Resources are entities required by an organism, the quantities of which can be reduced by the activity of the organism. Hence, organisms may compete with each other to capture a share of a limited resource.

Autotrophic organisms (green plants and certain bacteria) assimilate inorganic resources into packages of organic molecules (proteins, carbohydrates, etc.). These become the resources for heterotrophic organisms, which take part in a chain of events in which each consumer of a resource becomes, in turn, a resource for another consumer.

Solar radiation is the only source of energy that can be used in metabolic activities by green plants. Radiant energy is converted during photosynthesis into energy-rich chemical compounds of carbon, which will subsequently be broken down in respiration. But the photosynthetic apparatus is able to gain access to energy only in the waveband of 'photosynthetically active radiation'. We examine variations in the intensity and quality of radiation, and the responses of plants to such variations. We examine, too, the strategic and tactical solutions adopted by plants to resolve the conflicts between photosynthesis and water conservation.

Carbon dioxide is also essential for photosynthesis. We examine variations in its concentration, and their consequences, including global rises over time and those at the smallest spatial scales. There are three pathways to carbon fixation in photosynthesis: C3, C4 and CAM. The differences between the different pathways and the ecological consequences of them are explained.

Water is a critical resource for all organisms. For plants, we examine how roots 'forage' for water, and the dynamics of resource depletion zones around roots, for water and for mineral nutrients. Mineral nutrients, broadly divisible into macronutrients and trace elements, each enter a plant independently as an ion or a molecule, and have their own characteristic properties of absorption in the soil and of diffusion, which affect their accessibility to a plant.

Oxygen is a resource for both animals and plants. It becomes limiting most quickly in aquatic and waterlogged environments, and when organic matter decomposes in an aquatic environment, microbial respiration may so deplete oxygen as to constrain the types of higher animal that can persist.

Amongst heterotrophs, we explain the distinctions between saprotrophs, predators, grazers and parasites, and between specialists and generalists.

The carbon : nitrogen ratio of plant tissues commonly exceeds greatly that in bacteria, fungi and animals. The main waste products of organisms that consume plants are therefore carbon-rich compounds. By contrast, the main excretory products of carnivores are nitrogenous. The various parts of a plant have very different compositions. Hence, most small herbivores are specialists. The composition of the bodies of different herbivores is remarkably similar.

Most of the energy sources potentially available to herbivores comprise cellulose and lignins, but most animals lack cellulases - an evolutionary puzzle. We explain how, in herbivorous vertebrates, the rate of energy gain from different dietary resources is determined by the structure of the gut.

Living resources are typically defended: physically, by chemicals, or by crypsis, aposematism or mimicry. This may lead to a coevolutionary arms races between the consumer and the consumed.

Apparency theory and optimal defense theory seek to make sense of the distribution of different protective chemicals, especially those that are constitutive and those that are induced, in different plant species and plant parts.

Taking resources in pairs, plots for the consumers of zero net growth isoclines allow resource pairs to be classified as essential, perfectly substitutable, complementary, antagonistic or displaying inhibition. The zero net growth isoclines themselves define a boundary of a species' ecological niche.

Chapter 4

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