CK-12 Biology I - Honors
Page 67
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 below.
Figure 15.8
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.
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 in Figure 4, consume feces. In aquatic ecosystems, detritivores include “bottom feeders,” such as sea cucumbers and catfish.
Figure 15.9
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 below.
Figure 15.10
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 below.
Figure 15.11
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 above, 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 below.
Pyramid of Energy
Figure 15.12
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 below shows how biomass of organisms changes from first to higher trophic levels in a food chain.
Pyramid of Biomass
Figure 15.13
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 in ?
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.
Lesson Summary
Producers in ecosystems are autotrophs. They use energy from sunlight or chemical compounds to synthesize organic molecules from carbon dioxide and other simple inorganic molecules.
Consumers in ecosystems are heterotrophs, or organisms that consume other organisms for food. Consumers include herbivores such as deer, carnivores such as lions, and omnivores such as humans.
Decomposers break down dead organisms and other organic wastes in ecosystems. They resupply producers with the elements they need to synthesize organic compounds.
Food chains and food webs model feeding relationships in ecosystems. They show how energy and materials are transferred between trophic level when consumers eat producers or other organisms.
Review Questions
How do autotrophs use energy to produce organic molecules?
Define three different types of consumers, and name an example of each.
How do decomposers resupply elements to producers?
How is energy transferred between trophic levels in a food chain?
In Figure 6, identify two food chains containing gulls: one in which gulls are tertiary consumers, and one in which gulls are quaternary consumers.
If one million kilocalories of energy are stored in producers in an ecosystem, how many kilocalories can be transferred to tertiary consumers in the ecosystem? Show the calculations that support your answer.
Draw a terrestrial food chain that includes four trophic levels.
All organisms consist of carbon compounds. Infer how the amount of carbon stored in organisms changes from one trophic level to the next. Explain your answer.
Further Reading / Supplemental Links
Beebe, Alan and Brennan, Anne-Maria, First Ecology (3rd revised edition). Oxford University Press, 2007.
Biello, David, Humans Gobble One Quarter of Food Chain’s Foundation. Scientific American, July, 2007.
Parramon Studios (editor), Essential Atlas of Ecology. Barron’s Educational Series, 2005.
http://estrellamountain.edu/faculty/farabee/biobk/BioBookcycles.html
http://www.pmel.noaa.gov/vents/nemo/explorer/concepts/chemosynthesis.html
http://www.sciam.com/article.cfm?articleID=88D6F188-E7F2-99DF-33EF7C83A99DCD04
http://science-class.net/Ecology/energy_transfer.htm
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/F/FoodChains.html
http://www.arcytech.org/java/population/facts_foodchain.html
http://en.wikipedia.org
Vocabulary
Archaea
A prokaryotic domain of microorganisms that resemble bacteria; most archaea live in extreme environments, such as around hydrothermal vents in the deep ocean and are chemoautotrophs.
autotrophs
Organisms that produce organic compounds from energy and simple inorganic molecules; also known as producers.
carnivores
Organisms that eat a diet consisting mainly of herbivores or other carnivores.
chemoautotrophs
Organisms that use the energy stored in chemical compounds to make organic molecules by chemosynthesis.
chemosynthesis
The process by which carbon dioxide and water are converted to carbohydrates; uses energy from the oxidation of inorganic compounds.
consumers
Organisms that depend on producers or other types of organisms for food.
decomposers
Organisms that consume dead organisms and other organic waste.
detritivores
Organisms that consume the remains of dead plants (detritus).
detritus
Dead leaves and other plant remains that accumulate on the ground or at the bottom of a body of water.
food chain
A simple linear pathway through which energy and materials are transferred from one species to another in an ecosystem.
food web
A diagram of feeding relationships that includes multiple intersecting food chains.
herbivores
Organisms that consume only producers such as plants or algae; form a necessary link between producers and other consumers.
heterotrophs
Organisms that depend on producers or other types of organisms for food; also called consumers.
omnivores
Organisms that eat both plants and animals as primary food sources.
oxidation
An energy-releasing chemical reaction in which a molecule, atom, or ion loses electrons.
photoautotrophs
Organisms that use energy from sunlight to make food by photosynthesis; includes plants, algae, and certain bacteria.
photosynthesis
The process by which carbon dioxide and water are converted to glucose and oxygen, using sunlight for energy.
phytoplankton
All the tiny photoautotrophs found on or near the surface of a body of water; usually is the primary producer in aquatic ecosystems; includes both cyanobacteria and algae.
plankton
Large communities of producers and herbivores; made up of phytoplankton and zooplankton.
producers
Organisms that produce organic compounds from energy and simple inorganic molecules.
saprotrophs
Organisms that complete the breakdown of any remaining organic matter, such as bones, feathers, and fur of dead animals, and wood and other indigestible plant material.
scavengers
Carnivores that mainly eat the carcasses of dead animals.
trophic levels
The different feeding positions in a food chain or web.
zooplankton
All the small organisms that feed on phytoplankton; includes both single-celled organisms such as protozoa and multicellular organisms such as jellyfish.
Points to Consider
Matter recycles through the biotic components of ecosystems as producers synthesize organic compounds and other organisms consume the compounds.
Do you think abiotic components of ecosystems also play roles in recycling matter?
What abiotic components do you think might be involved? For example, what abiotic components do you think might be involved in the cycling of water?
Lesson 15.3: Recycling Matter
Lesson Objectives
Define and give examples of biogeochemical cycles that recycle matter.
Describe the water cycle and the processes by which water changes state.
Summarize the organic and geological pathways of the carbon cycle.
Outline the nitrogen cycle and state the roles of bacteria
in the cycle.
Introduction
Unlike energy, elements are not lost and replaced as they pass through ecosystems. Instead, they are recycled repeatedly. All chemical elements that are needed by living things are recycled in ecosystems, including carbon, nitrogen, hydrogen, oxygen, phosphorus, and sulfur. Water is also recycled.
Biogeochemical Cycles
A biogeochemical cycle is a closed loop through which a chemical element or water moves through ecosystems. In the term biogeochemical, bio- refers to biotic components and geo- to geological and other abiotic components. Chemicals cycle through both biotic and abiotic components of ecosystems. For example, an element might move from the atmosphere to ocean water, from ocean water to ocean organisms, and then back to the atmosphere to repeat the cycle.
Elements or water may be held for various lengths of time by different components of a biogeochemical cycle. Components that hold elements or water for a relatively short period of time are called exchange pools. For example, the atmosphere is an exchange pool for water. It holds water for several days at the longest. This is a very short time compared with the thousands of years the deep ocean can hold water. The ocean is an example of a reservoir for water. Reservoirs are components of a geochemical cycle that hold elements or water for a relatively long period of time.