Chapter 11 Modeling Species Distribution with GIS From the variety of checklists, atlases, and field guides available around the world it is easy to understand that distribution ranges are pieces of information that are seldom absent in a comprehensive description of species. | Chapter 11 Modeling Species Distribution with GIS Fabio Corsi Jan de Leeuw and Andrew Skidmore From the variety of checklists atlases and field guides available around the world it is easy to understand that distribution ranges are pieces of information that are seldom absent in a comprehensive description of species. Their uses range from a better understanding of the species biology to simple inventory assessment of a geographic region to the definition of specific management actions. In the latter case knowledge of the area in which a species occurs is fundamental for the implementation of adequate conservation strategies. Conservation is concerned mostly with fragmentation or reduction of the distribution as an indication of population viability Maurer 1994 given that for any species range dimension is considered to be correlated to population size Gaston 1994 Mace 1994 . Unfortunately animals move and this poses problems in mapping their occurrence. Traditional methods used to store information on species distributions are generally poor Stoms and Estes 1993 . Distributions have been described by drawing polygons on a map the blotch to represent with varying approximations a species ranges Gaston 1991 Miller 1994 . The accuracy of the polygons relies on the empirical knowledge of specialists and encloses the area in which the species is considered likely to occur although the probability level associated with this likelihood is seldom specified. A more sophisticated approach divides the study area into subunits . administrative units equal-size mesh grid with each subunit associated with information on the presence or absence of the species. In this case the distribution range of a species is defined by the total of all subunits in which presence is confirmed however blank areas are ambiguous as to whether the species is absent or no records were available Scott et al. 1993 . 390 CORSI de LEEUW AND SKIDMORE New approaches tend to overcome the concept of .
