PNW native-invasive hybrids

Invasive species are well known to have harmful impacts, including outcompeting native species and altering their surrounding habitat. Some of the most recognized invasive species in the US are the Asian carp in the Great Lakes (grass, bighead, and silver carp), famous for reaching high numbers and jumping out of the water and slamming into boaters (though only silver carp are jumpers).

Though rare, some species hybridize with natives. This can lead to genetic homogenization (where all species become genetically similar) or replacement of native genes, or a combination of the two. Hybrid swarms occur when hybrid individuals backcross with one or both of the parent species. This is a problem because once genetic diversity is lost, it is not simply regained. Biodiversity is key to helping species and habitats survive and have resilience in the face of change.

Invasive species hybrids occur in many groups, from plants to fishes to mammals to invertebrates such as crayfish. Here are two examples of invasive hybrids in the Pacific Northwest:

Brook trout (non-native) x Bull trout (native, endangered)

(Salvelinus fontinalis x Salvelinus confluentus)

Native bull trout and non-native brook trout co-occur throughout the West, however hybrids have only been found in certain areas in Montana and Oregon. Hybrids are often sterile, but some genetic evidence points to backcrosses between the hybrids and parent species. Hybridization occurs more often between female bull trout and male brook trout, which means greater wasted reproductive effort for the bull trout (Kanda et al. 2002)


A hybrid found at Sun Creek in Crater Lake National Park (Photo: National Park Service)

Eurasian watermilfoil (non-native) x Northern watermilfoil (native)

(Myriophyllum spicatum x Myriophyllum sibiricum)

The native Northern watermilfoil and the invasive Eurasian watermilfoil have overlapping ranges throughout much of North America. Hybrids have been found in many places where the two species co-occur. While the two species are morphologically distinct, the hybrid takes on qualities of both species and introgression has led to it being indistinguishable in most cases except through genetic testing. More information about both the native and invasive species and hybrid is available at Hybrids have been found in several locations in Washington and Oregon.


Photos from Draft Written Findings of the Washington State Noxious Weed Control Board. Left, leaf variability of Northern (top) vs the hybrid (middle) vs Eurasian watermilfoil (bottom); photo credit: S. Parks (Parks et al. 2014); right, native and invasive species leaves (Photo Jenifer Parsons).


Kanda, N., Leary, R.F. and Allendorf, F.W., 2002. Evidence of introgressive hybridization between bull trout and brook trout. Transactions of the American Fisheries Society 131(4): 772-782.

Parks, S., Thum, R., Pashnick, J. 2014. Incorporating genetic identifications of watermilfoils into
aquatic vegetation mapping to inform management decisions. The Michigan Riparian, Summer
2014: 22-25.



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