Occurrences of annual killifish (Rivulidae) in different bioregionalizations across the Neotropical domain

Aim Bioregionalization frameworks represent unique assemblages of species
resulting from geographic isolation and environmental heterogeneity.
Understanding how different bioregionalizations capture community
compositional variation is crucial, as underlying patterns and processes
are scale-dependent. This study aims to (1) explore the underlying
ecological processes through the decomposition of beta diversity (turnover
and nestedness); (2) identify which bioregionalization framework offers
the optimal spatial granularity for distinguishing between communities;
and (3) evaluate the effective number of compositionally distinct areas.
Location Neotropical domain Taxon Rivulidae - annual species Methods
Presence-absence data of fish species were analyzed using pairwise
β-diversity and hierarchical clustering methods (UPGMA) and compared with
14 comprehensive bioregionalization frameworks, including terrestrial
ecoregions (TEOW), freshwater ecoregions (FEOW), Neotropical provinces,
and watersheds (HydroBasins). Results The study revealed that (1) turnover
is the dominant component of β-diversity, surpassing nestedness across all
bioregionalization frameworks; (2) turnover increases non-linearly as
regionalization area decreases, with a threshold identified beyond which
further area reduction does not significantly increase turnover; and (3)
the optimal spatial granularity for bioregionalization is achieved at
smaller watershed scales (146–414 km²), where turnover is maximized and
the optimal number of bioregions (> 180) is identified. Main
Conclusions Turnover patterns are linked to factors such as high endemism,
low dispersal capacity, and the significant isolation of temporary
wetlands. The scale-dependence of β-diversity is influenced not only by
the area of bioregionalizations but also by the underlying design of these
units, such as those based on hydrogeomorphological features (HydroBasins)
or taxon distribution patterns (FEOW, TEOW). Finer spatial scales are more
effective for assessing biodiversity patterns for endemic taxa and in
habitats with low connectivity. These findings can enhance the
understanding of how bioregionalization frameworks reflect species
compositional variation, with important implications for interpreting
ecological patterns and developing scale-dependent conservation
strategies.