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CHAPTER FIFTEEN Linking Niches with Evolutionary Processes As methods for modeling and understanding ecological niches and geographic distributions of species have become increasingly robust and well-understood, evolutionary biologists have begun to pay attention. That is because a critical dimension of the evolutionary biology of species is precisely their ecological requirements, as biogeography, distribution, and genetic variation all hinge rather critically on the ecological niche. Evolutionary studies of ecological niches have thus begun to appear in numbers, amplifying the diversity of challenges to which these techniques have been applied. CHANGES IN THE AVAILABLE ENVIRONMENT Since the envelope of environmental space available to a species [i.e., environments represented within M or η(M)] changes through time, to avoid extinction a species must either track the geographic extent of its scenopoetic existing fundamental niche, or be able to change it via evolutionary responses in physiological or behavioral traits (Holt 1990). One of the important advantages of expressing Grinnellian niches as subsets of an E-space is that the issue of constancy or change of the environmental substrate on which the niches are manifested becomes apparent (Jackson and Overpeck 2000, Ackerly 2003). For example, Jackson and Overpeck (2000) showed changes in what they termed the “Realized Environmental Space,” our η(G) or E available in the study region , measured using two extreme temperatures in a G covering all of North America, from modern times back to 21,000 years before the present (figure 15.1). The actual, existing environmental combinations of a species’ niche will shift in spatial location and extent, and a species must track suitable conditions, adapt to suboptimal ones, or go extinct. The way in which species respond to these challenges is varied. Ackerly (2003) pointed out that the “leading” and “trailing” range edges may pose con- LINKING NICHES WITH EVOLUTION 239 trasting selective pressures during episodes of change. Imagine the retreat of glaciers in the Northern Hemisphere and the associated northward advance of vegetation. Along the northern edge of the range, populations of these plant species would encounter habitats with few competitors but novel environmental conditions; along the trailing edge, however, populations experience alreadyknown environmental conditions and combinations of species already coadapted but with environmental conditions becoming unsuitable (Ackerly 2003, Brown et al. 2003). The selective pressures are bound to be different in these scenarios . An extremely important question is whether populations can adapt quickly to changes in E-space or whether they must geographically “track” sites with the right conditions. When several correlated environmental variables that are Figure 15.1. Illustration of the changing climate conditions in North America at four points in time over the past 21,000 years. The black points in all panels show current-day conditions, while the gray points indicate the conditions at the given earlier period. Redrawn from Jackson and Overpeck (2000). Modern 6,000 yr BP 11,000 yr BP 21,000 yr BP July Temperature (°C) July Temperature (°C) January Temperature (°C) January Temperature (°C) 30 20 10 –10 –20 –30 –40 –50 30 20 10 –10 –20 –30 –30 –20 –10 0 10 20 30 40 –30 –20 –10 0 10 20 30 40 –40 –50 0 0 240 CHAPTER 15 indeed important for the species are considered, tracking them simultaneously may become impossible. Even without extinctions, when the scenopoetic fundamental niches of members of a community of species correlate in different ways with a suite of environmental variables, a change in climate may lead to wholesale rearrangements of species assemblies along gradients (Graham et al. 1996), as illustrated in figure 15.2. The preceding ideas illustrate the importance of understanding how fast species can adapt to environmental changes in G, which can take place over time spans of a few thousand years or even much shorter, over centuries or even decades (Balanyá et al. 2006). This process is that of niche evolution, although niches can evolve for other reasons not related to adaptation (e.g., owing to genetic drift or linkage with other traits under selection). This task would appear to be relatively innocuous: physiological tolerances and habitat associations are clearly features of the evolved phenotype of organisms (Angilletta et al. 2002). However, the concept of niche evolution as a consequence of the evolution of the broader phenotype forms the basis for many key insights from ecological niche modeling, and indeed, since some sort of conservatism in niche features would be required to make possible most of the predictions treated in the last several chapters of this book (Peterson...


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