Back To CourseGeneral Studies Science: Help & Review
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Jeff is a Biology teacher and has a Doctorate in Educational Leadership
All life needs energy. Whether living organisms make energy themselves or get it from the food they hunt, they need it to maintain and repair their bodies. Reproduction, hunting, growth, cell division, and metabolism are all processes that require energy.
The sun is the ultimate source of energy for life on Earth. Without it, nothing would be able to survive. As a result, living things have evolved special ways to harness the energy of the sun and use it for their own well-being. They have also developed special relationships and interactions that allow energy to be transferred. Once the energy has been captured, it gets passed around through the various organisms in a particular area. This transfer of energy is called a food web.
In their simplest form, food webs are made of food chains. Food chains show a direct transfer of energy between organisms. A chain might involve a mouse eating some seeds on the forest floor. Then, a snake comes along and eats the mouse. A while later, an owl eats the snake. With each step, some of the energy from the sun, which is trapped within the seeds, is getting passed on.
Food chains do not accurately portray the transfer of energy in an ecosystem. This is because there are often multiple organisms that can be eaten, and many that can do the eating. For example, the aforementioned mouse might eat seeds, but it also might eat some berries, or maybe even some grass. The mouse might be eaten by a snake, or the owl, or even a fox. The snake could be eaten be the owl, but also might get eaten by a fox or a coyote in the forest. Since each organism can eat multiple things and be eaten by multiple things, a food web is a much more realistic schematic of the transfer of energy within an ecosystem.
Often considered the bottom of the food chain, producers are the organisms that make their own food and serve as the foundation for all food chains and webs. Producers are organisms such as plants, algae, and even some bacteria. They make their own energy by converting sunlight into sugars through a process called photosynthesis. They use the sunlight as an energy source to convert carbon dioxide in the air into glucose (a simple sugar that can easily be broken down for energy). The producers then store this sugar and use it for energy later on.
In marine ecosystems, the producers are dominated by algae, plant-like organisms. Algae can be microscopic, like diatoms, or they can be quite large, like the giant kelp found off the coast of California. Either way, the oceans are the prime spot on the planet for producers. Since water covers 72 percent of the planet, it only makes sense that most of the oxygen on the earth comes from the oceans. In fact, the diatoms alone make about 30 percent of it!
Rainforests are another ecosystem that has many producers. Even though rainforests only cover about six percent of the earth's surface, they produce about 40 percent of the oxygen because of all the plants that live there. Since rainforests and the oceans are so plentiful in terms of the number of producers, there are many food webs that exist in each of these ecosystems. The plants and algae provide energy for many creatures living in these areas.
Primary consumers (also called herbivores) are those animals that eat producers. Many times, primary consumers are also considered prey species because they quite often get eaten by other animals. Primary consumers living on land include: deer, antelope, chipmunks, mice, many insects, many birds, horses, and even elephants. Marine primary consumers include: many types of fish, zooplankton (microscopic animals that float in the water), snails, sea urchins, and krill (small shrimp-like creatures that are part of the plankton).
Even though primary consumers are eating plants, they are still getting their energy from the sun. It was the sun that allowed the plants to grow. As the primary consumers are eating the plants, they break them down and release their energy. However, since some of the sun's energy was used by the plant itself, primary consumers do not get 100 percent of the sun's energy. In fact, they only get 10 percent of the energy. This is called the 10 percent rule , only 10 percent of the energy available gets passed onto the next level of consumers.
Secondary consumers are animals that eat other animals. These animals eat meat and are often called predators, because they hunt their prey. Terrestrial secondary consumers include: lions, snakes, hawks, hyenas, coyotes, wolves, and spiders. Those that live in the oceans include: killer whales, bluefish, sharks, lobsters, sea stars, sea anemones, and sea turtles.
Like the primary consumers, the secondary consumers are also getting energy from the sun, albeit indirectly. The secondary consumers are eating the primary consumers that ate the producers. As a result, the amount of energy transferred to the secondary consumers is even smaller than that obtained by the primary consumers. Due to the 10 percent rule, the amount of energy available to the secondary consumers is only 1 percent of the original sun's energy. This means that secondary consumers have to eat more often or take down larger prey in order to meet all of their energy demands.
A special group of consumers that bridges the primary and secondary consumers is called the omnivore group. These are animals that eat both plants and animals, so they fit into both categories. Most of the time, omnivores are meat-eaters, but when conditions are tough and meat is scarce (like during the winter), they can switch to eating plant matter. Bears, pigs, and even humans are considered omnivores.
The energy transfer in a food web for omnivores would be the same as for primary or secondary consumers. If the omnivore is eating plant matter, it is getting 10 percent of the sun's energy. If the omnivore is eating meat, it is only getting 1 percent. Obviously, it makes more sense for omnivores to eat plant matter, but they compensate by eating a lot of meat.
Sometimes, food webs can have more than two levels of consumers. You'll recall from the example of the forest ecosystem that the owl could serve as a secondary consumer, eating the mouse. However, because owls also eat snakes, it might eat that instead of the mouse. This changes the owl's position in the food web from a secondary consumer to a tertiary consumer, or an animal that eats secondary consumers (tertiary means 'third'). It is also possible that some food webs may have four or five levels of consumers. It all depends upon how much energy is available to be transferred up that high.
This image shows a food web of a marine ecosystem. The producers are the phytoplankton and the submerged aquatic vegetation. A simple transfer of energy (food chain) could look like this:
Phytoplankton --> Small Fish --> Wading Birds
The small fish is the primary consumer (the herbivore) and the wading bird is the secondary consumer (the carnivore). However, since organisms can get eaten by other things, it is possible to put together a food chain where the energy from the phytoplankton makes it to a tertiary consumer:
Phytoplankton --> Small Fish --> Large Fish --> Osprey
In another example, the bald eagle can be a quaternary consumer (fourth level):
Phytoplankton --> Zooplankton --> Small Fish --> Large Fish --> Bald Eagle
While it is possible, it is very rare to find a food web that shows a transfer of energy with more than five levels. Why? Remember that the amount of energy available to the next level up is only 10 percent of the level before it. At the quinary level (fifth), the amount of energy available would be only 0.001 percent of the original sun's energy—hardly enough to keep an animal alive. Animals that occupy this position of a food web will have to eat very large prey in order to maintain their metabolisms. A polar bear or a killer whale could be an animal that would occupy this position.
Decomposers are organisms that break down dead and decaying matter in an ecosystem. They are important in a food web because they are responsible for recycling all of the nutrients back into the system. When an organism dies, the decomposers break it down into its component parts. Carbon is released from the organism's tissues and placed back into the environment. It is then used by other organisms to grow. Decomposers impact all levels of the food web. Fungi, such as mushrooms, and bacteria play the role of decomposers.
Once energy from the sun has been captured by plants through photosynthesis, it gets passed around through the various organisms in a particular area. This transfer of energy is called a food web. These complex representations are different from food chains, which show a direct transfer of energy between organisms.
Often considered the bottom of the food chain, producers are the organisms that make their own food and serve as the foundation for all food chains and webs. Primary consumers (also called herbivores) are those animals that eat producers. They only get 10 percent of the energy. This is called the 10 percent rule; only 10 percent of the energy available gets passed onto the next level of consumers. Secondary consumers are animals that eat other animals. A special group of consumers that bridges the primary and secondary consumers is called the omnivore group. These are animals that eat both plants and animals. This can go on to tertiary, quaternary, and quinary consumers. Finally, decomposers are organisms that break down dead and decaying matter in an ecosystem.
|Food web||transfer of energy from the sun through the various organisms in a particular area|
|Food chain||shows a direct transfer of energy between various organisms|
|Producers||organisms that make their own food and serve as the foundation for all food chains and webs|
|Primary consumers/herbivores||those organisms that consume producers|
|Photosynthesis||the process of making energy by converting sunlight into sugars|
|10 percent rule||primary consumers only get and use 10% of the sun's energy|
|Secondary consumers||animals that eat other animals|
|Omnivores||animals that eat both plants and animals|
|Tertiary consumer||an animal that eats secondary consumers|
|Decomposers||organisms that break down dead and decaying matter in an ecosystem|
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Back To CourseGeneral Studies Science: Help & Review
24 chapters | 338 lessons | 1 flashcard set