Rarity in Astragalus: a California Perspective Philip W
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Aliso: A Journal of Systematic and Evolutionary Botany Volume 33 | Issue 2 Article 4 2015 Rarity in Astragalus: a California Perspective Philip W. Rundel University of California, Los Angeles Thomas R. Huggins University of California, Los Angeles Barry A. Prigge University of California, Los Angeles M. Rasoul Sharifi University of California, Los Angeles Follow this and additional works at: http://scholarship.claremont.edu/aliso Part of the Botany Commons, and the Ecology and Evolutionary Biology Commons Recommended Citation Rundel, Philip W.; Huggins, Thomas R.; Prigge, Barry A.; and Rasoul Sharifi, M. (2015) "Rarity in Astragalus: a California Perspective," Aliso: A Journal of Systematic and Evolutionary Botany: Vol. 33: Iss. 2, Article 4. Available at: http://scholarship.claremont.edu/aliso/vol33/iss2/4 Aliso, 33(2), pp. 111–120 ISSN 0065-6275 (print), 2327-2929 (online) RARITY IN ASTRAGALUS: A CALIFORNIA PERSPECTIVE PHILIP W. RUNDEL,1,2 THOMAS R. HUGGINS,1 BARRY A. PRIGGE,1 AND M. RASOUL SHARIFI1 1Department of Ecology and Evolutionary Biology, University of California, Los Angeles, California 90095 2Corresponding author ([email protected]) ABSTRACT Astragalus (Fabaceae), the largest genus of plants in the world with an estimated 3270 species, is known for large numbers of rare endemic species. An inventory of patterns of climatic, topographic, and edaphic diversity of Astragalus taxa in California (98 native species and 144 named taxa) provides a means to understand the occurrence of rarity in relation to climatic equitability and regional species richness of congeneric taxa. Most taxa in the genus have relatively small ranges of distribution, with 50% restricted geographically to a single Jepson Bioregion. The California Native Plant Society lists 51 Astragalus taxa (35% of the native Astragalus taxa) as rare, threatened, or endangered (RTE). Climate characteristics of geographic regions such as rainfall and temperature extremes show no obvious relationship to species richness or the proportion of listed taxa. Species richness is highest in the arid Great Basin (35 species and 53 taxa) combining both its components, followed by 29 species and 39 taxa in the Sierra Nevada East region that includes the White and Inyo Mountains. The Mojave Desert is also high in diversity with 32 species and 39 taxa, but in contrast the Sonoran Desert region is low with only 12 species and 14 taxa. Despite ranking highest in the number of Astragalus taxa present, the Great Basin regions are low in their proportion of RTE taxa (17%) compared to the South Coast Region (39.5%) and Mojave Desert (32%). Strong edaphic specialization is associated with the majority but not all RTE taxa. While no single ecophysiological adaptation can explain this pattern, it is significant that Astragalus taxa have the potential ability to develop symbiotic nitrogen fixation, and this trait is key to success in soils not conducive to growth of many potential competitors. Land use changes, alien grass invasion and grazing, among other threats, are increasing fragmentation of habitats for many rare taxa with consequent impacts on gene flow. The continued survival of rare and locally endemic taxa will require improved knowledge of their individual demographic traits and long-term population dynamics. Key words: Astragalus, California biodiversity, edaphic specialization, nitrogen fixation, rarity. INTRODUCTION life history traits or membership in rapidly speciating lineages, are positively associated with the persistence An understanding of rare plants and their relation to specific of naturally rare species (Harrison et al. 2008). Issues of habitats of occurrence form a fundamental component of conservation management of rare species are particularly biodiversity management. Although considerable effort has critical in California, a global hotspot of biodiversity gone into cataloguing rare taxa using a variety of criteria, (Norman 2003), where the large flora contains a high we nevertheless lack the unifying principles and underlying proportion of endangered, threatened, and rare species. critical data for the practical management of rare taxa The legume genus Astragalus L. with an estimated 3270 and an understanding of the traits that separate them from species of largely herbaceous perennials is the largest genus common taxa (Kunin and Gaston 1997; Me´dail and of vascular plant in the world (Frodin 2004) and presents classic Verlaque 1997; Bevill and Louda 1999; Domı´nguez Lozano examples of rarity. The distribution of Astragalus is widespread and Schwartz 2005). While there is a large body of published in semi-arid, arid, and cool temperate regions of the northern literature describing individual rare taxa, there is far less hemisphere, with very high species diversity in a band extending research on the nature of microhabitats and microclimates across the mountain steppes of the eastern Mediterranean Basin that support rare local endemics. The high frequency of across south-central Asia to the western Himalayan Plateau, rarity in regional floras leads to the question of how these where there are estimated to be about 1500–2000 species. naturally rare species have persisted through evolutionary time and avoided extinction in spite of what are often three- Examples of this extreme diversity can be seen in Afghanistan way problems of small geographic ranges, low population with 380 species (Breckle and Rafiqpoor 2010), Turkey with numbers, and specific habitat conditions (Rabinowitz 1981). 391 species (Gu¨nar et al. 2001), China with 401 species (Xu et al. Recent studies have suggested that benign climates with 2010), and Iran with 451 species (Mahmoodi et al. 2009). spatially extensive specialized habitats, rather than specific However, western North America is a secondary center of evolution of Astragalus with about 400–450 species (Barneby 1964). There is a small disjunct species complex in tropical East Africa (Gillett 1964). South America is home to about ’ 2015, The Author(s), CC-BY. This open access article is 100 species, with their diversity centered in the southern and distributed under a Creative Commons Attribution License, which allows unrestricted use, distribution, and reproduction in any medium, central Andean region. These South American species represent provided that the original author(s) and source are credited. Articles two distinct clades that arrived independently by long-distance can be downloaded at http://scholarship.claremont.edu/aliso/. dispersal from North America (Scherson et al. 2008). 112 Rundel, Huggins, Prigge, and Sharifi ALISO The geographic center of diversity and presumed origin of of several scales of rarity, including those taxa listed either Astragalus, as with most of its close relatives in the tribe federally or by the state of California as Rare, Threatened, or Galegeae Dumort., lies in arid and semi-arid regions of south- Endangered (RTE). The database also includes a Rare Plant central Asia (Polhill 1981). The Galegeae clade is entirely an Rank, with a designation of 1B signifying taxa that are rare, Old World group with the exception of the genera Astragalus threatened, or endangered (RTE) in California and elsewhere. and sister group Oxytropis DC. which also became established Plants listed in category 1B are further divided into three in North America. Although species-rich in northern temperate threat ranks. Plants constituting California Rare Plant Rank latitudes, Astragalus has relatively few arctic species, a trait 1B meet the definitions of California Endangered Species Act that it shares with other large temperate genera such as and are eligible for state listing. It is mandatory that they be Carex L., Senecio L., and Vaccinium L. (Hoffmann and fully considered during preparation of environmental docu- Ro¨ser 2009). ments relating to the California Environmental Quality Act One of the characteristic biogeographic features of Astra- (CEQA). galus over its range of occurrence is a remarkable level of Available information on RTE species was gathered from edaphic specialization, often leading to highly restricted our own field experience, the published literature, gray geographic ranges of distribution and rarity. This population literature in government reports including Federal Register trait is not only characteristic of the Irano-Turkish region and publications, and the California Native Plant Society’s online Himalayan Plateau in floras from those regions, but is also Inventory of Rare and Endangered Plants (CNPS 2015). This expressed in the Astragalus flora of the western United States. inventory includes considerable information on habitat, geo- The Great Basin and Mojave Desert are notable for diverse graphic and elevational distribution, population numbers, and examples of local endemism in Astragalus associated with threats, as available. edaphic specialization. These geographic patterns of isolated populations coupled with limited dispersal ability of seeds have RESULTS been hypothesized to promote rapid local differentiation and geographic speciation over the Quaternary (Sanderson and Biogeography and Diversity of California Astragalus Wojciechowski 1996). The California flora includes 98 native Astragalus species with Our objectives in this study are to examine the bio- a total of 144 described taxa (Baldwin et al. 2012, plus one new geographic and ecological patterns of rarity in the diverse record as described above). Following the Jepson Manual’s taxa of Astragalus present in the