Big cats are among the most widespread top predators on Earth. Lions stalk zebra in the African savanna tigers ambush antelope in the forests of Asia and jaguars hunt deer in the jungles of South America. They play a very important role in ecosystems by regulating the numbers of these herbivores in turn reducing the deterioration of vegetated habitats and maintaining species diversity altogether. Tigers lions and jaguars must cover large areas to find food so they need a lot of energy. As with many large carnivores big cats are under threat from habitat loss which leaves them with less to eat. Their spread out populations and slow reproductive rates make them particularly vulnerable. While the habitats in which big cats live range from boreal forests at high latitudes to tropical rainforests at the equator the causes of habitat loss are largely the same. Some of them are logging for wood and fuel plantations and urbanisation. Aside from depriving wildlife of a home forest loss also contributes to climate change. It is ironic then that hydroelectricity which is being developed worldwide especially in South American and Asian countries as an alternative energy source to fossil fuels is a big cause of deforestation also. Building hydroelectric dams has caused extinctions and spread diseases in rivers globally but the threat to ecosystems on land has largely been overlooked. In a recent study researchers from China and Portugal investigated how existing and planned hydroelectric dams might affect two carnivores the near threatened jaguar and the endangered tiger. The distribution of jaguars and tigers has shrunk considerably but the researchers found that hydroelectric plants pose a substantial risk of further declines. They found more than hundred dams that already intersect the jaguar range and other dams that do the same for tigers. This equates to one in five tigers that are likely to be affected by dams compared with one in two hundred jaguars. Since tiger numbers are already so low this could have a considerable influence on the capacity of population to recover and may even result in local extinctions. Large expanses of land are flooded to create reservoirs when building hydroelectric plants in low lying relatively flat areas. Although tigers and jaguars can and do swim they mainly hunt species such as deer that live on land. Sites chosen for dams typically incorporate floodplains and areas along rivers that are important for both species since they tend to contain lots of prey. The flooded area will force both predators and prey into surrounding areas. If the vegetation here can support the influx of herbivores tigers and jaguars will probably persist. But if it cannot the predators may be forced further afield in search of food potentially drawing them into conflict with people who may kill them. Relatively pristine habitats are under threat. The number of hydroelectric dams is set to quadruple across the range of jaguar where tigers roam. Aside from the direct loss of habitat these new constructions can also increase the presence of people in remote areas. Roads built to access new dam sites consequently open up areas that were previously impenetrable. Roads can be a barrier to some species and kill those that try to cross. Roads can also encourage new towns and villages which divide the habitat further.