Abstract
The introduction of non-native species can negatively impact native species through reduced genetic fitness resulting from hybridization. The lack of spatiotemporal data on hybrid occurrences makes hybridization risk assessment difficult. Here, we developed a spatially-explicit Hybridization Risk Model (HRM) between native Oregon bull trout, an Endangered Species Act-listed Oregon species, and introduced brook trout by combining an intrinsic potential model (IPM) of brook trout spawning habitat and existing bull trout distribution and habitat use datasets in Oregon, United States. We created an expert-based brook trout IPM classification score (0–1) of streams based on the potential of geophysical attributes (i.e., temperature, discharge, gradient, and valley confinement) to sustain spawning habitats. The HRM included a risk matrix based on the presence/absence of both species as well as the type of habitat (spawning versus other) at 100-m stream segment resolution. We defined the hybridization risk as “extreme” when stream reaches contained bull trout spawning habitat and brook trout were present with IPM moderate or greater scores (IPM >0.5). Conversely, “low” risk reaches contained historic or non-spawning bull trout habitat, brook trout were absent, and IPM scores were low (IPM <0.25). Our HRM classified 34 km of streams with extreme risk of hybridization, 115 km with high risk, 178 km with moderate risk, and 6,023 km with low risk. Our HRM can identify a differential risk of hybridization at multiple spatial scales when either both species coexist in bull trout spawning habitat or are absent. The model can also identify stream reaches that would have higher risk of hybridization, but where brook trout are not currently present. Our modeling approach can be applied to other species, such as cutthroat trout and rainbow trout, Chinook and coho salmon, or similar species occurring elsewhere that potentially hybridize in freshwaters.
Highlights
The tiger salamander (Ambystoma tigrinum), a nonnative species introduced in central California in the 1950s for use as bait in recreational fishing (Johnson et al, 2011), interbreeds with the rare native California tiger salamander (A. californiense), threatening its persistence (Riley et al, 2003)
The corresponding intrinsic potential model (IPM) scores were calculated at the 100-m reach scale and classified as Low (0.75) intrinsic potential for brook trout spawning use (Figure 5)
Major areas of overlap between bull trout habitat and brook trout spawning habitat were in the northeastern region—279 km (Figure 5B), followed by the northern Cascades—258 km (Figure 5A), and the Sprague River area—177 km (Figure 5C)
Summary
The introduction of non-native or invasive species poses several challenges to the conservation of native species. There is the risk of hybridization between native and non-native species, which has received less attention among researchers and natural resource managers (Dunham et al, 2004; Fausch et al, 2006; Gozlan et al, 2010; Al-Chokhachy et al, 2014; Kilshaw et al, 2016). Hybridization occurs naturally in wild populations as part of the evolutionary process of many taxa (Arnold 1997). When hybridization occurs between native and nonnative species it can lead to reduced genetic fitness and/or genetic extinction of native species (Rhymer and Simberloff 1996). In many western North American rivers, introgressive hybridization between salmonids (Bettles et al, 2005; Muhlfeld et al, 2009; Araujo et al, 2021) such as introduced rainbow trout (Oncorhynchus mykiss) and native cutthroat trout (O. clarkii lewisi and O. clarki clarki) has reduced the reproductive fitness of the latter (Bettles et al, 2005; Muhlfeld et al, 2009)
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