Large-Scale Diversity of Slope Fishes: Pattern Inconsistency between Multiple Diversity Indices

Abstract

Large-scale studies focused on the diversity of continental slope ecosystems are still rare, usually restricted to a limited number of diversity indices and mainly based on the empirical comparison of heterogeneous local data sets. In contrast, we investigate large-scale fish diversity on the basis of multiple diversity indices and using 1454 standardized trawl hauls collected throughout the upper and middle slope of the whole northern Mediterranean Sea (36°3′- 45°7′ N; 5°3′W - 28°E). We have analyzed (1) the empirical relationships between a set of 11 diversity indices in order to assess their degree of complementarity/redundancy and (2) the consistency of spatial patterns exhibited by each of the complementary groups of indices. Regarding species richness, our results contrasted both the traditional view based on the hump-shaped theory for bathymetric pattern and the commonly-admitted hypothesis of a large-scale decreasing trend correlated with a similar gradient of primary production in the Mediterranean Sea. More generally, we found that the components of slope fish diversity we analyzed did not always show a consistent pattern of distribution according either to depth or to spatial areas, suggesting that they are not driven by the same factors. These results, which stress the need to extend the number of indices traditionally considered in diversity monitoring networks, could provide a basis for rethinking not only the methodological approach used in monitoring systems, but also the definition of priority zones for protection. Finally, our results call into question the feasibility of properly investigating large-scale diversity patterns using a widespread approach in ecology, which is based on the compilation of pre-existing heterogeneous and disparate data sets, in particular when focusing on indices that are very sensitive to sampling design standardization, such as species richness.

Spatio-temporal trends in diversity of demersal fish assemblages in the Mediterranean

“…Areas with the highest diversity values on the continental shelf do not coincide with areas with the highest diversity values on the shelf break/ upper slope and lower slope. Although the assignment of depth strata was different in previous works and the comparison is not straightforward, a different pattern on shelf and slope areas was also observed for species richness (Gaertner et al 2007(Gaertner et al , 2013. This is likely due to differences in the distribution of cumulative threats to marine biodiversity, which are mainly concentrated in coastal areas and on the continental shelf of the Mediterranean (Coll et al 2012), and to the presence of particular habitats on the shelf break and slope bottoms, which may represent potential hot spots of biodiversity (Danovaro et al 2010).…”

“…data concerning only one guild of fishes or limited to specific depths, gear or habitat), some of regional inventories are useless for comparative studies (Psomadakis et al 2012). Recent studies based mainly on standardized time series data also question the previously considered west-east decreasing diversity trend in the Mediterranean (Gaertner et al 2013, Granger et al 2015, Peristeraki et al 2017.…”

Mapping diversity indices: not a trivial issue

“…; Gaertner et al . ). The second one reports woody plant species abundance in the central Western Ghats region, Karnataka, India (74·25°–75·5° E; 15·25°–13·5° N) in a network of 96 sampling sites. These data provide abundance on 334 tree species collected in 96 sampling sites during 1996–1997 (merged for this study) (Ramesh et al .…”

“…A total of 186 species properly sampled by the fishing gear were considered during this program (Gaertner et al . , ). Abundance was standardized to 1 km 2 , for each species caught (Morfin et al .…”

“…The 66 scientific bottom trawls analysed have been carried out in 2012, in the frame of the international MEDITS program (Bertrand et al 2002). A total of 186 species properly sampled by the fishing gear were considered during this program (Gaertner et al 2010(Gaertner et al , 2013. Abundance was standardized to 1 km 2 , for each species caught (Morfin et al 2012;Gaertner et al 2013).…”