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 2.1c). This is the case for sodium chloride – an essential resource for animals but lethal at high concentrations – and for the many elements that are essential micronutrient resources in the growth of plants and animals (e.g. copper, zinc and manganese), but that can become lethal at the higher concentrations sometimes caused by industrial pollution.
In this chapter, we consider responses to temperature in much more detail than other conditions, because it is the single most important condition that affects the lives of organisms, and many of the generalisations that we make have widespread relevance. We move on to consider a range of other conditions, before returning, full circle, to temperature because of the way that other conditions interact with it. We begin, though, by explaining the framework within which each of these conditions should be understood: the ecological niche.
2.2 Ecological niches
The term ecological niche is frequently misunderstood. It is often misused to describe the sort of place in which an organism lives, as in the sentence: ‘Woodlands are the niche of woodpeckers’. Strictly, however, where an organism lives is its habitat. A niche is not a place but an idea: a summary of the organism’s tolerances and requirements. The habitat of a gut microorganism would be an animal’s alimentary canal; the habitat of an aphid might be a garden; and the habitat of a fish could be a whole lake. Each habitat, however, provides many different niches: many other organisms also live in the gut, the garden or the lake – and with quite different lifestyles. The word niche began to gain its present scientific meaning when Elton wrote in 1933 that the niche of an organism is its mode of life ‘in the sense that we speak of trades or jobs or professions in a human community’. The niche of an organism started to be used to describe how, rather than just where, an organism lives.
niche dimensions
The modern concept of the niche was proposed by Hutchinson in 1957 to address the ways in which tolerances and requirements interact to define the conditions (this chapter) and resources (Chapter 3) needed by an individual of a species in order to practice its way of life. Temperature, for instance, limits the growth and reproduction of all organisms, but different organisms tolerate different ranges of temperature. This range is one dimension
