Assessing the geographic and ecological distribution of the Mexican endemic water lily Nymphaea gracilis Zucc. (Nymphaeales: Nymphaeaceae)

Nymphaea gracilis Zucc. (Nymphaeales: Nymphaeaceae) is a threatened Mexican endemic water lily with fewer than 100 verified localities reported since its description in 1832. Despite legal protection, its geographical distribution has not been formally assessed. We compiled occurrence records from herbaria, citizen science observations, and fieldwork conducted between 2018 and 2024, totaling 150 records. Of these, 105 met quality criteria for temporal analyses and 82 unique localities were used for distributional assessment. We evaluated distribution patterns across six chronological periods using four methods: grid (GR, 0.25° and 0.50°), convex hull (CH), mean propinquity (MP), and species distribution modelling (SDM) with Maxent. Ecological volume was estimated through principal component analysis (PCA) of 19 bioclimatic variables and a three-dimensional CH, complemented by rarefaction analyses to distinguish niche expansion from sampling artifacts. Methodological choice had a stronger effect on area estimates than record accumulation through time. GR produced the most conservative estimates, whereas CH, MP, and SDM were statistically equivalent in area but differed in spatial configuration. SDM and MP showed high predictive capacity from early periods onward, while GR required greater sampling effort to achieve similar coverage. Rarefaction analyses indicated that increases in ecological volume mainly reflected improved sampling rather than true niche expansion. Fieldwork documented the extirpation of populations in the Basin of Mexico and added two new localities. These results highlight the importance of integrating multiple analytical approaches and occurrence data to improve conservation assessments and management strategies for endemic aquatic plants.