Roles of land bridges in global biogeography and ecosystems

Abstract

Land Bridges: Ancient Environments, Plant Migrations, and New World Connections. By Alan Graham. University of Chicago Press, Chicago, IL, USA, 2018. XXIII + 310 ppUS $50.00 (paperback), $150.00 (clothbound). ISBN-10: 022654429X. ISBN-13: 978-0226544298. Available at http://www.press.uchicago.edu. Alan Graham is Curator of Paleobotany & Palynology at the Missouri Botanical Garden, St Louis, MO, USA. He is among the best palaeobotanists in the world (Manos, 2018) and has devoted about 60 years to the study of Late Cretaceous and Cenozoic vegetation history and terrestrial environments, and geological evolution of the New World ecosystems. He has published a number of scientific papers and several books including Floristics and Paleofloristics of Asia and Eastern North America (Graham, 1972), Vegetation and Vegetational History of Northern Latin America (Graham, 1973), Late Cretaceous and Cenozoic History of North American Vegetation (Graham, 1999), Late Cretaceous and Cenozoic History of Latin American Vegetation and Terrestrial Environments (Graham, 2010a), A Natural History of the New World: The Ecology and Evolution of Plants in the Americas (Graham, 2010b) and Academic Tapestries, Fashioning Teachers and Researchers out of Events and Experiences (Graham, 2014). Graham has received a number of awards including the Natural History Museum (London)/Marsh Trust Award for Best Earth Science Book of the Year 2018, Smithsonian Institution Cuatrecasas Medal for Research in Tropical Botany (2018), Festschrift, Paleobotany, and Biogeography for Alan Graham in his 80th Year by the Missouri Botanical Garden (2014), Contributions to Paleobotany, Service, and Outstanding Scholarship by Botanical Society of America Paleobotanical Section (2011), Asa Gray Award by American Society Plant Taxonomists (2009), and Merit Award/Distinguished Scholar by Botanical Society of America (2009). I am glad to see Graham's latest book focusing on land bridges. A land bridge is a physical connection of two otherwise separate landmasses. As stated in the Preface, biogeographically, land bridges have played important roles in shaping the patterns of the diversity and evolution of organisms. These roles include three aspects: (i) when land bridges existed, they allowed animals and plants to cross and colonize new lands, permanently changing the ecosystems into which they immigrate; (ii) after previously existing land bridges disappeared or fragmented, they separated organisms that were previously distributed continuously, causing reproductive isolation and promoting speciation; and (iii) land bridges alter moisture and heat transport at a global scale, impacting on the development of patterns of the diversity and evolution of organisms across geographically remote regions. In the Introduction, Graham reviews the history of the development and the biological consequences of land bridges. He argues that all relevant geological events and processes acting on organisms and communities of organisms collectively determine their movement, diversification and geographical affinities. He states that additional attention must be paid to plants as the conspicuous, conveniently stationary components of the Earth's ecosystems. The main body of the book is divided into three parts. The first part addresses Boreal Land Bridges: the Bering Land Bridge (BLB: West Beringia and East Beringia) and the North Atlantic Land Bridge (NALB). Graham states that from the Pliocene the BLB permitted episodic migrations of organisms between the biotas of boreal lands. In contrast, the NALB extended more or less continuously from Europe to North America until c. 59 Ma, when promulgation of the Mid-Atlantic Ridge reached the North Atlantic, partially disrupting movement of a wide array of organisms––and, thus, ecosystems––into and out of the New World over a long interval of time. The second part of the book is about the Equatorial Land Bridges: the Antillean Land Bridge (ALB) and the Central American Land Bridge (CALB). Graham defines the ALB as a series of discontinuous island stepping stones throughout the Tertiary characterized by having islands close together and land in the Greater and Lesser Antilles present since the latest Eocene and some islands in the Greater Antilles connected at various times. However, he views the CALB as a causeway for organisms migrating between North America and Central America as far south as eastern Panama, disjunctly into South America in the Late Cretaceous and throughout most of the Tertiary, then continuously with establishment of the Central American Land Bridge principally after c. 3.5 Ma (‘The Great American Biotic Interchange’). The third part of the book focuses on the Austral Land Bridge (the Magellan Land Bridge) between the Cono Sur and Antarctica. This land bridge served as a biotic gateway for dispersal across Gondwana throughout the Cretaceous and early Cenozoic. The dispersal was characterized by occurring mostly from west to east and south to north. The aim of the book as expressed in the Preface is “to trace the formation and disruption of New World land bridges and describe the biotic, climatic, and biogeographic consequences.” I found all these goals to have been achieved successfully in the book, as highlighted in the book's endorsements by Paul Manos (Duke University) and Henry Hooghiemstra (University of Amsterdam): it is “a synthesis of vast amounts of information to produce a paleo-perspective on plant diversity through the last hundred million years in a clear and timely, highly original work with great potential to contribute to the biogeographical history of the Americas; a fascinating story made out of such a large bulk of evidence; an impressive, integrated picture of Earth history. In other words, a unique and readable statement of an immensely complex subject.” Another value of the book for specialists is the considerable amount of supplementary, baseline data compiled in tables and appendices presented online at http://press.uchicago.edu/sites/graham. These include representative plants utilizing each of the land bridges through the different periods of geological time. The value of the book is further enhanced for those in related disciplines, students and general readers by the extensive bibliography of approximately 1400 references, and the inclusion under Additional References of related presentations in the public print and visual media. Widely used common/popular names for extant plants are provided in the book's online material. With the advent of molecular data, and particularly the use of molecular clocks when warranted, biogeographers have provided more and more evidence for the roles of land bridges played in global biogeography and ecosystems (Davis et al., 2002; Le Péchon et al., 2016; for reviews, see Givnish and Renner, 2004; Xiang et al., 2015; Wen et al., 2016). As noted in the book, “land bridges are important but they are not the only means of migration, and various plant and animals have other options. These include distributions that are residuals of ancient continental positions, crossings over the land by birds and/or wind, drift through or around the bridges by ocean currents, migration on land by means other than human transport (extant or extinct), more recent distributions by humans and the animals they followed, and combinations of all these.”