were released

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A natural ecosystem is a community of living and non-living beings that exist in the same environment. These beings have a close relationship in which the removal of one species can greatly impact the whole ecosystem. One example of this is the disappearance of wolves in Yellowstone National Park in the mid-1900s.

When the park was established in 1872, the wolf population in the area already decreased. Due to a lack of protection from park managers and the government's predator control programme, a large number of wolves were killed in the late 1800s and early 1900s, which led to the eradication of the last wolf in 1926.

Having eliminated an entire species from the ecosystem, park managers realised their big mistake. It was wolves that kept the population of deer under control. Because of the absence of their main predator, deer multiplied and the overpopulation destroyed plants and other species habitats.

Following the deteriorating condition of the park, many scientists and biologists suggested bringing back wolves to restore the balance of the park's ecosystem. Campaigns for the reintroduction of wolves began in the mid-1900s, but it wasn't until 1995 that fourteen wolves were released into the park. That was a big success bringing about greater biodiversity and a more balanced ecosystem.

The path was made by walkers…………the mountains each year A. crossed B. crossing C. they crossed D. which they crossed