The Energy Pyramid and Trophic Cascades

The Energy Pyramid

When talking about the food chain, it is important to recognize the relative abundance of each of the trophic levels. Producers take their energy straight from the sun, and produce sugars that let them grow and make food for primary consumers. However, there is a large difference between the energy put into each trophic level and the energy yielded. For example, only 10% of energy that producers consume is given to primary consumers. That results in a structure that looks like this:

Energy Pyramid:

Because of the decreasing amounts of available energy for each trophic level, the population respectively decreases. Plants are in abundance on this earth, even though only around 2% of the sunlight that hits the earth is taken up by plants. About 60-90% of energy intake for all animals is able to be used by the organism, but the main difference is the availability of each food source. Because there are fewer primary consumers than there are plants, there will be fewer secondary consumers and even fewer tertiary consumers. This is why food chains are only able to extend for 5 levels; there simply isn’t enough energy to provide for another level of organism.

Besides the energy pyramid, another way of understanding an ecosystem is by understanding the topic of trophic cascades.

Trophic Cascades

Trophic cascades occur when the abundance of certain organisms are greatly affected. A top-down trophic cascade occurs when there is a removal of a top predator. An example of this could be if the Great Horned Owl was removed from our ecosystem. As a result, rodent and rabbit populations would flourish, thereby decreasing the biomass of its food. (Biomass is defined as the total mass of a certain type of organism, and is important in understanding the relative abundance of different parts of the food chain). So the removal of just one predator affects the entire food chain, from the top down.

Here is a diagram representing a top-down cascade in a lake ecosystem:

Top-down trophic cascade:

As you can see, the reduction in the amount of predators in the lake allows the forage fish population to increase. This,in turn, decreases the population of the primary consumers, the zooplankton. Thus, the phytoplankton, the producers and foundation of the lake ecosystem, flourish.

The other type of trophic cascade is the bottom-up cascade. It involves the removal of a producer or primary consumer, which decreases population throughout the entire food chain. An example of this would be entirely removing many of the plants from our meadow in the garden. The insect/primary consumer population would decrease, therefore decreasing the population of secondary consumers and tertiary consumers. It is more self-explanatory than the top-down cascade, although it is just as detrimental.

As caretakers in our garden, it is important to understand the kind of natural balance we have. The elimination of one species or even the addition of another could be detrimental to the ecology of the garden. It is important to maintain the natural balance that would otherwise exist and allows plants to flourish because nature knows best.



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