Trophic Levels and Pyramids Mrs. A. Kay
Trophic Levels • Trophic = feeding level • refers to the organisms position in the food chain • Autotrophs are at the base (first trophic level). • Organisms that eat autotrophs are called herbivores or primary consumers (second trophic level). • An organism that eats herbivores is a carnivore and a secondary consumer. (third trophic level) • A carnivore that eats a carnivore that eats a herbivore is a tertiary consumer (forth trophic level).
Pyramids • Ecological Pyramids: represent graphically the structure of an ecosystem • 3 types: number, biomass, and energy flow • Pyramid of numbers: shows relative population size.
Why do these pyramids look so different?Hint: Look at how they begin (numbers!)
Answer • Since more energy is available along the bottom of the food chain, generally there are more numbers of producers
In our example, there is more energy for plankton, than there is for small fish; more energy for the larger fish than the seabirds. • Usually an animal eats organisms smaller than itself, so it makes sense that there needs to be more of the smaller species to support the larger ones.
Not always true! • Exceptions to the pattern. • Sometimes one large producer can feed many more organisms than itself. Ex: Oak tree • There are more caterpillars than birds, and less predator-type birds.
Science Inquiry Activity • What Eats What? • Pg 10 of your textbook
Pyramid of Biomass • Using numbers doesn’t take into consideration the sizes of the organisms. • Biomass tells us how much of a particular organism is there. • Biomass generally decreases as we rise in trophic levels
Exceptions • Ocean systems with algae eating zooplankton. • There can be more zooplankton than algae because algae reproduces more quickly . • They produce enough energy to support the system
Pyramid of Energy Flow • shows the total chemical energy that flows through each trophic level. • Always less energy available through each level. • Explains why food chains rarely have more than 4 links. Think!! You can’t support a population on a 0.000001% of stored energy.
notice that there is less energy at each higher level. increasing energy
Use the following to create your own pyramid of numbers, biomass and energy flow.What assumptions did you make?
Practice for Homework: • Page 17 # 1-6
Answers • 2nd trophic level can be any primary consumer = herbivore. Ex: rabbit • Rarely more than 4 links because as you move up a food chain there is less energy available. After 4 links there would only be 0.001% energy left to support that population. • Pyramid of numbers = population size Ex: 1500 cows Pyramid of biomass = the entire mass of the population. Ex: 20 000kg of cow
Must consider the # of foxes and their sizes. • Rabbits= more of them (b/c lower on the food chain, so more energy), but they weigh less than a fox • Fox = less of them (2nd consumer), but weigh more than a rabbit • There is usually significantly more rabbits within an ecosystem than foxes, so their difference in size would not affect our biomass pyramid. Rabbits would have a higher biomass than foxes
Fleas to dog: • Numbers: easier to count the fleas than weigh them • Biomass: doesn’t show the severity of the situation for the dog. • Ex: 100 fleas vs. 1 dog • If using biomass, ex: 20kg of dog vs 0.0001kg of fleas
BacteriaProtozoanZooplanktonFish • Pyramid of numbers: more bacteria on the bottom, less fish on top • Pyramid of biomass: bacteria are microscopic and have extremely small mass (also reproduce more quickly) so will have small biomass, but supports the system.
Pyramid of energy flow: Less Energy More energy