WERE MOUNTAIN PASSES HIGHER IN THE EOCENE?: PRELIMINARY RESULTS OF INSECT BETA DIVERSITY ACROSS THE OKANAGAN HIGHLANDS

Climatic factors and species diversity notably co-vary with latitude in the modern world. However, both mean annual temperature and cold month mean temperature have been shown to have had a much shallower latitudinal gradient in the Eocene, and various paleontological data have also shown much higher Eocene alpha diversity at middle and high latitudes. Here, we examine Eocene beta diversity. Just over four decades ago, Janzen hypothesized a relationship between modern beta diversity, topography, climate, and latitude in the evocatively titled paper: Why mountain passes are higher in the tropics (1967). He proposed that while warm valleys and cool mountain passes in seasonal temperate regions would have a temperature overlap at least part of the year, allowing dispersal of animals adapted to valley climates, the same elevation difference in the equable tropics would share no common temperatures over a year, and would then constitute a physiological dispersal barrier resulting in increased beta diversity. By this model, uplands in the globally less seasonal Eocene should have had high beta diversity even in higher latitudes. We test this with fossil insect beta diversity between localities across a thousand kilometer transect of the Early Eocene Okanagan Highlands of southern British Columbia, Canada and northern Washington, USA, and present our preliminary results.