style of feeding is that almost all plants are removed in distinct trails across the seagrass bed. When numbers of dugongs graze an area intensively, the meadow can appear devastated, ravenous sea cows consuming all but a small portion of the initial crop. However, paradoxically, this can be good for dugongs.

A heavily grazed seagrass bed looks like a lawn mown by a drunk. Dugongs graze apparently at random within a seagrass bed, their trails meandering in all directions across the bottom. This is rather an inefficient means of removing seagrass that results in numerous small tufts remaining. And this is where the dugongs derive some advantage from their inefficiency. The species that recover most quickly from this disturbance, spreading out vegetatively from the remaining tufts, are those that dugongs like to eat. In addition, the new growth found in these areas tends to be higher in nitrogen and lower in fibre, exactly what hungry dugongs like.

In effect, the dugongs are farming the seagrass. Several members of our group have conducted grazing experiments in subtropical and tropical sites in eastern Australia. The results show that if dugongs are prevented from grazing in part of a meadow, a progressive change in species present takes place, with the taller and less desirable (for dugongs at least) species eventually outcompeting the smaller species favoured by dugongs. In some areas, the dugongs’ strategy is to heavily graze a meadow, returning at intervals that are just long enough for the faster growing species to recover, but before the slow growing species take over. Thus an area of highly palatable food is maintained. It is the small tufts of seagrass left behind by the grazing dugongs that make this process work to their advantage.

Other processes that disturb the seagrass beds over large scales are not necessarily good for dugongs. Experience from various parts of northern Australia suggests that extreme weather, such as cyclones and floods, can result in the loss of hundreds of square kilometres of seagrass. For example, about 900 km2 of seagrass was lost in Hervey Bay in 1992, probably because of murky water from flooding of local rivers, and run-off turbulence from a cyclone three weeks later. Such events can cause extensive damage to seagrass communities through severe wave action, shifting sand and reduction in saltiness and light levels.

The effect of these losses on dugongs can be disastrous. Prior to the 1992 floods, the extensive seagrasses in Hervey Bay supported an estimated 1750 dugongs. Eight months after the floods the affected area was estimated to support only about 70 dugongs. Most animals presumably survived by moving to neighbouring areas. However, many died attempting to move to greener pastures, with emaciated carcasses washing up on beaches up to 900km away.

The major difference between the cultivation grazing of dugongs and other large-scale disturbances is the non-uniformity of the dugongs' impact. When, for example, sedimentation causes die-off, the effect is uniform, with all seagrasses dying off within the area, creating a single unvegetated patch. In contrast, the tufts left by the dugongs massively reduce the effective size of the impact. While the total area impacted might be great, the result is a matrix of ‘ungrazable reserves’ surrounding small patches of bare soil. The distance between bare soil and a surviving plant is rarely more than a metre, while in a similar area disturbed by sedimentation the distance may range from tens of metres to kilometres. In meadows disturbed by storms, recovery can be extremely slow, taking up to a decade to return to the original state. Our experiments and observations show that seagrass disturbed by dugongs recovers in a few months to a year depending on the season and the frequency of the disturbance.

The loss and variation in recovery time of seagrass meadows may explain the dugong’s roving tendencies. Our group has tracked more than 60 dugongs using satellite transmitters. Dugongs are caught using one of two techniques - hoop netting or "rodeo". The hoop net is much the same as those used for butterflies, though somewhat larger. The net is loosely attached to a frame that is plunged in front of the dugong’s head as it surfaces to breathe. The net breaks away from the frame and fits over the dugong’s head. To catch a dugong using the rodeo technique, involves rounding it up and tackling it by diving from the boat and grabbing its tail. This is very exciting but potentially stressful for the animal – and the scientists! We take great care to select only healthy animals and to handle them as quickly as possible.

Once the dugong is caught, we wrap a strap around its tail. The strap is attached to a nylon rod with the bouyant transmitter at its end. The transmitter floats to the surface while the dugong is feeding or resting in shallow water, making it more likely that a signal will be picked up while a satellite is passing overhead. Usually we are able to get between one and three location fixes each day for tagged dugongs.

Most of their movements within areas of seagrass beds and are dictated by the tides. At the southern limits of their range, dugongs also make seasonal journeys to seek refuge from the cold in deeper, warmer waters. In winter in Moreton Bay near Brisbane in eastern Australia, many dugongs make round trips of 15–40 km every few days between their foraging grounds on the shallow sand banks inside the bay and oceanic waters, which average up to 5°C warmer. All these movements are readily explained in terms of food availability or environmental conditions.

However, some dugongs undertake some puzzling long-distance journeys in which they may travel between 100 and up to 600 kilometres over a few days. Often they return to their starting point within a matter of one or two weeks. Such direct travel, with scarcely time to stop for a bite to eat, implies that they have both a specific goal in mind, and an impressive geographical knowledge. What could motivate an animal to make such a substantial effort to go somewhere, only to return a short time later? We can think of only two alternatives thus far: food and sex.

So far sex seems an unlikely explanation. It would seem sensible that to travel such a distance to mate would be advantageous in terms of finding an unrelated partner and increasing genetic diversity within the population. However, satellite tags only signal when the dugong is quite still and the transmitter can float at the surface. When they make these long movements they swim almost constantly and the transmitter is pulled along behind them and below the surface. All we know is that they've disappeared until they pop up somewhere hundreds of kilometres away a few days later.

For this reason we have so far not managed to observe whether they mate after these journeys. The only evidence we currently have is that of Dani Tikel's PhD study of dugong population genetics. Her findings were the opposite of what we expected. They show that the dugong population of the eastern coast of Queensland is in fact a series of separate sub-populations that interbreed very little. Clearly another explanation for the long distance swims of some dugongs is needed. If not sex, then food, perhaps?

We speculate that dugongs undertake long swims ‘to check out’ the status of the seagrass meadows in their region. Long-term studies of seagrass meadows around Townsville, North Queensland, indicate that the `weedy’ types of seagrasses that dugongs prefer come and go. A 400 kg herbivore like a dugong needs to spend most of the time eating if it is to consume the 30 kilograms or more of seagrass it requires every day. Dugongs need to know where they can find their preferred food and to check the results of their farming activities.

Better knowledge of dugongs' movements may be an important factor in their conservation. One of the significant human-related causes of dugong deaths is entanglement in fishing nets. In response to concerns about declines in dugong abundance along much of the urban coast of Queensland, the Australian and Queensland governments established a system of Dugong Protection Areas (DPAs) in the Great Barrier Reef region and Hervey Bay. The boundaries of these areas were based in part on our locations from satellite tracking. Gill netting is greatly restricted or banned in some of these areas. In others it is subject to lesser modifications designed to reduce the probability of a dugong drowning after entangling in nets.

Of course, dugongs swim in and out of these protection areas. The rationale is that they should be effective provided they consistently support high numbers of dugongs. This strategy will only work if these areas remain high quality dugong habitat. This means more than preventing pollution from damaging seagrass beds. It means giving the dugongs protection from the interruption of boat traffic. Dugongs need time and space if they are to ‘grow their own grass’.




Summary of dugong information

• Dugongs and manatees are the only surviving species in the order Sirenia. Dugongs are the only species in the family Dugongidae. The three species of manatees are included in the family Trichechidae, genus Trichechus. Dugongs can be distinguished from manatees by their whale-like tails (manatee tails are rounded, or paddle shaped).

• The only other modern dugongid, the much larger Steller's Sea Cow (up to 8m long and weighing some 6 tonnes), was exterminated by hunters within 30 years of its discovery off the coast of Siberia in 1741.

• Sirenians are unique in being the only entirely aquatic herbivorous mammals. Dugongs are the only living species that is wholly marine.

• Dugongs are specialised to feed on a diet of seagrass. They prefer pioneer species that are high in nitrogen and low fibre. In some parts of their range they may also supplement their diets with signficant amounts of animal material such as sea squirts (ascidians) and other soft bodied invertebrates.

• The closest terrestrial relatives of dugongs are elephants and hyraxes.

• Dugongs grow to approximately 3 metres in length and weight up to 400kg. They live for some 70 years or more. They are sexually mature at approximately 10 years of age and have one calf every 3-5 years. This means that dugong populations will go into decline if adult mortality exceeds 5% of the population per year.

• The distribution of dugongs extends across the Indo-Pacific from eastern Africa and Madagascar through Australia to Vanuatu and north to India and Japan.

• Dugong numbers have decreased throughout much of their range, and they have been listed as vulnerable to extinction by the IUCN.

• Australia remains the stronghold of the dugong. The total Australian population is estimated at approximately 85000 individuals. This is likely to be an underestimate because a conservative correction factor is used to allow for animals under water, and hence not visible, at the time of the survey. Also, large sections of the Australian coastline, principally in the northwest, have not been surveyed for dugongs, and likely support significant populations.

• Dugongs continue to be hunted by some Aboriginal and Torres Strait Islander communities in Australia. Hunting of dugongs is of major cultural importance to these people. Some communities are now teaming up with researchers and management agencies to develop plans that will ensure that such hunting is sustainable.

• One of the major requirements for effective conservation is knowledge of population size. Research into the diving behaviour and visibility of dugongs to observers in aircraft is ongoing. This will feed back into aerial surveys and allow more accurate population estimates.