Improved Total–Species Accumulation Curve for Reliable Estimation of Regional Species Richness: An Application to Macroalgae Diversity on Bioconstructions from the Northern Adriatic Sea (Mediterranean Sea)

Traditional species richness estimators often assume spatial homogeneity in species distribution, which can lead to underestimating biodiversity, especially in large, ecologically complex areas. The Total–Species (T–S) curve may provide an accurate framework for estimating γ-diversity by accounting for compositional variation across spatial subunits. Our study tested the T–S curve model, modified to account for species rarity and patterns of β-diversity, to estimate macroalgal richness in the northeast Adriatic (Mediterranean Sea), an area where the total macroalgal diversity is known and a comprehensive reference list is available (487 species). Uncertainty in species richness estimates from T–S curves was quantified as 95%CI based on bootstrapping, and a sensitivity analysis was also carried out to quantify changes in estimates under different settings. Other parametric and non-parametric estimators, including the classic T–S curve, largely under- or overestimated the total species richness if compared to the refined T–S model, which returned a realistic estimate of 393 species in total. Our results demonstrate that the T–S curve modified to consider species rarity, and refined for potential biases associated with erroneous quantification of small-scale patchiness and spatial variations in assemblage composition, allowed for more realistic extrapolations of γ-diversity over large areas.