EcD7: Ecological Pyramids

Unlock the secrets of ecological pyramids! See how energy, numbers, and biomass structure ecosystems.

Ecological Pyramids An ecological pyramid is an easy way to show the transfer of energy using the trophic (feeding) levels in an ecosystem.Each pyramid has the same basic setup with the autotrophs on the bottom followed by herbivorous heterotrophs (primary consumers) followed by carnivorous heterotrophs (secondary consumers).This activity will walk you through the relationship between each type of pyramid. The Pyramid of Energy The pyramid of energy was already discussed in lecture but here’s a reminder: The bottom of the pyramid (the autotrophs) receive the most energy. Remember, they are the producers for the rest of the ecosystem. Some of that energy is used by the autotrophs to grow, reproduce, and carry on their daily lives. When they’re eaten they pass whatever remains up the trophic levels. This amount is adjustable but is usually approximately 10%. Think about what this might mean about the level above. Where does an autotroph get its energy? Select all that apply the sun water inorganic compounds other organisms Why is only 10% of energy passed on to the next level? The Pyramid of Numbers If each level up receives less energy… it means you have to EAT MORE to get the energy you need! This means that each level must have more organisms than the level above so there’s more to actually eat.In the image you can see that there is a lot more grass than rabbits. That’s because the rabbits need to eat more grass to get enough energy. There are less snakes than rabbits and less hawks than snakes for the same reason.This makes a lot of sense if you think about what would happen if we flipped the pyramid upside down… could one blade of grass feed ten rabbits? Could ten rabbits feed 100 snakes? The fact that only a small fraction of energy gets transferred leads to the structure of the pyramid of numbers. The autotrophs tend to have the largest population while the apex predators have the smallest overall population size. The next diagram shows us a case where the pyramid isn’t set up exactly how we’d expect. Why is this pyramid still an accurate representation of the pyramid of numbers?This leads to our last pyramid example… Why is there a smaller population of consumers than producers? the producers eat all of the consumers food consumers do too much damage to the ecosystem consumers need to eat more of a producer to get enough energy The Pyramid of Biomass From looking at the image above showing one oak tree supporting a larger amount of insects you can probably already see why this is okay. The tree is physically larger providing more biomass for the level above. So even though it’s only one tree it provides a greater amount of potential energy. Biomass refers to the physical mass of the level, for example; the mass of all the grass in the bottom of the pyramid is greater than the mass of all the rabbits. What is biomass? the physical mass of a trophic level the mass of a singular organism a land mass a biological tool used to measure weight Explain why a biomass pyramid might look different from a typical energy pyramid and why it works.