BIOGEOGRAPHY, OVERVIEW Mark V. Lomolino Oklahoma Biological Survey, Oklahoma Natural Heritage Inventory, and University of Oklahoma I. Introduction BIOGEOGRAPHY HAS A LONG AND DISTIN- II. Biogeography in the Twentieth Century GUISHED HISTORY, and one inextricably woven into III. Biogeography and the Conservation of the historical development of evolutionary biology and Biodiversity ecology. Modern biogeography now includes an impres- sive diversity of patterns, each of which dealing with some aspect of the spatial variation of nature. Given this, few disciplines can be any more relevant to under- GLOSSARY standing and conserving biological diversity than bioge- ography. biogeography Study of the geographic variation of na- ture, including variation in any biological character- istics (e.g., body size, population density, or species richness) on a geographic scale. continental drift Model first proposed by Alfred Weg- I. INTRODUCTION ener that states that the continents were once united and then were displaced over the surface of the Traditionally, biogeography has been defined as the globe. study of patterns in distributions of geographic ranges plate tectonics Study of the origin, movement, and de- (Brown and Gibson, 1983). During the past three de- struction of the plates and how these processes have cades, however, this field has experienced a great surge been involved in the evolution of Earth’s crust. in development and sophistication, and with this devel- Pleistocene Geologic period from 2 million to 10,000 opment the scope of the field has broadened to include years before the present, which was characterized an impressive diversity of patterns. Simply put, modern by alternating periods of glaciation events and biogeographers now study nearly all aspects of the ‘‘ge- global warming. ography of nature.’’ Biogeography now includes studies species composition Types of species that constitute of variation in any biological feature (genetic, morpho- a given community or sample. logical, behavioral, physiological, demographic, or eco- species richness Number of species in a given commu- logical) across geographic dimensions such as distance nity or sample. among sites or along gradients of area, elevation, or depth (see Brown and Lomolino, 1998). Encyclopedia of Biodiversity, Volume 1 Copyright 2001 by Academic Press. All rights of reproduction in any form reserved. 455 456 BIOGEOGRAPHY, OVERVIEW A. Fundamentals of Biogeography selective pressures vary across space, and that all life- forms and their distributions are the product of natu- Despite the sometimes overwhelming complexity of the ral selection. natural world, all biogeographic patterns ultimately de- The early explorers and naturalists did far more than rive from two very general features of nature. First, just label and catalog their specimens. They soon, per- as we move along any dimension of the geographic haps irresistibly took to the task of comparing their template, environmental conditions tend to vary in a collections across regions, elevations, and other gradi- predictable manner. For example, more distant sites ents of the geographic template. At the same time, they tend to be more dissimilar than adjacent sites, environ- began to develop explanations for the similarities and ments at higher elevations tend to be cooler and wetter differences among the biotas they studied. In fact, most than those at lower elevations, and larger areas tend to of the persistent themes of the field of biogeography capture more solar energy and a greater diversity of (Table I) were well established during the eighteenth environmental conditions than smaller areas. Second, and nineteenth centuries. To be sure, biogeography has all forms of life differ in their abilities to adapt to geo- made great strides during the twentieth century to be- graphic variation in their environment. These differ- come a mature and sophisticated science. It is important ences, while including a great diversity of responses to acknowledge, however, that we owe a great deal to (e.g., physiological, behavioral, developmental, and the many visionary explorers and naturalists who evolutionary), ultimately influence the three fundamen- shared our fascination and asked the same questions tal processes of biogeography: immigration, extinction, about the geographic variation of nature. and evolution. All the biogeographic patterns we study derive from nonrandom variation in these processes 1. Historic Explorations of the across geographic gradients and across individuals, Eighteenth Century populations, and species. While motivated to a large degree by the quest for money and power, the Age of European Exploration B. Early History of the Field was also fueled by the call to serve God. It was widely believed that the Creator had placed on this earth a Biogeography has a long and distinguished history, and still unknown diversity of organisms—a divine zoo or one inextricably woven into the historical development garden of life. Accordingly, early European explorers of evolutionary biology and ecology. The historical de- believed that there was perhaps no greater way to wor- velopment of biogeography had its origins coincident ship God than to unlock the mysteries of creation. with the Age of World Explorations by Europeans dur- Yet with each new account of some distant biota came ing the eighteenth and nineteenth centuries. Yet the information that challenged the prevailing views of cre- study of geography of nature must be an ancient one. ation. Eventually, the growing body of knowledge The European explorers were not the first to ask ‘‘Where would overturn the long accepted view that the earth, did life come from, and how did it diversify and spread its climate, and its species were immutable, unchanging across the earth?’’ Aristotle asked these same questions, in both space and time. More immediately, however, as did many others before and after him, when faced biologists of the eighteenth century were struck by the with accounts of strange forms of life from foreign lands. The development of biogeography into a mature and respected field of science, however, required a much better understanding of variation in what we now call TABLE I the geographic template and the associated variation in Persistent Themes of Biogeographya the natural world. It is by no minor coincidence that both evolutionary biology and biogeography developed 1. Comparing and classifying geographic regions based on their biotas. in earnest during the Age of Exploration. Prior to this 2. Reconstructing the historical development of biotas, including time, biologists had ‘‘discovered’’ and described less their origin, spread, and diversification. than 1% of plant and animal forms that we know today. 3. Explaining the differences in numbers as well as types of spe- Each new voyage or expedition added to the accumu- cies among geographic areas. lated information on the earth’s environments and life- 4. Explaining geographic variation in the characteristics of individ- uals and populations of closely related species, including trends forms, and would eventually provide the raw material in morphology, behavior, and demography. for the disciplines of evolution and biogeography. These disciplines are interconnected by the knowledge that a After Brown and Lomolino (1998). BIOGEOGRAPHY, OVERVIEW 457 astounding diversity of species. Such diversity pre- Like Linnaeus, Buffon also concluded that there was sented them with two serious problems, one practical one ‘‘landing point,’’ one site where all animals origi- and the other conceptual. First, biologists urgently nated. However, this site, or region, was located far to needed a systematic and generally accepted scheme for the north of Mount Ararat, somewhere in the Arctic classifying the burgeoning wealth of specimens, one Circle where the early animals and their descendants that would reflect the similarities and differences among could gain ready access to both the Old and New the species. Second, it quickly became clear that there Worlds. This is where these life-forms survived the were just too many species to be carried by Noah’s Ark. Flood during some earlier period when the earth’s cli- How could all the forms of life, now adapted to many mate was much warmer, warm enough such that tropi- distant and distinct regions across the globe, have origi- cal environments could extend far poleward. Once the nated and then spread from that one landing point? floods receded, animals spread southward into the con- Carolus Linnaeus (1707–1778), certainly one of the tinents and began to diverge in form as they became most prominent biologists of all time, took on both of increasingly isolated on different landmasses. these challenges. In fact, his system of binomial nomen- Other biologists of the eighteenth century, including clature is the system we continue to use today to classify Joseph Banks and Johan Reinhold Forster, both of organisms. Linnaeus also attempted to rectify the Bibli- whom served as naturalist on voyages of Captain James cal doctrine with what he and his contemporaries knew Cook, were quick to confirm the generality of Buffon’s about the diversity and geography of nature. This was Law: environmentally similar but isolated regions have especially challenging because, like most of his col- distinct assemblages of plants and animals. Forster also leagues, Linnaeus was sure that species were immuta- discussed the relationship between regional floras and ble. Given this, how could species