Food Chains and Food Webs; Energy Transfer and Bioaccumulation

Module 2 (9/8–9/14) · Food Chains and Food Webs; Energy Transfer and Bioaccumulation · Blueprint Pillar 3 · PGCC Outcome 1 · Download .docx

Objectives

Distinguish producers, consumers, and decomposers.
Explain the 10% energy rule and construct a trophic pyramid.
Differentiate bioaccumulation from biomagnification using the DDT example.
Explain the role of indicator species.

Key terms

Producer (autotroph): Organism that makes its own food from sunlight or chemical energy — the base of every food web.
Consumer (heterotroph): Organism that obtains energy by eating other organisms.
Decomposer: Organism (bacteria, fungi) that breaks down dead organic matter and returns nutrients to the environment.
Food chain: A linear sequence showing who eats whom in an ecosystem.
Food web: The network of all food chains in an ecosystem; more accurate than a single chain.
Trophic level: A feeding position in a food web: producer, primary consumer, secondary consumer, etc.
10% rule: Only ~10% of energy at one trophic level passes to the next; the rest is lost as metabolic heat.
Biomass pyramid: Diagram showing decreasing total organic matter at each higher trophic level.
Bioaccumulation: Buildup of a toxic substance in a single organism's tissues over time.
Biomagnification: Increasing concentration of a toxin at each successive trophic level in a food web.
DDT: Pesticide whose biomagnification in food chains drove bald eagles near extinction; banned in the US in 1972.
Indicator species: A species whose population health reflects the overall health of its ecosystem.

The concept

Producers (autotrophs) make their own food through photosynthesis or chemosynthesis. Consumers eat other organisms and are classified by trophic level: primary consumers eat producers, secondary consumers eat primary consumers, and so on. Decomposers (bacteria, fungi) break down dead matter and return nutrients to the soil — closing the nutrient cycle.

The 10% rule governs energy flow: only about 10% of the energy at one trophic level transfers to the next. The rest is lost as metabolic heat. This is why biomass pyramids narrow toward the top, and why there are far fewer top predators than prey in any ecosystem.

Bioaccumulation is the buildup of a toxic substance (like DDT) in a single organism's tissues over its lifetime. Biomagnification is the increasing concentration of that toxin at each successive trophic level. Bald eagles almost went extinct because they ate many fish, each of which had accumulated DDT — so eagles received multiplied doses. Indicator species are sensitive organisms whose population health signals ecosystem change before broader damage is visible.

Self-check

Try each one before you look. A miss here costs nothing and tells you exactly what to reread.

1. The 10% rule in ecology states that:

2. Biomagnification differs from bioaccumulation in that biomagnification:

3. Why are there far fewer top predators than prey animals in an ecosystem?

4. An indicator species is most useful in environmental monitoring because:

5. A food web is more ecologically accurate than a food chain because:

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