Questions: Ecosystem Structure and Function

Question 1

Which of the following best explains why decomposers are as essential to ecosystem function as primary producers?

• A. Decomposers convert sunlight into chemical energy, supporting all other trophic levels
• B. Decomposers recycle nutrients from dead organic matter back into forms that producers can absorb, preventing permanent nutrient lock-up
• C. Decomposers are the most abundant organisms by biomass in all ecosystems
• D. Decomposers regulate consumer population sizes by removing diseased individuals

Answer: (B)

Without decomposers, nutrients like nitrogen and phosphorus would remain locked in dead organic matter indefinitely. Producers (plants, algae) can only absorb inorganic nutrient forms, so if decomposers stopped working, primary production would quickly halt from nutrient starvation, collapsing the entire ecosystem. Decomposers close the nutrient cycle — they are the recycling pathway that makes sustained life possible.

Question 2

An ecosystem can be fully described by cataloging most of the species present and their interactions.

• T. True
• F. False

Answer: False

Ecosystems are defined by both biotic (living) and abiotic (non-living) components. Temperature, light availability, precipitation, soil chemistry, and water availability are integral to ecosystem structure and function. A species list cannot predict ecosystem productivity, energy flow rates, or nutrient cycling dynamics — all of which depend critically on abiotic conditions. An ecosystem is not just a community of organisms; it is the community plus its physical environment, interacting as a system.

Question 3

Why does energy available to organisms decrease dramatically at each successive trophic level, and what does this imply about the number of top predators an ecosystem can support?

Model answer: At each trophic transfer, approximately 90% of energy is lost as heat through metabolic processes, movement, and excretion — only about 10% is incorporated into new biomass available to the next level. This progressive loss means each higher trophic level has far less total energy available, so ecosystems can support many herbivores but only a few top predators.

The '10% rule' is a useful approximation of ecological efficiency. Starting with 1,000 units of energy fixed by producers, herbivores can access ~100 units, secondary consumers ~10, and top predators ~1. This pyramid of energy explains why large carnivores are always rare relative to herbivores and why food chains rarely exceed four or five links — beyond that, insufficient energy remains to support a viable population.