Patterns of recent brook trout invasion in bull trout streams in relation to habitat, source connectivity, biotic resistance, and disturbance

Anticipating biological invasions by nonnative species is critical to effective conservation. Nonnative brook trout Salvelinus fontinalis represents one of the most widespread threats to native bull trout Salvelinus confluentus, but the factors allowing or preventing ongoing range expansions are poorly understood. We addressed this uncertainty by resampling 221 survey locations in bull trout streams in Idaho and relating shifts in brook trout occupancy to four controls on biological invasion (habitat suitability, source connectivity, disturbance, and biotic resistance to invasion). Brook trout detections increased substantially between the historical period (58 sites) and contemporary period (94 sites). Site colonizations were positively associated with water temperature and negatively associated with landscape resistance metrics (i.e., highest streamflow and gradient between a site and the nearest source) in all top models. In contrast, there was weak support for a positive association with wildfire and limited support for hydrologic distance and biotic resistance metrics. Brook trout invasions in bull trout habitat are ongoing, limited by cold temperatures, and highly influenced by dispersal barriers that may not inhibit more mobile native salmonids.