Revise Transfers of energy and matter for IB Biology (C4.2)

An ecosystem is an open system: it exchanges both energy and matter with its surroundings. But the two are exchanged in fundamentally different ways. Energy flows through an ecosystem in one direction — it enters (in most ecosystems) as light, is converted to chemical energy by producers, is passed between organisms, and is ultimately lost to the surroundings as heat. It is never recycled, so it must be continually resupplied by the Sun. Matter cycles within an ecosystem. The chemical elements that make up organisms — carbon, nitrogen, phosphorus and the rest — are finite. They pass from the environment into organisms and back again, circulating between the living (biotic) and non-living (abiotic) components, driven above all by decomposers. Holding these two ideas apart — flow versus cycle — is the single most important thing in this topic.

A trophic level is an organism's feeding position — how many steps its energy is from the original source. Producers (autotrophs such as plants and algae) form the first level: they make their own organic compounds, usually by photosynthesis, and are the entry point of energy into the ecosystem. Consumers (heterotrophs) obtain energy by feeding on other organisms. A primary consumer (herbivore) eats producers; a secondary consumer eats primary consumers; a tertiary consumer eats secondary consumers. Standing apart from this ladder are the decomposers — chiefly saprotrophic bacteria and fungi — which feed on dead organic matter and waste from every level, breaking it down and releasing inorganic nutrients back to the environment. Decomposers are what make matter cycling possible.

A food chain shows a single sequence of feeding relationships, for example: grass → grasshopper → shrew → owl. The arrows are critical: each arrow points in the direction that energy flows, from the organism being eaten to the organism that eats it (prey → predator). Real ecosystems are far more tangled than a single chain, because most organisms eat, and are eaten by, several species. A food web joins many food chains together to give a realistic map of who feeds on whom. Reading a web, you can trace how energy captured by producers is distributed through the whole community — and see why removing one species can ripple through many others.

The transfer of energy from one trophic level to the next is strikingly inefficient. As a rule of thumb, only about 10% of the energy at one level is passed on to the next; roughly 90% is lost. (The real figure varies, commonly between about 5% and 20%, so 10% is an approximation rather than a law.) The energy does not vanish — it goes to three places. Most is lost as heat from respiration: organisms respire much of their food to power movement, growth and, in birds and mammals, keeping warm. On top of that, not all of an organism is eaten (roots, bones, fur and wood are often left), and of what is eaten, not all is digested and absorbed — the indigestible remainder leaves in faeces. Only the small fraction built into new biomass is available to the next level.

Two consequences follow directly. First, food chains are short: after only a few ~90% losses, so little energy remains that no further trophic level can be supported — which is why chains rarely exceed four or five links. Second, a pyramid of energy (the energy, or rate of energy flow in kJ m⁻² yr⁻¹, at each level) is always upright, because each level must hold less than the one below. Pyramids of numbers or biomass can occasionally be inverted — a single tree feeding many insects, for instance — but a pyramid of energy never can.

While energy drains away, the atoms are conserved and cycled — and the clearest example is the carbon cycle. Carbon dioxide is removed from the atmosphere by photosynthesis, which fixes it into the organic compounds of producers. That organic carbon then moves along food chains by feeding, as consumers eat other organisms. Carbon returns to the atmosphere as CO₂ through respiration by every organism, and through decomposition, in which decomposers break down dead organisms and waste and respire their carbon. Finally, combustion — the burning of biomass, and of fossil fuels formed from ancient organisms — releases CO₂ as well. Decomposers are pivotal: by releasing carbon and mineral nutrients from dead matter, they return the raw materials of life to the environment so producers can take them up again. This is why matter cycles indefinitely while energy does not.

Human activity disturbs these balanced flows — most importantly by moving carbon faster than the cycle can absorb. Burning fossil fuels releases carbon that had been locked away for millions of years, and deforestation both removes photosynthesising producers and, when the timber is burned or left to decompose, adds still more CO₂. Because this input outpaces removal by photosynthesis and the oceans, atmospheric CO₂ rises, driving climate change. Human pressure on food webs — overharvesting top consumers, simplifying communities — also disrupts energy flow through ecosystems. At this level you need only the outline: human activity adds CO₂ to the atmosphere faster than the carbon cycle can remove it, unbalancing a cycle that was previously in near-equilibrium.

This is an EXPLAIN question, and explain marks are awarded analytically — each distinct valid biological point is worth one mark, up to the number of marks available. There is no need for a fixed wording: the engine looks for separate creditable ideas, awards the answer mark (A) for a correctly reasoned conclusion, applies error-carried-forward (ECF) so an earlier slip does not sink later points, and accepts equivalent phrasings. Study how each mark below is tied to a specific, named idea.

The two rules of this topic — energy flows, matter cycles — underpin the rest of ecology. The ten-percent ceiling explains why eating lower on a food chain feeds more people from the same land, and why top predators are always few. Nutrient cycling links straight to the nitrogen and phosphorus cycles and to conservation, while the carbon cycle connects your biology to the physics of the greenhouse effect and to the whole question of human-driven climate change. Hold the core distinction firmly — energy in as light and out as heat, atoms round and round forever — and the rest of ecosystem ecology falls into place.