The flow of energy in an ecosystem is

Concept. Energy and matter behave differently inside an ecosystem. Matter can be used again, but energy cannot: the second law of thermodynamics requires that every biological transfer degrade a large part of the transferred energy into heat, and no organism can recapture that heat for further work. A quantity that is degraded at every step and is never restored can move only in one sense — away from its source, forward through the successive stages, and finally out of the system. It may branch along many forward paths on the way; what it can never do is travel back to a stage it has already left. That fixed sense of travel, not the number of paths, is what a direction term for a flow describes.

Application. Trace the route energy actually takes through an ecosystem:

1. Solar radiation is the external source. Green plants (producers) convert part of the incoming light into chemical energy through photosynthesis.

2. Producers pass a part of that fixed energy on to herbivores (primary consumers) when they are eaten.

3. Herbivores pass a part on to carnivores (secondary consumers), and those in turn to top carnivores (tertiary consumers). In a real food web this happens along several branching chains at once — grazing chains and detritus chains — but every branch runs forward, away from the producers.

4. At each trophic transfer, only a fraction of the energy becomes biomass available to the next consumer level. Other portions are used in metabolism or enter waste, uneaten biomass and detrital pathways; through respiration across these pathways, energy is progressively dissipated as heat.

5. Decomposers release the energy still locked in dead tissue and waste, and this too finally leaves the ecosystem as heat.

Cross-check. No step of that chain runs backwards. A carnivore returns no usable energy to the grass that ultimately supported it, and the heat radiated to the atmosphere is not re-fixed by producers. Branching into several food chains does not change this, because each branch still carries energy forward and none of them carries it back. Every unit of energy therefore passes any given trophic level at most once, so the flow has a single fixed sense and is unidirectional.

Contrast — energy flows, matter cycles. The same ecosystem moves energy and matter in quite different ways:

Since the stem asks about energy and not about nutrients, the one-way description applies: the flow of energy in an ecosystem is unidirectional.