The presence of wolves in Yellowstone changes the patterns of movement of elk enough that it influences the course of streams. The sea otter example is a classic. The sea otters live in kelp beds, and eat urchins among other animals. If the otters are removed, the urchin population explodes, they eat the bases of all the kelp, and what is left is called "urchin barrens." I remember Grand Canyon had too many deer, they ate all the low branches on fir trees and killed any young trees, all because all the cougars had been shot. The thing I'm into is reef sharks- on super remote reefs, there are huge numbers of reef sharks, and that has a huge effect on the other fish. They have just discovered that places with more big predatory fish have less coral disease. Where there are predators there are fewer little butterflyfish, and some of the butterflyfish eat coral polyps, and apparently transfer disease from one colony to the next, spreading the disease. Those beautiful reefs you snorkeled and dove in Hawaii are radically different from a natural reef. No sharks, tons of little fish. They used to have "shark eradication" programs in Hawaii , killed thousands of sharks. Reef sharks that wouldn't hurt a flea.-Guest post by Douglas Fenner, Coral Reef Ecologist, Dept. Marine & Wildlife Resources, American Samoa. Also see his article at: http://www.sharksavers.org/en/education/sharks-are-in-trouble/399-loss-of-large-fish-on-coral-reefs.html
Aldo Leopold recognized the problem, and Robert Paine named it. Paine is at UW, and he is more famous for naming "keystone species." Both came from his studies of the starfish on rocky coasts that control mussel populations- the starfish can stand only so much dessication so they can only get so far up on the rocks. Above that the rocks can be covered with mussels, but below that there are few if any. Remove the starfish and a few years later the lower rocks are covered with mussels. The top predator controls the ecosystem, so it is a keystone species, and its removal causes a trophic cascade. UW Zoology is where Paine is, and he does research out on a tiny island called "Tatoosh" off Olympic Penn. where nobody else except the lighthouse operator goes.
The big problem is that humans love to remove the top predators, cougars, wolves, sharks, you name it. So even our national parks and wilderness areas are not pristine in at least one important way, they lack their top predators, just totally missing chunk of the ecosystem. Now you see what I'm fighting for on reefs, to get something even moderately close to a whole functioning ecosystem. When a piece of the machine is missing, driving it like it is in good repair could be trouble.
"If the biota, in the course of aeons, has built something we like but do not understand, then who but a fool would discard seemingly useless parts? To keep every cog and wheel is the first precaution of intelligent tinkering." Aldo Leopold
Also see this related item from High Country News, in an article on wolves in Colorado...
“Biologists have long recognized the power of predators in ecosystems. In the 1930s, Aldo Leopold, who advocated wolf extirpation early in his career, began to realize that the killing of predators had helped create what he called "the modern curse of excess deer and elk." In 1980, ecologist Robert Paine coined the term "trophic cascades" to describe the ripple effects of predators on herbivores, and herbivores on plants. Researchers continue to investigate and debate exactly how trophic cascades operate, but they find these so-called top-down effects at work throughout the natural world: Predators ranging from mountain lions to otters to sea stars have dramatic impacts on the ecosystems they inhabit.”
