6 Broken Chains-Mid

Broken Chains FOCUS Question How does a disrupted food chain affect an ecosystem?

Everything's Connected In the brief Alaskan summer, tiny plants use energy from the Sun to quickly sprout and bloom. Busy little rodents called lemmings gather up and eat these plants. Arctic foxes hunt and eat the lemmings. This is an example of a food chain—the movement of energy from one living organism to another. Arctic plants support more than just lemmings. These plants also provide food for insects, hares, and caribou. In addition to nourishing foxes, lemmings are food for snowy owls, weasels, and wolves. These connected food chains make up a food web. If lemmings disappear, what happens to the foxes, wolves, plants, hares, and other organisms in the food web? If a food chain breaks, the effects can reach far and wide.

Natural Cycles Lemmings in the Alaskan tundra make up a population— all the members of a species living in one place. Populations rise and fall naturally. For example, during an especially warm Alaskan summer, more plants grow. With this extra food, the lemmings have more offspring. Now Arctic foxes, which prey on the lemmings, are able to have more young and their population rises. Before long, with too many predators and not enough food to go around, the lemming population crashes. Then many foxes starve or move to another location to find food. Without hungry lemmings, the plants soon recover With more plants and fewer predators, the lemming population explodes again. While these populations go up and down, the food chain stays in balance over the long term. However, if a species disappears entirely, the food chain breaks.

Missing Links Species sometimes disappear for natural reasons. Diseases and natural disasters can leave missing links in food chains. But more often, people are the cause of broken food chains. In the 1800s, people hunted sea otters for their warm, waterproof fur. The animals came close to extinction. Otters eat sea urchins, and sea urchins eat kelp—a large seaweed. Without otters, the sea urchin population exploded. Urchins devoured entire kelp "forests." These forests provided food and shelter to hundreds of other species in the food web. These animals now had nothing to eat and nowhere to go. Today, sea otters are protected. Their numbers have rebounded enough to keep the sea urchin population in check. Many kelp forests have recovered, too.

Invasive Links The sea otter example shows what happens when a species disappears. But food chains also break when new species come in. Invasive species are organisms that move into a new area and disrupt the natural food chain. These new links in the chain show up in ecosystems all over the world. But their effect may be greatest in Australia. Australia is a continent surrounded by water. For millions of years, food chains there had a healthy balance of native species—organisms that evolved naturally in the area. When people from Europe arrived in the 1700s, they brought many invasive, or nonnative species. These new plants and animals quickly disrupted many food chains.

When people from Europe started moving to Australia in the late 1700s, pet cats began running wild. Native prey animals had never seen cats. They had no defenses against them. Even today, cats continue to overhunt Australian birds and mammals. In 1859 a hunter released several rabbits into the Australian countryside. With no predators, the rabbit population exploded. They devoured native plants, turning many areas into barren desert. In 1935, people brought cane toads to Australia to eat insects that damaged sugar-cane crops. Big mistake! Instead of eating the insects, the toads ate native rodents, snakes, and birds. Do You Know? In 1890, a man brought European starlings to New York City. He wanted Central Park to have all the birds mentioned in plays written by Shakespeare. European starlings now live across the United States.