Costa, J. H. A. d. | Search bioRxiv

bioRxiv2026

Biodiversity knowledge and conservation shortfalls in Rivulidae fishes

The biodiversity crisis is exacerbated by persistent gaps in taxonomic, geographic, and conservation knowledge. This study provides a comprehensive assessment of Linnean, Wallacean, and conservation shortfalls in Rivulidae (Cyprinodontiformes), one of the most diverse families of Neotropical freshwater fishes. To this end, an extensive dataset was compiled, comprising 494 valid species, 51 synonyms, 3,419 occurrence records, and information on life cycle, distribution, and conservation status. The Linnean shortfall remains open: after 1975, the rate of species description increased sharply, and estimates indicate that 103 species remain undescribed (95% CI: 53-212). The year of species description was influenced by detectability and accessibility factors, with larger species, more widely distributed species, and species occurring in more densely populated areas being described earlier. The Wallacean shortfall was broad and spatially uneven: only 5.34% of the region was considered adequately sampled. The conservation shortfall was also substantial: 179 species are threatened with extinction, 69 species remain Not Evaluated, and 62 are Data Deficient. The mean time between taxonomic description and first IUCN extinction-risk assessment was 25.7 years. Moreover, 59.9% of species have no records within protected areas, including 69.3% of threatened species. These findings synthesize an urgent challenge: biodiversity knowledge and conservation shortfalls must be overcome simultaneously to protect species that are still being discovered, remain poorly documented spatially, and are restricted to habitats under intense anthropogenic pressure. The conservation of Rivulidae cannot wait for complete knowledge; action amid uncertainty is necessary to prevent both known and unknown species from disappearing.

ecology↗

bioRxiv2025

Drivers of fish beta diversity vary by habitat and rainfall period in ephemeral aquatic habitats of the Atlantic Forest

Ephemeral aquatic habitats, such as temporary pools and roadside ditches, are characterized by cyclical flooding and drying that create highly dynamic conditions for fish community assembly. These environments can exhibit high spatial and temporal variability in community structure driven by local topography, distance to source habitats and interannual variation in rainfall, which affects connectivity dynamics. As such, they provide unique opportunities to test hypotheses about the processes shaping metacommunities, particularly through the partitioning of beta diversity into turnover and nestedness components. Here, we investigate these processes in ephemeral aquatic habitats of the Atlantic Forest, where we sampled 36 temporary pools and 36 roadside ditches throughout 2024, recording 1,545 individuals from 20 species. Fish communities composition did not differ significantly between habitats or hydrological periods, but beta diversity was consistently high and predominantly influenced by turnover. This suggests spatial processes such as dispersal limitation and colonization history are key drivers of community structure in those temporary aquatic habitats. We tested whether beta diversity components were associated with local environmental (temperature, dissolved oxygen, pH, volume) and spatial (distance to the nearest stream) predictors. During the dry period, nestedness was positively correlated with differences in volume for temporary pools, indicating that greater habitat availability promotes species accumulation under low connectivity, where dispersal occurs possibly only by overland displacement. Additionally, turnover was associated with differences in distance to the nearest stream in roadside ditches during the dry period, likely reflecting differential colonization by stream-associated species. During the wet period, differences in pH were correlated with turnover in roadside ditches, consistent with Mass Effect dynamics under higher connectivity conditions. Meanwhile, in temporary pools during the wet period, turnover increased with differences in distance to the nearest stream, suggesting colonization by highly tolerant and capable of dispersal species in more isolated sites. These contrasting relationships may also reflect the spatial organization of habitats, with linear ditches facilitating directional dispersal along roads, whereas spatially dispersed pools depend on more stochastic colonization. Thus, pools act as seasonal refugia for tolerant or amphibious species during low connectivity, and for stream-dwelling species during high connectivity, whereas ditches function as more persistent nodes that facilitate dispersal during wetter periods. This seasonal complementarity helps explain the observed temporal and habitat-related variation in beta diversity, highlighting that effective conservation must target the entire floodable mosaic, rather than isolated sites, to maintain this dynamic metacommunity.

ecology↗