An approach based on the total-species accumulation curve and higher taxon richness to estimate realistic upper limits in regional species richness

Abstract

Most of accumulation curves tend to underestimate species richness, as they do not consider spatial heterogeneity in species distribution, or are structured to provide lower bound estimates and limited extrapolations. The total-species (T-S) curve allows extrapolations over large areas while taking into account spatial heterogeneity, making this estimator more prone to attempt upper bound estimates of regional species richness. However, the T-S curve may overestimate species richness due to (1) the mismatch among the spatial units used in the accumulation model and the actual units of variation in β-diversity across the region, (2) small-scale patchiness, and/or (3) patterns of rarity of species. We propose a new framework allowing the T-S curve to limit overestimation and give an application to a large dataset of marine mollusks spanning over 11 km² of subtidal bottom (W Mediterranean). As accumulation patterns are closely related across the taxonomic hierarchy up to family level, improvements of the T-S curve leading to more realistic estimates of family richness, that is, not exceeding the maximum number of known families potentially present in the area, can be considered as conducive to more realistic estimates of species richness. Results on real data showed that improvements of the T-S curve to accounts for true variations in β-diversity within the sampled areas, small-scale patchiness, and rarity of families led to the most plausible richness when all aspects were considered in the model. Data on simulated communities indicated that in the presence of high heterogeneity, and when the proportion of rare species was not excessive (>2/3), the procedure led to almost unbiased estimates. Our findings highlighted the central role of variations in β-diversity within the region when attempting to estimate species richness, providing a general framework exploiting the properties of the T-S curve and known family richness to estimate plausible upper bounds in γ-diversity.

Keywords: Mollusca; habitat mapping; higher taxa; multivariate dispersion; species‐accumulation curves; β‐diversity.

Figure 1

Logical sequence of the stepwise procedure described in the Methods section, which progressively integrate the T–F curve to account for spatial heterogeneity among subareas, among habitats within subareas, small‐scale patchiness, and rarity (Steps 1–4). The last step (5) refers to the application of the fully improved model to species richness estimates

Figure 2

T–F curves accounting progressively for spatial heterogeneity due to (a) subareas only; subareas and habitats (b); subareas, habitats and patchiness (c). The T–F curves (dotted black lines) through the terminal points (red circles) of nested accumulation curves are showed. Nested accumulation curves were obtained for each combination of 1, 2,…, 6, 7 subareas (a) and of 1, 2,…, 10, 11 subareas × habitat units (b, c) within the total area. Replicate samples (0.25 m²) were used to build accumulation curves in (a) and (b) (n = 216), whereas in (c), samples (n = 72) were the sum of three replicates in each station (4 m²)

Figure 3

Extrapolation of total family richness (${\widehat{F}}_{\mathrm{Tot}}$) over the total area (11 × 10⁶ m²) from the T–F curves (dotted lines) accounting for spatial heterogeneity due to subareas only (orange triangles), subareas and habitats (blue squares), subareas, habitats and patchiness (green circles). Note that x‐axis is log‐scaled

Figure 4

Randomized accumulation curves of common, intermediate, rare families (a) and species (b). The number of common ($F_{\mathrm{Obs}}^{\mathrm{Com}}$) and intermediate ($F_{\mathrm{Obs}}^{\mathrm{Interm}}$) families and species ($S_{\mathrm{Obs}}^{\mathrm{Com}}$, $S_{\mathrm{Obs}}^{\mathrm{Interm}}$) at saturation were reported along with the number of samples to achieve saturation (respectively, n _Com and n _Interm)