An ecosystem is a structural and functional unit of nature comprising abiotic and biotic components.

Abiotic components. The inorganic materials — air, water and soil. In a pond, this is the water with all the dissolved inorganic and organic substances, and the rich soil deposit at the bottom. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond.

Biotic components. These are of three kinds.

Producers, or autotrophs. In a pond, the phytoplankton, some algae and the floating, submerged and marginal plants at the edges. They carry out the conversion of inorganic into organic material with the help of the radiant energy of the sun.

Consumers, or heterotrophs. In a pond, the zooplankton and the free-swimming and bottom-dwelling forms. They consume the autotrophs, directly or indirectly.

Decomposers. In a pond, the fungi, bacteria and flagellates, especially abundant at the bottom. They carry out decomposition and mineralisation of dead matter, releasing it back for reuse by the autotrophs.

The structure that results. Interaction of biotic and abiotic components produces a physical structure characteristic of each type of ecosystem, described by two features: species composition, obtained by identification and enumeration of the plant and animal species, and stratification, the vertical distribution of different species occupying different levels.

How the components function as a unit. Through four aspects: productivity, decomposition, energy flow and nutrient cycling. These events are repeated over and over again, while energy moves unidirectionally towards higher trophic levels and is dissipated and lost as heat.

It would be an inverted pyramid of numbers — narrow at the base and broad above.

Why. A single large tree is one individual, yet it supports thousands of insects feeding on it. Those insects support a smaller but still substantial number of small birds, which support fewer larger birds. Counting individuals therefore gives one producer at the base, thousands at the second level, and fewer again above — a shape quite unlike the upright pyramid of a grassland, where nearly 6 million plants support only three top carnivores.

What this reveals. That a count of individuals ignores size, and size is what determines how much living material and energy a trophic level actually holds. One tree may weigh more than every insect, bird and mammal in the ecosystem combined. This is the same problem met in the previous chapter, where stating that the population density of a banyan is low relative to carrot grass underestimates the enormous role of the banyan.

The remedy. Express the pyramid in biomass or in energy instead. A pyramid of energy is always upright and can never be inverted, because some energy is always lost as heat when it flows from one trophic level to the next — so energy is the measure that never misleads.

What structure describes. Interaction of biotic and abiotic components results in a physical structure characteristic for each type of ecosystem. Species composition is what you get by identification and enumeration of the plant and animal species; stratification is the vertical distribution of different species occupying different levels — trees at the top of a forest, then shrubs, then herbs and grasses. Both are descriptions of a state: what is present, and where. They could in principle be recorded from a single photograph and a species list.

What function describes. Productivity, decomposition, energy flow and nutrient cycling are all rates — processes measured per unit area per unit time. They describe what the ecosystem is doing, and none of them can be seen at an instant.

What structure alone cannot tell you. Two forests with identical species lists and identical layering can differ several-fold in productivity, depending on nutrients, temperature and moisture. Structure cannot tell you whether an ecosystem is gaining or losing organic matter, how fast nutrients are being returned, or how much energy is available to consumers.

What function alone cannot tell you. Which species are doing the work, and therefore what would happen if one were removed. A productivity figure is silent about resilience.

Why the chapter needs both. The components of an ecosystem are seen to function as a unit only when the four functional aspects are considered — but those functions are carried out by the particular species arranged in the particular way that the structure describes. Structure is the machine; function is its operation.

What makes it self-sustaining: the nutrients are recycled. The pond performs all the functions of any ecosystem, and of the biosphere as a whole — conversion of inorganic into organic material by the autotrophs; consumption of the autotrophs by heterotrophs; and decomposition and mineralisation of the dead matter to release it back for reuse by the autotrophs. These events are repeated over and over again. Because the decomposers return to the producers exactly what the producers need, the same atoms can circulate indefinitely and nothing has to be imported.

What it is not independent of: energy. There is a unidirectional movement of energy towards the higher trophic levels and its dissipation and loss as heat to the environment. Energy leaves and does not come back, so the pond requires a constant input of solar energy to keep running. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond.

Why the word 'fairly' is doing real work. No natural pond is closed. Rain and run-off bring in water, silt and nutrients; a stream may carry material out; birds, amphibians and insects move in and out, carrying matter and energy with them. And the pond's rate of function is set by climate, which it does not control at all.

The general lesson. Self-sustaining means self-sustaining in matter, given a continuous supply of energy. No ecosystem anywhere, including the biosphere, is self-sustaining in energy.

Why it is divided. Many ecologists regard the entire biosphere as a global ecosystem, a composite of all local ecosystems on Earth. But this system is too big and complex to be studied at one time, so it is convenient to divide it into two basic categories, terrestrial and aquatic — forest, grassland and desert on the one hand; pond, lake, wetland, river and estuary on the other.

Where the man-made cases fit. Crop fields and an aquarium may also be considered as ecosystems — man-made ones. A crop field has all four components and all four functions, but its species composition is chosen, its productivity is subsidised with fertiliser, and its organic matter is exported at harvest rather than decomposed in place. An aquarium is an aquatic ecosystem small enough to hold, and depends on the keeper for the functions it cannot perform for itself.

Is the division scientific or convenient? Explicitly convenient — the chapter says so. And the boundaries leak in both directions: an estuary is listed as aquatic but is defined by its mixing with the land; a wetland is neither fully one nor the other; a forest's productivity depends on its water and a lake's on the soil that drains into it.

Why the convenience is still justified. Because the two categories genuinely differ in the things this chapter measures. In an aquatic ecosystem the grazing food chain is the major conduit for energy flow, whereas in a terrestrial ecosystem a much larger fraction of energy flows through the detritus food chain. The division is a tool, but a tool that cuts along a real joint.