5 Energy Flow through Ecosystems

Learning Objectives

After studying this chapter, you should be able to:

  • List the levels of organization of living things in the environment
  • Differentiate between food chains and food webs
  • Describe how organisms acquire energy in a food web and in associated food chains
  • Explain how the efficiency of energy transfers between trophic levels affects ecosystem
  • Describe the overall process of cellular respiration
  • Describe the overall process of photosynthesis

Levels of Organization in the Environment

Living things and the environments in which they live are highly organized and structured, following a hierarchy on a scale from small to large (Figure 1). Organisms are individual living entities. For example, antelope on an African savanna is an organism. All the individuals of one species living within a specific area are collectively called a population. For example, a savanna may be home to many antelopes. All of these antelopes represent the population of antelopes in this area. Different populations may live in the same specific area. For example, the savanna with the antelopes includes populations of trees and also insects and microbial populations.

Levels of organization in an african savanna include organisms, populations, communities, ecosystems, and the biosphere
Figure 1. Living things and the environments in which they live can be organized into increasing levels of complexity, from organisms to the entire biosphere.

A community is all the populations inhabiting a particular area. For instance, all of the antelopes, lions, vultures, insects, and other populations in a savanna form the savanna’s community. The savanna itself is an ecosystem. An ecosystem consists of all the biotic, or living, things in a particular area together with the abiotic, or non-living, parts of that environment such as nitrogen in the soil or rainwater. At the highest level of organization, the biosphere is the collection of all ecosystems, and it represents the zones of life on Earth. It includes land, water, and portions of the atmosphere.

Ecosystems can be small, such as the tide pools found near the rocky shores of many oceans, or large, such as those found in the tropical rainforest of the Amazon in Brazil (Figure 2).

Left photo shows a shallow pool of water with seaweed and snails, right photo shows a forest
Figure 2. A (a) tidal pool ecosystem in Matinicus Island, Maine, is a small ecosystem, while the (b) Amazon rainforest in Brazil is a large ecosystem.

Ecology of Ecosystems

Life in an ecosystem often involves competition for limited resources, which occurs both within a single species and between different species. Organisms compete for food, water, sunlight, space, and mineral nutrients. These resources provide the energy for metabolic processes and the matter to make up organisms’ physical structures. Other critical factors influencing community dynamics are the components of its physical environment: a habitat’s climate (seasons, sunlight, and rainfall), elevation, and geology. These can all be important environmental variables that determine which organisms can exist within a particular area.

There are three broad categories of ecosystems based on their general environment: freshwater, marine, and terrestrial. Within these three categories are individual ecosystem types based on the environmental habitat and organisms present.

Freshwater ecosystems are the least common, occurring on only 1.8 percent of Earth’s surface. These systems comprise lakes, rivers, streams, and springs; they are quite diverse, and support a variety of animals, plants, fungi, protists and prokaryotes.

Marine ecosystems are the most common, comprising 75 percent of Earth’s surface and consisting of three basic types: shallow ocean, deep ocean water, and deep ocean bottom. Shallow ocean ecosystems include extremely biodiverse coral reef ecosystems, yet the deep ocean water is known for large numbers of plankton and krill (small crustaceans) that support it. These two environments are especially important to aerobic respirators worldwide, as the phytoplankton perform 40 percent of all photosynthesis on Earth. Although not as diverse as the other two, deep ocean bottom ecosystems contain a wide variety of marine organisms. Such ecosystems exist even at depths where light is unable to penetrate through the water.

Terrestrial ecosystems, also known for their diversity, are grouped into large categories called biomes. A biome is a large-scale community of organisms, primarily defined on land by the dominant plant types that exist in geographic regions of the planet with similar climatic conditions. Examples of biomes include tropical rainforests, savannas, deserts, grasslands, temperate forests, and tundras. Grouping these ecosystems into just a few biome categories obscures the great diversity of the individual ecosystems within them. For example, the saguaro cacti (Carnegiea gigantea) and other plant life in the Sonoran Desert, in the United States, are relatively diverse compared with the desolate rocky desert of Boa Vista, an island off the coast of Western Africa (Figure 3).

Left photo shows a desert with many cacti and other plants, right photo shows a desert with only rocks and sand
Figure 3. Desert ecosystems, like all ecosystems, can vary greatly. The desert in (a) Saguaro National Park, Arizona, has abundant plant life, while the rocky desert of (b) Boa Vista island, Cape Verde, Africa, is devoid of plant life.

Food Chains and Food Webs

A food chain is a linear sequence of organisms through which nutrients and energy pass as one organism eats another (Figure 4). Each organism in a food chain occupies a specific trophic level (energy level), its position in the food chain or food web. The trophic levels in the food chain are producers, primary consumers, and higher-level consumers. These levels are used to describe ecosystem structure and dynamics. There is a single path through a food chain.

In many ecosystems, the base, or foundation, of the food chain consists of photosynthetic organisms (plants or phytoplankton), which are called producers. The organisms that consume the producers are herbivores: the primary consumers. Secondary consumers are usually carnivores that eat the primary consumers. Tertiary consumers are carnivores that eat other carnivores. In some ecosystems, quaternary consumers eat tertiary consumers. Higher-level consumers feed on the next lower trophic levels, and so on, up to the organisms at the top of the food chain: the apex consumers, or apex predators. An apex predator is a species that is not eaten by any other consumers. In the Lake Ontario food chain, shown in Figure 4, the Chinook salmon is the apex consumer at the top of this food chain.

Image of organisms that make up trophic levels in a food chain. The trophic levels are producer, primary consumer, secondary consumer, and tertiary consumer.
Figure 4. These are the trophic levels of a food chain in Lake Ontario at the United States–Canada border. Energy and nutrients flow from photosynthetic green algae at the base to the top of the food chain: the Chinook salmon.

One major factor that limits the number of steps in a food chain is energy. Energy is lost at each trophic level and between trophic levels as heat and in the transfer to decomposers (Figure 5). In general, about 10% of the energy from one trophic level passes to the next trophic level. Thus, after a limited number of trophic energy transfers, the amount of energy remaining in the food chain may not be great enough to support viable populations at yet a higher trophic level.

Energy pyramid diagram shows only 10% of energy passes from one trophic level to the next. Each level loses energy as heat.
Figure 5. An energy pyramid shows the relative amount of energy that passes from one trophic level to the next. Each trophic level has only about 10% of the energy available to the level below it.

There is one problem when using food chains to describe most ecosystems. Even when all organisms are grouped into appropriate trophic levels, some of these organisms can feed on more than one trophic level; likewise, some of these organisms can also be fed on from multiple trophic levels. In addition, species feed on and are eaten by more than one species. In other words, the linear model of ecosystems, the food chain, is a hypothetical, overly simplistic representation of ecosystem structure. A holistic model—which includes all the interactions between different species and their complex interconnected relationships with each other and with the environment—is a more accurate and descriptive model for ecosystems.

A food web is a concept that accounts for the multiple trophic (feeding) interactions between each species and the many species it may feed on, or that feed on it. In a food web, the several trophic connections between each species and the other species that interact with it may cross multiple trophic levels. The matter and energy movements of virtually all ecosystems are more accurately described by food webs (Figure 6).

Food web diagram shows animals eat plants, other animals eat those animals, and all organisms decompose
Figure 6. This food web shows the interactions between organisms across trophic levels. Arrows point from an organism that is consumed to the organism that consumes it. All the producers and consumers eventually become nourishment for the decomposers (fungi, earthworms, bacteria, and other organisms in the soil). Notice how some organisms, such as the owl, can be considered members of multiple trophic levels.

Two general types of food webs are often shown interacting within a single ecosystem. A grazing food web has plants or other photosynthetic organisms at its base, followed by herbivores and various carnivores. A detrital food web consists of a base of organisms that feed on decaying organic matter (dead organisms), including decomposers (which break down dead and decaying organisms) and detritivores (which consume organic detritus). These organisms are usually bacteria, fungi, and invertebrate animals that recycle organic material back into the biotic part of the ecosystem as they themselves are consumed by other organisms. As ecosystems require a method to recycle material from dead organisms, grazing food webs have an associated detrital food web. For example, in a meadow ecosystem, plants may support a grazing food web of different organisms, primary and other levels of consumers, while at the same time supporting a detrital food web of bacteria and fungi feeding off dead plants and animals. Simultaneously, a detrital food web can contribute energy to a grazing food web, as when a robin eats an earthworm.

 

Knowledge Check

How Organisms Acquire Energy in a Food Web

All living things require energy in one form or another. Energy is used by most complex metabolic pathways, especially those responsible for building large molecules from smaller compounds. Living organisms would not be able to assemble complex molecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their building blocks without a constant energy input.

Food-web diagrams illustrate how energy flows directionally through ecosystems. They can also indicate how efficiently organisms acquire energy, use it, and how much remains for use by other organisms of the food web.

An autotroph is an organism that can produce its own food. The Greek roots of the word autotroph mean “self” (auto) “feeder” (troph). Plants are the best-known autotrophs, but others exist, including certain types of bacteria and algae (Figure 7). Oceanic algae contribute enormous quantities of food and oxygen to global food chains. Plants are technically photoautotrophs, a type of autotroph that uses sunlight and carbon from carbon dioxide to synthesize chemical energy in the form of carbohydrates using the process of photosynthesis. All organisms carrying out photosynthesis require sunlight.

Photo a shows a green fern leaf. Photo b shows a pier protruding into water filled with algae. Photo c shows green colored bacteria.
Figure 7. (a) Plants, (b) algae, and (c) certain bacteria, called cyanobacteria, are photoautotrophs that can carry out photosynthesis. Algae can grow over enormous areas in water, at times completely covering the surface.

Heterotrophs are organisms incapable of photosynthesis that must therefore obtain energy and carbon from food by consuming other organisms. The Greek roots of the word heterotroph mean “other” (hetero) “feeder” (troph), meaning that their food comes from other organisms. Even if the food organism is another animal, this food traces its origins back to autotrophs and the process of photosynthesis. Humans are heterotrophs, as are all animals. Heterotrophs depend on autotrophs, either directly or indirectly. Deer and wolves are heterotrophs. A deer obtains energy by eating plants. A wolf eating a deer obtains energy that originally came from the plants eaten by that deer. The energy in the plant came from photosynthesis, and therefore it is the only autotroph in this example (Figure 8). Using this reasoning, all food eaten by humans also links back to autotrophs that carry out photosynthesis.

Photo shows two deer running through tall grass
Figure 8. The energy stored in carbohydrate molecules from photosynthesis passes through the food chain. The predator that eats these deer is getting energy that originated in the photosynthetic vegetation that the deer consumed.

Consequences of Food Webs: Biomagnification

One of the most important consequences of ecosystem dynamics in terms of human impact is biomagnification. Biomagnification, also known as bioaccumulation, is the increasing concentration of persistent, toxic substances in organisms at each successive trophic level. These are substances that are fat soluble, not water soluble, and are stored in the fat reserves of each organism. Many substances have been shown to biomagnify, including classical studies with the pesticide dichlorodiphenyltrichloroethane (DDT), which were described in the 1960s bestseller, Silent Spring by Rachel Carson. DDT was a commonly used pesticide before its dangers to apex consumers, such as the bald eagle, became known. In aquatic ecosystems, organisms from each trophic level consumed many organisms in the lower level, which caused DDT to increase in birds (apex consumers) that ate fish. Thus, the birds accumulated sufficient amounts of DDT to cause fragility in their eggshells. This effect increased egg breakage during nesting and was shown to have devastating effects on these bird populations. The use of DDT was banned in the United States in the 1970s.

Other substances that biomagnify are polychlorinated biphenyls (PCBs), which were used as coolant liquids in the United States until their use was banned in 1979, and heavy metals, such as mercury, lead, and cadmium. These substances are best studied in aquatic ecosystems, where predatory fish species accumulate very high concentrations of toxic substances that are at quite low concentrations in the environment and in producers. As illustrated in a study performed by the NOAA in the Saginaw Bay of Lake Huron of the North American Great Lakes (Figure 9), PCB concentrations increased from the producers of the ecosystem (phytoplankton) through the different trophic levels of fish species. The apex consumer, the walleye, has more than four times the amount of PCBs compared to phytoplankton. Also, based on results from other studies, birds that eat these fish may have PCB levels at least one order of magnitude higher than those found in the lake fish.

Graph shows PCB pollutants increase in fish higher in the food chain
Figure 9. This chart shows the PCB concentrations found at the various trophic levels in the Saginaw Bay ecosystem of Lake Huron. Notice that the fish in the higher trophic levels accumulate more PCBs than those in lower trophic levels.

Other concerns have been raised by the biomagnification of heavy metals, such as mercury and cadmium, in certain types of seafood. The United States Environmental Protection Agency (EPA) recommends that pregnant women and young children should not consume any swordfish, shark, king mackerel, or tilefish because of their high mercury content. If they choose to eat fish, these individuals are advised to eat those lower in mercury: salmon, shrimp, pollock, and catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even influencing the food we eat.

 

Photosynthesis

All living organisms on earth consist of one or more cells. Each cell runs on the chemical energy found mainly in carbohydrate molecules (food), and the majority of these molecules are produced by one process: photosynthesis. Through photosynthesis, certain organisms convert solar energy (sunlight) into chemical energy, which is then used to build carbohydrate molecules. The energy used to hold these molecules together is released when an organism breaks down food. Cells then use this energy to perform work, such as cellular respiration.

The energy that is harnessed from photosynthesis enters the ecosystems of our planet continuously and is transferred from one organism to another. Therefore, directly or indirectly, the process of photosynthesis provides most of the energy required by living things on earth.

Photosynthesis also results in the release of oxygen into the atmosphere. In short, to eat and breathe, humans depend almost entirely on the organisms that carry out photosynthesis.

Main Structures and Summary of Photosynthesis

Photosynthesis requires sunlight, carbon dioxide, and water as starting reactants (Figure 10). After the process is complete, photosynthesis releases oxygen and produces carbohydrate molecules, most commonly glucose. These sugar molecules contain the energy that living things need to survive.

Illustration shows water and carbon dioxide are used by plants to make sugars and oxygen using sunlight energy
Figure 10. Photosynthesis uses light, carbon dioxide, and water to release oxygen and to produce energy-storing sugar molecules.

The complex reactions of photosynthesis can be summarized by the chemical equation shown in Figure 11. In a typical chemical equation, the items to the left of the arrow are known as reagents. These are the elements and/or compounds that react with each other in a chemical reaction. The items to the right of the arrow are called products. These are the elements and/or compounds that are produced during a chemical reaction. In photosynthesis, carbon dioxide and water are reagents, while glucose (a type of sugar) and oxygen are products. Light provides energy to allow the reactions to occur, so it is not technically a reagent. The process of photosynthesis can be represented by an equation, wherein six carbon dioxide (CO2) molecules and six water (H2O) molecules produce one glucose (C6H12O6) molecule and six oxygen (O2) molecules using energy from light.

[latex]6CO_2\:+6H_2O\:+light\:\rightarrow\:C_6H_{12}O_6\:+6O_2[/latex]

Although the equation looks simple, the many steps that take place during photosynthesis are actually quite complex, as in the way that the reaction summarizing cellular respiration represented many individual reactions. Before learning the details of how photoautotrophs turn sunlight into food, it is important to become familiar with the physical structures involved.

In plants, photosynthesis takes place primarily in leaves, which consist of many layers of cells and have differentiated top and bottom sides. The process of photosynthesis occurs not on the surface layers of the leaf, but rather in a middle layer called the mesophyll (Figure 12). The gas exchange of carbon dioxide and oxygen occurs through small, regulated openings called stomata (singular: stoma).

Plants, animals, and fungi belong to a group of organisms called eukaryotes, which contain a nucleus and other organelles inside their cells. In all autotrophic eukaryotes, photosynthesis takes place inside an organelle called a chloroplast. In plants, chloroplast-containing cells exist in the mesophyll. Chloroplasts have a double (inner and outer) membrane. Within the chloroplast is a third membrane that forms stacked, disc-shaped structures called thylakoids. Embedded in the thylakoid membrane are molecules of chlorophyll, a pigment (a molecule that absorbs light) through which the entire process of photosynthesis begins. Chlorophyll is responsible for the green color of plants. The thylakoid membrane encloses an internal space called the thylakoid space. Other types of pigments are also involved in photosynthesis, but chlorophyll is by far the most important. As shown in Figure 11, a stack of thylakoids is called a granum, and the space surrounding the granum is called stroma (not to be confused with a stoma, the opening on a leaf).

Diagram shows leaves are made of layers of cells. In some cells are chloroplasts where photosynthesis occurs.
Figure 11. Not all cells of a leaf carry out photosynthesis. Cells within the middle layer of a leaf have chloroplasts, which contain the photosynthetic apparatus.

The Two Parts of Photosynthesis

Photosynthesis takes place in two stages: the light-dependent reactions and the Calvin cycle, also known as the light-independent reactions. In the light-dependent reactions, which take place at the thylakoid membrane, chlorophyll absorbs energy from sunlight and then converts it into chemical energy with the use of water. The light-dependent reactions release oxygen from the splitting of water as a byproduct. In the Calvin cycle, which takes place in the stroma, the chemical energy derived from the light-dependent reactions drives both the capture of carbon in carbon dioxide molecules and the subsequent assembly of glucose molecules. The two reactions use carrier molecules to transport the energy from one to the other. The carriers that move energy from the light-dependent reactions to the Calvin cycle reactions can be thought of as “full” because they bring energy. After the energy is released, the “empty” energy carriers return to the light-dependent reactions to obtain more energy.

Glucose is useful as a short-term source of energy for plants. For longer-term storage, the glucose molecules are combined to form starches, cellulose, and other compounds that make up the cells of the plant. The glucose can also be used as a building block for other complex molecules like fats and proteins.

Knowledge Check

Cellular Respiration

All living organisms require energy, and for all organisms this energy comes from the chemical energy found in compounds that they acquire from their environment. The mitochondrion is the site of energy generation in eukaryotes (organisms whose cells contain organelles). As you read in the previous section, the process of photosynthesis uses solar energy (sunlight) and converts this energy into chemical energy in the form of carbohydrates like glucose. In order for the chemical energy in the carbohydrates to be made available to do cellular work, the energy must be converted into a useable form known as adenosine triphosphate (ATP). ATP is the energy currency of the cell, and everything you do from walking down the street to reading this book requires energy in the form of ATP. Organisms need a constant supply of ATP, and the potential energy stored in food is the source of energy to meet this need. By connecting all this together, you should realize that your daily activities are fueled by the energy from the sun and that even on the cellular level nutrients cycle and energy flows (Figure 12).

Diagram shows photosynthesis generates molecules that are used in cellular respiration, and vice versa
Figure 12. This image illustrates the relationship between photosynthesis and cellular respiration. Compounds generated in photosynthesis are used in cellular respiration, and vice versa.

All organisms need ATP, but not all organisms use the same pathways to generate ATP from the food that is consumed. Aerobic cellular respiration uses oxygen (O2) and glucose to generate ATP. Organisms (plants, animals, fungi and microbes) that live in an oxygen-rich environment use this process to generate ATP.

As the aerobic cellular respiration equation shows, an organism needs to acquire oxygen (O2) to combine with glucose (C6H12O6) in order to transfer the energy in glucose to ATP. In the process, carbon dioxide (CO2) and water (H2O) are given off as waste products.

[latex]C_6H_{12}O_6\:+6O_2\:\rightarrow\:6CO_2\:+6H_2O\:+energy\:\left(ATP\right)[/latex]

 

Like photosynthesis, aerobic cellular respiration is a series of linked chemical reactions rather than a single reaction. This series can be best understood if it is separated into four stages. These are glycolysis, pyruvate oxidation, the Krebs Cycle, and oxidative phosphorylation. During the reactions in these stages, electrons are stripped from the chemical bonds of the original glucose molecule and eventually added to oxygen, via a series of intermediate steps. This series of reactions releases small amounts of energy at each step; this energy is used to drive the formation of ATP. Eventually, the carbon in the glucose molecule is converted into six CO2 molecules to be expelled by the organism as a waste product.

The acquisition of O2 and the release of CO2 is accomplished in a variety of ways. In single-celled organisms, the movement of O2 and CO2 (gas exchange) is done by simple diffusion. However, in complex organisms there are specialized organs that allow for gas exchange; for example, gills in aquatic organisms and lungs in terrestrial animals.

A common misconception is that plants do not undergo cellular respiration because they make their own energy by photosynthesis. Plants do perform cellular respiration using the carbohydrates produced via photosynthesis; this occurs in tissues that are not photosynthetically active (e.g., roots), as well as in leaves and stems. The plant consumes approximately half of the glucose produced by photosynthesis, mostly to generate ATP during aerobic cellular respiration. Other uses of glucose in the plant include synthesis of cell walls, starch, and other plant carbohydrates. So, plants harvest light energy via photosynthesis, making carbohydrates, and then they use the energy stored in those carbohydrates to perform various cellular functions. This is the reason why they are called autotrophs, or self-feeders.

Some single-celled organisms use anaerobic metabolism to extract energy from biological molecules; this process occurs in the absence of oxygen and is known as fermentation. You may already be familiar with one type of fermentation, lactic acid fermentation, especially if you have recently over-exerted your muscles. Anaerobic metabolism is used by many organisms to produce ATP when oxygen is not available and thus pathways that require oxygen cannot be used. The amount of ATP produced by fermentation is much less than that produced by aerobic cellular respiration, so there are costs and benefits associated with organisms utilizing fermentation.

Knowledge Check

Attribution

“Waterford’s Energy Flow through Ecosystems” in OpenStax Concepts of Biology, modified by Sean Whitcomb. License: CC BY

“Overview of Cellular Respiration” by Robert Bear and David Rintoul, modified by Sean Whitcomb. License: CC BY

“Overview of Photosynthesis” in OpenStax Concepts of Biology, modified by Sean Whitcomb. License: CC BY

Media Attributions

  • Levels_of_organization © Siyavula Education is licensed under a CC BY (Attribution) license
  • Tide pool and Amazon rainforest © Jim Kuhn; Ivan Mlinaric adapted by OpenStax is licensed under a CC BY (Attribution) license
  • Deserts © Jay Galvin; Ingo Wölbern adapted by OpenStax is licensed under a CC BY (Attribution) license
  • Food chain trophic levels © NOAA is licensed under a Public Domain license
  • Energy Pyramid © Epipelagic adapted by Sean Whitcomb is licensed under a CC BY-SA (Attribution ShareAlike) license
  • Food_web © "fox": modification of work by Kevin Bacher, NPS; "owl": modification of work by John and Karen Hollingsworth, USFWS; "snake": modification of work by Steve Jurvetson; "robin": modification of work by Alan Vernon; "frog": modification of work by Alessandro Catenazzi;"spider": modification of work by "Sanba38"/Wikimedia Commons; "centipede": modification of work by “Bauerph”/Wikimedia Commons;"squirrel": modification of work by Dawn Huczek; "mouse": modification of work by NIGMS, NIH; "sparrow": modification of work by David Friel; "beetle": modification of work by Scott Bauer, USDA Agricultural Research Service; "mushrooms": modification of work by Chris Wee; "mold": modification of work by Dr. Lucille Georg, CDC; "earthworm": modification of work by Rob Hille; "bacteria": modification of work by Don Stalons, CDC adapted by OpenStax is licensed under a CC BY (Attribution) license
  • Autotrophs © Steve Hillebrand (US FWS); NASA is licensed under a CC BY (Attribution) license
  • Deer © Steve VanRiper, U.S. Fish and Wildlife Service is licensed under a Public Domain license
  • Bioaccumulation
  • Photosynthesis illustration is licensed under a CC BY (Attribution) license
  • Leaf_structure © Cory Zanker adapted by OpenStax is licensed under a CC BY (Attribution) license
  • Photosynthesis respiration relationship © Eva Horne and Robert Bear (used with permission) is licensed under a CC BY (Attribution) license

License

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Environmental Science Copyright © by Sean Whitcomb is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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