Marine biogeographic realms and species endemicity

Abstract

Marine biogeographic realms have been inferred from small groups of species in particular environments (e.g., coastal, pelagic), without a global map of realms based on statistical analysis of species across all higher taxa. Here we analyze the distribution of 65,000 species of marine animals and plants, and distinguish 30 distinct marine realms, a similar proportion per area as found for land. On average, 42% of species are unique to the realms. We reveal 18 continental-shelf and 12 offshore deep-sea realms, reflecting the wider ranges of species in the pelagic and deep-sea compared to coastal areas. The most widespread species are pelagic microscopic plankton and megafauna. Analysis of pelagic species recognizes five realms within which other realms are nested. These maps integrate the biogeography of coastal and deep-sea, pelagic and benthic environments, and show how land-barriers, salinity, depth, and environmental heterogeneity relate to the evolution of biota. The realms have applications for marine reserves, biodiversity assessments, and as an evolution relevant context for climate change studies.

Fig. 1

Classification hierarchy and number of species belonging to the mapped biogeographic realms. The classification hierarchy from this analysis is compared to the clustering of seas and oceans as shown on a blue background. The red text in column 1 denotes the realms defined by pelagic-only species and clustered at 1% similarity. The red numbers in parentheses in columns 2 and 3 indicate further similarity index levels, e.g., 3 = 3% similarity between 5^o areas in that region. The percentage of unique species indicates how distinct the realm was from the others; cells with yellow background represent > 40%, while cells with peach colored background represent > 50%. Comparisons with previous studies are in Supplementary Table 7. E, east; N, north; S, south; Spp, species; W, west

Fig. 2

The biogeographic realms as numbered 1–30 in Fig. 1. a Shows realms (denoted by solid lines) overlaid on the original 5^o latitude-longitude cells. Realm boundary smoothing included following the Southern Ocean 10^oC annual average sea surface temperature sub-Antarctic Front. b Shows the 1000 m depth contour as a dashed line

Fig. 3

Alternative analyses of marine biogeographic realms with alternative dissimilarity metrics and cell sizes. a–c Used Beta SIM index and d Sorensen’s index of dissimilarity. a, b Used 50,000 km² and 600,000 km² hexagons, respectively. c, d Used 5° latitude-longitude cells. e Used Infomaps to construct the realms