Integrating abundance and functional traits reveals new global hotspots of fish diversity

Abstract

Species richness has dominated our view of global biodiversity patterns for centuries. The dominance of this paradigm is reflected in the focus by ecologists and conservation managers on richness and associated occurrence-based measures for understanding drivers of broad-scale diversity patterns and as a biological basis for management. However, this is changing rapidly, as it is now recognized that not only the number of species but the species present, their phenotypes and the number of individuals of each species are critical in determining the nature and strength of the relationships between species diversity and a range of ecological functions (such as biomass production and nutrient cycling). Integrating these measures should provide a more relevant representation of global biodiversity patterns in terms of ecological functions than that provided by simple species counts. Here we provide comparisons of a traditional global biodiversity distribution measure based on richness with metrics that incorporate species abundances and functional traits. We use data from standardized quantitative surveys of 2,473 marine reef fish species at 1,844 sites, spanning 133 degrees of latitude from all ocean basins, to identify new diversity hotspots in some temperate regions and the tropical eastern Pacific Ocean. These relate to high diversity of functional traits amongst individuals in the community (calculated using Rao's Q), and differ from previously reported patterns in functional diversity and richness for terrestrial animals, which emphasize species-rich tropical regions only. There is a global trend for greater evenness in the number of individuals of each species, across the reef fish species observed at sites ('community evenness'), at higher latitudes. This contributes to the distribution of functional diversity hotspots and contrasts with well-known latitudinal gradients in richness. Our findings suggest that the contribution of species diversity to a range of ecosystem functions varies over large scales, and imply that in tropical regions, which have higher numbers of species, each species contributes proportionally less to community-level ecological processes on average than species in temperate regions. Metrics of ecological function usefully complement metrics of species diversity in conservation management, including when identifying planning priorities and when tracking changes to biodiversity values.

Global conservation outcomes depend on marine protected areas with five key features

“…1a), as expected 12,17 . However, when only the number of large fishes sighted along transects was considered (Fig.…”

“…Surveys were based on Reef Life Survey methodology 12,30 , with support from volunteer SCUBA divers trained individually to scientific data collection standards. All fishes observed within 50 m 3 5 m transect blocks were counted, and total fish lengths estimated, during swims on adjoining blocks up one side and down the other side of 50-m lines.…”

“…Eight community-level metrics were assessed using data from 40 nations on shallow reef fish densities and sizes provided by researchers and trained volunteer divers participating in the Reef Life Survey (RLS) programme 12 . A total of 964 sites in 87 MPAs were surveyed (Extended Data Fig.…”

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“…Although it is widely recognized that this focus on species richness overlooks the contributions of species abundances and trait diversity to ecosystem functioning, documenting global patterns in these metrics has been hindered by a paucity of data. In a recent paper published in Nature, Stuart-Smith et al [4] claim to make a significant step forward in this regard. Using standardized reef fish surveys from temperate and tropical reefs around the world, the authors describe a hitherto unnoticed latitudinal gradient in community evenness -a measure of species' relative abundances -which contributes to the identification of new functional diversity hotspots [4].…”

“…In a recent paper published in Nature, Stuart-Smith et al [4] claim to make a significant step forward in this regard. Using standardized reef fish surveys from temperate and tropical reefs around the world, the authors describe a hitherto unnoticed latitudinal gradient in community evenness -a measure of species' relative abundances -which contributes to the identification of new functional diversity hotspots [4]. Neither of these findings withstands scrutiny.…”