A global assessment of endemism and species richness across island and mainland regions Gerold Kier a,1 , Holger Kreft a,b,1,2 , Tien Ming Lee b , Walter Jetz b , Pierre L. Ibisch c , Christoph Nowicki c , Jens Mutke a , and Wilhelm Barthlott a a Nees Institute for Biodiversity of Plants, University of Bonn, Meckenheimer Allee 170, D-53115 Bonn, Germany; b Division of Biological Sciences, University of California San Diego, 9500 Gilman Drive MC 0116, La Jolla, CA 92093-0116; and c Faculty of Forest and Environment, University of Applied Sciences Eberswalde, Alfred-Mo ¨ ller Strasse 1, 16225 Eberswalde, Germany Edited by Peter R. Crane, University of Chicago, Chicago, IL, and approved April 14, 2009 (received for review October 13, 2008) Endemism and species richness are highly relevant to the global prioritization of conservation efforts in which oceanic islands have remained relatively neglected. When compared to mainland areas, oceanic islands in general are known for their high percentage of endemic species but only moderate levels of species richness, prompting the question of their relative conservation value. Here we quantify geographic patterns of endemism-scaled richness (‘‘endemism richness’’) of vascular plants across 90 terrestrial biogeographic regions, including islands, worldwide and evaluate their congruence with terrestrial vertebrates. Endemism richness of plants and vertebrates is strongly related, and values on islands exceed those of mainland regions by a factor of 9.5 and 8.1 for plants and vertebrates, respectively. Comparisons of different measures of past and future human impact and land cover change further reveal marked differences between mainland and island regions. While island and mainland regions suffered equally from past habitat loss, we find the human impact index, a measure of current threat, to be significantly higher on islands. Projected land-cover changes for the year 2100 indicate that land-use-driven changes on islands might strongly increase in the future. Given their conservation risks, smaller land areas, and high levels of endemism richness, islands may offer particularly high returns for species conservation efforts and therefore warrant a high priority in global biodiversity conservation in this century. biodiversity | conservation | human impact | terrestrial vertebrates | vascular plants W orldwide loss of biodiversity requires global conservation priority setting to channel limited international conservation resources to regions of highest conservation value and need for action (1–5). Approaches for using biological data as a component of priority setting vary but can largely be divided into (i) algorithm- based assessments such as minimum-area sets or gap analyses (2, 6–8) and (ii) index-based assessments using indices such as ende- mism or species richness as surrogates for the conservation value of a region (1, 4, 9). While many theoretical arguments underpin the strengths of algorithm-based assessments (2, 8, 10, 11), they require detailed distribution data that are only available for few taxonomic groups—almost exclusively terrestrial vertebrates (6, 9, 12–15), on which systematic conservation planning has thus relied increasingly in the past years. Such detailed data are not available for the vast majority of taxonomic groups on the global scale including vascular plants. Although great effort is being made in digitizing existing data from natural history collections for conservation purposes (16, 17), biodiversity loss is arguably proceeding more rapidly than the documentation of species distributions. Hence, an inventory-based approach, which forms a main basis for the present study, is a workable solution if global conservation planning is to be informed by vascular plants (18), a group of organisms which is of outstanding ecological and economical importance for human well-being (19). Islands are well-known centers of range-restricted species and thus see high levels of endemism (20). However, they are simulta- neously acknowledged for their lower species richness compared to mainland areas (20). Hence, an index combining both endemism and species richness (‘‘endemism richness’’) can provide insight into the question of relative conservation value of islands and mainlands. Such an index has been calculated in a number of regional-to- continental-scale studies (21–25), which, however, have not focused on the differences between mainlands and islands. The basic concept of calculating endemism richness is to give each species the same value that is equally distributed across its range (21). For instance, when 100% of a species’ range, i.e., 1 range equivalent, falls into a mapping unit (such as a grid cell or, as in the present study, a biogeographic region), its entire value is attributed to this area. When half of the species’ distribution area falls into a mapping unit, only 50% of its value, or 0.5 range equivalents, are attributed to the mapping unit and so on. Summing up the fractions of range equivalents for all species within each mapping unit would then reflect both endemism and species richness (21, 26). In contrast to species richness, values of endemism richness on average show a linear relation with area at a given sampling scale and can thus be corrected for area disparity by linear conversion (21). Furthermore, unlike species richness or species endemism, the combined metric of endemism richness shows another useful property: The sum of all range equivalents of all mapping units yields the total number of species in the analysis—in our case, the global number of species in the studied taxa. Consequently, endemism richness can be interpreted as the specific contribution of an area to global biodiversity (21). Here we calculated endemism richness of vascular plants for 90 biogeographic regions covering the entire terrestrial realm (except for Antarctica where the value is negligible). We compared our plant-based results to the spatial pattern of endemism richness of terrestrial vertebrates (amphibians, reptiles, birds, mammals) to quantify, for the first time, how well vascular plants capture the diversity of this charismatic group (and vice versa) and how taxon-specific differences might affect prioritization rankings. In this study, we specifically focused on comparing biodiversity of mainlands and islands because of the as yet undecided question of their relative conservation importance. Furthermore, islands are centers of past and imminent species extinction (27, 28), stressing even more the need for information on both biodiversity and specific threats in this part of the world. Results and Discussion Summing up the range equivalents of vascular plants for all 90 regions yields a total of 315,903. Given that the sum of range Author contributions: G.K., H.K., and T.M.L. designed research; G.K., H.K., and T.M.L. performed research; W.J., P.L.I., C.N., J.M., and W.B. contributed analytical data; G.K., H.K., and T.M.L. analyzed data; and G.K., H.K., T.M.L., W.J., P.L.I., C.N., J.M., and W.B. wrote the paper. The authors declare no conflict of interest. This article is a PNAS Direct Submission. Freely available online through the PNAS open access option. 1 G.K. and H.K. contributed equally to this work. 2 To whom correspondence should be addressed. E-mail: hkreft@ucsd.edu. This article contains supporting information online at www.pnas.org/cgi/content/full/ 0810306106/DCSupplemental. 9322–9327 | PNAS | June 9, 2009 | vol. 106 | no. 23 www.pnas.orgcgidoi10.1073pnas.0810306106 Downloaded by guest on May 30, 2020