On Tuesday people around the globe celebrated World Water Day – a day designated by the United Nations as an opportunity to reflect on the importance of water to the health of humans and the environment.
By Dr. Douglas McCauley, University of California Santa Barbara
Hippopotamus have an obligate dependence on water – that is they just can’t do without it. Hippos graze on land, typically at night. But like gigantic, slobbery, vegetarian vampires, hippos must get back to their watery resting places every morning before sunrise. Their closest living relatives of hippos are in fact cetaceans – whales and dolphins. Like these distant cousins, they can’t survive without reliable access to water. If a hippo spends too long out of the water it overheats, dehydrates, its sensitive skin dries, cracks, and sunburns.
The intense dependency of hippos on water is a tough card to draw in Africa, the most water stressed continent on the planet. The impacts of water scarcity on human health in Africa are widely known, but lack of water also profoundly affects wildlife.
Our research group, with support from Safari Club International Foundation, has been intensely studying the interplay between water and the ecology of hippo populations in Tanzania’s Ruaha National Park. The Great Ruaha River meanders through the arid Ruaha landscape and is a life giving force for the ecosystems and wildlife of the region. But, the Great Ruaha River is not as great as is used to be. Intense development of rice plantations in the headwaters of the Ruaha have increased more than tenfold in recent years and have diverted massive amounts of freshwater from the river. The Ruaha consequently has been transformed from a river that flowed year round to a river that now dries for multiple months every year. This scenario is repeating itself across the continent where often highly inefficient use of water for rapidly expanding agriculture projects accounts to close to 90% of freshwater withdrawals.
We have been studying how reduced flow in the Ruaha shapes the impact that hippos have on river ecosystems and affects hippo behavior. Hippos are critically important conveyor belts of nutrients and energy from land ecosystems to aquatic ecosystems. Over the course of a year they consume tons of grass and other vegetation during their nocturnal feeding forays. Much of this gets deposited as dung into their watery refuges. By probing the chemistry of the tissue of fish and aquatic insects, our research showed that river animals rely on this hippo-vectored fertilizer. These findings highlight a hidden bonus of keeping around a healthy hippo population – you may get more fish in your river or lake. Fish are in fact a major source of protein and nutrition to communities that share these same landscapes.
When we alter the hydrology of these rivers, we create a situation where we have too much of a good thing. Our work in Ruaha has shown that when the river dries down to a series of swimming pool-sized water holes, the dung of hippos eking out and existing in these shrinking pools gets so highly concentrated that it can pollute the waters, causing algal blooms and mass losses of oxygen. By taking too much water out of the river, our science indicates that we are transforming hippos from a life giving force, to a force that poisons river life. We hope that this research will help highlight the importance of hippos to the environment and society in Africa, and will inspire more efficient and responsible use of the continent’s precious freshwater resources.
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