Spatially congruent sites of importance for global shark and ray biodiversity

explore the spatial distribution and overlap of different biodiversity measures in the terrestrial realm and even fewer in the ocean. The terrestrial studies have all found a lack of spatial over- lap occurring throughout a variety of different taxa (i.e. birds, insects, plants) [22,32,33]. While marine studies yield comparable patterns to the terrestrial realm, most focus on relatively ses- sile species (i.e. coral reefs) or on other dimensions of biodiversity (i.e. functional diversity) [5,10]. Here, we use a global database of all known shark and ray distributions to explore the spatial congruency among three species richness measures: total number of species, ED species rich- ness, and endemic species richness. Spatial congruence is defined here as the spatial overlap between hotspot areas. We also explore the level of spatial congruency of the species richness measures for threatened shark and ray species only because of their greater conservation urgency. Specifically, we examine the (1) overall spatial congruency among all species richness measures and the subset of threatened species, and (2) changes in spatial congruency accord- ing to different definitions of hotspot used, as well as different levels of spatial resolution. Methods We obtained distribution maps for all known sharks, rays, and chimaeras in the class Chon- drichthyes from the IUCN [25,34]. All maps were projected with Lambert equal area for analy- sis. A global grid map was overlain at a cell resolution of 1˚ by 1˚, equating to an approximate

distance at the equator of 110 km. The global grid contains 44,181 cells after excluding terres- trial land masses, which are any cells containing land from the Environmental Systems Research Institute (ESRI) vector map of the world [35]. Across all species richness measures evaluated, each species is scored as present within a grid cell if any part of their distribution range falls within the grid cell boundaries. Total species richness (n = 1,083 spp.) was calcu- lated as the total number of unique species within each grid cell. We consider all marine spe- cies together rather than separate coastal and pelagic species because many pelagic species are also neritic–occurring on the continental shelf. Hence, we have retained the pelagic species to capture the true richness and evolutionary distinctness of shelf seas. Evolutionary distinc- tiveness scores were calculated as the sum of the branch lengths of a species down to the root of the phylogenetic tree, with each branch inversely weighted by the number of species that it subtends [36,37]. Species with longer branches and fewer relatives have higher evolutionary distinctiveness scores. ED species richness (n = 264 spp.) was defined as those species with the highest quartile of evolutionary distinctiveness scores (represented as age in millions of years) and is calculated as the total number of unique species per cell that are within the evolution- arily distinct upper quartile. Endemic species richness (n = 527 spp.) was calculated as the total

number of unique species within each grid cell that have range sizes below the median of the range sizes of all species (i.e. 419,659 km2) [10,38,39]. To quantify total threatened species rich- ness (n = 178 spp.), we counted the number of species within each grid cell that are currently listed as Vulnerable, Endangered, or Critically Endangered (i.e. threatened) according to the IUCN Red List Categories and Criteria [40]. Threatened endemic richness (n = 70 spp.) was calculated in the same way as endemic species richness, but subset to the IUCN threatened spe- cies only. Finally, threatened ED species are those ED species that have been classified by the IUCN as threatened (n = 49 spp.). We defined richness hotspots as those containing the top 5% of richest cells for each of the biodiversity measures. Previous research has shown that the richest 1–5% of total land area can capture a substantial proportion of species [18,41,42]. We tested the extent of spatial con- gruency between shark and ray hotspots derived for all three species richness measures (i.e. total species, ED species, and endemic species), and between all three threatened subsets of the PLOS ONE Spatially congruent sites of importance for global shark and ray biodiversity PLOS ONE | https://doi.org/10.1371/journal.pone.0235559 July 6, 2020 3 / 19