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According to Tilman (1982), all things consumed by an organism are resources for it. But consumed does not simply mean ‘eaten’. Bees and squirrels do not eat holes, but a hole that is occupied is no longer available to another bee or squirrel. Similarly, females that have already mated may be unavailable to other mates. All these things have been consumed in the sense that their stock or supply can be reduced by the activities of the organisms concerned.

autotrophs and heterotrophs

There is a fundamental distinction between autotrophic and heterotrophic organisms. Autotrophs assimilate simple inorganic resources into packages of organic molecules (proteins, carbohydrates, etc.). These become the resources for the heterotrophs (decomposers, parasites, predators and grazers), which take part in a chain of events in which each consumer of a resource becomes, in turn, a resource for another consumer. At each link in this food chain, the most obvious distinction is between saprotrophs and predators. Saprotrophs – bacteria, fungi and detritivorous animals (see Chapter 11) – use other organisms as food but only after they have died, or they consume another organism’s waste or secretory products. Predators, defined broadly, feed on other living organisms, or parts of other living organisms (see Section 3.7).

photoautotrophs and chemoautotrophs

Autotrophs may themselves be divided into photoautotrophs and chemoautotrophs. The photoautotrophs – green plants and algae, and photosynthetic protists and bacteria – utilise solar radiation, carbon dioxide (CO₂), water and mineral nutrients as resources. Through photosynthesis, they use the radiation as a source of energy to reduce CO₂ to obtain the organic compounds and energy that they need for growth and reproduction. Directly or indirectly, photosynthesis is the source of all energy in terrestrial and most aquatic ecosystems. Its evolution has led to the current 21% levels of oxygen in the atmosphere, driving down the levels of CO₂. By contrast, chemoautotrophs – certain bacteria and archaea – use chemical energy from the oxidation of inorganic substances such as hydrogen sulphide, elemental sulphur, ferrous iron or ammonia to reduce CO₂ and so obtain the organic compounds and energy that they need. They typically live in ‘extreme’ environments such as hot springs and deep‐sea vents.

For both autotrophs and heterotrophs, resources, once consumed, are no longer available to another consumer. This has the important consequence that organisms may compete with each other to capture a share of a limited resource – a topic to which we turn in Chapter 5.

In this chapter we start (Sections 3.2–3.6) with the resources that fuel the growth of individual plants, and so, collectively, determine the primary productivity of whole areas of land or volumes of water: the rate, per unit area or volume, at which plants produce biomass. Broad‐scale patterns of primary productivity are examined in Chapter 20. Relatively little space in this chapter (Section 3.7) is given to food as a resource for animals, simply because a series of later chapters (Chapters 9–13) is devoted to the ecology of predators, grazers, parasites and saprotrophs (the consumers and decomposers of dead organisms). This chapter then closes with sections on two important topics, drawing on material from the present chapter and the last – one (Section 3.8) on the ecological niche and resource classification, and a second (Section 3.9) on a so‐called metabolic theory of ecology.