Food Webs and Energy — Answer Key
Part A: Multiple Choice
Circle the best answer for each question.
1. If ten thousand units of energy are available in the producers, about how much energy reaches the secondary consumers?
A) 10,000 units
B) 1,000 units
C) 100 units
D) 10 units
Apply the ten percent rule twice: 10,000 units in producers becomes 1,000 in primary consumers (10%), then 100 in secondary consumers (10% again). Each level keeps only a tenth of the energy from the level below.
2. Which statement best explains why energy cannot be recycled in a food web?
A) Animals store all the energy they eat
B) Energy is used up and lost as heat at each level
C) Producers create unlimited energy from the sun
D) Decomposers send energy back to producers
Once organisms use energy for movement, growth, and body heat, that energy escapes into the environment as heat and cannot be captured again. This is why ecosystems need a constant supply of new energy from the sun.
3. A pond ecosystem has algae, tadpoles, fish, and herons. An oil spill kills most of the algae. What happens first?
A) Herons have more food available
B) Tadpole populations decrease because they lose their food source
C) Fish populations increase rapidly
D) Decomposer populations vanish immediately
Algae are the producers that tadpoles eat directly, so tadpoles would be the first to suffer when algae disappear. Fish and herons are higher up the chain and would feel the effects later.
4. Which organism would have the MOST energy available to it in a grassland food web?
A) eagle
B) snake
C) grasshopper
D) grass
Grass is a producer that captures energy directly from the sun, so it holds the most total energy in the food web. Every level above it has less energy because of the losses at each step.
Part B: Fill in the Blank
Write the correct answer on each line.
1. Unlike nutrients, energy does not cycle through an ecosystem.
Nutrients like carbon and nitrogen get recycled by decomposers, but energy cannot be reused. It flows one way, from the sun through organisms, and exits the ecosystem as heat.
2. A top predator sits at the top of the food web with no natural enemies.
A top predator like an eagle or a shark has no animals that regularly hunt it. It sits at the highest level of the food web and helps control the populations of animals below it.
3. Organisms use most of the energy they consume for growth, movement, and maintaining body temperature.
Animals spend most of their energy keeping their bodies at the right temperature, moving around, and growing. This is why so little energy is left over to pass to the next consumer.
4. When one food source disappears, animals that eat many types of food can survive more easily.
Animals with a varied diet can switch to other food sources when one disappears. This flexibility gives them a much better chance of surviving changes in the ecosystem.
5. The process by which dead organisms are broken down into simpler substances is called decomposition.
Decomposition is nature's recycling process, where bacteria and fungi break down dead organisms into simple nutrients. These nutrients then return to the soil for producers to use.