Distinguishing range expansions from previously undocumented populations using background data from museum records

Knowledge of the distribution of a species is fundamental to understanding the species' biology, and changes in known distribution may reflect response to environmental change. However, in many cases, purported range expansions are not adequately justified and extra-limital records may simply reflect previously undocumented populations. Thus, it is critical to distinguish between these hypotheses. This study aimed to provide a simple, conceptual framework for evaluating these hypotheses that depends on the use of museum background data to assess the adequacy of historical sampling. American Southwest and worldwide. Background data are records of species other than the taxon of interest that are likely to be documented using the same sampling methods. If evaluation of the background group determines that the taxon of interest would have been documented if present, then the extra-limital record can be considered a range expansion. Conversely, if evaluation of the background group indicates that historical sampling was inadequate to have documented the taxon of interest, then the most parsimonious explanation for the extra-limital record is an undocumented population, which should be presented as a range extension. As an example, I applied the methods to a purported range expansion of the yellow-nosed cotton rat (Sigmodon ochrognathus) in the Rincon Mountains in south-eastern Arizona. Evaluation of the background group revealed that sampling was inadequate to have documented S. ochrognathus in the Rincon Mountains if present. Thus, the record of S. ochrognathus in the Rincon Mountains was interpreted as a previously undocumented population rather than a range expansion. Range expansions can reflect factors that are of important conservation concern, such as climate change or habitat modification. Thus, it is essential that the cause of extra-limital records be accurately interpreted. The method provides a simple means for testing between hypotheses of range extension and range expansion.