An integrated approach to balance freshwater connectivity restoration and invasion risk

Habitat fragmentation negatively affects many native migratory fish populations. However, this fragmentation can also prevent the spread and establishment of invasive species. We modified the dendritic connectivity index (DCI), a commonly used freshwater connectivity metric, to account for multiple invasive dispersal pathways in estimates of invasive species connectivity. We considered dispersal from source populations in the outlet by limiting the diadromous form of the DCI to consider dispersal paths from the outlet to uninvaded segments. We additionally considered dispersal from already established populations by modifying the potamodromous DCI to consider dispersal paths from invaded segments to uninvaded ones. We applied this method to the case of the Grand River watershed in Ontario, Canada. We focused on the round goby (Neogobius melanostomus), a pervasive invasive species in the region with established populations in our studied watershed. We found that invasive connectivity was closely linked to native species' connectivity. In single barrier removal simulations, gains in native connectivity were in most cases nearly completely matched by gains in invasive connectivity. When multiple barrier removals were considered, invasive connectivity further constrained restorative actions. Specifically, connectivity restoration through barrier removals that did not consider invasive dispersal increased the risk of these populations spreading at all levels of barrier removal. Even when both native and invasive priorities were considered, increases in invasive connectivity could not be avoided because round goby populations are established in mainstem regions. We found that accounting for this dispersal risk from established populations, in addition to dispersal from the outlet, was essential to fully consider the risk of further spread to the watershed's tributaries. In addition, we identified areas where native connectivity was high relative to invasive connectivity, which could inform selective habitat restoration. Taken together, we demonstrated the importance of considering invasive connectivity from multiple sources when undertaking connectivity restoration.