7th Grade Science

FOOD WEBS AND CHAINS

OBJECTIVES

• Define and give examples of organisms at different trophic levels

• Describe how energy flows in a food chain.

• Explain why producers must outnumber consumers in a balanced ecosystem.

PRODUCERS• Food chains begin with producers• organism that makes its own food

• Green Plants use energy from the Sun to create food through photosynthesis

• Make up Trophic Level in an ecosystem

• Examples: Grass, Trees, and Algae

CONSUMERS• Organism that get their energy by eating other organisms

• “YOU” cannot make your own food, no matter how hard you try!

• Must obtain energy from other sources to live and grow (EAT!!!!!)

• All animals, including humans are consumers

PRIMARY CONSUMER

• First consumer in Food Chain

• Obtain energy directly from “Producers”

• a.k.a—”HERBIVORES” or plant eaters

• Land Ecosystem Herbivores include:

• Cows, Deer, and Grasshoppers

• Aquatic Ecosystem Herbivores include:

• Shrimp, Minnows, and Snails

LAND ECOSYSTEM HERBIVORES

AQUATIC HERBIVORES

SECONDARY CONSUMERS

• Eat Primary Consumers

• Include “Carnivores” (meat eaters)

• Lions, Tigers, Coyotes, and Wolves

TERTIARY CONSUMER

• Eats “Secondary Consumer”

• Includes: Carnivores and Omnivores

• Carnivores:

• Only eat meat

• Omnivores

• Eats both meat and plants

• Examples of Omnivores:

• Humans and Bears

DECOMPOSERS• Last link in a food chain

• Break down nutrients in dead organisms and return essential chemicals to earth:

• Nitrogen (N), Carbon (C), and Phosphorous (P)

• In land ecosystems, chemicals are released into soil.

• In aquatic ecosystems, chemicals are dissolved in water.

• Essential part of an ecosystem because:

• Producers absorb nutrients to make food

• Most common Decomposers are:

• Bacteria and Fungi

ENERGY FLOW• Where do you get energy to read, come to school, hunt, and play ball?

• Eating what? Food!!

• All energy as you already know, comes from the Sun

• Producers absorbs the energy and uses “Photosynthesis” to make food

• 50% of energy absorbed from Sun is used to live and grow

• Unfortunately not all energy is transferred to consumer, due to heat “loss”

• Only around 10% of energy is transferred from producer to consumer

BALANCED ECOSYSTEM• Producers outnumber consumers greatly

• More Specifically:

• Producers outnumber Primary Consumers

• Primary Consumers outnumber Secondary Consumers

• Secondary Consumers outnumber Tertiary Consumers and so on…….

• Since energy decreases 10% with each Trophic level, it is very important for higher level consumers to eat a lot to get enough energy to survive.

• Example:

• Even though a mouse only has to eat a few stalks of grass to have energy, a snake must eat several mice to survive. To take this one step further, a hawk has to eat even more snakes!

TROPHIC PYRAMID

FOOD WEBS

• Used to represent the complex food relationships between organisms in an ecosystem.

• Network of interconnecting food chains

FOOD WEB #2

END OF CLASS REVIEW• How much energy is transferred from one trophic level to the next?

• Describe a Balanced Ecosystem.

• Where do producers get their food and how?

• What are the 3 types of consumers? How do they get their food?

• Describe a food web