Dugongs starve as climate change destroys their food supply
In Queensland, Australia, dugong populations are crashing as extreme weather events combined with increased river sediments from human activities are depleting the seagrasses they live on. Gabrielle Ahern reports.
The ocean has always attracted the curious, its hidden depths the subject of folklore and many adventurous tales. One marine creature that has captured the imagination are the sirens of the sea, better known as the dugongs (Order Sirenia). These mammals are quiet companions to shallow waters throughout the Indo-Pacific, including the northern coasts of Australia.
Dugongs are listed as vulnerable on the International Union for the Conservation of Nature’s Red List of Threatened Species. Their species decline is obvious from numbers along the Queensland coast; compared to about 72,000 in the 1960s, just 500 were recorded between Cairns to Rockhampton in 2011. So what has caused their decline over a relatively short space of time?
Scott Wooldridge is an ecological modeller and spatial (GIS) analyst. His research focuses on developing management-focused decision-support tools that assess the combined impact of local and global stress factors on the health and resilience of coral and seagrass communities. He has written a paper that investigates how impacts to water quality from river run-off are negatively affecting conditions for the growth of seagrass on which dugongs graze.
To determine water quality in the study zone – the Townsville region of the Great Barrier Reef, which is affected by run-off from the Burdekin River – he integrated remote satellite imagery measuring the depth of sunlight penetration and linked it to the abundance of seagrass recorded during field surveys off the coast. Traditionally scientists have dropped secci discs (devices that measure water clarity) to measure water quality, he says, “whereas in recent years we have been able to link those secci depth measurements with the observations from the satellite”.
The innovative technology provides an enhanced view of the Great Barrier Reef that will help marine scientists predict stressors to coastal waters and advise marine park managers how to reduce impacts to ecosystems. Seagrasses are sensitive to environmental disturbance, Wooldridge says, and a good proxy indicator species for water quality.
His study notes the sediment load carried into the lagoon by the Burdekin River is about eight times higher than before European settlement. The material produced from from agricultural, pastoral and development activities decreases coastal water quality markedly. This together with climate change leading to more extreme events is a bad combination for the seagrasses, and thus the dugong. The most direct effect of climate change on dugong populations is the damage to seagrass caused by floods, storms and cyclones. “The big events have the biggest influence on the declines of seagrass and associated declines on dugong,” Wooldridge says.
On average, about 75% of the fine sediment flowing into the Great Barrier Reef comes from the Burdekin (about 100 km south-east of Townsville) along with the Fitzroy River, which meets the ocean near Rockhampton, about 800 km south-east of Townsville. During a major flood event, you can imagine the devastating affect both rivers have on seagrass growing over 1,100 km of coastline, stretching from Rockhampton to Cairns.
Floodwaters scour the seabed and strip them of seagrass meadows and valuable seed banks, while the sediment that settles from the flood plume smothers the growth of established meadows. Seagrasses tolerate reduced light conditions for a short time but not over the long term. Similar to flowering plants, they require other seagrasses to cross-pollinate and grow. “Obviously as you get smaller and smaller seagrass beds because of poor water quality, the chances of that happening take longer,” Wooldridge says.
The species biodiversity of the Great Barrier Reef Marine Park highlights the successful impact marine park areas have made to the replenishment of wild plant and animal populations. But interruptions to the growth of seagrasses may overwhelm those benefits for dugongs. The animals are slaves to the impacts caused by climate change and man-made activities that directly affect their only source of food. Even though they can migrate over long distances, the quality of seagrass in Cairns might be poorer compared to those growing in waters off Rockhampton.
Because of this, Wooldridge says, dugong populations are slowly starving to death. “In some terrestrial models, animals are declining because they’ve lost habitat and are being killed by other mechanisms, but this one’s a starvation thing.”
He hopes that improving land management practices can halt major mortality events like the one in 2011 that followed Cyclone Yasi. Other impacts causing dugong fatality, such as boat strikes and net drownings, are “low-hanging fruit”, simpler to manage and controllable through better regulation. But given the dugong is only able to produce just one calf every three to five years, the greater question is whether anything can be done to save the dwindling dugong population from the increased intensity of climatic events eroding seagrass abundance.