Why are there fewer apex predators than primary producers represented in a trophic pyramid?

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Please note that we are covering these topics and questions over the next few weeks. You should be able to answer Questions #1 through 4 now, if not soon.

What is the definition of a food chain?
Why does a food chain always start with a producer?
How do plants, algae and cyanobacteria make their own food?
What is the definition of a food web?
Why are there fewer organisms (less biomass) at the top of the trophic pyramid than at the bottom?
Why are apex predators important?
Why are scavengers not considered decomposers?
If a cheetah regularly eats gazelle, impalas, rabbits and other grazing animals, what trophic level does it typically occupy?
Using the food web illustration below, what is the highest trophic level of the fox, owl and snake?
You should be able to draw a food chain, food web and trophic pyramid with examples of organisms - this is something we will practice in class and with worksheets.

The transfer of energy from one organism to another

All living things (plants, animals, bacteria, etc.) need energy to live, and all living things get this energy from food. Food provides energy in the form of chemical energy. But, we learned that the energy stored in food has to be converted to a form of energy that organisms use to live. It must be changed into usable energy by the organism using a process called respiration. The energy is used by the organism to carry out metabolic activity (all the things that cells do to keep you alive).

Food chain: The definition of a food chain is, "the transfer of energy from one organism to another." The arrows point to the organism that receives the energy. These arrows are called strands.

Producers: Producers (aka autotrophs) are organisms that make their own food. Some, but not all, organisms make their own food using a process called photosynthesis. They use the sun's energy to combine carbon dioxide (CO2) and water into a food called glucose. Plants, algae and some types of bacteria (cyanobacteria) produce their own food this way. Producers also

include certain types of bacteria that use chemical energy (instead of the sun) to make their own food. These bacteria live near underwater volcanoes that are so deep in the ocean that no sunlight can penetrate. They live in total darkness. But the volcanic vents put out chemicals that the bacteria can use to make their own food.

Consumers: Consumers (aka heterotrophs) are organisms that obtain their food by consuming other organisms.

Even though all organisms need food for energy, many organisms do not have the adaptation (i.e.mouths, beaks, teeth) to "eat." Some organisms consume by absorbing food (ex. fungi and bacteria) like a sponge absorbs water. Some organisms consume by eating with special adaptations such as a mouth (and sometimes teeth or beaks) or a proboscis. So all organisms that "eat" are consumers, but not all consumers "eat."

Organisms that consume producers are called

primary consumers. Organisms that consume primary consumers are called secondary consumers. Organisms that consume secondary consumers are called tertiary consumers, while organisms that consume tertiary consumers are called quaternary consumers and so on.

Trophic level: The highest position an organism occupies on a food chain.

Decomposers: Decomposers are Nature's recyclers! They are a special group of organisms

that obtain their food by consuming dead or decaying organisms and break them down into smaller molecules called "nutrients." Some of these nutrients enter the soil and are dissolved in water where they can be taken up by plants through their roots or by fungi through their root-like mycelium. Decomposers can be fungi, bacteria, insects and small animals such as crabs.

Decomposers that have mouths are also called detritivores. Examples include worms, crabs, fly maggots (yuck!)

Decomposers that don't have mouths to eat with, break down or digest dead organisms using special enzymes and then absorb the nutrients (like a sponge absorbs water). Some examples of these decomposers include fungi and bacteria.
Decomposers can be primary, secondary, and tertiary consumers depending on which level of the trophic pyramid they are consuming at. A worm that eats a dead plant is a primary consumer, while a fly maggot that eats a dead deer is a secondary consumer. So, on the trophic pyramid, we have a special place where we put the decomposers. There is an illustration at the bottom of this page to show you.

Organisms that eat....All organisms that eat are consumers, but not all consumers eat because they do not have special adaptations (such as mouths, teeth, beaks, proboscis etc.) to eat with. "Vore" means "to eat." We use special terms for organisms that eat. Fungi and many bacteria are consumers but they absorb their food without mouths

Herbivores: Organisms that eat plants are not only called primary consumers. They are also called herbivores (herb = plant, vore = to eat)

Cows and deer are herbivores, as are many insects.

Carnivores: Carnivores are a type of consumer that eats meat (carne = meat, vore = to eat). Owls are carnivores because they eat rodents and birds. Some insects are carnivores. If a carnivore eats an herbivore, it is also called a secondary consumer. Depending on what organism it eats, a carnivore may also be a secondary, tertiary, quaternary (and so on) consumer. If you look at the food chain illustration above, you'll see that owls and shrews are both carnivores. Because the owl eats the shrew, this is an example of a tertiary consumer eating a secondary consumer.

Omnivores: Organisms that eat both producers and consumers are called omnivores. People are omnivores, and so are rats, racoons, chickens & skunks. So... is an omnivore a primary, secondary or tertiary consumer? Well, it depends on what it's eating.. If it's eating grass, it is a primary consumer. But when it's eating a rabbit, it's a secondary consumer. And when it eats a salmon (that eats insects and crustaceans), it's a tertiary or even quaternary consumer. But, we rank them at their highest level of consumption. So a deer will always be a primary consumer, and an owl can be as high as a 5th level consumer.

Detritivores: are a special kind of decomposer that eats dead or decaying organisms. Detritivores (detrit = wear down into bits, vore = to eat; have mouths and eat dead bits of plants and animals. All detritivores are decomposers because they both consume dead organisms. But not all decomposers are detritivores. Some examples include worms, crabs and certain insects (pill bugs, millipedes).

Scavengers, like decomposers, consume (and typically eat) dead organisms. But unlike decomposers, scavengers consume large quantities and do not break down the food into small molecules called nutrients like decomposers do. Typical examples of scavengers are racoons, vultures, polar bears, and hyenas.

Remember: Each consumer in the food chain gets their energy in the form of food by consuming another organism - except producers, who make their own food.
When you draw a food chain, you then always begin with a producer. Without the producers, there would be no food to pass on to the consumers. Don't forget, the arrows should point to the organism doing the consuming because the energy is transferred into that organism. See the picture above for an example.

Click on the image to play games and learn about food chains and webs.

Click here for lots of good interactives and games that help you learn about almost everything covered on this page. It's on the Legends of Learning website. Please ask your parent first if it's okay to sign upon this website.

Click on this image for another explanation of food chains. Then go to the "Decomposer" game on the same website and see if you can identify the decomposers. Please note that this website says that decomposers "eat." This is not necessarily true, as mentioned in our definitions.

Use this food web to answer Question #9 above (and in your Google Classroom document).

A food web is defined as "interconnecting food chains."

Because organisms in a habitat generally consume more than one thing, the energy produced in plants connects with several organisms living together. For example, an owl eats many types of rodents, including rats, voles, shrews and birds. Since shrews eat insects, and rats and birds are omnivores, you can see that the food chains become interconnected and more complex.

You can learn about energy transfer of the organisms in different habitats when you click on this link to order food webs in several habitats.

1. Each stage/level of the pyramid is called a "trophic level," or the highest position an organism typically occupies in a food chain. Each trophic level is shared by organisms that occupy the same position in a food chain.

2. As you go up the trophic pyramid, the total number of organisms (or biomass) at the next level decreases because much of the original energy captured from the sun during photosynthesis is lost at each level of the trophic pyramid (see explanation below). Consumers at the highest trophic level are called apex predators.

3. The trophic pyramid shows that some, but not all, the original food energy made by photosynthesizers is transferred from one trophic level to the next.

a. Organisms use food to obtain nutrients, grow and have energy for metabolic activity. Metabolic activity is all the things your cells do to keep you alive.

b. MOST (90%) of the food an organism makes (in the case of a producer) or consumes (in the case of a consumer) is changed back into energy for metabolic activity through the process called "respiration." The remaining 10% of the food is used to grow (or make biomass), SO the next organism that consumes it only receives 10% of the original energy. This is called the "10% rule."For example, calories is a measure of food energy. A plant might make 10,000 calories of its own food by photosynthesis. It will use 9,000 (90% of 10,000 calories) calories to live and 1,000 to grow. When an grasshopper eats that plant, it will only get 1,000 of the original calories that the plant made. That grasshopper will use 900 of those 1,000 calories for its own metabolism; and 100 of those calories become part of the grasshopper (its biomass). If a frog eats that grasshopper, it will receive only 100 of the original calories produced by the plant, 90 of which the frog will use to live and 10 that will become part of the frog. So, as you go up the trophic pyramid, you'll see that you need a LOT of plants to provide energy to all the other organisms up the trophic pyramid.

Because energy is lost as you go up the trophic pyramid, there are fewer numbers (less biomass) of consumers as you go up the pyramid. This means that if you gathered all the organisms at each level of the trophic pyramid and weighed them, you'll see a LOT more plants, by weight (or mass) than primary consumers; a lot more primary consumers than secondary consumers; a lot more secondary consumers than tertiary consumers, etc. That's why there are fewer top predators.

On a trophic pyramid , we place the decomposers in a special place along the side of the pyramid (as seen in your homework and notes) because they are responsible for breaking down the dead organisms at all trophic levels into small molecules called nutrients. Often these nutrients enter the soil and can dissolve in water where they are then taken up by the roots of plants or the root-like mycelium of fungi, or they can be absorbed through the water by algae and cyanobacteria. These essential nutrients then re-enter the food chain through the producers (plants, algae and cyanobacteria).

We often refer to the top trophic level in a trophic pyramid as the "apex consumer" or "top predator." Each trophic level is responsible for controlling the population of organisms in the level below. An apex predator plays a very important role by making sure that the organisms lower down in the trophic don't consume all the producers. For example, if there are too many zebras, they might eat all the grass. So lions would make sure there aren't too many zebras. Apex predators also help maintain biodiversity. We'll learn more about biodiversity next year, but it essentially means the number of different species in an ecosystem. Because of this, apex predators are sometimes also called "keystone" species for their role in making sure that no species is lost in the ecosystem. They keep the ecosystem in balance. Interestingly, keystone species do not necessarily have to be predators. Sometimes, the role of an herbivore is essential to sustaining an ecosystem's balance. The concept of keystone species and trophic cascades is something you'll learn when you study 8th grade biology.

For some extracurricular reading on keystone species you may read "The Ecologist Who Threw Starfish." It's pretty advanced reading, so it's completely optional. There's a one hour Nature program called Serengeti Rules that talks about this, too. It's wonderful, but also optional.

* Did you ever wonder what the 4th level is called? Quaternary!! What about the levels after that????
5th: quinary6th: senary7th: septenary8th: octonary9th: nonary

10th: denary