Energy Flow

What is happening inside each leaf and blade of grass?

Photosynthesis. Maybe the most important biochemical reaction of Earth. As sunlight shines down on this forest, the sunlight is being absorbed, and the energy from that sunlight is being transformed into chemical energy. That chemical energy is then distributed to all other living organisms in the ecosystem.

Flow of Energy

To survive, ecosystems need a constant influx of energy. Energy enters ecosystems in the form of sunlight or chemical compounds. Some organisms use this energy to make food. Other organisms get energy by eating the food.

Producers (Autotrophs)

Producers are organisms that produce food for themselves and other organisms. They use energy and simple inorganic molecules to make organic compounds. The stability of producers is vital to ecosystems because the are the first step of energy conversions within ecosystems and they are able to produce many of the organic molecules needed by organisms that feed upon them. Producers are also called autotrophs. There are two basic types of autotrophs: photoautotrophs and chemoautotrophs.

1. Photoautotrophs use energy from sunlight to make food by photosynthesis. They include plants, algae, and certain bacteria (see Figure below).

2. Chemoautotrophs use energy from chemical compounds to make food by chemosynthesis. They include some bacteria and also archaea. Archaea are microorganisms that resemble bacteria.

[Figure 2]

Different types of photoautotrophs are important in different ecosystems.

Consumers (Heterotrophs)

Consumers are organisms that depend on other organisms for food. They take in organic molecules by essentially “eating” other living things. They include all animals and fungi. (Fungi don't really “eat”; they absorb nutrients from other organisms.) They also include many bacteria and even a few plants, such as the pitcher plant shown in Figure below. Consumers are also called heterotrophs. Heterotrophs are classified by what they eat:

• Herbivores consume producers such as plants or algae. They are a necessary link between producers and other consumers. Examples include deer, rabbits, and mice.

• Carnivores consume animals. Examples include lions, polar bears, hawks, frogs, salmon, and spiders. Carnivores that are unable to digest plants and must eat only animals are called obligate carnivores. Other carnivores can digest plants but do not commonly eat them.

• Omnivores consume both plants and animals. They include humans, pigs, brown bears, gulls, crows, and some species of fish.

[Figure 3]

Pitcher Plant. Virtually all plants are producers. This pitcher plant is an exception. It consumes insects. It traps them in a sticky substance in its “pitcher.” Then it secretes enzymes that break down the insects and release nutrients. Which type of consumer is a pitcher plant?

Decomposers

When organisms die, they leave behind energy and matter in their remains. Decomposers break down the remains and other wastes and release simple inorganic molecules back to the environment. Producers can then use the molecules to make new organic compounds. The stability of decomposers is essential to every ecosystem. Decomposers are classified by the type of organic matter they break down:

• Scavengers consume the soft tissues of dead animals. Examples of scavengers include vultures, raccoons, and blowflies.

• Detritivores consume detritus—the dead leaves, animal feces, and other organic debris that collects on the soil or at the bottom of a body of water. On land, detritivores include earthworms, millipedes, and dung beetles (see Figure below). In water, detritivores include “bottom feeders” such as sea cucumbers and catfish.

• Saprotrophs are the final step in decomposition. They feed on any remaining organic matter that is left after other decomposers do their work. Saprotrophs include fungi, bacteria and single-celled protozoa. Fungi are the only organisms that can decompose wood.

[Figure 4]

Dung Beetle. This dung beetle is rolling a ball of feces to its nest to feed its young.

Food Chains, Food Webs, Ecological Pyramids

Who eats whom?

Describing the flow of energy within an ecosystem essentially answers this question. To survive, one must eat. Why? To get energy. Food chains and webs describe the transfer of energy within an ecosystem, from one organism to another. In other words, they show who eats whom.

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

[Figure 2]

This food chain includes producers and consumers. 

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. In the terrestrial food chain shown in the Figure above, grasses are the producers. 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 frogs, which are called secondary consumers. Frogs are consumed by snakes (called tertiary consumers), and snakes are consumed by owls (called quaternary consumers).

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 multiple feeding relationships within an ecosystem that includes multiple intersecting food chains. Examples of food webs are shown below.

[Figure 3]

Examples of food webs.

[Figure 4]

Food Web. This food web consists of several different food chains. Which organisms are producers in all of the food chains included in the food web?

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 the Figure below.

[Figure 2]

Ecological Pyramid. This pyramid shows how energy and biomass decrease from lower to higher trophic levels. Assume that producers in this pyramid have 1,000,000 kilocalories of energy. How much energy is available to primary consumers?

Energy and Biomass Pyramids

[Figure 6]

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 biomass pyramid in the Figure below shows how biomass of organisms changes from first to higher trophic levels in a food chain.

Biomass Pyramid

[Figure 7]

This pyramid shows the total biomass, or mass of organisms, at each trophic level in an ecosystem. How does this pyramid relate to the energy pyramid?

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.