Flow of Energy in Ecosystems

Energy enters most ecosystems from sunlight. However, some ecosystems, such as hydrothermal vent ecosystems at the bottom of the ocean, receive no sunlight and obtain energy instead from chemical compounds. Energy is used by some organisms in the ecosystem to make food. These organisms are called primary producers, or autotrophs, which include small plants, algae, photosynthetic prokaryotes and chemosynthetic prokaryotes. From primary producers, energy eventually is transferred to all the other organisms in the ecosystem through consumers or decomposers known as heterotrophs.

Producers

Producers are organisms that produce organic compounds from energy and simple inorganic molecules. Producers are also called autotrophs, which literally means “self nutrition.” This is because producers synthesize food for themselves. They take energy and materials from the abiotic environment and use them to make organic molecules. Autotrophs are a vital part of all ecosystems. The stability of the producers is vital to the survival of every ecosystem; without this stability an ecosystem may not thrive; in fact, the ecosystem may collapse. The organic molecules the producers make are needed by all the organisms in the ecosystem. There are two basic types of autotrophs: photoautotrophs and chemoautotrophs. They differ in the type of energy they use to synthesize food.

Photoautotrophs

Photoautotrophs are organisms that use energy from sunlight to make food by photosynthesis. Photosynthesis is the process by which carbon dioxide and water are converted to glucose and oxygen, using sunlight for energy. Glucose, a carbohydrate, is an organic compound that can be used by autotrophs and other organisms for energy. As shown in Figure 1, photoautotrophs include plants, algae, and certain bacteria.

Figure 1: Different types of photoautotrophs are important in different types of ecosystems. Each type of photoautotroph pictured here is an important producer in some ecosystem.

Plants are the most important photoautotrophs in land-based, or terrestrial, ecosystems. There is great variation in the plant kingdom. Plants include organisms as different as trees, grasses, mosses, and ferns. Nonetheless, all plants are eukaryotes that contain chloroplasts, the cellular “machinery” needed for photosynthesis.

Algae are photoautotrophs found in most ecosystems, but they generally are more important in water-based, or aquatic, ecosystems. Like plants, algae are eukaryotes that contain chloroplasts for photosynthesis. Algae include single-celled eukaryotes, such as diatoms, as well as multicellular eukaryotes, such as seaweed.

Photoautotrophic bacteria, called cyanobacteria, are also important producers in aquatic ecosystems. Cyanobacteria were formerly called blue-green algae, but they are now classified producers in terrestrial as well as aquatic ecosystems.

Both cyanobacteria and algae make up phytoplankton. Phytoplankton refers to all the tiny photoautotrophs found on or near the surface of a body of water. Phytoplankton usually is the primary producer in aquatic ecosystems.

Chemoautotrophs

In some places where life is found on Earth, there is not enough light to provide energy for photosynthesis. In these places, producers called chemoautotrophs use the energy stored in chemical compounds to make organic molecules by chemosynthesis. Chemosynthesis is the process by which carbon dioxide and water are converted to carbohydrates. Instead of using energy from sunlight, chemoautotrophs use energy from the oxidation of inorganic compounds, such as hydrogen sulfide (\(H_2 S\)). Oxidation is an energy-releasing chemical reaction in which a molecule, atom, or ion loses electrons.

Chemoautotrophs include bacteria called nitrifying bacteria. Nitrifying bacteria live underground in soil. They oxidize nitrogen-containing compounds and change them to a form that plants can use.

Chemoautotrophs also include archaea. Archaea are a domain of microorganisms that resemble bacteria. Most archaea live in extreme environments, such as around hydrothermal vents in the deep ocean. Hot water containing hydrogen sulfide and other toxic substances escapes from the ocean floor at these vents, creating a hostile environment for most organisms. Near the vents, archaea cover the sea floor or live in or on the bodies of other organisms, such as tube worms. In these ecosystems, archaea use the toxic chemicals released from the vents to produce organic compounds. The organic compounds can then be used by other organisms, including tube worms. Archaea are able to sustain thriving communities, like the one shown in Figure 2, even in these hostile environments.

Figure 2: Red tube worms, each containing millions of archaea microorganisms, grow in a cluster around a hydrothermal vent in the deep ocean floor. Archaea produce food for themselves (and for the tube worms) by chemosynthesis.

Consumers

Consumers are organisms that depend on producers or other types of organisms for food. They are also called heterotrophs, which literally means “other nutrition.” Heterotrophs are unable to make organic compounds from inorganic molecules and energy. Instead, they take in organic molecules by consuming other organisms. All animals and fungi and many bacteria are heterotrophs. A few insect-eating plants are also heterotrophic. Heterotrophs can be classified on the basis of the types of organisms they consume. They include herbivores, omnivores, and carnivores.

Herbivores

Herbivores are organisms that consume only producers such as plants or algae. In most ecosystems, herbivores form a necessary link between producers and other consumers. Herbivores transform the energy stored in producers to compounds that can be used by other organisms.

In terrestrial ecosystems, many animals and fungi and some bacteria are herbivores. Herbivorous animals include deer, rabbits, and mice. Herbivores may specialize in particular types of plants, such as grasses, or specific plant parts, such as leaves, nectar, or roots. Examples of herbivores are shown in Figure 3.

Figure 3: Deer browse on leaves. A hummingbird sips nectar from a flower. A bee gathers pollen from a flower. Many bats, including this one, primarily eat fruit. Some birds mainly eat seeds. A rabbit eats grasses. Beetle larvae like this one eat plant roots.

In aquatic ecosystems, the main herbivores are the heterotrophic organisms that make up zooplankton. Zooplankton refers to all the small organisms that feed on phytoplankton. These organisms include both single-celled organisms such as protozoa and multicellular organisms such as jellyfish. Phytoplankton and zooplankton together make up large communities of producers and herbivores called plankton.

Carnivores

Carnivores are organisms that eat a diet consisting mainly of herbivores or other carnivores. Carnivores include lions, wolves, polar bears, hawks, frogs, fish, and spiders. Animals that eat only meat are called obligate carnivores. They generally have a relatively short digestive system that cannot break down the tough cellulose found in plants. Other carnivores, including dogs, can digest plant foods but do not commonly eat them. Certain carnivores, called scavengers, mainly eat the carcasses of dead animals. Scavengers include vultures, raccoons, and blowflies.

A tiny minority of plants—including Venus flytraps and pitcher plants—are also carnivorous. These plants trap and digest insects. Some fungi are carnivorous as well. Carnivorous fungi capture and digest microscopic protozoan organisms such as amoebas.

Omnivores

Omnivores are organisms that eat both plants and animals as primary food sources. Humans are an example of an omnivorous species. Although some humans eat foods derived only from plants or only from animals, the majority of humans eat foods from both sources. Other examples of omnivorous animals are pigs, brown bears, gulls, and crows. Aquatic omnivores include some species of fish, such as piranhas.

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.

Detrivores

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 in Figure 4, consume feces. In aquatic ecosystems, detritivores include “bottom feeders,” such as sea cucumbers and catfish.

Figure 4: 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. Two examples of food chains are shown in Figure 5.

Figure 5: In the terrestrial food chain on the left, 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 mice, which are called secondary consumers. Mice are consumed by snakes (called tertiary consumers), and snakes are consumed by hawks (called quaternary consumers). In the aquatic food chain on the right, phytoplankton is the producer. Phytoplankton is eaten by zooplankton, which is the primary consumer. Zooplankton, in turn, is eaten by fish (secondary consumers). Fish are eaten by seals (tertiary consumers), and seals are eaten by white sharks (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 feeding relationships that includes multiple intersecting food chains. An example of a food web is shown in Figure 6.

Figure 6: This aquatic food web consists of several intersecting food chains. Which organisms are producers in all the food chains included in the food web?

In the food web in Figure 6, phytoplankton is the producer, and zooplankton is the primary consumer. Secondary consumers, which eat zooplankton, include salmon, cephalopods, sand lances, and auklets. Puffins, kittiwakes, and gulls are tertiary consumers. Foxes, rats, and gulls are quaternary consumers. Two possible food chains within this food web are:

Fox → Puffin → Cephalopod → Zooplankton → Phytoplankton;

Rat → Kittiwake → Sand lance → Zooplankton → Phytoplankton.

Can you identify other food chains in this 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 Figure 7.

Figure 7: 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 biomass pyramid in Figure 8 shows how biomass of organisms changes from first to higher trophic levels in a food chain.

Figure 8: Pyramid of Biomass.This pyramid shows the total biomass, or mass of organisms, at each trophic level in an ecosystem.

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.