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Efficiency of Biomass Transfer Between Trophic Levels

Instructor: Matthew Bergstresser

Matthew has a Master of Arts degree in Physics Education. He has taught high school chemistry and physics for 14 years.

Energy is transmitted through food chains within food webs. This lesson will go into detail regarding the efficiency of these energy transfers between the different levels of food chains and food webs.

Food Chains and Webs

Do you know that you eat sunlight? Even though you may not remember ever biting at sunbeams, it's true. The food we eat has energy, and that energy can be traced back to the sun. Let's start with organisms that can create their own food using sunlight, then follow the energy through the different trophic levels that make up food chains and food webs.

Energy Transfer through Biomass

Plants

The first trophic level is made up of plants. Plants have the ability to absorb light energy and transform it into chemical energy through a process called photosynthesis, which is why they are called producers. Most plants use only 3-6% of the solar energy available to them. An enormous amount of sunlight hits the plant, so what happens to the rest of it? Plants are generally green in color which means they reflect green light and absorb the other wavelengths of light. Shading, reflections and the availability of water and nutrients can also affect how much energy plants can take in.


A leaf reflecting green light.
leaf

Absorbing only 3% of the light available may not seem like a lot, but it is plenty for plants to generate food as long as there is ample water and soil. A lot of that energy is stored in the chemical bonds in glucose and starches, which make up the plant's biomass. Plants also use some to respire, or exchange gases with the environment. Some energy is also used to make secondary metabolites, which is any compound not directly involved in growth or reproduction. Nicotine -- a poison that discourages insects from eating plants -- is a good example of a secondary metabolite. Of all these uses for energy, only the biomass (plus a small amount of metabolites) is passed on to the next step in the food chain.

Herbivores

The second trophic level is herbivores, or primary consumers, which are organisms that only eat plant material. They eat a lot of it too! Think about what crickets, cows, horses, and deer do for most of the day. They wander around looking for food, and then eat it. The energy in plant biomass is relatively non-dense compared to animal biomass. This is why herbivores have to eat a lot.

If we start with an arbitrary value of 1000 units of energy in plant biomass that a herbivore eats, the herbivore will only pass on 100 units of energy to the next step in the food chain. This is 100/1000, which is only 1/10, or 10%! Energy can't just disappear, so where does it go? Animals respire and move around. As well as finding food, they need to reproduce and hide from predators. Some of the food they eat is not easily digestible and is excreted as feces which still contains energy, and a lot of their bodies are not edible, including bone, teeth and fur.

Predators

The third trophic level is occupied by secondary consumers, which are usually small predators such as mice, frogs, and fish. Secondary consumers eat herbivores, which has some advantages. Animal cells do not have a cell wall, making them easy to digest, and protein (muscle tissue) is high in energy. However, out of the 100 units of energy available to secondary consumers, they only pass along 10 units. Again, this is only 10/100, which is 1/10, or 10%. Of the total energy eaten by the herbivore, only 10/1000 -- 1/100, or 1% -- is available to the next trophic level.

The 90/100 units of energy that are not passed along are used by the secondary consumer. A lot of it is used for hunting. It takes a lot of energy to stalk and chase prey, and they're not always successful. Just like herbivores, they also have to respire, reproduce, hide, and grow bones and other inedible body parts. For secondary predators, this can include claws and sharp teeth, as well as protective gear like scales. They also have to eliminate all the parts of the herbivores that are edible, but unusable, such as small bones.

Tertiary consumers, such as snakes and raccoons, have 10 units of energy available to them in biomass, and they pass on only 1/10 (10%) of it to the top-level predators (quaternary consumers). 9/10 of the energy available to them (9/1000 total units) are used for respiration, movement and bone-building.

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