study guide

ECOLOGY TODAY: APPLYING THE CONCEPTS

The California sea otter. This once abundant marine mammal has experienced large fluctuations in numbers as a result of human activities during the past three centuries.

In this first chapter, we have examined a wide range of topics, including the hierarchy of perspectives in ecology, the biological and physical principles that govern natural systems, the variety of roles that different species play, the multiple approaches to studying ecology, and the influence of humans on ecological systems. To help you see how these topics interconnect, let’s examine a case study of the sea otter (Enhydra lutris) off the Pacific coast. Humans have affected sea otter populations for hundreds of years. Several scientific approaches have been taken to understand these effects and to help reverse them.

The sea otter was once abundant, with a geographic range that extended around the northern Pacific Rim, from Japan up to Alaska and down to Baja California. However, in the 1700s and 1800s, intense hunting for otter pelts reduced the population to near extinction, which caused the fur industry to subsequently collapse. When a small population was discovered off the coast of central California in the 1930s, the otters were placed under protection. As a result, the population increased to several thousand individuals by the 1990s, though in more recent years, the otter has again experienced population declines. These changes in the size of otter populations presented an opportunity for scientists to examine a natural experiment in action.

Ecologists quickly realized that to understand the causes and consequences of the sea otter’s fluctuations in abundance, they needed to use a range of ecological approaches, from the individual to the ecosystem. Taking an individual approach, ecologists established that the sea otter was a predator on a wide range of prey species, including abalone, spiny lobsters, small fish, crabs, sea urchins, and small snails. Among these prey items, observations of otter feeding behavior revealed that otters prefer certain prey such as abalone, a large species of sea snail. They will only eat other small species of snails when their preferred prey becomes rare.

Once scientists understood the sea otter’s niche, they were better able to protect it. However, not everyone was happy about the resurgence of sea otters after they became protected. California anglers became upset; they argued that the growing otter population would cause a dramatic

change in the marine community, including a drastic reduction in the populations of commercially valuable fish, clams, abalone, and spiny lobsters — all harvested for human consumption. While otters do have negative effects on their prey — especially clams — scientists who took a community approach to ecology found that an increasing otter population was also having other dramatic effects on the marine community that were positive for many species of commercially harvested fish and shellfish. For example, otters eat a lot of sea urchins, which consume giant algae known as kelps. Kelps can grow up to 100 m long, and regions of the ocean containing large amounts of kelps are called kelp forests. As the growing otter population caused sea urchins to decrease, the reduction in sea urchins caused predation of kelp forests to decrease, providing young fishes greater refuge from predators and providing them areas in which they could feed. Kelps also can be harvested by humans to make fertilizer, food, and

pharmaceuticals, so the increase in otters also allowed an increase in the commercial harvesting of kelps. Thus, the sea otter plays a key role in determining the community composition of coastal marine ecosystems.

Sea otters and the species with which they interact. Once scientists determined the major species in the ocean that affected the abundance of otter populations, they could better protect the otter from extinction. Solid arrows indicate consumption of one species by another.

In the 1990s, the sea otter population mysteriously began to decline. To understand these declines, scientists used individual, community, and ecosystem approaches. In 1998, researchers showed that populations of otters in the vicinity of the Aleutian Islands, Alaska, had declined

precipitously during the 1990s. The reason was that killer whales, or orcas (Orcinus orca), which previously had not preyed on otters, had begun to come close to shore where they consumed large numbers of otters. Why did killer whales adopt this new behavior? The researchers pointed out that populations of the principal prey of killer whales — seals and sea lions — collapsed during the same period, perhaps causing the whales to hunt the otters as an alternative food source. Why did the seals and sea lion populations decline? One can only speculate at this point, but intense human fisheries have reduced fish stocks exploited by the seals and sea lions to levels low enough to seriously threaten their populations.

There also were declines in otter populations along the California coast. Initially, declines in sea otters were attributed to the use of gill nets along the coast to exploit a new fishery that inadvertently killed otters in substantial numbers. Subsequent legislation moved the fishery farther offshore to help protect the otters. In this same region, the otters were also dying from infections by two protist parasites, Toxoplasma gondii and Sarcocystis neurona. These parasites cause a lethal inflammation of the brain. In 2010, for example, 40 dead and dying sea otters were found near Morro Bay, California, and 94 percent were infected with S. neurona. This was a surprising observation because the only known hosts of these parasites are opossums (Didelphis virginiana) and several species of cats. Given that these mammals live on land, how did sea otters become infected?

Scientists hypothesized links between the terrestrial and marine ecosystems that allowed the parasites to infect sea otters. To date, two potential links have been suggested. First, house cats that spend time outside defecate on land and their feces contain the parasites. When it rains, the parasites get washed into local streams and rivers and eventually end up in the ocean. Second, when humans flush cat feces and kitty litter down the toilet and into the sewer system, the wastewater eventually enters the ocean. Although manipulative experiments found that the protists do not infect marine invertebrates and cause illness, the invertebrates can take the parasites into their bodies inadvertently while feeding. When invertebrates infested with parasites are consumed by otters, the otters get infected. New research indicates that abalone do not carry the parasites, whereas small marine snails do. Thus, when otters have an abundance of their preferred food, such as abalone, they have a low risk of being infected by the deadly parasite. When abalone is scarce, however, the otters are forced to feed on small snails that carry the parasite, which dramatically increases the risk of infection and death.

The story of the sea otter highlights the importance of understanding ecology from multiple approaches using both manipulative and natural experiments. It also underscores the multiple roles that species can play in communities and ecosystems and how humans can dramatically influence these roles. This understanding can then be used to take action to reverse harmful effects on the environment. In the case of the sea otter, education campaigns now encourage the public to keep their cats inside more and to put used cat litter into the trash rather than flushing it down the toilet.