Every time energy is transferred from one organism to another, there is a net loss of energy. This loss of energy can be shown in an energy pyramid. An example of an energy pyramid is shown in Figure 6. Due to the energy loss in food chains, it takes many producers to support just a few carnivores in a community. For example, there are far fewer hawks than acorns in this food chain.
Each step of the food chain reflected in the ecological pyramid is called a trophic level. Plants or other photosynthetic organisms are found on the first trophic level, at the base of the pyramid. The next level would be the herbivores, then the carnivores that eat the herbivores. The energy pyramid in Figure below shows only three levels of a food chain, from plants (producers) to hawks (carnivores). Because of the high rate of energy loss in food chains, there are usually only 4 or 5 levels in the chain or energy pyramid.
Figure 24.7
As illustrated by this ecological pyramid, it takes a lot of phytoplankton to support the carnivores of the oceans.
Lesson Summary
Producers, which include photosynthetic organisms like plants and algae, can make their own food from simple inorganic compounds.
Consumers must obtain their nutrients and energy by eating other organisms, while decomposers break down animal remains and wastes to obtain energy.
Food chains and food webs are visual representations of feeding patterns in an ecosystem.
As energy is transferred along a food chain, energy is lost as heat.
Review Questions
How do decomposers obtain energy?
What happens to 90% of the energy that passes from one step in the food chain to the next step?
For #’s 3 - 5, Analyze the following food chain: algae -> fish -> herons
What is the producer in the food chain?
What is the herbivore in the food chain?
What is the carnivore in the food chain?
In a food chain, does the prey or predator have a greater biomass?
In an ecological pyramid, which level would have the greatest biomass?
What is the term for the visual representation of complex feeding interactions in a community?
In a forest community, caterpillars eat leaves, and birds eat caterpillars. Draw the food chain.
What’s the term for a consumer that eats both plants and animals?
Further Reading / Supplemental Links
http://www.eelsinc.org/id43.html
http://science-class.net/Ecology/energy_transfer.htm
http://en.wikipedia.org/wiki/Energy_pyramid
http://curriculum.calstatela.edu/courses/builders/lessons/less/biomes/calcpy.html
http://science-class.net/Ecology/energy_transfer.htm
http://en.wikipedia.org/wiki/Food_chain
Vocabulary
biomass
The total dry weight of all the individuals of one type of organism.
carnivore
An organism that eats other animals.
consumer
An organism that must eat other organisms to obtain energy and nutrients.
decomposer
An organism that breaks down animal remains or wastes to gain energy and nutrients.
ecological pyramid
A visual representation of the energy content or biomass at various levels in a food chain.
food chain
A visual representation of the flow of energy from producers to consumers in a community.
food web
A visual representation of the complex eating relationships in a community; a cross-linking of food chains.
herbivore
A consumer of producers in a community; often organisms that eat plants.
omnivore
A consumer in a community that eat both producers and consumers; usually eaters of both plants and animals.
producer
An organism that can absorb the energy of the sun and convert it into food through the process of photosynthesis; i.e. plants and algae.
trophic level
A level of the food chain reflected in the ecological pyramid.
Points to Consider
Animals are carbon-based organisms. When animals decompose, what happens to the carbon? Discuss this with your class.
We need nitrogen to make our DNA. Where does it come from? Where does it go? What would happen to nitrogen released from decaying organisms?
Water is essential for photosynthesis. Water moves through both the living and non-living parts of an ecosystem. How does water move through the living parts of an ecosystem?
Lesson 24.2: Cycles of Matter
Lesson Objectives
Describe the key features of the water cycle.
Describe the key features of the nitrogen cycle.
Describe the key features of the carbon cycle.
Check Your Understanding
What types of organisms break down animal remains and wastes to release nutrients?
What are the main chemical elements that are essential for life?
Introduction
What happens to all the plants and animals that die? Do they pile up and litter ecosystems with dead remains? Or do they decompose? The role of decomposers in the environment often goes unnoticed, but these organisms are absolutely crucial for every ecosystem. Imagine if the decomposers were somehow taken out of an ecosystem. The nutrients, such as carbon and nitrogen, in animal wastes and dead organisms would remain locked in these forms if there was nothing to decompose them. Overtime, almost all the nutrients in the ecosystem would be used up. However, these elements are essential to build the organic compounds necessary for life and so they must be recycled. The decomposition of animal wastes and dead organisms allows these nutrients to be recycled and re-enter the ecosystem, where they can be used by living organisms.
The pathways by which chemicals are recycled in an ecosystem are biogeochemical cycles. This recycling process involves both the living parts (biotic) of the ecosystem and the non-living (abiotic) parts of the ecosystem, such as the atmosphere, soil, or water. The same chemicals are constantly being passed through living organisms to non-living matter and back again, over and over. Through biogeochemical cycles, inorganic nutrients that are essential for life are continually recycled and made available again to living organisms. These recycled nutrients contain the elements carbon and nitrogen. Water is obviously an extremely important aspect of every ecosystem. Life could not exist without water. Water is also cycled through the biotic and abiotic factors of an ecosystem.
The Water Cycle
Since many organisms contain a large amount of water in their bodies, and some even live in water, the water cycle is essential to life on earth. Water is continually moving between living things and non-living things such as clouds, rivers, or oceans. The water cycle is also important because water is a solvent, so it plays an important role in dissolving minerals and gases and carrying them to the ocean. Therefore, the composition of the oceans is also dependent on the water cycle (Figure below).
The water cycle does not have a real starting or ending point, since it is an endless circular process; however, we will start with the oceans. Water evaporates from the surface of the oceans, leaving behind salts. As the water vapor rises, it collects and is stored in clouds. As water cools in the clouds, it condenses into precipitation such as rain, snow, hail, sleet, etc. The precipitation allows the water to return again to the Earth’s surface. On land, the water can sink into the ground to become part of our underground water reserves, also known as groundwater. Much of this underground water is stored in aquifers, which are porous layers of rock that can hold water. Most precipitation that occurs over land, however, is not absorbed by the soil and is called runoff. This runoff collects in streams and rivers and moves back into the ocean.
Water also moves through the living organisms in the ecosystem. Plants are especially significant to the water cycle because they soak up large amounts of water through their roots. The water then
moves up the plant and evaporates from the leaves in a process called transpiration. The process of transpiration, like evaporation, returns water back into the atmosphere.
Figure 24.8
The water cycle. See for an animation of the water cycle.
The Carbon Cycle
Carbon is one of the most abundant elements found in living organisms. Carbon chains form the backbones of carbohydrates, proteins, and fats. Carbon is constantly cycling between living things and the atmosphere (Figure below).
In the atmosphere, carbon is in the form of carbon dioxide. Producers capture this carbon dioxide and convert it to food through the process of photosynthesis (discussed in the chapter titled Cells and Their Structures). As consumers eat producers or other consumers, they gain the carbon from that organism. Some of this carbon is lost, however, through the process of cellular respiration. When our cells burn food for energy, carbon dioxide is released. We exhale this carbon dioxide and it returns to the atmosphere. Also, carbon dioxide is released to the atmosphere as an organism dies and decomposes.
Figure 24.9
The carbon cycle.
Millions of years ago there was so much organic matter that it could not be completely decomposed before it was buried. As this buried organic matter was under pressure for millions of years, it formed into fossil fuels such as coal, oil, and natural gas. When humans excavate and use fossil fuels, we have an impact on the carbon cycle (Figure below). The burning of fossil fuels releases more carbon dioxide into the atmosphere than is used by photosynthesis. Therefore the net amount of carbon dioxide in the atmosphere is rising. Carbon dioxide is known as a greenhouse gas since it lets in light energy but does not let heat escape, much like the panes of a greenhouse. The increase of greenhouse gasses in the atmosphere is contributing to a global rise in Earth’s temperature, known as global warming (see the Environmental Problems chapter for additional information).
Figure 24.10
Human activities like burning gasoline in cars are contributing to a global change in our climate.
The Nitrogen Cycle
Nitrogen is also one of the most abundant elements in living things. It’s important for constructing both proteins and nucleic acids like DNA. The great irony of the nitrogen cycle is that nitrogen gas (N2) comprises the majority of the air we breathe, and yet is not accessible to us or plants in the gaseous form (Figure below). In fact, plants often suffer from nitrogen deficiency even through they are surrounded by plenty of nitrogen gas!
In order for plants to make use of nitrogen, it must be converted into compounds with other elements. This can be accomplished several different ways. First, Nitrogen gas can be converted to nitrate (NO3 -) through lightning strikes. Alternatively, special nitrogen-fixing bacteria can also convert nitrogen gas into useful forms, a process called nitrogen fixation. These bacteria live in nodules on the roots of plants in the pea family. In aquatic environments, bacteria in the water can fix nitrogen gas into ammonium (NH4 +), which can be used by aquatic plants as a source of nitrogen.
Nitrogen also is released to the environment through decaying organisms or decaying wastes. These wastes often take on the form of ammonium. Ammonium in the soil can be converted to nitrate by a two-step process completed by two different types of bacteria. In the form of nitrate, it can be used by plants through a process called assimilation.
The conversion of nitrate back into nitrogen gas happens through the work of denitrifying bacteria. These bacteria often live in swamps and lakes. The release of nitrogen gas would equal the amount of nitrogen gas taken into living things if human activities did not influence the nitrogen cycle. These human activities include the burning of fossil fuels, which releases nitrogen oxide gasses into the atmosphere, leading to problems like acid rain.
Figure 24.11
The nitrogen cycle includes assimilation, or uptake of nitrogen by plants; nitrogen-fixing bacteria that make the nitrogen available to plants in the form of nitrates; decomposers that convert nitrogen in dead organisms into ammonium; nitrifying bacteria that convert ammonium to nitrates; and denitrifying bacteria that convert help convert nitrates to gaseous nitrogen.
Lesson Summary
During the water cycle, water enters the atmosphere through evaporation, and water returns to land through precipitation.
During the carbon cycle, animals add carbon dioxide to the atmosphere through respiration and plants remove carbon dioxide through photosynthesis.
During the nitrogen cycle, gaseous nitrogen is converted into water-soluble forms that can be used by plants, while denitrifying bacteria convert nitrate back to gaseous nitrogen.
Review Questions
What human activities have thrown the carbon cycle off balance?
What biological process “fixes” carbon, removing it from the atmosphere?
What is the significance of nitrogen-fixing bacteria?
What is the term for the remains of organisms that are burned for energy?
How does water in the atmosphere return to the ground?
What biological process releases carbon back into the atmosphere?
What are some examples of fossil fuels?
Why is carbon dioxide referred to as a “greenhouse gas”?
What must happen for plants to use nitrogen in the atmosphere?
What is the significance of denitrifying bacteria?
Further Reading / Supplemental Links
http://earthobservatory.nasa.gov/Library/CarbonCycle
http://www.cosee-ne.net/resources/documents/OceanLiteracyWorkshopIReport.pdf
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookcycles.html
http://earthobservatory.nasa.gov/Library/CarbonCycle
http://earthguide.ucsd.edu/earthguide/diagrams/watercycle/index.html
http://en.wikipedia.org/wiki
Vocabulary
assimilation
The uptake of nitrogen by plants.
aquifers
Layers of porous rock that can hold water underground.
biogeochemical cycles
The pathway of elements like carbon and nitrogen through the non-living and living parts of the ecosystem.
denitrifying bacteria
Bacteria that convert nitrates or nitrites back to nitrogen in the gaseous form.
fossil fuels
Fuels made from partially decomposed organic matter that has been compressed underground for millions of years; examples are: coal, natural gas, and oil.
global warming
Global increase in the Earth’s temperature due to human activities that release greenhouse gasses into the atmosphere.
groundwater
Underground water reserves.
nitrogen fixation
Process by which gaseous nitrogen is converted into chemical forms that can be used by plants.
precipitation
Water that falls to the earth in the form of rain, snow, sleet, hail.
runoff
Water that is not absorbed by the soil that eventually returns to streams and rivers.
transpiration
Process by which water leaves a plant by evaporating from the leaves.
Points to Consider
Do ecosystems change over time? Why or why not?
Can you think of an example of a ecosystem changing over time?
Lesson 24.3: Ecosystem Change
Lesson Objectives
Explain the process of ecological succession.
Distinguish between secondary and primary succession.
Describe a climax community.
Check Your Understanding
What is a biome?
What is the most abundant element in living things?
How do humans obtain nitrogen?
Introduction
When you see an established forest, it’s easy to picture that the forest has been there forever. This is not the case, however. Ecosystems are dynamic and change over time. That forest may lie on land th
at was once covered by an ocean millions of years ago. Or the forest may have been cut down at one point for agricultural use, then abandoned and allowed to re-establish itself over time. During the ice ages, glaciers once covered areas that are tropical rainforests today. Due to both natural forces and the influence of humans, ecosystems are constantly changing.
Primary Succession
If conditions of an ecosystem change drastically due to natural forces or human impact, the community of plants and animals that live there may be destroyed or be forced to relocate. Over time a new community will be established, and then that community may be replaced by another. You may see several changes in the plant and animal composition of the community over time. Ecological succession is the continual replacement of one community by another that occurs after some disturbance of the ecosystem.
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