By Don Hazel
You may have heard of the parlor game called "six degrees of Kevin Bacon." The premise is that any actor, in any movie, can be linked through movie roles to Kevin Bacon in six steps or less. For example, Kevin Bacon was in a movie with Julia Roberts, and Julia Roberts was in a movie with Denzel Washington, so Kevin Bacon is linked to Denzel Washington, and so on. You have to be a real movie buff to excel at this game.
The linkages of different species in nature can be just as complicated to figure out. For instance, how can wolves increase the number of songbirds in Yellowstone National Park; or, how are lake trout reducing elk populations? Better yet, how can more mountain lions mean more butterflies?
Scientists have known for a long time that the removal or introduction of one species from an environment can have consequences for several other species. The term for this is a trophic cascade. A trophic cascade means that a predator change has a cascading effect on many species down the food chain.
When the last wolf was killed in 1924 in Yellowstone National Park, elk populations began to flourish. With their main predator gone, elk browsed without interruption, and they consumed nearly all the willow along the river banks. They also consumed young cottonwoods and aspen. Without these trees along the rivers, beaver disappeared from many parts of the national park.
When wolves were re-introduced in Yellowstone in 1995, after a 70 year absence, elk changed their behavior. They began to eat on the move rather than hanging out in the river bottoms and eating every willow sprout. As streamside vegetation began to grow again, beaver came back. In just several years, beaver in northern Yellowstone Park increased from only one colony, to 12. With beaver dams creating wetlands, even more willow sprouted, and, with more vegetation, the number and diversity of songbirds increased. The wolf re-introduction seems to have put a balance back in nature in that area.
While I was watching an elk herd along the Madison River in Yellowstone last month, a fellow visitor pointed out that there were very few elk calves in the herd. He said that the wolves should have never been re-introduced, and that they were killing all the elk calves. Yes, wolves eat lots of elk, but they eat mainly adult elk. Studies are showing that there is another, very interesting, reason that elk calf survival rates are down.
Scientists have discovered that the illegal introduction of lake trout into Yellowstone Lake (approximately 30 years ago) has contributed to the lower elk calf survival rates. No, lake trout are not eating elk calves; lake trout are eating cutthroat trout. Non-native lake trout have reduced the populations of native cutthroat trout by up to 90 percent. For many years grizzly bears have relied on cutthroat trout spawning in streams for critical food in the spring. Lake trout spawn in deeper water and are unreachable to bears. So, with few cutthroat trout to eat, grizzly bears have turned to elk calves for protein. It appears that the introduction of lake trout has caused a significant increase in bear predation on elk calves. Studies also point out that 28 different species rely, to some extent, on spawning cutthroat trout. The thoughtless introduction of lake trout into an ecosystem has had an unforeseen cascade effect on many other species.
By now, you are probably close to figuring out how a mountain lion can save a butterfly. In Zion National Park, studies are showing that there are fewer, and less diverse species of butterflies in areas where mountain lions have been reduced through human interference. Without mountain lions, mule deer flourish. And, just like the elk in Yellowstone, without predators to worry about, the deer mow down much of the vegetation along the streams. Without plants, butterflies disappear. In areas where there are more mountain lions, there are more butterflies.
Maybe we should invent a new game called the six degrees of trophic cascade. Actually, it is probably more important than just a game, and often too complicated for us humans to fully understand. I guess we'll stick to Kevin Bacon. Can you link Kevin Bacon to Lassie in six steps or less?
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