Taxonomic distinctness of demersal fishes of the California current: moving beyond simple measures of diversity for marine ecosystem-based management

Abstract

Background: Large-scale patterns or trends in species diversity have long interested ecologists. The classic pattern is for diversity (e.g., species richness) to decrease with increasing latitude. Taxonomic distinctness is a diversity measure based on the relatedness of the species within a sample. Here we examined patterns of taxonomic distinctness in relation to latitude (ca. 32-48 degrees N) and depth (ca. 50-1220 m) for demersal fishes on the continental shelf and slope of the US Pacific coast.

Methodology/principal findings: Both average taxonomic distinctness (AvTD) and variation in taxonomic distinctness (VarTD) changed with latitude and depth. AvTD was highest at approximately 500 m and lowest at around 200 m bottom depth. Latitudinal trends in AvTD were somewhat weaker and were depth-specific. AvTD increased with latitude on the shelf (50-150 m) but tended to decrease with latitude at deeper depths. Variation in taxonomic distinctness (VarTD) was highest around 300 m. As with AvTD, latitudinal trends in VarTD were depth-specific. On the shelf (50-150 m), VarTD increased with latitude, while in deeper areas the patterns were more complex. Closer inspection of the data showed that the number and distribution of species within the class Chondrichthyes were the primary drivers of the overall patterns seen in AvTD and VarTD, while the relatedness and distribution of species in the order Scorpaeniformes appeared to cause the relatively low observed values of AvTD at around 200 m.

Conclusions/significance: These trends contrast to some extent the patterns seen in earlier studies for species richness and evenness in demersal fishes along this coast and add to our understanding of diversity of the demersal fishes of the California Current.

Figure 1. West coast of the USA.

Bathymetry represents the 200 m and 1200 m depth contours.

Figure 2. Taxonomic distinctness versus latitude and depth.

(A,B) two views of average taxonomic distinctness (AvTD), and (C,D) two views of the variation in taxonomic distinctness (VarTD). Data were analyzed with Generalized Additive Models.

Figure 3. Average taxonomic distinctness versus latitude for five depth zones.

Data were analyzed with Generalized Additive Models. Solid lines represent the smoothed trend. Broken lines are ±1 s.e. The data points shown are the residuals around the smoothed term. The y-axis label is the smoothed parameter and its estimated degrees of freedom from the GAM. Trends for the 50–150 m and 601–900 m depth bins were better represented by linear models. *  = p<0.05, **  = p<0.01, ***  = p<0.001.

Figure 4. Variation in taxonomic distinctness versus latitude for five depth zones.

Data were analyzed with Generalized Additive Models. Solid lines represent the smoothed trend. Broken lines are ±1 s.e. The data points shown are the residuals around the smoothed term. The y-axis label is the smoothed parameter and its estimated degrees of freedom from the GAM. *  = p<0.05, **  = p<0.01, ***  = p<0.001.

Figure 5. Taxonomic distinctness versus depth for different latitude zones.

(A–D) AvTD and (E–H) VarTD. Latitude zone given in panes (A–D) corresponds with panes (E–H). Data were analyzed with Generalized Additive Models. Solid lines represent the smoothed trend. Broken lines are ±1 s.e. The data points shown are the residuals around the smoothed term. The y-axis is the smoothed parameter and its estimated degrees of freedom from the GAM. *  = p<0.05, **  = p<0.01, ***  = p<0.001.

Figure 6. Taxonomic trees for the region between Point Conception and Cape Mendocino for five depth zones.

Species are those found in at least 50% of the trawls in a given depth zone. Numbers indicate species identified in Table 2.

Figure 7. AvTD versus depth and latitude.

AvTD_diff for (A) Scorpaeniformes, (B) Pleuronectiformes, (C) Perciformes, (D) Gadiformes, (E) Chondrichthyes, and (F) AvTD for Actinopterygii only. All GAMs are significant at the α = 0.05 level.

Figure 8. VarTD versus depth and latitude.

VarTD_diff (A) Scorpaeniformes, (B) Pleuronectiformes, (C) Perciformes, (D) Gadiformes, (E) Chondrichthyes, and (F) VarTD for Actinopterygii only. All GAMs are significant at the α = 0.05 level.