Course: BIS 105 Environmental Biology

Unit 2 Learning Module

Decomposers

Decomposers

When a plant or animal dies, it leaves behind energy and matter in the form of the organic compounds that make up its remains. Decomposers are organisms that consume dead organisms and other organic waste. They recycle materials from the dead organisms and waste back into the ecosystem. These recycled materials are used by the producers to remake organic compounds. Therefore, decomposers, like producers, are an essential part of every ecosystem, and their stability is essential to the survival of each ecosystem. In essence, this process completes and restarts the "circle of life." As stated above, scavengers consume the carcasses of dead animals. The remains of dead plants are consumed by organisms called detritivores.

Detritivores

When plants drop leaves or die, they contribute to detritus. Detritus consists of dead leaves and other plant remains that accumulate on the ground or at the bottom of a body of water. Detritus may also include animal feces and other organic debris. Heterotrophic organisms called detritivores feed on detritus. Earthworms, millipedes, and woodlice are detritivores that consume rotting leaves and other dead plant material in or on soil. Dung beetles, like the one shown below, consume feces. In aquatic ecosystems, detritivores include “bottom feeders,” such as sea cucumbers and catfish.

Dung beetle rolling a ball of feces to its nest to feed its offspring.

Saprotrophs

After scavengers and detritivores feed on dead organic matter, some unused energy and organic compounds still remain. For example, scavengers cannot consume bones, feathers, and fur of dead animals, and detritivores cannot consume wood and other indigestible plant material. Organisms called saprotrophs complete the breakdown of any remaining organic matter. The main saprotrophs that decompose dead animal matter are bacteria. The main saprotrophs that decompose dead plant matter are fungi. Fungi are also the only organisms that can decompose dead wood. Single-celled protozoa are common saprotrophs in aquatic ecosystems as well as in soil.

Saprotrophs convert dead organic material into carbon dioxide and compounds containing nitrogen or other elements needed by living organisms. The elements are then available to be used again by producers for the synthesis of organic compounds.

Food Chains and Food Webs

Food chains and food webs represent the feeding relationships in ecosystems. They show who eats whom. Therefore, they model the flow of energy and materials through ecosystems.

Food Chains

A food chain represents a simple linear pathway through which energy and materials are transferred from one species to another in an ecosystem. In general, food chains show how energy and materials flow from producers to consumers. Energy and materials also flow from producers and consumers to decomposers, but this step usually is not included in food chains.

As an example terrestrial food chain, grasses are a common producer. Grasses, in turn, are consumed by grasshoppers. Because grasshoppers directly consume producers, they are called primary consumers. At the next level of the food chain, grasshoppers are consumed by mice, which are called secondary consumers. Mice are consumed by snakes (called tertiary consumers), and snakes are consumed by hawks (called quaternary consumers).

A musical summary of food chains can be heard at http://www.youtube.com/watch?v=TE6wqG4nb3M (2:46).

Food Webs

Food chains tend to be overly simplistic representations of what really happens in nature. Most organisms consume multiple species and are, in turn, consumed by multiple other species. A food web represents these more complex interactions. A food web is a diagram of feeding relationships that includes multiple intersecting food chains. An example of a food web is shown below.

Trophic Levels and Energy Transfer

The different feeding positions in a food chain or web are called trophic levels. The first trophic level consists of producers, the second of primary consumers, the third of secondary consumers, and so on. There usually are no more than four or five trophic levels in a food chain or web. Humans may fall into second, third, and fourth trophic levels of food chains or webs. They eat producers such as grain, primary consumers such as cows, and tertiary consumers such as salmon.

Energy is passed up the food chain from one trophic level to the next. However, only about 10 percent of the total energy stored in organisms at one trophic level is actually transferred to organisms at the next trophic level. The rest of the energy is used for metabolic processes or lost to the environment as heat. As a result, less energy is available to organisms at each successive trophic level. This explains why there are rarely more than four or five trophic levels. The amount of energy at different trophic levels can be represented by an energy pyramid like the one in below.

Pyramid of Energy

This pyramid shows the total energy stored in organisms at each trophic level in an ecosystem. Starting with primary consumers, each trophic level in the food chain has only 10 percent of the energy of the level below it. The pyramid makes it clear why there can be only a limited number of trophic levels in a food chain or web.

Because there is less energy at higher trophic levels, there are usually fewer organisms as well. Organisms tend to be larger in size at higher trophic levels, but their smaller numbers still result in less biomass. Biomass is the total mass of organisms in a trophic level (or other grouping of organisms).

The materials in dead organisms and wastes at all trophic levels are broken down by decomposers. Organisms such as detritivores and saprotrophs return needed elements to the ecosystem and use up most remaining energy. Because of the reduction in energy at each trophic level, virtually no energy remains. Therefore, energy must be continuously added to ecosystems by producers.