Difficulty: Introductory | Prerequisites: Biological Organisation and Cell Biology notes (Doc 1 in this series)
This material covers how energy moves through living systems and how organisms interact with each other and their environment. It builds on the five unifying themes from the previous set of notes, particularly energy and matter and interactions. You should already understand the basic hierarchy of biological organisation. This is where the course starts to connect individual organisms to the larger systems they live in.
Energy enters ecosystems through producers (mainly via photosynthesis), flows one way through consumers, and exits as heat. Chemicals, by contrast, cycle: they are used, returned to the environment by decomposers, and reused. Organisms interact through mutualism, predation, and competition, and human activity is now a major force reshaping these interactions.
Producers (autotrophs)
Organisms that make their own food from inorganic sources. Plants and some bacteria convert sunlight into chemical energy via photosynthesis. Think of them as the energy entry point for an ecosystem.
Consumers (heterotrophs)
Organisms that obtain energy by eating other organisms. Animals are the obvious example, but fungi and many bacteria are heterotrophs too.
Decomposers
Organisms that break down dead organic material and return chemicals to the environment. In simple terms, they close the loop on chemical cycling.
Mutualism
An interaction in which both species benefit. Example: cleaner fish removing parasites from sea turtles.
Predation
An interaction in which one organism (the predator) benefits by consuming another (the prey).
Competition
An interaction in which both species are harmed because they compete for the same limited resource. Example: two plant species competing for soil nutrients.
Biodiversity
The variety of life in a given area, at every level from genes to ecosystems. In simple terms, it is a measure of how many different forms of life coexist in one place.
Life depends on a continuous transfer of energy. There is one critical distinction to internalise: energy flows one way, chemicals cycle.
Producers (e.g. plants) convert sunlight into chemical energy through photosynthesis. They are the base of virtually every food chain.
Consumers (e.g. animals) obtain energy by eating producers or other consumers.
Energy flows unidirectionally through ecosystems. At each transfer (producer to herbivore, herbivore to predator), most energy is lost as heat. It does not loop back.
Chemicals (carbon, nitrogen, phosphorus, water) do cycle. They are absorbed by organisms, used in metabolic processes, and returned to the environment when organisms die and decomposers break them down.
This combination of one-way energy flow and circular chemical cycling sustains life and maintains ecosystem stability.
Organisms are in constant interaction with each other and with their physical surroundings. Three types of species interaction are most commonly tested.
Mutualism – Both species benefit. Cleaner fish remove parasites from sea turtles: the fish gets food, the turtle loses its parasites.
Predation – One species benefits at the expense of another. Lions hunting zebras is the textbook example. Predation is a major driver of natural selection, as prey species evolve defences and predators evolve better hunting strategies.
Competition – Both species are harmed because they draw on the same limited resource. Two plant species growing side by side compete for soil nutrients, light, and water. Competition tends to limit the population size of both.
These interactions are not isolated events. They shape community structure, drive adaptation, and influence which species persist in a given ecosystem over time.
Humans are now a dominant force in nearly every ecosystem on the planet. The key impacts covered in this material are:
Increased greenhouse gas emissions, driving global warming
Habitat loss through deforestation, urbanisation, and agriculture
Species extinctions, accelerating as habitats shrink and climate shifts
These changes threaten biodiversity and destabilise ecosystem functions that all species, including humans, depend on. This topic connects directly to the interactions section above: when humans remove one species or alter a habitat, the ripple effects travel through mutualistic, predatory, and competitive relationships across the ecosystem.
Energy flow explains why food chains rarely exceed four or five levels: so much energy is lost as heat at each step that there is not enough left to sustain another tier of predators. Chemical cycling is the principle behind composting and wastewater treatment. Competition is the reason farmers weed their fields, as crops and weeds compete for the same resources.
Students often think energy cycles through ecosystems the way chemicals do. It does not. Energy flows in one direction and is lost as heat at every transfer. Only chemicals cycle.
Mutualism is sometimes confused with commensalism (where one species benefits and the other is unaffected). In mutualism, both species benefit.
"Competition" in biology does not require direct conflict. Two plant species can compete intensely without ever touching each other, simply by drawing down the same pool of soil nutrients.
Students sometimes assume decomposers are unimportant because they are not "dramatic" like predators. Without decomposers, chemical cycling would stall and nutrients would be locked in dead matter indefinitely.
⚠️ Be able to explain the difference between energy flow (one-way) and chemical cycling (circular). This distinction is heavily tested.
⚠️ Know the three interaction types (mutualism, predation, competition) and be able to identify each from a novel example.
⚠️ Understand the role of decomposers in closing the chemical cycle.
⚠️ Be prepared to discuss at least two ways human activity disrupts ecosystems and why these disruptions matter for biodiversity.
True or false: Energy cycles through ecosystems just as chemicals do. (False)
Fill in the blank: Organisms that make their own food from sunlight are called ______. (Producers / autotrophs)
True or false: In competition, one species benefits and the other is harmed. (False – both are harmed.)
Fill in the blank: ______ return nutrients to the environment by breaking down dead organic material. (Decomposers)
True or false: Removing one species from an ecosystem can have ripple effects on other species. (True)
Q: Explain why energy flow in an ecosystem is described as "one-way" while chemical cycling is described as "circular."
A: Energy enters ecosystems as sunlight, is converted to chemical energy by producers, and is lost as heat at every transfer between organisms. It cannot be recycled. Chemicals, by contrast, are absorbed by organisms, used in metabolic processes, released when organisms die and decompose, and then taken up again by new organisms.
Q: Give an example of mutualism, predation, and competition, and explain the effect on each species involved.
A: Mutualism: cleaner fish and sea turtles (both benefit). Predation: lions and zebras (lions benefit, zebras are harmed). Competition: two plant species in the same soil (both harmed, as each gets fewer resources).
Q: What role do decomposers play in ecosystem stability?
A: Decomposers break down dead organic material and return nutrients to the soil and atmosphere, allowing producers to take them up again. Without decomposers, nutrients would accumulate in dead matter and become unavailable to living organisms.
Q: Name two ways human activity threatens biodiversity.
A: Habitat destruction (deforestation, urbanisation) reduces the space available for species to live. Greenhouse gas emissions drive global warming, shifting climates faster than many species can adapt.
Energy flow connects to cellular respiration and photosynthesis, which you will study in detail in later chapters. Biological interactions link directly to evolution and natural selection (Doc 3 in this series), since predation and competition are major selective pressures. Human impact on ecosystems is a recurring theme throughout ecology and conservation biology.
Energy flow, chemical cycling, nutrient cycling, producers, autotrophs, consumers, heterotrophs, decomposers, food chain, food web, trophic levels, mutualism, predation, competition, commensalism, parasitism, biodiversity, ecosystem stability, greenhouse gases, global warming, habitat loss, species extinction, ecology, human impact on environment, BIO 101, intro biology