8.2Ecology

Energy Flow Through Ecosystems — AP BIOLOGY Revision Notes & Topical Questions

5 min read 9 parts Lesson 3 of 8 1 quiz

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Lesson summary

8.2 Energy Flow Through Ecosystems — Notes — AP Biology notes: Describe the strategies organisms use to acquire and use energy and how energy flows through ecosystems.Use of energy — organisms use it to organise, maintain homeostasis, grow and reproduce.Balance — net energy gain …

Heterotroph
Saprotroph
Detritivore
Trophic level
Small endotherms — have a larger surface area-to-volume ratio and greater heat loss
Ectotherms — rely on behaviour such as basking, seeking shade or huddling

Part 1 of 7Energy use in organisms

Use of energy — organisms use it to organise, maintain homeostasis, grow and reproduce.

Balance — net energy gain gives growth, storage and reproduction; net energy loss leads to loss of mass, reduced reproduction and potentially death; energy spent on reproduction is a trade-off with survival.

Metabolic rate = energy expended by an organism in a given time; per unit mass it is inversely related to body size,

so smaller organisms have higher rates.

Small endotherms — have a larger surface area-to-volume ratio and greater heat loss.

Part 2 of 7Body temperature and reproduction strategies

Endotherms — use metabolic heat to keep body temperature within limits (thermoregulation, a homeostasis example) by shivering, sweating, vasodilation and vasoconstriction; they need more food per unit mass.

Ectotherms — rely on behaviour such as basking, seeking shade or huddling.

Asexual reproduction — less energy-intensive and allows rapid population growth.

Sexual reproduction — needs more energy but increases genetic variation for changing environments.

Seasonal reproduction — ensures offspring are born in favourable conditions, such as abundant food.

Photoperiodism — day length regulates growth and reproduction, e.g. plants flower and fruit with daylight changes.

Biennial plants — grow in year one and reproduce in year two; sugar beet stores energy in year one.

Reproductive diapause — suspension of reproduction to conserve energy, e.g. monarch butterflies during migration.

Part 3 of 7Autotrophs and heterotrophs

Autotrophs and heterotrophs

Autotrophs — make organic molecules from simple inorganic substances and are the producers.

Photosynthetic organisms (plants, algae and some bacteria) use sunlight to convert CO₂ into carbohydrates.
Chemosynthetic organisms oxidise small inorganic molecules for energy without sunlight or oxygen (e.g. deep-sea vents).

Heterotrophs — rely on organic molecules made by others, consuming and digesting carbohydrates, lipids and proteins by hydrolysis and incorporating the matter into their tissues.

Primary consumers eat producers;
- secondary consumers eat primary consumers;
- higher consumers feed on other consumers.
Scavengers feed on dead animals (vultures).
Detritivores ingest dead organic matter (earthworms, woodlice).
Saprotrophs digest dead matter externally with enzymes and absorb the nutrients (fungi, bacteria);
- decomposers are heterotrophs.

Part 4 of 7Energy flow and productivity

Direction — energy enters as sunlight captured by autotrophs and flows one way: producers, primary, secondary, tertiary consumers, leaving as heat; decomposers recycle matter, not energy.

Gross primary productivity (GPP) = total rate of energy captured by producers.

Net primary productivity (NPP) = GPP − energy used in respiration (R), the energy available to consumers (units such as kJ m⁻² yr⁻¹).

Transfer — only about 10% of energy passes to the next level; the rest is lost as heat from respiration, incomplete digestion, excretion or uneaten material.

Efficiency (%) = (energy at the higher level ÷ energy at the lower level) × 100; the 10% refers to energy, not numbers.

Consequences — food chains are limited to about 4–5 trophic levels and a pyramid of energy is always pyramid-shaped.

Pyramids — pyramids of energy show energy stored in biomass at each trophic level; pyramids of numbers or biomass can look inverted in some cases,

but a pyramid of energy never inverts.

Matter — carbon, nitrogen, phosphorus and water are recycled through biogeochemical cycles, but energy is not.

Part 5 of 7Food chains and energy availability

Food chains and energy availability

Food chains, webs and pyramids — show energy transfer, with arrows pointing from the food to the organism that eats it and gains the energy.

Decreased energy — less sunlight or fewer producers reduces population sizes at all higher trophic levels.

Increased energy — more sunlight raises stored chemical energy in producers and supports more consumers.

Trophic level	Role	Example
1	Primary producer	Grass
2	Primary consumer	Rabbit, grasshopper
3	Secondary consumer	Fox, frog
4	Tertiary consumer	Snake

Part 6 of 7Energy through trophic levels

Level	Example	Energy (kJ)	% passed on
Producers	Grass	10 000	10%
Primary consumers	Grasshopper	1 000	10%
Secondary consumers	Frog	100	10%
Tertiary consumers	Snake	10	10%
Quaternary consumers	Hawk	1	—

Part 7 of 7More from the specification

Mechanism

The sequence of steps by which something happens

Fitness

Ability of an organism to survive and pass on its alleles to its offspring.

Species

A group of organisms that can reproduce to produce fertile offspring.

Natural selection

Survival and reproduction of the best-adapted individuals.

Key terms

Term	Meaning
Autotroph	organism making organic molecules from inorganic substances
Heterotroph	organism using organic molecules made by others
GPP	total energy captured by producers
NPP	GPP minus respiration
Endotherm	organism using metabolic heat to regulate body temperature
Ectotherm	organism relying on environmental heat
Saprotroph	organism digesting dead matter externally
Detritivore	organism that ingests dead organic matter
Trophic level	feeding position of an organism in a food chain

Avoid these mistakes

Saying energy is recycled — matter is recycled but energy flows one way.

Forgetting NPP = GPP − respiration.

Saying 90% of energy is passed on — about 10% is, and the rest is lost mostly as heat.

Pointing food-chain arrows from predator to prey — arrows point from the food to the consumer.

Thinking only plants are autotrophs, or that decomposers are not heterotrophs — algae and some bacteria are autotrophs, and decomposers are heterotrophs.

Last updated: 14 October 2026