Questions: Trophic Levels and Food Webs

Question 1

In a grassland ecosystem, grasses are eaten by grasshoppers, grasshoppers are eaten by frogs, and frogs are eaten by hawks. A disease eliminates most of the frog population. Which outcome is most likely?

• A. Grasshopper populations decline because they lose a food source
• B. Hawk populations increase because hawks can now eat more grasshoppers directly
• C. Grasshopper populations increase and grass biomass decreases
• D. The ecosystem is unaffected because hawks can eat other prey

Answer: (C)

Removing frogs releases grasshoppers from predation pressure (a trophic cascade), causing grasshopper populations to grow. More grasshoppers consume more grass, reducing grass biomass. This illustrates how a perturbation propagates through trophic levels — removing a middle predator can have large cascading effects on both the level above (less food for hawks) and below (more consumption at lower levels).

Question 2

Decomposers such as bacteria and fungi occupy a separate, parallel energy pathway and do not interact with the main food web.

• T. True
• F. False

Answer: False

Decomposers are integral to the food web, not separate from it. They break down dead organic matter (detritus) from every trophic level — dead plants, animals, and waste products — and release nutrients back into the soil and water. In most terrestrial ecosystems, the detrital pathway processes more energy than the grazing (live-plant) pathway. Nutrients released by decomposers are taken up by producers, closing the cycle.

Question 3

Why are food chains in nature rarely longer than four or five trophic levels?

Model answer: Because energy is lost at each trophic transfer — typically only about 10% of the energy at one level is available to the next. After four or five transfers, so little energy remains that sustaining a viable population of top predators becomes impossible.

This is the 10% rule of ecological efficiency. When a herbivore eats a plant, roughly 90% of the plant's energy is lost to respiration, heat, and indigestible material. The same occurs at every subsequent level. Starting with, say, 10,000 units of plant energy: ~1,000 reach primary consumers, ~100 reach secondary consumers, ~10 reach tertiary consumers. A fifth trophic level would have only ~1 unit of energy available — too little to sustain a population.