There are a number of factors to which the species richness of a community can be related, and these are of several different types. First, there are factors that can be referred to broadly as 'geographic', notably latitude, altitude and, in aquatic environments, depth. These have often been correlated with species richness, as we shall discuss below, but presumably they cannot be causal agents in their own right. If species richness changes with latitude, then there must be some other factor changing with latitude, exerting a direct effect on the communities.
A second group of factors does indeed show a tendency to be correlated with latitude (or altitude or depth), but they are not perfectly correlated. To the extent that they are correlated at all, they may play a part in explaining latitudinal and other gradients. But because they are not perfectly correlated, they serve also to blur the relationships along these gradients. Such factors include climatic variability, the input of energy, the productivity of the environment, and possibly the 'age' of the environment and the 'harshness' of the environment.
A further group of factors vary geographically but quite independently of latitude (or altitude, island location or depth). They therefore tend to blur or counteract relationships between species richness and other factors. This is true of the amount of physical disturbance a habitat experiences, the isolation of the habitat and the extent to which it is physically and chemically heterogeneous.
Finally, there is a group of factors that are biological attributes of a community, but are also important influences on the structure of the community of which they are part. Notable amongst these are the amount of predation or parasitism in a community, the amount of competition, the spatial or architectural heterogeneity generated by the organisms themselves and the successional status of a community. These should be thought of as 'secondary' factors in that they are themselves the consequences of influences outside the community. Nevertheless, they can all play powerful roles in the final shaping of community structure.
A number of these factors have been discussed in previous chapters (disturbance and successional status in Chapter 16, competition, predation and parasitism in Chapter 19). In this chapter we continue by examining the relationships between species richness and factors that can be thought of as exerting an influence in their own right. We do this first by considering factors whose variation is primarily spatial (productivity, spatial heterogeneity, environmental harshness - Section 21.3) and, second, those whose variation is primarily temporal (climatic variation and environmental age - Section 21.4). We will then be in a position to consider patterns in species richness related to habitat area and remoteness (island patterns - Section 21.5), before moving to gradients in species richness related to latitude, altitude, depth, succession and position in the fossil record (Section 21.6). In Section 21.7, we take a different tack by asking whether variations in species richness themselves have consequences for the functioning of ecosystems (e.g. productivity, decomposition rate and nutrient cycling). We begin, though, by constructing a simple theoretical framework (following MacArthur (1972), probably the greatest macroecologist, although he did not use the term) to help us think about variations in species richness.
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