| Summary: | Of all the characteristics of biodiversity, the most noteworthy is its variability. Recognition that the significance of the mechanisms underlying this variation changes as the scale of interest is altered, and that variation at one level may cascade up (or down) to affect many others in the ecological and genealogical hierarchies, are hallmarks of modern ecology (Wiens, 1989). For example, it is clear that both local-andregional-scale processes affect the identity and richness of species at any given site (Ricklefs, 1987, 2004; Hawkins & Porter, 2003; Witman et al., 2004; Kreft & Jetz, 2007) and that local–regional interactions can profoundly affect the properties of assemblages (Gaston, 2000; Blackburn&Gaston, 2001a; Leibold et al., 2004; Rangel & Diniz-Filho, 2005; Thomas et al., 2008), even in circumstances where life history characteristics have little influence over the demographic rates of their constituent species (Hubbell, 2001;He, 2005). Likewise, genetic-level variation in primary producers can cascade up through individuals to affect the functioning of whole ecosystems, including feedback loops to plant performance (Treseder & Vitousek 2001; Whitham et al., 2003). For example, genetic variation among pinyon pines in resistance to a stem-boring moth, whose feeding activity on susceptible pines can lead to cone elimination, has effects on seed-feeding birds and mammals, and also on fungi in the decomposer community (Whitham et al., 2003). In consequence, understanding the determinants of biodiversity requires investigation of processes at a variety of spatial and temporal scales and, as a first step, the identification of the patterns which are the reflection, though sometimes beguiling, of these processes (Gaston & Blackburn, 1999). Doing so is essential, not only because of the insight into the natural world that such understanding brings, but also because it is only in this way that appropriate interventions can be recommended to slow the extraordinary impact humansare ...
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