Empirical perspectives on species borders: from traditional biogeography to global change

In this paper we will outline several empirical approaches to developing and testing hypotheses about the determinants of species borders. We highlight environmental change as an important opportunity - arguing that these unplanned, large-scale manipulations can be used to study mechanisms which limit species distributions. Our discussion will emphasize three main ideas. First, we review the traditional biogeographic approach. We show how modern analytical and computer techniques have improved this approach and generated important new hypotheses concerning species' range determinants. However, abilities to test those hypotheses continue to be limited. Next we look at how the additions of temporal data, field and lab experimentation, biological details and replication, when applied to systems that have been the subject of classical biogeographic studies, have been used to support or refute hypotheses on range determinants. Such a multi-faceted approach adds rigor, consistency and plausible mechanisms to the study of species ranges, and has been especially fruitful in the study of climate and species' ranges. Lastly, we present an alternative avenue for exploration of range-limiting mechanisms which has been under-utilized. We argue that carefully designed comparisons and contrasts between groups of species or systems provide a powerful tool for examining hypotheses on species' borders. The seasonality hypothesis as an explanation for Rapoport's rule serves as a model of this approach. A test is constructed by comparing patterns of seasonality and range size among marine and terrestrial systems. The seasonality hypothesis is not supported.