Egorova, Y. | Search bioRxiv

bioRxiv2025

Rapoport s rule in the marine realm: wrong axis, right pattern

AimRapoports rule posits that species range size increases with distance from benign conditions along environmental gradients. We asked whether, and along which axis, marine species obey Rapoports rule when ranges are quantified in three dimensions. LocationGlobal ocean. Taxon> 20,000 marine species spanning pelagic and benthic habitats across major animal phyla and fishes. MethodsWe combined AquaMaps 2.0 / AquaX modelled distributions with independently curated depth limits from FishBase and SeaLifeBase to estimate latitudinal and vertical (bathymetric) ranges for each species. We quantified latitudinal range versus absolute mid-latitude and depth range versus mid-depth, and repeated analyses by habitat and taxonomic group. We used linear and polynomial regressions and Gaussian mixture models in range-gradient space to identify and compare alternative Rapoport regimes. ResultsMarine biodiversity exhibits a classic latitudinal diversity gradient and strong concentration of richness in the upper ocean, with broader ranges at higher latitudes and greater depths. Support for Rapoports rule is weak and inconsistent for latitudinal ranges, but strong and pervasive with depth, with correlations between vertical range and mid-depth frequently exceeding 0.8 across habitats and taxa. Latitudinal and vertical ranges are positively, but only moderately, coupled, with a small subset of "3-D generalists" spanning both large latitudinal and depth extents. Main conclusionsThe marine realm appears to obey Rapoports rule primarily along the vertical, rather than latitudinal, axis. Depth-structured environmental tolerance thus emerges as a dominant constraint on marine range limits and three-dimensional biodiversity gradients.

ecology↗

bioRxiv2025

AquaX: An enhanced and revised AquaMaps framework to model marine species distributions and biodiversity

Marine biodiversity underpins ecosystem health and is critical for the provision of essential ecological services. Global efforts to mitigate biodiversity loss are underway but require comprehensive knowledge on the biogeography of species to be effective. However, key challenges limit comprehensive mapping of species distributions, including the complexity of ocean ecosystems and the difficulty of sampling the marine realm. Global initiatives such as AquaMaps pioneered large-scale marine species mapping using species distribution models or Ecological Niche models and provided the knowledge base for effective marine conservation and management. Recently, methodological and data advances have enabled a more modern and robust approach that enables higher resolution outputs more suited to conservation applications at all scales. Building on AquaMaps, we develop a next-generation marine species habitat suitability modelling platform called AquaX, providing a suite of advances that include an ensemble of ten machine learning algorithms, enabling spatial uncertainty assessments, validation indices, and ecological niche representation at a ten-fold improved spatial resolution of 0.05{degrees}. Furthermore, AquaX integrates (i) accepted taxonomy from the World Register of Marine Species, (ii) species-specific ecological, physiological, and biogeographical information (D3-Ocean system), (iii) updated occurrence records validated through expert input, and (iv) refined species range maps using expert knowledge and biogeographical divisions. AquaX also projects species habitat suitability for both present and future conditions based on two time periods and three climate scenarios. This work provides species range maps for numerous species compared to previously available datasets and improves the accurate use of observational data. The approaches described here improve predictive accuracy at scales more relevant to marine biodiversity conservation and offer an openly accessible tool to support marine biodiversity research and conservation planning under accelerating environmental change. AquaX represents an important step forward in species distribution modeling, enabling researchers and policymakers to better understand marine biodiversity patterns and develop more effective conservation strategies.

ecology↗