Food Chains, Food Webs & Energy Flow

1. Food Chains, Food Webs & Energy Flow IB Topic 5.1

2. Recap … Some common problems • In food chains & food webs, arrows () are used to show which organism eats which • For example: • Grass  grasshopper  toad • The grasshopper eats the grass • NOT the grass eats the grasshopper

3. Food webs • Food webs are interconnected food chains • Why? • Since one organism often eats more than just one type of food • A food chain does not tell the whole story • Can be very complex …

4. There is no set number • Some food chains have six trophic levels, but most have four • The number of levels is limited to how much energy enters the ecosystem • A lot of energy is lost at each level • Determining trophic levels in a food web can be difficult … why?

6. Difficulties … • Some organisms occupy more than one trophic level or take their food from multiple trophic levels • Many ecosystems are not fully understood • Ditto for feeding preferences • Food webs do not show change over time as a population changes • Seasons • Disturbances

7. Energy! That’s why organisms eat, right? • Producers are the most important organisms … why? • Sunlight is the initial source of energy for all vegetation • Some food chains can start without sunlight (deep sea hydrothermal vents rely on chemicals instead of light) • Think about the food you like to eat – can you trace it back to the sun?

8. Energy Flow • Light is absorbed by producers (photosynthesis) • Energy is transferred from one organism to another as carbohydrates, lipids, and proteins are digested

9. Energy Transfer • Only chemical energy can be used by the next trophic level (remember ATP?) • Only a small amount of energy which an organism absorbs is converted into chemical energy • No organism can utilize 100% of energy present in the organic molecules of the food it eats • Only 10-20% of the energy is used

10. Why only 10%? • Where does the energy go? • Not all of the organism is swallowed as a food source • Not all swallowed food can be absorbed/used (bones, hair, seeds) • Lost as fecal matter; spit up • Some organisms die before being eaten • Considerable heat loss due to cellular respiration • Moving, running,

11. Pyramid of Energy • Used to show how much and how fast energy flows from one trophic level to the next in a community • Units are: • Energy per unit area per unit time • Kilojoules per square meter per year • kJ m-2 yr-1

12. Why the shape? • Take a look at figure 5.3 page 130 • Because energy is lost, each level is always SMALLER than the one before it • Organisms cannot create energy, they can only transfer it • Each level is smaller by ~10%

13. Diagrams

14. Energy vs. Nutrients • Energy: • Enters in the form of light  chemical energy  most is lost as heat • Not recycled (you can’t give energy back to the burger you ate for lunch) • Nutrients: • Recycled • There is not enough in the biosphere to meet all the needs • So, elements like carbon and nitrogen are recycled • Locked up in cells of organisms until they die • Only available by eating and decomposition

15. Lets hear it for the decomposers • How do we get those precious nutrients stored in the cells of plants and animals? • Decay • Detritivores & saprotrophs • Break down the body parts of dead organisms • Important for soil (nitrogen – key element for plants)

16. Review • Which diagram shows the flow of energy through a community with three trophic levels?

17. Review Which is the best prediction about biomass? A. The biomass of X is more than the biomass of W. B. The biomass of X is less than the biomass of Y. C. The biomass of V + X + Z is equal to the biomass of W. D. The biomass of Y is less than the biomass of Z.

18. Review Which of the species feed both as secondary consumers and as tertiary consumers? A. I, II and IV only B. I, III and V only C. I, IV and V only D. III, IV and V only