Temporal patterns in species richness and occupancy in mountain-prairie transitional watersheds

Our understanding of biodiversity loss and change in many regions, especially riverine headwaters, remains limited. Using multi-species occupancy models, we investigate the temporal trends in species richness, species turnover and occupancy probability across the mountain-prairie transitional watersheds of several major Canadian rivers in southern Alberta, Canada, over a 25-year period (1996-2020; years binned into 5-year periods). Forty-five species were identified in 12,863 backpack electrofishing surveys across 382 unique watersheds. Jaccard’s similarity index showed an overall similarity of 0.87 across the 25 years (a change of five species). Of the 45 species observed, 39 of those were observed in multiple time periods. The trend in occupancy probability showed 9 species increasing in occupancy probability and 12 declining. The largest declines in occupancy probability were observed for cold water fishes, with Arctic grayling, longnose sucker and mountain whitefish showing the largest declines. Optimal temperature for species occupancy ranged from 5 °C to 18 °C. Where conservation recovery action has targeted specific species, such as bull trout and westslope cutthroat trout, positive trends in occupancy probability are observed.