Reproductive and Trophic Patterns Associated with Non-Native Fish Dominance in a Mexican Spring Ecosystem

Biological invasions are among the main threats to freshwater biodiversity, yet ecological patterns associated with assemblage structure and high relative abundances of non-native fishes in spring ecosystems remain insufficiently documented. We evaluated seasonal variation in community composition, reproductive traits, and trophic interactions in La Zarcita springs, part of the Natural Protected Area Laguna de Zacapu, central Mexico. Bimonthly sampling was conducted, including stomach content analysis and reproductive trait assessment. A total of 14 fish taxa were recorded (seven native and seven non-native), with the assemblage numerically dominated by Oreochromis niloticus (30%), Pseudoxiphophorus bimaculatus (24%), and Xiphophorus hellerii (14%). Overall diet composition did not differ significantly between taxa classified as native and non-native (PERMANOVA, p > 0.05), consistent with overlap in resource use within the assemblage. Exploratory assemblage-level analyses detected differences in omnivory index values among taxa grouped according to species origin (LMM, p < 0.05). Reproductive analyses detected variation in fertility values (GLMM, p < 0.05), reproductive activity (Wilcoxon test, p < 0.05), gonadosomatic index values, and Fulton’s condition factor values (LMM, p < 0.01) among taxa within the assemblage. Physicochemical variables varied seasonally but were not significantly associated with trophic composition, condition factor values, or reproductive traits in the statistical analyses performed. Overall, the results document variation in reproductive characteristics and trophic patterns among taxa within this urbanized spring system and highlight the value of assemblage-level ecological studies for understanding fish community structure in small freshwater habitats.