Competition & Hybridization and System Capacity

a. Competition by Non-Native Species
Brook Trout is a wide-spread, invasive species that may compromise populations of the three native trout species through competition (Warnock 2012, McMahon et al. 2007, Rieman et al. 2006, Peterson et al. 2008, 2004; Shepard et al. 2002, Magoulick and Wilzbach 1998; Popowich 2005; Donald 1987). If successful, Brook Trout may displace or replace, native salmonids (Behnke 1992; Peterson et al. 2004; Fausch 2007; McGrath and Lewis Jr. 2007; Peterson et al. 2008; Earle et al. 2010a, b).
While the mechanisms of non-native Rainbow Trout invasiveness have not been fully explored, it is assumed that niche overlap with Athabasca Rainbow Trout results in the replacement, but not displacement of the native trout species. In this theoretical framework, Athabasca rainbow trout are initially lost because of another threat (e.g., overfishing), and the direct stocking or migration of stocked Rainbows results in the replacement of native fish with non-native fish. This assumed no competitive advantage from non-native fish but is simply a factor of few native fish being in the area to allow a natural recovery, in area, versus abundant, locally stocked fish. This follows the overall theory of competition/replacement/exclusion of Volpe et al. (2000). At this time, there is no known system where native fish were abundant yet were replaced by non-native fish.
Competition only occurs when resources are limited, or the system is near carrying capacity (Dunham et al. 2002). Therefore, researchers should carefully examine available evidence to determine if Brook Trout, non-native Rainbow Trout or Lake Trout are competing with the three native trout species, or if they are taking advantage of resources made available as a result of declining native trout density due to other stressors (e.g., habitat changes, over-exploitation). Relatively high densities of Brook Trout, non-native Rainbow Trout or Lake Trout may indicate that the system is near or at carrying capacity, and therefore, competition may be occurring.
b. Hybridization
Currently there is an ongoing effort to understand the hybridization risk and extent of hybridization of Athabasca Rainbow Trout and non-native Rainbow Trout. Previous genetic sampling and analysis indicate that there are watersheds in the range that have only pure genetics while other watersheds contain a mix of pure and hybridized fish, this information will be refined using current genetic techniques and analysis.
The stressor-response curve to depict the impacts of competition and hybridization by non-native species on the three native trout species is a simple linear relationship between the ability of a system to hold adult native trout (system capacity) and the capacity used by Brook Trout, non-native Rainbow, Lake Trout, and hybrids (carrying capacity). Native trout system capacity and non-native/hybridized trout carrying capacity must add to 100% (Figure 1). We have not accounted for the additional adverse effects of hybridization with pure native trout. Rather, this dose-response curve only accounts for competition and replacement related to habitat, so the true negative impacts of these non-native species is likely underestimated.