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CHAPTER THIRTEEN Species’ Invasions Invasive species are a global phenomenon with massive consequences, both in biological and economic realms (Williamson 1996, NAS 2002). In human economic arenas, invasive species affect agricultural productivity, transportation systems, communication systems, disease transmission, recreational fishing, hunting, and birdwatching, and many other dimensions (Pimentel et al. 2004), with economic costs that mount into the billions of U.S. dollars annually. Indeed , a recent calculation was that the annual cost of invasive species in the United States alone reaches $120 billion annually (Pimentel et al. 2004). In natural systems, invasive species can be transformational, affecting not only ecosystem services (Zavaleta et al. 2001) but also endangering or extirpating native species (Chapin et al. 2000, Clavero and García-Berthou 2005).As such, invasive species—and the population-level and biogeographic processes that lead to invasions—are of considerable interest and importance. In the field of ecological niche modeling, invasive species are of particular interest. This application makes use of the “type three” predictions illustrated in figure 10.1: areas are identified that are currently unoccupied by the species but are likely to be susceptible to invasion if limitations on dispersal are removed. Niche modeling has been applied most commonly at broad spatial extents (Sober ón 2007)—as such, it is difficult to test with experimental manipulations, due to the considerable effort required to perform ecological experiments at biogeographic extents. The broadening degree of human movements and activities , however, provides what is effectively a series of experiments—what happens to a species’ geographic distribution when its dispersal capabilities (M in the BAM diagram) are expanded? ENM and associated theory encapsulated in the BAM diagram yield clear, quantitative predictions that can be tested by means of such unplanned experiments, so species’ invasions offer a fascinating arena in which to explore the processes underlying geographic ecology. In effect, both the practical (applied) interest and the theoretical (conceptual ) interest in species’ invasions distill down to the same question: to what degree can the geographic course of species’ invasions be anticipated based on scenopoetic variables and biotic interactions? As will become clear in this 216 CHAPTER 13 chapter, the answer is neither simple nor straightforward, but the effort has certainly been informative and educational. CONNECTION TO THEORY Several points from chapters 2 and 3 earlier in this book are central to the invasive species question. First, species do not generally occupy the entire spatial footprint of their potential distributions (GP), but rather are limited to some subset of that potential by historical barriers to dispersal (related to M) and potentially by biotic limitations (related to B). This subset is GO, and we expect that GO ⊆ GP. Indeed, when the spatial extent of the analysis (G) is broad, the expectation is that GO will be much smaller than GP, because species’ distributions are highly constrained by dispersal limitations, especially at continental or global extents. A key insight is that understanding species’ invasions in a niche modeling context may require inclusion of the role of biotic interactions in shaping species ’ distributions. That is, B in the BAM diagram plays a critical role in determining whether a model based on a species’ distribution and ecology in one region will be able to anticipate its distribution in a novel region (e.g., a different continent). If B is so broad as essentially not to limit the species’ distribution (the Eltonian Noise Hypothesis), or if the environmental structure of B (i.e., the community context) is comparable between regions, then a niche model calibrated in one region should be predictive of its distribution and ecology in another. This assertion is of course subject to the conditions that the models are well-calibrated and not substantially biased, and that the genetically determined elements of the scenopoetic existing fundamental niche EA of the species do not differ between the native-range population and the invasive populations (meaning that scenopoetic niches are conserved). The fact that many niche models have shown excellent predictivity regarding the geographic potential of species’ invasions suggests that, at biogeographic extents and coarse spatial resolutions, B does not frequently present a strong constraint on species’ distributions . On the other hand, the massive and sometimes pervasive effects that some invasive species have on elements of native biotas attest to the opposite. PRACTICAL CONSIDERATIONS The basic approach to understanding species’ invasions using ecological niche modeling is simple. The idea is to use occurrence records of the species in one SPECIES’ INVASIONS 217 region to calibrate models, and to...


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