In the 1800s, people hunted and trapped animals for their fur. Many people profited from the fur trade. Fur clothing was in high demand all over the world. In North America, people traveled farther and farther west in search of animal fur.

However, the fur trade had a huge effect on wildlife. It affected populations of hunted animals, like sea otters (Enhydra lutris). But it also affected entire food chains. A food chain is a group of organisms linked together in the order of the food they eat. Animals can be part of many different food chains. The chains connect to form a food web.

The fur trade took a toll on sea otters and also affected their entire food web. People hunted sea otters to the point of near-extinction. Sea otters are a top predator or apex predator. Top predators sit at the top of the local food chain. They hunt other animals for food.

The removal of sea otters from their environment had a negative effect. This effect trickled down through the ecosystem. Due to the lingering effects on the food web, sea otter populations have since had a hard time bouncing back.

This is just one example of how humans have changed ecosystems. In many ecosystems, overhunting has caused problems. It has left top predators unable to regain their position in the ecosystem.

A Loss of Sea Otters Changed an Entire Ecosystem

In the 1700s, more than 10,000 sea otters lived off the United States' California coast. Due to overhunting, fewer than 100 were left by the early 1900s. With hardly any otters left, the ecosystem began to change.

Sea otters eat sea urchins. Without any otters to eat them, the sea urchin population went way up. The sea urchins ate kelp, which is a type of seaweed. Many fish and other animals used the kelp forests for shelter. Without the kelp forests, these animals were forced to leave.

Without the otters, the entire ecosystem became a lot less diverse. It also resulted in a trophic cascade. This can happen when a top predator is removed from an ecosystem. It affects the prey population and other organisms in the food web. When a top predator is removed, everything else will be affected in some way.

With Fewer Wolves, the Elk Population Exploded

Something similar happened with gray wolves (Canis lupus) in the 1900s. Settlers hunted large animals like bison and elk (Cervus canadensis), but wolves hunted these same animals. The hunters and wolves competed. The wolves had fewer big prey to catch, so the hungry wolves started to attack livestock. Ranchers hunted the wolves to protect their animals.

The loss of wolves led to an elk population explosion. The elks ate too many plants, including young trees. The loss of these plants hurt beaver and bird populations. Without enough plants to hold the soil together, the river banks crumbled.

Reintroduction Can Help Ecosystems

Scientists have learned more about what happens when top predators go missing. They have tried to bring some top predators back. For example, the U.S. National Park Service reintroduced wolves into Yellowstone National Park in the 1990s. The wolves had a big effect on the ecosystem and food web. The elk population decreased. With fewer elk, there was a lot more plant growth. Small animal populations increased too.

In Yellowstone, the reintroduction of predators has been largely successful. In the case of the sea otters, bringing back the predators has been more complicated.

In Central California, there are now almost 3,000 sea otters. However, the otters have not expanded their range. Ecosystem changes may be to blame. When the sea otters vanished, the kelp forests disappeared. When otters today swim into these empty areas, sharks can spot and catch them.

The Results Are not Always Successful

In other cases, the reintroduction of top predators has had less successful results. For example, people reintroduced brown bears (Ursus arctos) in the Pyrenees mountain range in Europe. They ended up attacking livestock.

This shows that trophic cascades are complicated. Predators can affect ecosystems and food webs in unpredictable ways. Reintroducing top predators might not always solve an ecosystem's problems.