Food Chains and Webs

Chain: grass -> grasshopper -> frog -> snake -> hawk (example). Arrow means 'eaten by' / energy to. Trophic levels: producer, primary consumer (herbivore), secondary, tertiary. Web: many chains linked. Omnivore sits on two levels. Remove a keystone (named if the book) and the web shifts. Energy is lost as heat at each step (next page). Food chains simplify feeding relationships but food webs better represent the complexity of real ecosystems. Energy is lost at each trophic level as heat. Arrows point from the food organism to the eater.

FOOD CHAINS AND WEBS is an independent FBISE learning unit because students must connect its structures and processes with their biological functions and examples.

CHAIN RULES: Start with a producer (or detritus in a detrital chain). Length is short because energy dies as heat. Do not start with the sun as an organism. Always start with producer (green plant or alga). Arrows point from food to eater (energy flow direction). Each step loses ~90% energy as heat. Examples: Herbivore = 2nd trophic; Carnivore higher; Decomposer on all dead. WEB: One animal, many foods. More links can mean more stability in the school tale (not always). Bioaccumulation: toxin concentrates up the chain (named warning, not a poisoning method). Omnivores connect multiple trophic levels. Removing one species may cascade through the web. Keystone species have disproportionate impact (sea otter example if named). Examples: Omnivore; Keystone named; Toxin tale: named chemicals, no recipes. Worked chain labels: Grass: producer. Grasshopper: primary consumer. Frog: secondary consumer. Arrows point to the eater.