Every ecosystem depends on organisms that capture energy, organisms that consume organic matter, and organisms that recycle nutrients back into usable forms. Understanding these three feeding strategies is the foundation of ecosystem ecology.
A forest ecosystem contains trees, deer, wolves, fungi, and bacteria. If you removed all the fungi and bacteria, what would happen to the forest over 50 years? Think about where nutrients come from and where they go when organisms die.
Type your initial response below — you will revisit this at the end of the lesson.
Write your initial response in your book. You will revisit it at the end of the lesson.
Content from this lesson that appears directly in HSC Biology exams
Identifying autotrophs, heterotrophs and saprotrophs in ecosystem contexts. Regularly tested in Section I and Section II short answer — 2–3 marks.
Explaining why decomposers are essential for ecosystem productivity. Common in 4–5 mark extended responses on ecosystem dynamics.
Core Content
Wrong: Plants are the only autotrophs.
Right: Algae, cyanobacteria and some bacteria are also autotrophs. In aquatic ecosystems, algae and cyanobacteria are often the primary producers rather than plants.
Wrong: Saprotrophs and detritivores are the same thing.
Right: Saprotrophs digest dead matter externally and absorb nutrients (e.g. fungi, bacteria). Detritivores ingest dead matter and digest it internally (e.g. earthworms, some crustaceans). Both are decomposers, but they use different feeding mechanisms.
How every organism in an ecosystem obtains matter and energy
All organisms in an ecosystem can be classified by how they obtain the organic molecules they need for growth, repair and energy. There are three fundamental strategies: making your own (autotrophs), eating other organisms (heterotrophs), and recycling the dead (saprotrophs).
Classification tree showing the three feeding strategies that drive every ecosystem.
Autotrophs make organic molecules from inorganic inputs. Most ecosystems rely on photosynthetic producers such as plants, algae and cyanobacteria, which capture light energy and convert it into chemical energy stored in glucose. A smaller group of autotrophs — chemoautotrophs — obtain energy from chemical reactions rather than light, and are important in deep-sea vent communities and some soil bacteria.
Because autotrophs do not need to consume other organisms, they form the entry point for energy into almost all food webs. Every joule of energy that passes through a food web originated as light energy captured by an autotroph. This is why ecosystem productivity — the total biomass produced per unit area per year — is ultimately limited by the photosynthetic capacity of producers.
Heterotrophs obtain organic molecules by consuming other organisms or their products. This category includes herbivores (plant-eaters), carnivores (meat-eaters), omnivores (both) and detritivores (dead matter-eaters). All animals are heterotrophs, as are fungi that form mycorrhizal relationships and many bacteria.
Unlike autotrophs, heterotrophs cannot add new energy to an ecosystem. They only transfer energy that was originally captured by autotrophs. At each trophic level transfer, approximately 90% of energy is lost as heat through metabolic processes. This is why food webs rarely extend beyond four or five trophic levels — there is simply not enough energy remaining to support higher predators.
Saprotrophs externally digest dead organic matter and absorb the soluble products. Fungi and many bacteria are essential decomposers. Without them, dead organisms and waste would accumulate, and nutrients would remain locked in organic compounds instead of being released back into the environment for producers to use.
The enzymes secreted by saprotrophs — cellulases, ligninases, proteases, lipases — break down the complex polymers in plant and animal tissues into simple monomers: glucose, amino acids, fatty acids and nucleotides. These monomers are absorbed by the saprotroph and used for its own growth, but much of the mineral content — nitrogen, phosphorus, potassium, calcium — is released into the soil or water as inorganic ions.
Energy flows through ecosystems; matter is recycled
Energy flows through ecosystems and is eventually lost as heat, but matter is recycled. Saprotrophs release nutrients such as nitrogen and phosphorus from dead organisms and waste. Those nutrients can then be taken up by producers, supporting new biomass and maintaining food webs.
Without decomposers, dead matter would accumulate and nutrients would remain locked away from living producers. Productivity would fall even if sunlight and water were available. A forest without decomposers would gradually become a cemetery of locked nutrients — the carbon, nitrogen and phosphorus in dead trees would be inaccessible to living plants.
Energy enters via producers (sunlight) and is lost as heat at each transfer. Matter cycles continuously through producers, consumers and decomposers.
Summarise the key ideas from this lesson in your notebook
Make organic molecules from inorganic inputs (CO₂ + H₂O + light → glucose). Plants, algae, cyanobacteria. Form the base of food webs. Entry point for energy.
Obtain organic molecules by consuming other organisms. Herbivores, carnivores, omnivores, detritivores. Cannot add new energy to the ecosystem — only transfer it.
Externally digest dead organic matter and absorb nutrients. Fungi and bacteria. Essential decomposers that recycle nutrients (N, P, K) back to producers.
Energy flows through ecosystems and is lost as heat. Matter cycles continuously through producers → consumers → decomposers → producers.
At the start of this lesson you predicted what would happen to a forest if all fungi and bacteria were removed. Now, using what you have learned about saprotrophs and nutrient cycling, refine your answer.
Type your revised response below.
Write your revised response in your book.
1. Which group forms the main entry point for energy in most food webs?
2. A scientist removes all fungi and bacteria from a forest ecosystem. What is the most likely long-term consequence?
3. Which statement correctly distinguishes a saprotroph from a detritivore?
4. In an aquatic ecosystem, algae are the primary producers. A disease kills 90% of the algae. Which prediction is best supported by ecosystem principles?
1 Explain the difference between a heterotroph and a saprotroph. 2 marks
2 Predict what would happen to nutrient availability in a forest if fungal decomposers were removed. Explain your prediction using the concept of nutrient cycling. 3 marks
3 Contrast how energy and matter move through an ecosystem. In your answer, refer to autotrophs, heterotrophs and saprotrophs. 4 marks
Test your knowledge of autotrophs, heterotrophs and saprotrophs in a rapid-fire quiz battle.