Chapter 10 Patterns in Species Richness Recognizing and conserving the world’s biological resources are becoming increasingly important. To conserve biodiversity we must understand why species richness varies widely across the face of the Earth. Why do some communities contain more species than others? Are there patterns or gradients in this biodiversity? If so, what are the reasons for these patterns? Chapter Contents 10.1 Introduction 10.2 A simple model of species richness 10.3 Spatially varying factors that influence species richness 10.4 Temporally varying factors that influence species richness 10.5 Gradients of species richness 10.6 Patterns in taxon richness in the fossil record 10.7 Appraisal of patterns in species richness Key Concepts In this chapter you will n understand the meanings of species richness, diversity indices, and rank–abundance diagrams n appreciate that species richness is limited by available resources, the average portion of the resources used by each species (niche breadth), and the degree of overlap in resource use n recognize that species richness may be highest at intermediate levels of both productivity and predation intensity but tends to increase with spatial heterogeneity n appreciate the importance of habitat area and remoteness in determining richness, especially with reference to the equilibrium theory of island biogeography n understand richness gradients with latitude, altitude, and depth, and during community succession, and the difficulties of explaining them n appreciate how theories of species richness can also be applied to the fossil record Patterns in Species Richness Chapter 10 331 10.1 Introduction Why the number of species varies from place to place, and from time to time, are ques- tions that present themselves not only to ecologists but to anybody who observes and ponders the natural world. They are interesting questions in their own right—but they are also questions of practical importance. The number of species in a community is a crucial aspect of that community’s biodiversity. The exact meaning of biodiversity is dis- cussed in Chapter 14, but for now it is clear that if we wish to conserve or restore bio- diversity, then we must understand how species numbers are determined and how it comes about that they vary. We will see that there are plausible answers to the ques- tions we ask, but these answers are by no means conclusive. Yet this is not so much a disappointment as a challenge to ecologists of the future. Much of the fascination of ecology lies in the fact that many of the problems are blatant, whereas the solutions are not. We will see that a full understanding of patterns in species richness must draw on our knowledge of all the areas of ecology discussed so far in this book. The number of species in a community is referred to as its species richness. determining species richness Counting or listing the species present in a community may sound a straightforward procedure, but in practice it is often surprisingly difficult, partly because of taxonomic problems, but also because only a subsample of the organisms in an area can usually be counted. The number of species recorded then depends on the number of samples that have been taken, or on the volume of the habitat that has been explored. The most common species are likely to be represented in the first few samples, and as more samples are taken, rarer species will be added to the list. At what point does one cease to take further samples? Ideally, the investigator should continue to sample until the number of species reaches a plateau. At the very least, the species richness of different communities should be compared only if they are based on the same sample sizes (in terms of area of habitat explored, time devoted to sampling, or, best of all, number of individuals included in the samples). An important aspect of the structure of a community is completely ignored, though, diversity indices and when its composition is described simply in terms of the number of species present— rank–abundance diagrams namely, that some species are rare and others common. Intuitively, a community of ten species with equal numbers in each seems more diverse than another, again consisting of ten species, with 91 percent of the individuals belonging to the most common species and just 1 percent in each of the other nine. Yet, each community has the same species richness. Diversity indices are designed to combine both species richness and the even- ness or equitability of the distribution of individuals among those species (Box 10.1). Moreover, attempts to describe a complex community structure by one single attribute, such as richness, or even diversity, can still be criticized because so much valuable information is lost. A more complete picture of the distribution of species abundance in a community is therefore sometimes provided in a rank–abundance diagram (Box 10.1). Nonetheless, for many purposes, the simplest measure, species richness, suffices. In the following sections, therefore, we examine the relationships between species richness and a variety of factors that may, in theory, influence richness in ecological communities. It will become clear that it is often extremely difficult to come up with unambiguous predictions and clean tests of hypotheses when dealing with something as complex as a community. 332 Part III Individuals, Populations, Communities, and Ecosystems Box 10.1 Quantitative Aspects Diversity Indices and Rank–Abundance Diagrams The measure of the character of a community that is most commonly used to take into account both 3 Control species richness and the relative abundances of those ) species is known as the Shannon or the Shannon– H Weaver diversity index (denoted by H). This is 2 calculated by determining, for each species, the proportion of individuals or biomass (Pi for the ith species) that that species contributes to the total in the 1 Fertilized sample. Then, if S is the total number of species in the Species diversity ( community (i.e., the richness): 0 1860 1900 1940 Σ diversity, H = – Pi lnPi, Σ Figure 10.1 where the (“summation”) sign indicates that the Species diversity (H) of a control plot and a fertilized plot in the Rothamstead product (Pi lnPi) is calculated for each of the S species Parkgrass experiment. (After Tilman, 1982.) in turn and these products are then summed. As required, the value of the index depends on both the species richness and the evenness (equitability) with against “rank”; i.e., the most abundant species takes which individuals are distributed among the species. rank 1, the second most abundant rank 2, and so on, Thus, for a given richness, H increases with equitability, until the array is completed by the rarest species of all. and for a given equitability, H increases with richness. Rank–abundance diagrams, like indices of richness An example of an analysis using diversity and diversity, should be viewed as abstractions of indices is provided by the unusually long-term study the highly complex structure of communities that that commenced in 1856 in an area of pasture at may nonetheless sometimes be useful when making Rothamsted in England. Experimental plots received a comparisons. For example, the steeper the slope of a fertilizer treatment once every year, and control plots rank–abundance diagram, the greater the dominance did not. Figure 10.1 shows how the species diversity of common species over rare species in the (H) of the grass species changed between 1856 and community (a steep slope means a sharp drop in 1949. While the unfertilized area remained essentially relative abundance, Pi, for a given drop in rank). Thus, unchanged, the fertilized area showed a progressive in the case of the Rothamsted experiment, Figure 10.2 decline. This is discussed in Section 10.3.1. shows how the dominance of commoner species Rank–abundance diagrams, on the other hand, steadily increased (steeper slope) while species make use of the full array of Pi values by plotting Pi richness decreased over time. Patterns in Species Richness Chapter 10 333 Box 10.1 (cont’d) 1.0 10–1 10–2 10–3 Relative abundance 1949 1919 10–4 1903 1872 1862 1856 Species rank Figure 10.2 Change in the rank–abundance pattern of plant species in the Parkgrass fertilized plot from 1856 to 1949. (After Tokeshi, 1993.) 10.2 A Simple Model of Species Richness To try to understand the determinants of species richness, it will be useful to begin with a simple model (Figure 10.3). Assume, for simplicity, that the resources available to a community can be depicted as a one-dimensional continuum, R units long. Each species uses only a portion of this resource continuum, and these portions define the niche breadths (n) of the various species: the average niche breadth within the com- munity is C. Some of these niches overlap, and the overlap between adjacent species can be measured by a value o. The average niche overlap within the community is then D. With this simple background, it is possible to consider why some communities should contain more species than others. First, for given values of C and D, a community will contain more species the larger the value of R, i.e., the greater the range of resources (Figure 10.3a). Second, for a given range of resources, more species will be accommodated if C is smaller, i.e., if the species are more specialized in their use of resources (Figure 10.3b). Alternatively, if species overlap to a greater extent in their use of resources (greater D), then more may coexist along the same resource continuum (Figure 10.3c).