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, just as an atom of nitrogen, a sip of nectar or a mouthful of acorn are no longer available to other consumers. Similarly, females that have already mated may be unavailable to other mates. All these things have been consumed in the sense that the stock or supply has been reduced. Thus, resources are entities required by an organism, the quantities of which can be reduced by the activity of the organism.

Green plants photosynthesize and obtain both energy and matter for growth and reproduction from inorganic materials. Their resources are solar radiation, carbon dioxide (CO2), water and mineral nutrients. 'Chemosynthetic' organisms, such as many of the Archaebacteria, obtain energy by oxidizing methane, ammonium ions, hydrogen sulfide or ferrous iron; they live in environments such as hot springs and deep sea vents and use resources that were much more abundant during early phases of life on earth. All other organisms use as their food resource the bodies of other organisms. In each case, what has been consumed is no longer available to another consumer. The rabbit eaten by an eagle is no longer available to another eagle. The quantum of solar radiation absorbed and photosynthesized by a leaf is no longer available to another leaf. This has an important consequence: organisms may compete with each other to capture a share of a limited resource - a topic that will occupy us in Chapter 5.

A large part of ecology is about the assembly of inorganic resources by green plants and the reassembly of these packages at each successive stage in a web of consumer-resource inter actions. In this chapter we start with the resources of plants and focus especially on those most important in photosynthesis: radiation and CO2. Together, plant resources fuel the growth of individual plants, which, collectively, determine the primary productivity of whole areas of land (or volumes of water): the rate, per unit area, at which plants produce biomass. Patterns of primary productivity are examined in Chapter 17. Relatively little space in this chapter is given to food as a resource for animals, simply because a series of later chapters (9-12) is devoted to the ecology of predators, grazers, parasites and saprotrophs (the consumers and decomposers of dead organisms). This chapter then closes where the previous chapter began: with the ecological niche, adding resource dimensions to the condition dimensions we have met already.

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