A vision for historical biogeography

Abstract

Ecological similarity generally decreases with distance, but exceptions to this pattern command the attention of biologists and natural historians. Prior to the development of plate tectonic theory, some biogeographers postulated the existence of lost land bridges that linked distant regions to explain surprising patterns of similarity between distant communities. Classic examples include plant groups such as hickory (Carya), mayapple (Podophyllum), and ginseng (Panax) that are found in the deciduous forests of Eastern Asia and eastern North America. Fossils of extinct species that have been found in unexpected places also signal an ancient and complex history of life on Earth. Dramatic examples include the discovery of Jurassic monkey puzzle trees (Araucaria) in Utah, Eocene bananas (Ensete, Musaceae) in Oregon, and Pleistocene camels (Camelops) in Alaska, none of which are native to North America. Although our understanding of the interaction between climate change and plate tectonics in explaining species distributions has improved, the imagery of the land bridge persists. Making sense of the role of geologic history, climate change, vicariance, and long-distance dispersal in shaping species distributions and community composition over geologic time is the domain of historical biogeography. The role of terrestrial connections between continents in shaping American biotas is the focus of Alan Graham’s latest book Land Bridges: Ancient Environments, Plant Migrations, and New World Connections. Graham is Curator of Paleobotany and Palynology at the Missouri Botanical Garden; he has made many contributions to our understanding of the terrestrial environments of the Americas and their history through his studies of the plant fossil record. In addition to having written numerous journal articles, he has compiled much of the available information on plant fossils into a series of three books that serve as indispensable references. In Land Bridges, he focuses on the connections between the Americas and other continents. Readers are introduced to the geology, physiography, climate, vegetation, and cultural history of each of the five regions of connection, setting up discussion of the latest research into the assembly of the American biotas we know today. The tour begins with the boreal connections. Graham argues that migration between North America and Asia has been possible for many organisms intermittently over the last 100 million years, and this argument is supported by the similarity of fossils that have been found on either side of the Bering Strait and across both continents. Although the dataset is admittedly littered with outdated identifications, he argues that the general pattern is nonetheless evident. Rates of migration and the identity of the migrants have depended on the interaction between global climate and sea level. By contrast, exchange between Europe and eastern North America has weakened over the last 100 million years as the distances between major land masses have increased. Exchange was common prior to the opening of the North Atlantic because the continents were continuous, but since then exchange has only occurred via high-latitude island hopping and long-distance dispersal; thus the analogy of the land bridge is rather tenuous. The next stops on the tour are in the American tropics: the Antilles and then the Isthmus of Panama. Students of biogeography will no doubt already be familiar with the Great American Biotic Interchange. With the recent expansion of the Panama Canal, research in the region has surged and Graham brings the latest findings to his audience by discussing some of the newest discoveries and ongoing debates. The modern and fossil biotas of the Greater Antilles include some animals unlikely to survive oceanic dispersal. Some researchers have argued that these occurrences might be explained by the formation of a short-lived continuous landspan from northern South America to the Greater Antilles via the Aves Ridge about 34 million years ago. Graham reviews the debate and summarizes the relevant paleobotanical evidence, concluding that, although the plant fossils do not show the similarity among the Greater Antillean paleofloras that one would expect under this model, any additional discoveries in the region might greatly improve our ability to evaluate this hypothesis. Graham also shares some of the exciting recent discoveries of plant and animal fossils of South American affinity from early Miocene deposits that were north of what was once the Central American Seaway. The Great American Biotic Interchange of ∼3.5 million years ago was preceded by a complex tropical biotic interchange. The ongoing question is whether these ‘early arrivals’ reached North America via long-distance dispersal or whether there were short-lived Miocene land connections to aid them. Graham provides an up-to-date and evenhanded discussion of the alternative hypotheses from both biological and geological perspectives. Finally, readers are transported to the southern connection between Patagonia and Antarctica. Here, as with the North Atlantic, the land bridge imagery is stretched to its limit. Fossil evidence demonstrates that interchanges involving South America go back 700 million years to the formation of Gondwana. As Gondwana fragmented, interchange persisted until South America’s final link to Antarctica was severed during the Late Cretaceous to early Cenozoic. While some vascular plants persisted on the Antarctic Peninsula after the Oligocene, there was little potential for the so-called Magellan Land Bridge to play a role in the exchange of South American and Antarctic floras. Cooling and drying across much of Patagonia and the establishment of the Antarctic Circumpolar Current caused the range of many plant and animal species to contract northward. Graham’s discussions combine a command of the literature with personal experience. Stories of international scientific collaborations and pictures personalize what might otherwise be straightforward literature review and synthesis. Some outdated perspectives persist in the writing. For example, the idea that there were ‘Paleozoic residues’ in Early Cretaceous communities confounds morphological similarity with continuity of species associations over 150 million years or more. The title of the book is another example, but it is easy to see how this language makes for compelling imagery. Much of the value of this work lies in introducing readers to ongoing scientific debates in topics such as geology, archaeology, and island biogeography. By bringing these topics together in a single book, Graham emphasizes the interdisciplinary nature of historical biogeography. Interested readers will find some of these discussions helpful, but they will also notice minor errors. For example, the dates of the Early Eocene Climatic Optimum and the Paleocene–Eocene Thermal Maximum are confounded in some places. Although the author includes very recent discoveries, he also occasionally fails to include some revisions to outdated identifications. Geographers will have to forgive the occasional map in Mercator projection in the chapters on high-latitude connections. Most importantly, throughout this ambitious project, Graham emphasizes how high-quality information from the fossil record can be combined with modern methods of molecular phylogenetics to provide an elegant and often surprising history of life on Earth. Fossils provide direct evidence of ancient distributions and morphologies that are largely (if not entirely) hidden by extinction from molecular phylogenetic studies. By contrast, studies of extant species provide a wealth of character data, information about physiological requirements, and detailed spatial information. Graham thoughtfully argues for an integration of molecular phylogenetics, paleontology, climatology, and macroecology. He dreams of a large coordinated and collaborative effort by paleobotanists, plant taxonomists, and molecular systematists to add value to the world’s natural history collections by updating identifications and digitizing collections simultaneously. Rapid growth in the availability of sequence data and development of new analytical methods makes the pace of paleontological research feel steady by comparison. Fossil-based attempts at biogeographic analyses are sometimes criticized for being limited by a fundamentally incomplete record that provides only minimum age constraints. On the other hand, molecular-based attempts to interpret historical biogeography tend to rest on the similarity of results that have been obtained using different genes or models of sequence change, despite the fact that the modern biota is also an incomplete sample of information on species diversity and distributions because of range shifts and extinction. As Graham makes clear in this book, the problem with the fossil record is not that it is incomplete but that it is desperately undersampled. The limiting factor is not an incomplete fossil record but the pace of field work, collections work, and lab research in paleontology. Many surprising discoveries in the field and in existing museum collections simply await the attention of investigators. This book is intended for a wide audience of interested biologists. It is worth reading for the perspective of a paleobotanist with decades of experience who conducted pioneering work in tropical paleobotany and created large and widely used reference collections. Readers will also gain insight into how climate and biotas have changed in the past, and this is important for making sense of the changes we all face ahead.