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Sustainable Human Interactions With Appendix 3.2 The following material is Appendix 3.2 some species characteristics compensate for others for Chapter 3 of: Fowler, C.W. 2009. to reduce the risk of extinction for a particular Systemic Management: Sustainable species. The structure and function of ecosystems Human Interactions with Ecosystems and and other species groups are then described as the the Biosphere. Oxford University Press result of selective extinction and speciation, in con- cert with all other contributing factors (Fig. 1.4)— 1 Evolutionary contributions to the the emergence of ecosystems involves evolution formation of species-level patterns at various levels. Another section highlights the (frequency distributions) relative importance of evolutionary changes, par- Extinction has only defined the groups: it has by no ticularly selective extinction and speciation, in the means made them . formation of frequency distributions. —Charles Darwin 1.0 Evolutionary development of species-level This appendix further develops the concept that patterns (frequency distributions) patterns among species are simply natural phe- nomena that emerge from the complexity both of When considering the extinction, speciation, and the systems within which they occur and of which evolution of species, the ways species function they are composed. Chapter 2 began consideration within ecosystems is as important as their morph- of what is involved in this explanatory complexity, ology (the cornerstone of taxonomy). The function with examples including the abiotic environment, of species includes their coevolutionary interac- and the evolutionary processes of natural selection, tions (“ . .life is a dense web of genetic interac- speciation and extinction, and ecological mechan- tions . “; Lederberg 1993). From an evolutionary/ ics. Chapter 3 continued that process. This appen- ecological perspective, species may be classified dix elaborates through consideration of how the according to characteristics independent of pedi- mechanics of evolutionary processes, particularly gree or taxonomic relationship to other species. speciation and extinction, contribute to species- In other words, they may be placed in categories level patterns and, in fact, may be the most import- based on measurements like those of the species ant contributing factors. distributions shown in Chapter 2. Characteristics The bulk of what are presented here are hypo- such as body size, trophic level, geographic range, thetical examples to explore the effects of extinction metabolic rates, population variability, population and speciation to include the evolution of species size, and generation time are shared by all species, (the latter involving natural selection as it acts on just as are their taxonomic links. Among higher individuals, genes, and gene combinations). These trophic levels selectivity in consumption (e.g., processes are examined in their contribution to the selectivity by size or sex) are measurable charac- formation of species-level patterns, particularly as teristics regarding interspecific interactions. frequency distributions among single and multiple Limits to variation among species are key to the species characteristics. Early sections compare the information in each species-level pattern. These effects of selectivity or nonselectivity in one or more limits are observable in the form of frequency evolutionary process. A later section describes how distributions—often “bell” curves with tails 41 noam.indb 41 3/9/2009 12:44:55 PM 42 SYSTEMIC MANAGEMENT representing declines in the number of species each side of a mode. In this section, the influence of extinction and speciation in the formation of fre- quency distributions is described using hypothet- ical examples, illustrated graphically. The purpose here is to show how selective extinction and speci- Portion of species Species characteristic ation combine in a trial-and-error process to prod- uce examples of sustainability among species. This section begins by illustrating how select- Pseudo- extinction ive extinction and speciation (coupled with the Extinction Extinction microevolution involved in speciation) influence Replication Replication frequency distributions for a single species-level characteristic. This requires making the unrealistic assumption that individual characteristics affect evolutionary processes independently. In nature, the simultaneous actions of selectivity along mul- tiple dimensions create a complex of factors that Portion of species contribute to the formation of species frequency distributions. Examples seen in the following are Species characteristic presented for patterns resulting from two or more Appendix Figure 3.2.1 The effects of selective extinction and interacting evolutionary factors in an effort to bet- speciation on the relative number of species in two categories ter understand this complexity. Such examples are of one species-level characteristic (see Table 3.1). Relative, not an oversimplification of nature but reveal critical absolute, rates are indicated by the width of the bars on the arrows. insights. Other examples further illustrate the Over a unit of time, the group represented by the white bar on the left of the top panel experiences more cladogenic replication. The interplay of evolutionary processes, showing how group on the right (black bar) experiences more extinction and extinction, speciation, and/or evolution may be pseudoextinction (i.e., loss through anagenesis). After the combined selective for some species characteristics and non- effects of this selectivity (bottom panel), the group of species on selective for others. the left outnumber those on the right. 1.1 Effects on relative abundance of two The top panel of Appendix Figure 3.2.1 repre- groups for one species-level characteristic sents the starting point of a hypothetical set of Species of any group (set, or sample) can be divided species divided into two groups with the same into subsets represented by bars corresponding to number of species in each group. Selective extinc- the portion of the sample they represent (Appendix tion, as shown in this example, removes a greater 1.3, Fowler and Perez 1999). Examples were shown fraction of those on the right than those on the left. in Chapters 1 and 2 where we begin to see first If extinction were acting alone, the result would be approximations of probability distributions of a redistribution of the relative numbers of species practical utility in implementing Management so that the group on the left would be a larger por- Tenet 5 (Chapter 1). Species that can be divided tion of the total; the number of species on the left into two categories for a single species-level char- would change to outnumber those on the right. By acteristic can be treated the same way, as shown in contrast, nonselective extinction (which would be Appendix Figure 3.2.1. The species-level character- shown by arrows of equal width on the right and istic in this illustration is generic but could apply to left of a graph like Appendix Fig. 3.2.1) would not, a specific case (e.g., one group might be species that on the average, change the relative numbers of spe- reproduce sexually and the other asexually, or one cies. Each species would have the same probability might be of high trophic level and another low, on of extinction and the same1 fraction of each group each side of an arbitrary midpoint). of species would, on average, suffer extinction. noam.indb 42 3/9/2009 12:44:55 PM SELECTIVE EXTINCTION AND SPECIATION 43 Extinction, however, is only part of what deter- are reinforced by cladogenic replication. The loss mines relative species numbers. Speciation is also of species in the group on the right is an example of involved. This includes species replication2 as Combination 4 wherein the negative effects of ana- shown by the vertical arrows between the panels of genesis and extinction are reinforced by selective Appendix Figure 3.2.1. The species in the category replication. The replication rate (vertical arrows) of on the left have a higher species-level replication the group on the left is larger than the extinction rate than those on the right. Some speciation may rate for the same group. The extinction rate of the include anagenesis but with insufficient change group on the right is larger than on the left. Finally, to move to a different category. In the absence of the rate of conversion of species from the group on extinction or anagenic interchange, the category the right to that on the left is the larger of the two with the highest replication rate would eventually anagenic exchange rates. Clearly, the number of be represented by the larger (and growing) fraction combinations of potential rates is infinite, as are of the total number of species (which itself would the outcomes, even though the options fall into the be increasing). eight categories of Table 3.1. Species-level dynamics also include anagenesis One set of combined dynamics is noteworthy. If or evolution to change category (pseudoextinction). the extinction rates of two groups of species (as in In Appendix Figure 3.2.1 such a change is illus- Appendix Fig. 3.2.1) are quite different but equal trated by the central diagonal arrows. The num- to the replication rates in each case, the differences ber of species in the category that loses a larger between extinction and replication rates are zero fraction of its (new) species to the other category for each group. Consequently, there would be a lar- exhibits a relative decline
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