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CHAPTER FIVE Species’ Occurrence Data Although most of biological diversity is poorly known (Wilson 1988, Erwin 1991), one commonality among species is that something is generally known about where they occur on Earth. That is, an integral part of every scientific description of a species is information about the geographic provenance of the available type specimen material (e.g., for animals, Article 76, ICZN 1999). Therefore, all known species should have at least one geographic occurrence locality available. In many cases, of course, the situation is better than just the type locality, with data from specimens or observations documenting many additional occurrences also available. This book focuses on the process of transforming such primary occurrence data into a synthetic understanding of the geographic and ecological conditions under which a species occurs. As noted previously, an alternative set of methods for characterizing ecological niches takes a mechanistic approach to the challenge (Porter et al. 2002, Natori and Porter 2007). However, our focus is on correlative models based on occurrence data, since such models can take advantage of the near-universal availability of some form of occurrence data, are easily implemented for large numbers of species, and hence can have quite broad applicability. TYPES OF OCCURRENCE DATA Although the concept of occurrence data seems quite straightforward—dots on maps showing places where the species has been found—the details can be much more complicated. Starting with the very basics, occurrence data for use in ecological niche modeling would ideally be drawn from the accessible areas that are environmentally suitable for the species to maintain long-term stable, source populations (Pulliam 2000, Araújo and Guisan 2006)—that is, GO in the discussions in chapter 3. Absence data would ideally also be carefully considered— in this case, these points would come from areas that have been accessible to the species (M), and where the environment is unsuitable for the species to SPECIES’ OCCURRENCE DATA 63 maintain populations, or the area outside of GP (or in some cases the area outside GA), although in practice they will frequently be drawn from the area outside of GO (see chapter 5). With occurrence data and absence information of this nature (i.e., presences from GO and absences from outside of GA but within M and B), niche modeling would be relatively straightforward. All, however, is not as simple as it might seem, or as one might wish, as the numerous considerations of biases and problems in this and succeeding chapters will illustrate. What Makes an Occurrence Record? In reality, a long chain of events connects the suitability of a site to the existence of a data record documenting the species’ presence or absence at that site. Among these factors are the following: 1. The area may be unsuitable, and for that reason the species is not present. 2. The area may be unsuitable, but the species is present (at least occasionally ) owing to dispersal from suitable areas. 3. The area may be suitable, but the species has never been able to disperse to it. 4. The suitable area was at one point occupied by the species, but the species has since been extirpated from the area. (Note that this idea can include temporal variation in presence of a species at a site, which may, in some cases, be important.) 5. The suitable area may be occupied by the species, but no researcher has ever visited the place to sample. 6. The suitable area may be occupied and may have been visited and sampled by researchers, but they did not detect the species. 7. The suitable area may be occupied and may have been visited and sampled , and the species may also have been detected, but the record is not among those available to the researcher. 8. The suitable area may be occupied and may have been visited and sampled , the species detected, and a record is available to the researcher. This set of factors can be considered quite usefully in the form of a probability tree diagram, in which nodes are arranged sequentially to represent factors , and branches denote the possible ways each factor can result (figure 5.1). These factors can be divided into at least two groups: biological factors (i.e., mobility, abiotic suitability, and biotic suitability, as discussed in the context of the BAM diagram in chapter 3) and factors related to exploration, detection, and data (e.g., the species must have been identified correctly). For a presence record to occur at a...


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