Functional diversity and redundancy of freshwater fish communities across biogeographic and environmental gradients

Aim: Functional redundancy occurs when species share overlapping ecological functions and is considered an important component of ecosystem resilience. However, much of what we know about functional redundancy comes from relatively species-rich terrestrial and marine environments. Here, we examined patterns of functional redundancy among Ontario freshwater fish communities with species richness levels ranging from 4 to 30 species across lakes of differing size, depth, productivities and thermal characteristics. Location: Six thousand nine hundred and seventy-seven lakes in Ontario, Canada. Methods: We examined functional redundancy by quantifying the relationship between functional diversity and species richness in lakes across Ontario and within smaller biogeographic regions. We used null models to test whether fish communities had greater redundancy than expected from random assemblages. We then used generalized additive models (GAMs) to predict how patterns of redundancy vary across environmental variables. At last, we compared species-level functional rarity metrics across fish thermal preference groups, body sizes and species occurrence rates. Results: The functional diversity and species richness relationship were saturating among fish communities at the provincial scale but varied between smaller regions with differing biogeographic histories. Most communities fell within expectations from weighted null models of the functional diversity and species richness relationship. The GAMs indicated that fish communities in the largest, deepest and warmest lakes showed the greatest overall functional redundancy. No differences were observed in functional rarity measures between thermal preference groups, across body sizes or across species occurrence rates. Main conclusions: Although lakes in this study were relatively depauperate of fish species, Ontario fish communities exhibited functional redundancy at the provincial scale, with variation regionally. North-eastern communities showed the least saturating relationship overall as predicted by historical biogeographic patterns of freshwater fish colonization. Overall, this study provides a broad perspective of freshwater fish diversity patterns and highlights the importance of investigating redundancy from different perspectives and multiple spatial scales.