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CHAPTER ELEVEN Discovering Biodiversity Ecological niche models may be exciting principally because they provide a predictive basis for novel inferences about biodiversity and its distribution in space, time, and environment. One way in which this predictive understanding can be put to good use is that of anticipating distributions of new elements of biodiversity (populations and species) that are not as-yet known or documented . Conceptually, the idea is quite straightforward, and a few initial applications have been developed; however, this application of niche models begs further exploration. If the initial promise continues to translate into further success , this application may rank among the most interesting uses to which these tools can be applied, enabling further discovery and documentation of biodiversity . In this chapter, we describe the conceptual basis of using ecological niche modeling for discovering new elements of biodiversity, and review applications of this approach undertaken to date. We then outline both limitations and frontiers for this field. Generally, current knowledge of the diversity and distribution of biological species on Earth is remarkably poor, with the great majority of species yet to be described and catalogued scientifically. This problem has two key elements , which may be termed the “Linnaean” and “Wallacean” shortfalls (Whittaker et al. 2005). The “Linnaean Shortfall” refers to lack of knowledge of how many and what kind of species exist—the term is a reference to Carl Linnaeus (1707–1778), who laid the foundations of modern taxonomy in the eighteenth century. The “Wallacean Shortfall” refers to inadequate and incomplete knowledge of geographic distributions of species, referring to Alfred Russel Wallace (1823–1913), who, as well as contributing to the early development of evolutionary theory, studied geographic distributions of species long before it became popular. Ecological niche modeling offers a powerful tool with which to address both of these shortfalls. 190 CHAPTER 11 DISCOVERING POPULATIONS The idea of using ecological niche models to guide searches for and discovery of unknown populations of species is perhaps the simplest of the “discovering biodiversity” applications. Given that few or no species have been sampled exhaustively, one of the original motivations behind the development of niche modeling tools was exactly this: filling in gaps among known occurrence sites by means of interpolation that is informed by environmental conditions. Hence, discovering as-yet undocumented populations that are disjunct from known populations (i.e., more complete documentation of GO) is another important functionality of ecological niche models. This approach takes advantage of “type two” model predictions illustrated in figure 10.1: the model identifies areas environmentally similar to sites where the species has already been found, but from which no occurrence records are available. New surveys targeting these areas should have increased chances of discovering unknown populations, in comparison with unguided, randomly placed surveys. The idea of niche suitability providing an indication of presence of unsampled populations links to questions of inventory completeness that have been the focus of a suite of studies (e.g., Moreno and Halffter 2000). As biodiversity knowledge accumulates for a particular site, existing knowledge can be used to anticipate how many additional species remain to be detected there, and consequently how complete the inventory is at any point (Colwell and Coddington 1994, Soberón et al. 2007). Niche models can provide an independent source of information on the question of how many additional species remain to be detected at a site, by superimposing individual niche models for each of multiple species. Ideally, such analyses should be conducted after processing distributional predictions to consider the limitations of M and B and, as a consequence, estimate GO more closely (see chapter 8). Several analyses have tested the predictive nature of niche projections for anticipating community composition with some degree of success (e.g., Feria and Peterson 2002, Graham and Hijmans 2006). Conversely, the existence of barriers and interruptions in landscape suitability , combined with absence of a particular species from isolated suitable regions , can be a means of discovering dispersal barriers. When a species could be present (i.e., conditions are suitable) but is not (and this absence is demonstrated via tests of sampling adequacy, as discussed in chapter 8; Anderson 2003), we may interpret the situation as one of dispersal limitation (i.e., site is within GP, but not GO; in other words, it is in GI) or biotic limitation. A few explorations of these ideas have now been developed (Anderson et al. 2002a, DISCOVERING BIODIVERSITY 191 Kambhampati and Peterson 2007), but these ideas will be treated in greater detail in...


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