Trophic cascade

A trophic cascade is the effect that organisms on one trophic level of an ecosystem have on organisms at other trophic levels. A trophic level of an ecosystem includes all the organisms that occupy the same position or serve the same function in a food chain. For example, plants are producers that make their own food. The effects of some trophic cascades move from the top down, while the effects of other trophic cascades move from the bottom up. These effects may be direct or indirect and positive or negative. Scientists have studied trophic relationships for decades, but it was not until 1980 that American ecologist Robert Paine first introduced the term trophic cascade.

Background

Scientists have long studied the connections among organisms in different environments, identifying producers (organisms that make their own food), consumers (organisms that consume other organisms), prey (the hunted), and predators (the hunters). In 1880, Italian scientist Lorenzo Camerano crafted the first food web, an illustration showing food relationships among different species in an ecosystem. Later, in the twentieth century, English zoologist Charles Elton introduced a number of concepts that form the foundation of trophic ecology.

The word trophic comes from a Greek word that means "nourishment," which is why it is used to describe the food relationships among organisms. According to Elton, all the organisms in an ecosystem belong to one or more food chains. At the bottom of a food chain are plants, which make their own food. Above plants are herbivores, or animals that eat plants. Above herbivores are carnivores, or meat eaters, which eat herbivores. The various food chains cross one another to form a food cycle, or what is more commonly known as a food web. Each organism in a food chain occupies a specific position and fills a certain role. In addition, a "pyramid of numbers" exists, with a large number of plants at the bottom and a small number of carnivores at the top. In the twenty-first century, this is known as an ecological (or trophic) pyramid.

American ecologist Raymond Lindeman further categorized organisms in an ecosystem into specific trophic levels. He noted that organisms at each level obtain their energy from organisms in the preceding level. Lindeman identified plants as producers because they make their own food. They form the bottom level. Organisms at all other levels are consumers. Animals that consume plants (the herbivores) are primary consumers. Animals that consume herbivores (carnivores) are secondary consumers. Tertiary consumers are carnivores that consume other carnivores. In 1980, American ecologist Robert Paine coined the phrase trophic cascade to describe a change at one trophic level that affected organisms at other levels.

During the 1960s, Paine had conducted experiments on intertidal ecosystems. After watching a species of sea star prey on mussels, he decided to see what would happen if sea stars were removed from the ecosystem. He subsequently observed that when sea stars were no longer present, the mussel population exploded, and the mussels crowded out other species. From this, he concluded that sea stars, in preying on mussels, were necessary for maintaining the balance and diversity of the ecosystem. By removing sea stars, he created a trophic cascade that affected all other organisms in the food chain.

Overview

Paine's experiments examined a saltwater environment, but trophic cascades also may occur in freshwater or terrestrial ecosystems. Trophic cascades may move through trophic levels from the top down or from the bottom up. The presence or absence of an organism at one level may directly or indirectly affect the organisms at all other levels. In some cases, these effects may be positive, meaning they bring some type of benefit to organisms at another level. In other cases, these effects may be negative, meaning they cause harm to organisms at another level.

At its most basic level, a trophic cascade involves three organisms: a producer (plant), a primary consumer (herbivore/prey), and a secondary consumer (carnivore/predator). Removing any one of these organisms from its place within the food chain will result in a trophic cascade affecting the other levels. For example, in Yellowstone National Park, hunting led to the near-extinction of wolves in the 1920s. The wolves were predators that preyed on elk populations. Elk were herbivores that fed on aspen and willow plants. When the wolves began to disappear, elk populations ballooned. As a result, the elk overtaxed the aspen and willow plants, which then began to disappear. In this top-down trophic cascade, the disappearance of wolves had a direct positive effect on the population of elk and an indirect negative affect on the population of aspen and willow plants. When wolves were reintroduced to Yellowstone in 1995, they brought elk populations back under control, which, in turn, allowed the aspen and willow plants to return. In this trophic cascade, the wolves had a direct negative effect on the elk and an indirect positive effect on the aspen and willows.

In a bottom-up scenario, a trophic cascade begins at a lower trophic level and moves upward. For example, coastal regions often experience harmful algal blooms that can affect organisms at every trophic level. Algae are plants that grow in water, which makes them producers. A number of organisms, such as small fish and crustaceans, depend on algae for food. During some algal blooms, algae grow at an alarming rate and produce harmful toxins. When fish and crustaceans eat the algae, they ingest these toxins. Later, when larger fish ingest the smaller fish and crustaceans, they also ingest the toxins. The toxins work their way up through the food chain, affecting fish, shellfish, birds, animals, and even people. Harmful algal blooms have a direct negative effect on the fish and crustaceans that are the primary consumers of algae and an indirect negative effect on consumers at all other levels of the food chain.

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