Ecosystem Pyramids 318-4 Explain why different ecosystems respond differently to short-term stresses and long-term changes Pyramid of Numbers • Why do you think some organisms are very common and others are rare? • Which organisms do you think are most common? • Which organisms do you think are least common? Remember back to trophic levels… • The greatest amount of energy is in the first trophic level/the producers. They are the first organisms to use the energy from the sun. In the second trophic level there is less energy available and even less in the third trophic level and so on. • As a result of being less energy as you go up the trophic levels, there are generally more organisms in the lower trophic levels. This pattern results in a pyramid of numbers. • Most animals are larger than the food they eat. However, some animals are smaller than the food they eat (e.g. beetle eating a tree). This results in an inverted pyramid of numbers. With the example of the beetle and some birds the pyramid is turned upside down. Why are there other pyramids? • Counting organisms is a useful way to estimate the energy that is present in each trophic level, but organisms come in different sizes and it would take a long time to count all of the organisms in an ecosystem. Pyramid of Biomass A different measure of energy uses the number of organisms and the size. This is called biomass. Biomass is calculated as the total dry mass of a given population of organisms. It may include a few large number organisms or a large number of small organisms. Biomass is a good indicator of the amount of energy that is present in living matter. A pyramid of biomass shows that biomass decreases from each trophic level to the one above. Exceptions … • There is an exception to this pyramid. In the ocean, zooplankton eat phytoplankton (algae). At any given time, there may be more zooplankton than phytoplankton, so why does this ecosystem survive? • The reason is that phytoplankton grow and reproduce much faster than zooplankton, so they can support the much larger zooplankton population. Zooplankton • Phytoplankton Pyramid of Energy Flow • A third type of pyramid is called the pyramid of energy flow. It measures the chemical energy (kJ) that flows through each trophic level. • It is impossible to invert a pyramid of energy flow because there is always less energy available for each successive trophic level in the pyramid. • Complete the following questions: 1. Give an example of an organism in the second trophic level of an ecosystem. 2. Why are there rarely more than four links in a food chain? 3. What is a pyramid of numbers? 4. Which would you expect to have the greatest biomass, a population of rabbits or a population of foxes living in the same ecosystem? Explain your answer. 5. THINKING CRITICALLY – Ecosystem A has plant biomass of x. Ecosystem B has plant biomass of 2x. Suggest which ecosystem has the largest populations of consumers, giving reasons for your answer.