In this section, you will explore the following questions:
- How do organisms acquire energy in a food web and associated food chains?
- How does the efficiency of energy transfer between trophic levels affect ecosystem structure and dynamics?
- What are the characteristics of each trophic level in an ecosystem, and how can ecological pyramids be used to model them?
Connection for AP® Courses
As we learned when we explored concepts in earlier chapters, all living organisms require energy in one form or another, usually ATP, to carry out cellular processes. It is important to understand how organisms in an ecosystem acquire free energy and how that energy is passed among organisms through food webs and their constituent food chains. Energy takes a one-way path through ecosystems because energy conversions result in a loss of usable (free) energy through the release of heat. In addition, matter cycles and recycles as it moves from organism to organism. We also have learned that the biotic and abiotic components of an ecosystem interact. Autotrophs, chemoautotrophs, heterotrophs and decomposers comprise the living components, whereas abiotic factors include nutrients, temperature, pH, availability of sunlight, and type of soil. In this section, a variety of ways to depict this movement of energy through an ecosystem will be presented. Utilizing multiple representations of data as well as understanding the movement of matter and energy through systems are significant concepts in the AP® Biology course.
Information presented and the examples highlighted in the section support concepts outlined in Big Idea 4 of the AP® Biology Curriculum Framework. The AP® Learning Objectives listed in the Curriculum Framework provide a transparent foundation for the AP®Biology course, an inquiry-based laboratory experience, instructional activities, and AP® exam questions. A learning objective merges required content with one or more of the seven science practices.
|Big Idea 4||Biological systems interact, and these systems and their interactions possess complex properties.|
|Enduring Understanding 4.A||Interactions within biological systems lead to complex properties.|
|Essential Knowledge||4.A.6 Interactions among living systems and with their environment result in the movement of matter and energy.|
|Science Practice||2.2 The student can apply mathematical routines to quantities that describe natural phenomena.|
|Learning Objective||4.14 The student is able to apply mathematical routines to quantities that describe interactions among living systems and their environment, which result in the movement of matter and energy.|
|Essential Knowledge||4.A.6 Interactions among living systems and with their environment result in the movement of matter and energy.|
|Science Practice||1.4 The student can use representations and models to analyze situations or solve problems qualitatively and quantitatively.|
|Learning Objective||4.15 The student is able to use visual representations to analyze situations or solve problems qualitatively to illustrate how interactions among living systems and with their environment result in the movement of matter and energy.|
All living things require energy in one form or another. Energy is required by most complex metabolic pathways (often in the form of adenosine triphosphate, ATP), especially those responsible for building large molecules from smaller compounds, and life itself is an energy-driven process. Living organisms would not be able to assemble macromolecules (proteins, lipids, nucleic acids, and complex carbohydrates) from their monomeric subunits without a constant energy input.
It is important to understand how organisms acquire energy and how that energy is passed from one organism to another through food webs and their constituent food chains. Food webs illustrate how energy flows directionally through ecosystems, including how efficiently organisms acquire it, use it, and how much remains for use by other organisms of the food web.
How Organisms Acquire Energy in a Food Web
Energy is acquired by living things in three ways: photosynthesis, chemosynthesis, and the consumption and digestion of other living or previously living organisms by heterotrophs.
Photosynthetic and chemosynthetic organisms are both grouped into a category known as autotrophs: organisms capable of synthesizing their own food (more specifically, capable of using inorganic carbon as a carbon source). Photosynthetic autotrophs (photoautotrophs) use sunlight as an energy source, whereas chemosynthetic autotrophs (chemoautotrophs) use inorganic molecules as an energy source. Autotrophs are critical for all ecosystems. Without these organisms, energy would not be available to other living organisms and life itself would not be possible.
Photoautotrophs, such as plants, algae, and photosynthetic bacteria, serve as the energy source for a majority of the world’s ecosystems. These ecosystems are often described by grazing food webs. Photoautotrophs harness the solar energy of the sun by converting it to chemical energy in the form of ATP (and NADP). The energy stored in ATP is used to synthesize complex organic molecules, such as glucose.
Productivity within Trophic Levels
Productivity within an ecosystem can be defined as the percentage of energy entering the ecosystem incorporated into biomass in a particular trophic level. Biomass is the total mass, in a unit area at the time of measurement, of living or previously living organisms within a trophic level. Ecosystems have characteristic amounts of biomass at each trophic level. For example, in the English Channel ecosystem the primary producers account for a biomass of 4 g/m2 (grams per meter squared), while the primary consumers exhibit a biomass of 21 g/m2.
Because all organisms need to use some of this energy for their own functions (like respiration and resulting metabolic heat loss) scientists often refer to the net primary productivity of an ecosystem. Net primary productivity is the energy that remains in the primary producers after accounting for the organisms’ respiration and heat loss. The net productivity is then available to the primary consumers at the next trophic level. In our Silver Spring example, 13,187 of the 20,810 kcal/m2/yr were used for respiration or were lost as heat, leaving 7,632 kcal/m2/yr of energy for use by the primary consumers.
Ecological Efficiency: The Transfer of Energy between Trophic Levels
Ecologists have many different methods of measuring energy transfers within ecosystems. Some transfers are easier or more difficult to measure depending on the complexity of the ecosystem and how much access scientists have to observe the ecosystem. In other words, some ecosystems are more difficult to study than others, and sometimes the quantification of energy transfers has to be estimated.
Another main parameter that is important in characterizing energy flow within an ecosystem is the net production efficiency. Net production efficiency (NPE) allows ecologists to quantify how efficiently organisms of a particular trophic level incorporate the energy they receive into biomass; it is calculated using the following formula:
Net consumer productivity is the energy content available to the organisms of the next trophic level. Assimilation is the biomass (energy content generated per unit area) of the present trophic level after accounting for the energy lost due to incomplete ingestion of food, energy used for respiration, and energy lost as waste. Incomplete ingestion refers to the fact that some consumers eat only a part of their food. For example, when a lion kills an antelope, it will eat everything except the hide and bones. The lion is missing the energy-rich bone marrow inside the bone, so the lion does not make use of all the calories its prey could provide.
Thus, NPE measures how efficiently each trophic level uses and incorporates the energy from its food into biomass to fuel the next trophic level. In general, cold-blooded animals (ectotherms), such as invertebrates, fish, amphibians, and reptiles, use less of the energy they obtain for respiration and heat than warm-blooded animals (endotherms), such as birds and mammals. The extra heat generated in endotherms, although an advantage in terms of the activity of these organisms in colder environments, is a major disadvantage in terms of NPE. Therefore, many endotherms have to eat more often than ectotherms to get the energy they need for survival. In general, NPE for ectotherms is an order of magnitude (10x) higher than for endotherms. For example, the NPE for a caterpillar eating leaves has been measured at 18 percent, whereas the NPE for a squirrel eating acorns may be as low as 1.6 percent.
The inefficiency of energy use by warm-blooded animals has broad implications for the world's food supply. It is widely accepted that the meat industry uses large amounts of crops to feed livestock, and because the NPE is low, much of the energy from animal feed is lost. For example, it costs about $0.01 to produce 1000 dietary calories (kcal) of corn or soybeans, but approximately $0.19 to produce a similar number of calories growing cattle for beef consumption. The same energy content of milk from cattle is also costly, at approximately $0.16 per 1000 kcal. Much of this difference is due to the low NPE of cattle. Thus, there has been a growing movement worldwide to promote the consumption of non-meat and non-dairy foods so that less energy is wasted feeding animals for the meat industry.
Modeling Ecosystems Energy Flow: Ecological Pyramids
The structure of ecosystems can be visualized with ecological pyramids, which were first described by the pioneering studies of Charles Elton in the 1920s. Ecological pyramids show the relative amounts of various parameters (such as number of organisms, energy, and biomass) across trophic levels.
- Pyramid of biomass, as biomass will always be found at the base and there is loss and not gain of biomass through the trophic levels.
- Pyramid of number, as the numbers of producers is always more than the number of consumers in every ecosystem.
- Pyramid of energy, as energy will always be found at the base and there is loss and not gain of energy through the trophic levels.
- Pyramid of predators, as predators are always fewer in an ecosystem than the producers for the ecosystem to work efficiently.
Consequences of Food Webs: Biological Magnification
One of the most important environmental consequences of ecosystem dynamics is biomagnification. Biomagnification is the increasing concentration of persistent, toxic substances in organisms at each trophic level, from the primary producers to the apex consumers. Many substances have been shown to bioaccumulate, including classical studies with the pesticide dichlorodiphenyltrichloroethane (DDT), which was published in the 1960s bestseller, Silent Spring, by Rachel Carson. DDT was a commonly used pesticide before its dangers became known. In some aquatic ecosystems, organisms from each trophic level consumed many organisms of 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 adverse effects on these bird populations. The use of DDT was banned in the United States in the 1970s.
Other concerns have been raised by the accumulation 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. These individuals are advised to eat fish low in mercury: salmon, tilapia, shrimp, pollock, and catfish. Biomagnification is a good example of how ecosystem dynamics can affect our everyday lives, even influencing the food we eat.
Many people enjoy eating swordfish, but pregnant women and young children should avoid it due to its high mercury content resulting from biomagnification.
- Larger animals consume many small organisms, which leads to accumulation of oxygen in their body over time.
- Larger animals consume many small organisms, which leads to accumulation of mercury in their body over time.
- Larger animals consume many small organisms, which leads to accumulation of hydrogen in their body over time.
- Larger animals consume many small organisms, which leads to accumulation of polyglucose in their body over time.
AP® Biology Investigation Labs: Inquiry-Based Approach, Investigation 10: Energy Dynamics. This inquiry-based investigation provides an opportunity for you to explore factors that govern energy capture, allocation, storage, and transfer between producers and consumers in a terrestrial ecosystem composed of Wisconsin Fast Plants (producers) and cabbage white butterfly (Pieris rapae) larvae.
An ecosystem consists of earthworms, heterotrophic soil bacteria, grass, deer, beetles, and a lion. Create a mini-poster to describe the trophic structure of the ecosystem, how each organism receives inputs of energy and nutrients, where outputs (e.g., wastes) go, and the effect(s) each organisms has on the others. Include all energy transformations and transfers based on the hypothetical assumption that 9,500 J of net energy is available at the producer level.