Continents as Units for the Study of Floristic Assembly and Biodiversity: Focus on North America

Abstract

Species are the fundamental units of biodiversity and, consequently, records of species, such as preserved specimens and details of observations, are among the fundamental units for studies on biodiversity (Claridge et al., 1997). However, species and their records do not comprise biodiversity in isolation. Instead, we study them at scales that are inclusive of many species or records, such as across a taxonomic lineage, throughout a geological stratum, or within a geographic area (Willis & Whittaker, 2000). The scale for studying biodiversity has no single fundamental unit, and units are contingent upon the scientific objectives or downstream applications at hand. Therefore, many different scales are highly suitable for a wide range of important questions on biodiversity (Whittaker, 2004; McGill, 2010; Schierenbeck, 2014). Within this special volume, we wish to convey the value of studying plant biodiversity at the continental scale and showcase, as an example, recent work on North America. Each continent on earth is a geographically well-defined landmass and has a distinct geological history. By comparison to many smaller land areas, the geological histories of continents are often relatively well-known. Thus, continents represent natural, cohesive units for biodiversity studies, which may also benefit from clear continental boundaries (see Nagalingum et al., 2002 for an example from Australia). Moreover, the long, shared history of the land comprising each continent facilitates integrative studies of biodiversity that account for evolutionary origins as well as changing patterns of plant distributions from ancient to modern times. There is an acute need for biodiversity studies of all kinds in the Anthropocene, in which humans have increasing, unique capabilities to permanently alter the distributions and abundances of other organisms cohabiting the planet (Crutzen & Stoermer, 2000; Steffen et al., 2007; Smith & Zeder, 2013). However, biodiversity studies of plants remain challenging in regions without road or trail access in both developed and developing nations (Margules & Austin, 1994; Funk & Richardson, 2002; Kadmon et al., 2004; Huettmann & Ickert-Bond, 2017), the latter of which may face additional hurdles, such as funding for exploration, safety for field workers, and so on (Kanyamibwa, 1998; Gómez-Pompa, 2004; Fjeldså et al., 2005; Padilla-Pérez & Gaudin, 2014). Therefore, the most impactful biodiversity studies are those which capitalize on the available data and can be used to extrapolate or interpolate for a clearer understanding of areas that are presently less accessible or are under-surveyed for any reason. We believe that biodiversity studies at the continental scale may prove to be valuable for interpolation at finer scales and that continental-scale studies may stand in to inform conservation and policy decisions until higher resolution studies can be conducted (Whittaker et al., 2001; Henle et al., 2014). North America lends itself particularly well to continental-scale studies on plant biodiversity. This is because, broadly, its flora is relatively well-known and many species records exist in a myriad of forms, especially published floras and checklists, databases of vegetation plots, and physical and digitized specimens maintained and mobilized by herbaria, of which there are c. 700–1000 in the United States, Canada, and Mexico (Thiers; Gray, 1856; Alroy et al., 2004; Peet et al., 2012; Kartesz, 2013; Palmer, 2014; Funk, 2017; USDA, 2017). Such records of species facilitate studies on a vast scale; not just in terms of the continental land area but also in terms of the data records involved. The utility of species may be especially realized in combination with continued traditional surveys of plant biodiversity, which remain of the utmost importance as evidenced by regular discoveries of new species in plain sight (e.g., Wen et al., 2018) and from underexplored areas (e.g., Kyrkjeeide et al., 2018), the ongoing spread of alien species (e.g., Marshall et al., 2012), and changing patterns in distributions of species under climate change (e.g., Gamache & Payette, 2005). While records of species remain at the core of biodiversity science, new discoveries are increasingly made possible using these records in combination with new technologies. New technologies emerging in florsitics research include remote sensing and drones, bioinformatics inferences that guide choices for field surveys, integrative and multidisciplinary studies such as in ecology and taxonomy, automated workflows, and utilizing genomic and environmental information for revealing morphologically cryptic diversity (Palmer et al., 2002; Raxworthy et al., 2007; Reginato, 2016; Wen et al., 2017; Beszteri et al., 2018; Zheng et al., 2018). These and other cutting-edge methodologies are already in wide use in biodiversity studies in North America, and promise to continue to enhance efforts to document diversity of the continent (Loera et al., 2012; Kriebel, 2016). The studies presented in this volume highlight the importance of emerging and traditional methodologies for exploring and documenting plant life on the North American continent. Within North America, as elsewhere, alpha taxonomy and taxonomic revisions continue to comprise the core of biodiversity discovery. Here, Svoboda & Harris (2018) re-evaluated the taxonomy of morphologically challenging species of Passiflora L. from Baja California, and Xiang et al. (2018) clarified infrageneric sections of the fir genus, Abies Mill. While these studies represent classic taxonomy, the study authors also applied modern methods, such as morphometrics using image analysis and detection of environmental differences among species using Global Information Systems (GIS) data in the case of Svoboda & Harris (2018), and adapting classification to reflect robust molecular phylogenies in the case of Xiang et al. (2018). Similarly, Garroutte et al. (2018) coupled a classic study in floristics of the Aleutian Islands with newly-available machine learning models to better understand the drivers of observed patterns in plant diversity across the archipelago, and Rodríguez et al. (2018) used specimens to detect areas of endemism of flowering plants in Mexico on a big-data scale. Moreover, several papers in this volume represent spatial phylogenetic approaches to studying biodiversity. Spatial phylogenetics integrates evolutionary history with ecological information, such as preferences of species for particular abiotic and biotic habitat features. These studies are at the forefront of advancing understanding of the assembly and patterns of biodiversity, because biodiversity has emerged over deep, evolutionary time, but is maintained on ecological time scales of season-to-season or year-to-year (Eriksson, 1993; Harris, 2015). Papers presented include spatial phylogenetic approaches to studying biodiversity within the taxonomically vast Cyperaceae family in North America (Spalink et al., 2018), the extensive pteridophyte biotia of the Pacific Northwest (Link-Pérez & Laffan, 2018), the vascular plant diversity throughout Mexico (Sosa et al., 2018), and the Aleutian Islands (Garroutte et al., 2018). Although continents may be appealing units for the study of plant diversity, they, like species, do not exist in isolation, and global scale processes, such as continental drift, oceanic currents, and wind patterns, have led to many, varied opportunities for floristic elements to become shared across continents (de Candolle, 1855; Darwin, 1859; Wallace, 1876; Wegener, 1912; Chaney, 1936). Although, the eastern North American-eastern Asian disjunction may be one of the earliest-discovered, and best-known cases of an intercontinental flora (Gray, 1846; Li, 1952; Graham, 1966; Wen, 1999, 2001), many other disjunctions exist, such as more broadly between North America and Eurasia, between North America and South America, and between neotropical North America and the Old World tropics (Raven, 1972). In this volume several authors utilized the broader context of continental disjunction as the framework for their studies of plant diversity in North America. Notably, Graham (2018) reviewed historical geological connections via land bridges between North America and adjacent areas and compiled a list of fossil plants that occurred within the landbridge regions at various times in the past. Edwards et al. (2018) used the geographic and environmental ranges of tundra and steppe species to infer ancient climatic conditions of the Bering land bridge, which once facilitated migrations between North America and eastern Asia. In contrast to land bridges, Harris et al. (2018) discussed the roles of long distance dispersal in the assembly of the North American flora and proposed means to explicitly test for dispersal histories of taxa using methods ranging from field work, to population biology, to integrating GIS data on abiotic dispersal vectors such as wind patterns, to radio-tagging potential biological dispersal vectors. Xiang et al. (2018) focused on the disjunct lineage, Abies, and showed that North American Abies is more diverse infragenerically and morphologically than Abies of eastern Asia, which has more species. Each of these studies potentially provides a link between finer resolution studies of diversity within the continent and studies within a wider, global extent. The study of biodiversity at the continental scale is made challenging by their vast geographic extent, which may necessitate extremely broad physiogeographic, floristic and taxonomic knowledge and can make representative sampling daunting, in terms of either digital records or museum or new collections (e.g., Mutke & Barthlott, 2005). Thus, continental-scale biodiversity studies benefit from cooperation among collaborators, who can contribute distinct expertise as well as, when needed, aid in sampling from their own local regions. Many of the papers in this volume represent collaborations across disciplines and across continents, such as informatics and taxonomic experts collaborating on the spatial phylogenetic study of Cyperaceae (Spalink et al., 2018). Moreover, papers by Xiang et al. (2018) and Edwards et al. (2018) illustrate that advancing the continental-scale study of the North American flora is a large undertaking that benefits from the work of scientists globally. International, professional meetings undoubtedly facilitate collaborations on plant biodiversity at large continental scales. Notably, this special volume was conceived as a symposium for the International Botanical Congress, which occurs every six years and took place in Shenzhen, China in 2017 (Crane et al., 2017; Shenzhen Declaration Drafting Committee, 2017). The symposium not only catalyzed the special volume but led to additional collaborative opportunities for participants (e.g., Harris et al., 2018) on the topic of the assembly and biodiversity of North America. The symposium was made possible by generous financial support from the Journal of Systematics and Evolution, to which we are grateful for both contributing to the work presented here and the continued advancement of the study of the North American flora. We expect that the efforts herein will lead to additional discoveries in biodiversity within North America and beyond.