Energy Flow

Energy Flow Through Ecosystems

Unlike nutrients, which cycle repeatedly through an ecosystem, energy flows through it in one direction only — entering as sunlight, being fixed by producers, and being progressively lost as heat at every subsequent transfer, ultimately leaving the system permanently. This one-way flow is what obligates ecosystems to a continuous external energy input (sunlight) to keep functioning.

Trophic Levels and Food Chains

Organisms in an ecosystem can be arranged into trophic (feeding) levels:

• Producers (autotrophs, mainly green plants and algae) — fix solar energy into chemical energy via photosynthesis, forming the base of the system.
• Primary consumers (herbivores) — feed directly on producers.
• Secondary consumers (primary carnivores) — feed on primary consumers.
• Tertiary consumers and beyond (top carnivores) — feed on secondary consumers.
• Decomposers/detritivores — break down dead organic matter at every level.

A linear sequence of who-eats-whom is a food chain. Ecosystems support two broad types:

• Grazing food chain (GFC) — begins with living green plants (producers), which are eaten by herbivores, which in turn are eaten by carnivores. This is the familiar "producer → herbivore → carnivore" sequence.
• Detritus food chain (DFC) — begins not with living plants but with dead organic matter (detritus) — fallen leaves, dead animals, faecal matter — which is broken down by detritivores and decomposers, releasing nutrients back into the system. In many ecosystems (especially forests), the DFC processes a larger share of total energy and biomass than the GFC.

In a real community, feeding relationships are rarely a single straight chain — most organisms eat, and are eaten by, more than one other species, so the many interconnected and overlapping food chains of a community together form a food web. A food web gives a far more realistic and resilient picture of energy flow than any single food chain, since if one prey or predator species is lost, energy can still move through alternative pathways in the web.

Ecological Pyramids

The quantitative relationship between trophic levels can be represented as ecological pyramids, with producers forming the base and each successive trophic level stacked above:

• Pyramid of numbers — represents the number of individual organisms at each trophic level; usually upright (many producers, fewer top carnivores) but can be inverted in some systems (for instance, a single large tree supporting many insect herbivores).
• Pyramid of biomass — represents the total dry mass of organisms at each trophic level; typically upright on land, but can be inverted in some aquatic ecosystems where the standing crop of producers (phytoplankton) is small at any instant despite very high turnover.
• Pyramid of energy — represents the amount of energy present/flowing at each trophic level over a given time; because energy is progressively lost as heat at each transfer, the pyramid of energy is always upright and can never be inverted, making it the most fundamentally reliable of the three.

The Second Law of Thermodynamics in Ecosystems

Every transfer of energy from one trophic level to the next is inefficient: a large part of the energy an organism consumes is used up in respiration and lost as heat, or is not consumed at all (unconsumed parts, faeces). This is a direct ecological expression of the second law of thermodynamics — no energy transfer is 100% efficient, and usable energy diminishes at every step. As a rough working figure, only about ten percent of the energy at one trophic level is typically incorporated into the biomass of the next level (the rest being lost as heat or in wastes), which is why food chains are rarely longer than four or five trophic levels — there simply is not enough energy left to support a further level.

Productivity

• Primary productivity is the rate at which producers (autotrophs) convert solar energy into organic matter via photosynthesis. Gross primary productivity (GPP) is the total rate of organic matter production, while net primary productivity (NPP) is what remains after subtracting the plants' own respiratory losses (NPP = GPP − respiration); NPP is the energy actually available to primary consumers.
• Secondary productivity is the rate at which consumers (heterotrophs) convert the food they assimilate into their own new biomass.

Decomposition …