Natural and anthropogenic drivers of the broad-scale geographic distributions of species

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The most fundamental -- and practical -- questions about ecological systems are : what determines how many organisms there are, and where they are found? Complete answers to these questions would be impossibly complex. Useful preliminary answers would involve two parts: (1) predictive models that statistically relate the observed variance in nature of how many organisms there are, and where they are, to characteristics of organisms' habitat; coupled with (2) simple, process-based models that predict those statistical patterns, as well as other aspects of their biology and geography. The present study has two main goals. The first is to develop a simple, predictive model of the continental biogeography of species, beginning with trees and terrestrial vertebrates. I hypothesize that, for any given species, the probability of occupying a given geographic region, the probability of going locally extinct, and the probability of dispersing to neighboring regions, are all simple functions of climate, and the function form is largely constant among species. Summed over all species in a clade, these processes generate highly predictable patterns of species richness, species turnover through space, and species range sizes. The model should be able to predict the continental biogeography of species assemblages, and how they will react to climate change. The second goal is to develop models to predict how many, and which, species will persist in regional landscapes, in the face of human activity. It is generally believed that habitat destruction is the greatest cause of species loss. However, many species persist in landscapes with a mixture of natural land and human activity. Human activities will continue to convert natural habitat to human purposes. I propose to attempt to determine which habitat characteristics account for the majority of the variance in the persistence of species (beginning with birds and amphibians). I hope to be able to identify mixtures of natural- human-dominated landscapes that maximize the persistence of species.