Ecology. Michael Begon

8 and 18).

      pairs of species

      It is important to note that the existence of one type of organism in an area immediately diversifies it for others. Over its lifetime, an organism may increase the diversity of its environment by contributing dung, urine, dead parts (e.g. skin or leaves) and ultimately its dead body. During its life, its body may serve as a place in which other species find homes. Indeed, some of the most strongly developed matches between organisms and their environment are those in which one species has developed a dependence upon another. This is the case in many relationships between consumers and their foods. Whole syndromes of form, behaviour and metabolism constrain the animal within its narrow food niche, and deny it access to what might otherwise appear suitable alternative foods. Similar tight matches are characteristic of the relationships between parasites and their hosts. The various interactions in which one species is consumed by another are the subject matter of Chapters 8–10 and 12.

      Where two species have evolved a mutual dependence, the fit may be even tighter. We examine such ‘mutualisms’ in detail in Chapter 13. The association of nitrogen‐fixing bacteria with the roots of leguminous plants, and the often extremely precise relationships between insect pollinators and their flowers, are two good examples. When a population has been exposed to variations in the physical factors of the environment, for example a short growing season or a high risk of frost or drought, a once‐and‐for‐all tolerance may ultimately evolve. The physical factor cannot itself change or evolve as a result of the evolution of the organisms. By contrast, when members of two species interact, the change in each produces alterations in the life of the other, and each may generate selective forces that direct the evolution of the other. In such a coevolutionary process the interaction between two species may continually escalate. What we then see in nature may be pairs of species that have driven each other into ever‐narrowing ruts of specialisation – an ever closer match.

      coexistence of similar species

While it is no surprise that species with rather different roles coexist within the same community, it is also generally the case that communities support a variety of species performing apparently rather similar roles. Do these species compete with one another? Do competing species need to be different if they are to coexist? If so, how different do they need to be: is there some limit to their similarity? Do species like Darwin's finches interact with one another at the present time, or has evolution in the past led to the absence of such interactions in contemporary communities? We return to these questions about coexisting, similar species in Chapter 8, and take them up again in Chapters.

      Even at this stage, though, we may note that coexisting species, even when apparently very similar, commonly differ in subtle ways – not simply in their morphology or physiology but also in their responses to their environment and the role they play within the community of which they are part. The ‘ecological niches’ of such species are said to be differentiated from one another. The concept of the ecological niche is itself explained in the next two chapters.

Chapter 2 Conditions

      2.1 Introduction

      In order to understand the distribution and abundance of a species we need to know its history (Chapter 1), the resources it requires (Chapter 3), the individuals’ rates of birth, death and migration (Chapters 4 and 6), their interactions with their own and other species (Chapters 5 and 8–13) and the effects of environmental conditions. This chapter deals with the limits placed on organisms by environmental conditions.

      conditions may be altered – but not consumed

      A condition is an abiotic environmental factor that influences the functioning of living organisms. Examples include temperature, relative humidity, pH, salinity and the concentration of pollutants. A condition may be modified by the presence of other organisms. For example, temperature, humidity and soil pH may be altered under a forest canopy. But unlike resources, conditions are not consumed or used up by organisms.

For some conditions we can recognise an optimum concentration or level at which an organism performs best, with its activity tailing off at both lower and higher levels (Figure 2.1a). But what is meant by ‘performs best’? From an evolutionary point of view, ‘optimal’ conditions are those under which individuals leave most descendants (are fittest), but these are often impossible to determine in practice because measures of fitness should be made over several generations. Instead, we more often measure the effect of conditions on some key property like the activity of an enzyme, the respiration rate of a tissue, the growth rate of individuals or their rate of reproduction. However, the effect of variation in conditions on these various properties will often not be the same; organisms can usually survive over a wider range of conditions than permit them to grow or reproduce (Figure 2.1a).

Figure 2.1 Response curves illustrating the effects of a range of environmental conditions on individual survival (S), growth (G) and reproduction (R). (a) Extreme conditions are lethal, less extreme conditions prevent growth, and only optimal conditions allow reproduction. (b) The condition is lethal only at high intensities; the reproduction–growth–survival sequence still applies. (c) Similar to (b), but the condition is required by organisms, as a resource, at low concentrations.

      The precise shape of a species’ response will vary from condition to condition. The generalised form of response, shown in Figure 2.1a, is appropriate for conditions like temperature and pH in which there is a continuum from an adverse or lethal level (e.g. freezing or very acid conditions), through favourable levels of the condition to a further adverse or lethal level (heat damage or very alkaline conditions). There are, though, many environmental conditions for which Figure 2.1b is a more appropriate response curve: for most toxins, including radioactive emissions and chemical pollutants, a low‐level intensity or concentration of the condition has no detectable effect, but an increase begins to cause damage and a further increase may be lethal. There is also a different form of response to conditions that are toxic at high levels but essential for growth at low levels (Figure