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By Sonya Angelica Diehn
Dams are often touted as environmentally friendly. Although they do represent a renewable source of energy, a closer look reveals that they are far from green. DW lays out the biggest environmental problems of mega-dams.
1. Dams Alter Ecosystems
Water is life — and since dams block water, that impacts life downstream, both for ecosystems and people. In the case of the Grand Ethiopian Renaissance Dam (GERD), which is being built in Ethiopia and is set to be Africa's largest source of hydroelectric power, Egypt is concerned it will receive less water for things like agriculture.
Downstream ecosystems rely not only on water, but also on sediment, both of which are held back by big dams. As solid materials build up in a manmade reservoir, downstream land becomes less fertile and riverbeds can become deeper or even erode away. Emilio Moran, a professor of geography and environment at Michigan State University in the US, described sediment loss of 30 to 40% as a result of large dams.
"Rivers carry sediment that feeds the fish, it feeds the entire vegetation along the river. So, when you stop sediment flowing freely down the streams, you have a dead river."
And ecosystems may have adapted to natural flooding, which dams take away.
Mega-dams also often have a large footprint on land upstream. Aside from displacing human communities, flooding to create a reservoir also kills plants, and leaves animals to drown or find new homes. Reservoirs can also further fragment valuable habitat and cut off migratory corridors.
2. Dams Reduce Biodiversity and Cause Extinction
Aquatic species, particularly fish, are vulnerable to the impacts of dams. Moran says the Itaipu Dam, which was constructed on the border between Paraguay and Brazil in the 1970s and 1980s, resulted in a 70 percent loss of biodiversity.
"On the Tucuruí Dam that was built in the 80s in the Amazon," he added, "there was a 60% drop in productivity of fish."
Many fish species rely on the ability to move about freely in a river, be it to seek food or return to where they were born. Migratory species are badly affected by the presence of dams. In 2016, the International Union for the Conservation of Nature (IUCN) reported a 99% drop in catches of sturgeon and paddlefish — both of which are migratory — over a period of three decades. Overfishing and river alteration were cited as major threats to the species' survival.
A 2018 study predicted that fish stocks on Asia's Mekong River could drop by 40% as a result of dam projects – with consequences not only for biodiversity, but for the people whose lives and livelihoods depend on those fish.
The stakes for biodiversity are particularly high for animals threatened with extinction. And not only for aquatic species. The Tapanuli orangutan — the Earth's rarest ape, with only 500 individuals left — could finally be pushed to the brink if a planned hydroelectric project in Sumatra, Indonesia, is completed. Dams can literally snuff out species.
3. Dams Contribute to Climate Change (and Are Affected by It)
As reservoirs fill, upstream forests are flooded, eliminating their function as carbon sinks. As the drowned vegetation decomposes, decaying plants in manmade reservoirs release methane, a powerful greenhouse gas. That makes reservoirs sources of emissions — particularly those in tropical forests, where there is dense growth. It's estimated that greenhouse gas emissions from dams amount to about a billion tons annually, making it a significant global source.
And as the climate changes, more frequent and prolonged drought means dams will capture less water, resulting in lower electricity production. Countries dependent on hydropower will be especially vulnerable as temperatures keep rising.
Moran described a vicious circle, for example in Brazil, which gets 60 to 70% of its energy from hydropower: "If you wipe out half the rainforest, there will a loss of half the rainfall. And then there won't be enough water to provide the amount of power from those dams," he explained.
4. Dams Reduce Water Quality
Manmade reservoirs trap fertilizers that run into the water from surrounding land. In addition, in some developing countries, sewage flows directly into the reservoirs. This kind of pollution can result in algae blooms that suck the oxygen out of the water, making it acidic and potentially harmful to people and animals.
Still water in large manmade lakes is warm at the top and cold at the bottom, which can also affect water quality. While warm water promotes the growth of harmful algae, the cold water that is often released through turbines from the bottom of a reservoir may contain damagingly high mineral concentrations.
In some cases, water in manmade reservoirs is of such bad quality that it is not even fit to drink.
5. Dams Waste Water
Since more surface area of the water gets exposed to the sun, reservoirs result in much more evaporation than the natural flow of the river before that dam existed. It's estimated at least 7% of the total amount of freshwater needed for human activities evaporates from the world's reservoirs every year.
This effect is made worse in hot regions, Moran pointed out. "Certainly if you had a reservoir in a tropical area with high temperatures, there is going to be a lot of evaporation," he said. And big reservoirs "are, of course, evaporating constantly."
Reservoirs are also a haven for invasive plant species, and weed-covered reservoir banks can lead to evapotranspiration — or the transfer of water from the land to the atmosphere through evaporation from soil and transpiration from plants. Such evapotranspiration amounts to six times more than the evaporation from the water's surface. And there is even evidence that dams increase water use and promote water waste by creating a false sense of water security.
In the face of dwindling global freshwater resources, some question whether dams should be reconsidered.
So What Are the Alternatives?
The evidence is damning. But if mega-dams have so many harmful environmental effects, what are the alternatives? Although some green groups point to small hydropower as being more ecologically sound, Moran is skeptical. "A dam is a dam - it's blocking the fish, it's blocking the sediment."
He pointed to the need to consider not just how to maximize energy production, but also maintain ecological productivity. One option he cited is the use of in-stream turbines.
And many environment advocates agree that other renewable energies such as solar and wind can provide clean electricity at a far lower environmental cost.
Reposted with permission from Deutsche Welle.
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By Gavin Naylor
Sharks elicit outsized fear, even though the risk of a shark bite is infinitesimally small. As a marine biologist and director of the Florida Program for Shark Research, I oversee the International Shark Attack File – a global record of reported shark bites that has been maintained continuously since 1958.
A Big, Diverse Family<p>Not all sharks are the same. Only a dozen or so of the roughly 520 shark species pose any risk to people. Even the three species that account for almost all shark bite fatalities – the <a href="https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharodon-carcharias/" target="_blank">white shark</a> (<em>Carcharodon carcharias</em>), <a href="https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/galeocerdo-cuvier/" target="_blank">tiger shark</a> (<em>Galeocerdo cuvier</em>) and <a href="https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharhinus-leucas/" target="_blank">bull shark</a> (<em>Carcharhinus leucas</em>) – are behaviorally and evolutionarily very different from one another.</p><p>The tiger shark and bull shark are genetically as different from each other as a dog is from a rabbit. And both of these species are about as different from a white shark as a dog is from a kangaroo. The evolutionary lineages leading to the two groups split 170 million years ago, during the age of dinosaurs and before the origin of birds, and <a href="https://www.ck12.org/book/CK-12-Human-Biology/section/7.2/" target="_blank">110 million years before the origin of primates</a>.</p>
White, tiger and bull sharks are distinct species that diverged genetically tens of millions of years ago. Gavin Naylor / CC BY-ND<p>Yet many people assume all sharks are alike and equally likely to bite humans. Consider the term "shark attack," which is scientifically equivalent to "mammal attack." Nobody would equate dog bites with hamster bites, but this is exactly what we do when it comes to sharks.</p><p>So, when a reporter calls me about a fatality caused by a white shark off Cape Cod and asks my advice for beachgoers in North Carolina, it's essentially like asking, "A man was killed by a dog on Cape Cod. What precautions should people take when dealing with kangaroos in North Carolina?"</p>
Know Your Species<p>Understanding local species' behavior and life habits is one of the best ways to stay safe. For example, almost all shark bites that occur off Cape Cod are by white sharks, which are a large, primarily cold-water species that spend most of their time in isolation feeding on fishes. But they also aggregate near seal colonies that provide a reliable food source at certain times of the year.</p><p>Shark bites in the Carolinas are by warm-water species like bull sharks, tiger sharks and <a href="https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/carcharhinus-limbatus/" target="_blank">blacktips</a> (<em>Carcharhinus limbatus</em>). Each species is associated with particular habitats and dietary preferences.</p><p>Blacktips, which we suspect are responsible for most relatively minor bites on humans in the southeastern United States, feed on schooling bait fishes like menhaden. In contrast, bull sharks are equally at home in fresh water and salt water, and are often found near estuaries. Their bites are more severe than those of blacktips, as they are larger, more powerful, bolder and more tenacious. Several fatalities have been ascribed to bull sharks.</p><p>Tiger sharks are also large, and are responsible for a significant fraction of fatalities, particularly off the coast of volcanic islands like Hawaii and Reunion. They are tropical animals that often venture into shallow water frequented by swimmers and surfers.</p>
<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="1cfa1c9590e386ecc9455b4762b87518"><iframe lazy-loadable="true" src="https://www.youtube.com/embed/yDqzGOa-adc?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span>
Humans Are Not Targets<p>Sharks do not "hunt" humans. Data from the International Shark Attack File compiled over the past 60 years show a tight association between shark bites and the number of people in the water. In other words, shark bites are a simple function of the probability of encountering a shark.</p><p>This underscores the fact that shark bites are almost always cases of mistaken identity. If sharks actively hunted people, there would be many more bites, since humans make very easy targets when they swim in sharks' natural habitats.</p><p>Local conditions can also affect the risk of an attack. Encounters are more likely when sharks venture closer to shore, into areas where people are swimming. They may do this because they are following bait fishes or seals upon which they prey.</p><p>This means we can use environmental variables such as temperature, tide or weather conditions to better predict movement of bait fish toward the shoreline, which in turn will predict the presence of sharks. Over the next few years, the Florida Program for Shark Research will work with colleagues at other universities to monitor onshore and offshore movements of tagged sharks and their association with environmental variables so that we can improve our understanding of what conditions bring sharks close to shore.</p>
More to Know<p>There still is much to learn about sharks, especially the 500 or so species that have never been implicated in a bite on humans. One example is the tiny <a href="https://www.newsweek.com/one-worlds-rarest-sharks-also-one-most-adorable-325280" target="_blank">deep sea pocket shark</a>, which has a strange pouch behind its pectoral fins.</p><p>Only two specimens of this type of shark have ever been caught – one off the coast of Chile 30 years ago, and another more recently in the Gulf of Mexico. We're not sure about the function of the pouch, but suspect it stores luminous fluid that is released to distract would-be predators – much as its close relative, the <a href="https://sharkdevocean.wordpress.com/2015/04/23/second-ever-pocket-shark-discovered-in-gulf-of-mexico/" target="_blank">tail light shark</a>, releases luminous fluid from a gland on its underside near its vent.</p>
<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="5783b39d0838d6e410344a852ed0dcc3"><iframe lazy-loadable="true" src="https://www.youtube.com/embed/UTO5debfmsg?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span><p>Sharks range in form from the bizarre <a href="https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/mitsukurina-owstoni/" target="_blank">goblin shark</a> (<em>Mitsukurina owstoni</em>), most commonly encountered in Japan, to the gentle filter-feeding <a href="https://www.floridamuseum.ufl.edu/discover-fish/species-profiles/rhincodon-typus/" target="_blank">whale shark</a> (<em>Rhincodon typus</em>). Although whale sharks are the largest fishes in the world, we have yet to locate their nursery grounds, which are likely teeming with thousands of <a href="https://www.earthtouchnews.com/oceans/sharks/baby-whale-shark-rescued-from-gillnet-in-india-video/" target="_blank">foot-long pups</a>. Some deepwater sharks are primarily known from submersibles, such as the giant <a href="https://twitter.com/gavinnaylor/status/1146144452681113601" target="_blank">sixgill shark</a>, which feeds mainly on carrion but probably also preys on other animals in the deep sea.</p><p>Sharks seem familiar to almost all of us, but we know precious little about them. Our current understanding of their biology barely scratches the surface. The little we do know suggests they are profoundly different from other vertebrate animals. They've had 400 million years of independent evolution to adapt to their environments, and it's reasonable to expect they may be hiding more than a few tricks up their gills.</p>
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