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Modeling herpetile range distributions from species occurrences and landscape variables.
Henebry, Geoffrey1, Putz, Brian1, Merchant, James1, 1
ABSTRACT- In order to provide a transparent and durable modeling framework for the range distributions of vertebrate species, the Nebraska Gap Analysis Project has used recursive partitioning to develop "objective" semi-empirical models. Recursive partitioning algorithms predict membership of individual cases in classes of a categorical dependent variable from measurements of one or several independent variables. The motivation for using this strategy is two-fold: the resulting trees of decision points and values that form the models are readily understandable, debatable, and tunable; and the non-parametric modeling handles the multimodality common to regional species occurrence data. Although the best-known recursive partitioning algorithm is CART (Classification and Regression Trees), we have used QUEST (Quick, Unbiased, and Efficient Statistical Trees), a recent improvement on CART which greatly speeds up searching of the data space and which is more robust in the face of categorical variables with many levels. State Museum voucher specimens collected in Nebraska since 1969 served for the occurrence data. Explanatory factors included land cover class/vegetation alliance composition, surficial soils characteristics, climatic means, variance, and extremes, and terrain data. A main result of the modeling effort was the importance of interannual climatic variability in establishing range limits for these ectotherms. The poster illustrates the modeling procedure, provides the model trees and resulting range distributions for several representative amphibian and reptiles species, and discusses the weaknesses and strengths of the framework.
KEY WORDS: GAP, amphibians, reptiles, range distributions