CONCEPTS IN BIOLOGY

PART IV. EVOLUTION AND ECOLOGY

15. Ecosystem Dynamics. The Flow of Energy and Matter

15.2. Trophic Levels and Food Chains

One of the broad concepts of ecology is that organisms fit into categories, based on how they satisfy their energy requirements. There is a pattern in the way energy moves through an ecosystem. In general, energy flows from the Sun to plants and from plants to animals. However, there are recognizable steps in this flow of energy. Each stage in the flow of energy through an ecosystem is known as a trophic level. The series of organisms feeding on one another is known as a food chain (figure 15.3).

FIGURE 15.3. Trophic Levels in a Food Chain

As one organism feeds on another organism, energy flows from one trophic level to the next. This illustration shows six trophic levels.

Producers

Producers are organisms that trap sunlight and use it to produce organic molecules from inorganic molecules through the process of photosynthesis. Green plants and other photosynthetic organisms, such as algae and cyanobacteria, in effect, convert sunlight energy into the energy contained within the chemical bonds of organic compounds. Because producers are the first step in the flow of energy, they occupy the first trophic level.

Consumers

Only producers are capable of using sunlight to make organic molecules. All other organisms are directly or indirectly dependent on the organic molecules generated at the producer trophic level to meet their energy needs. Because these organisms must consume organic matter as a source of energy, they are called consumers. Consumers can be subdivided into several categories, based on how they obtain food.

Herbivores are animals that obtain energy by eating plants. Because herbivores obtain their energy from eating plants, they are also called primary consumers, and they occupy the second trophic level.

Carnivores are animals that eat other animals. They are also referred to as secondary consumers. Carnivores can be subdivided into different trophic levels, depending on what animals they eat. Animals that feed on herbivores occupy the third trophic level and are known as primary carnivores. Animals that feed on the primary carnivores are known as secondary carnivores, and they occupy the fourth trophic level. For example, a human may eat a fish that ate a frog that ate a spider that ate an insect that consumed plants for food.

Omnivores are animals that have generalized food habits and act as carnivores sometimes and herbivores other times. They are classified into different trophic levels depending on what they are eating at the moment. Most humans are omnivores.

Decomposers

Decomposers are a special category of consumers that obtain energy when they decompose the organic matter of dead organisms and the waste products of living organisms. Decomposers are usually not assigned to a trophic level, because they break down the organic matter produced by all trophic levels. Furthermore, the decomposer category includes a wide variety of organisms, such as bacteria, fungi, and other microorganisms, that feed on one another. Thus, there are food chains of decomposers.

Decomposers efficiently convert nonliving organic matter into simple inorganic molecules, which producers can reuse in the process of trapping energy. Thus, decomposers are very important components of ecosystems that cause materials to be recycled. As long as the Sun supplies the energy, elements are cycled repeatedly through ecosystems. Table 15.1 summarizes the categories of organisms within an ecosystem. Outlooks 15.1 describes changes in food chains that result from the introduction of exotic, invasive species.

TABLE 15.1. Categories in an Ecosystem

OUTLOOKS 15.1

Changes in the Food Chain of the Great Lakes

Many kinds of human activity aid the distribution of species from one place to another. Today there are over 100 exotic species in the Great Lakes. Some, such as smelt, brown trout, and several species of salmon, were purposely introduced. However, most exotic species entered accidentally as a result of human activity.

Prior to the construction of locks and canals, the Great Lakes were effectively isolated from invasion of exotic fish and other species by Niagara Falls. Beginning in the early 1800s, the construction of canals allowed small ships to get around the falls. This also allowed some fish species such as the sea lamprey and alewife to enter the Great Lakes. The completion of the St. Lawrence Seaway in 1959 allowed ocean-going ships to enter the Great Lakes. Because of the practice of using water as ballast, ocean-going vessels are a particularly effective means of introducing species. They pump water into their holds to provide ballast when they do not have a full load of cargo. (Ballast adds weight to empty ships to make their travel safer.) Ballast water is pumped out when cargo is added. Since these vessels may add water as ballast in Europe and empty it in the Great Lakes, it is highly likely that organisms will be transported to the Great Lakes from European waters. Some of these exotic species have caused profound changes in the food chain of the Great Lakes.

The introduction of the zebra and quagga mussels is correlated with several changes in the food web of the Great Lakes. Both mussels reproduce rapidly and attach themselves to any hard surface, including other mussels. They are very efficient filter feeders that remove organic matter and small organisms from the water. Measurements of the abundance of diatoms and other tiny algae show that they have declined greatly—up to 90% in some areas where zebra or quagga mussels are common. There has been a corresponding increase in the clarity of the water. In many places, people can see objects two times deeper than they could in the past.

Diporeia is a bottom-dwelling crustacean that feeds on organic matter. Populations of Diporeia have declined by 70% in many places in the Great Lakes. Many feel that this decline is the result of a reduction in their food sources, which are being removed from the water by zebra and quagga mussels. Since Diporeia is a major food organism for many kinds of bottom-feeding fish, there has been a ripple effect through the food chain. Recently, whitefish that rely on Diporeia as a food source have shown a decline in body condition. Other bottomfeeding fish that eat Diporeia serve as a food source for larger predator fish and there have been recent declines in the populations and health of some of these predator fish.

Another phenomenon that is correlated with the increase in zebra and quagga mussels is an increased frequency of toxic algal blooms in the Great Lakes. Although there are no clear answers to why this is occurring, two suggested links have been tied to mussels. The clarity of the water may be encouraging the growth of the toxic algae or the mussels may be selectively rejecting toxic algae as food, while consuming the nontoxic algae. Thus, the toxic algae have a competitive advantage.

Finally, wherever zebra or quagga mussels are common, species of native mussels and clams have declined. There may be several reasons for this correlation. First, the zebra and quagga mussels are in direct competition with the native species of mussels and clams. Zebra and quagga mussels are very efficient at removing food from the water and may be out-competing the native species for food. Secondly, since zebra and quagga mussels attach to any hard surface, they attach to native clams, essentially burying them.

A new threat to the Great Lakes involves the potential for exotic Asian carp (bighead and silver carp) to enter through a canal system that connects Lake Michigan at Chicago to the Mississippi River. These and other species of carp were introduced into commercial fish ponds in the southern United States. However, they soon escaped and entered the Mississippi River and have migrated upstream and could easily enter Lake Michigan. Both bighead and silver carp are filter-feeders that consume up to 40% of their body weight in plankton per day. They could have a further impact on the base of the Great Lakes food web, which has already been greatly modified by zebra and quagga mussels.

15.2. CONCEPT REVIEW

4. Describe two ways that decomposers differ from herbivores.

5. Name an organism that occupies each of the following trophic levels: the producer trophic level, the second trophic level, and the third trophic level.

6. How does each of the following organisms satisfy its energy needs: decomposer, plant, herbivore, omnivore, carnivore?