Carnivores

Apparently large predators are big news right now…I mean, one lousy Science paper comes out and suddenly everyone is chanting “Lions and tigers and bears, oh my!” No, seriously. That is how NPR led off the story that aired this morning. Actually, it was not just the Science paper this week. There was also a study out this week in PLOS ONE discussing the decline of lion populations in Africa and the endangered status of the large cats. Anyhow, the overall point being made by most of these stories is an important one: large predators play an important role in regulating ecosystems, but many of them are endangered and their natural habitat is shrinking.

There are many complicated interactions and interdependencies in any ecosystem, but you do not have to be a trained ecologist to have a basic understanding of how the food chain works. In the simplest version of the food chain, carnivores tend to occupy a position at the top of the chain, herbivores beneath them, and plants at the bottom. Visualizing it as a pyramid can be helpful, because that also provides an indication of the relative populations of those groups. The amount of available vegetation determines the size of a herd of grazing animals, and the size of the herd determines the number of predators that prey upon it. These numbers fluctuate, of course. This is known as the predator-prey relationship: as the number of predators increases, the number of prey dwindle. At some point, there are not enough to support the predator population, so predators die off. As the predator population decreases, the population of the prey increases, and the cycle repeats.

Of course the same cycle can happen with herbivores and vegetation as well, which is why predators are such an important part of the system. Without predators to help control the population, herbivore populations will continue to grow and consume more resources. This becomes a problem when the population begins to exceed the carrying capacity of the environment. Vegetation gets stripped away, resulting in erosion that can compound the problem by causing loss of habitat. With insufficient food available, especially during the winter, animals are forced into closer contact with humans. An excellent example is the deer population in much of the United States. With so few predators to control their numbers, deer become a nuisance to farmers and gardeners, and also a danger to drivers as they cross roadways in search of food. Larger populations lead to greater incidence of disease, which can effect humans since animals like deer often serve as reservoirs for viral diseases, which can then be transmitted to humans by vectors such as ticks and mosquitoes.

When thinking about such issues, we often consider the direct effects on human beings, but fail to go a step beyond that and look at the effects that such interactions will have on other species as well (which often lead back to humans anyways). Continuing the example of deer, if the population is too large, there will be more competition for food resources not only between deer themselves, but with other species as well. If the deer outcompete those other species, the numbers will likely decline, which in turn affects everything upstream (things that eat that species) and downstream (things eaten by that species). Getting more complicated, right? These things are easy to understand when examined in isolation, but very challenging when considering all of the variables and interactions that exist in nature.

So what is the big deal? We just need to relocate some predators so that the balance can be restored, right? If only that were the case. As usual, we are the problem. Most of us are going to feel sad at the thought of sick and dying deer, starving to death in the winter. Unfortunately, most of us are also going to feel a bit nervous about the idea of having wolves and mountain lions running around in our backyard. That is part of the reason that we have a shortage of predators in the first place. When humans move in, we build things like farms and towns, and cut down things like forests. Habitat disappears for predator and prey alike, but there is another problem as well. We introduce much easier food sources for large carnivores…things like livestock, pets, and children. I mean, if you were a mountain lion, which would you rather do: spend all day stalking and trying to bring down a deer, or find a backyard where some tiny human is playing? Tiny humans are not especially fast, or alert. Who knows? Maybe there will be a yapping little dog to serve as an appetizer. This is like fast food for large predators. See the problem?

So that is one of the many challenges that we humans have in store for us. How do we balance our desire for expansion and consumption of natural resources with nature, especially given how little we really understand the interactions between species in our environment? We often discover important relationships only when it is too late to do anything about it. To quote William Ripple, “Promoting tolerance and coexistence with large carnivores is a crucial societal challenge that will ultimately determine the fate of Earth’s largest carnivores and all that depends upon them, including humans.

Thanks for reading!

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Category(s): Science Stuff

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