U.S. Department of Agriculture: Animal and Plant Health Inspection Service


Date of this Version



Biol Invasions (2015) 17:1713–1727


The range of the barred owl (Strix varia) has expanded westward over the past century and now entirely overlaps the range of the federally threatened northern spotted owl (S. occidentalis caurina) in the Pacific Northwest. We compared Haemoproteus blood parasite assemblages among northern spotted owls in their native range and barred owls in both their native and invasive ranges to evaluate predictions of five hypotheses about parasites and biological invasions: (1) Enemy Release, where hosts benefit from a loss of parasites in their invasive range, (2) Enemy of My Enemy, where invasive hosts introduce parasites to naı¨ve native hosts, (3) Parasite Spillback, where invasive hosts act as a new reservoir to native parasites, (4) Increased Susceptibility, where native hosts introduce parasites to naı¨ve invasive hosts, and (5) Dilution Effect, where invasive species act as poor hosts to native parasites and decrease the density of potential hosts in their invasive range. We used haplotype network analyses to identify one haplotype common to both owl species throughout North America, three more haplotypes that appeared to be isolated to the barred owl’s historic range, and a fifth haplotype that was only found in California. Based on infection status and parasite diversity in eastern and western barred owl populations, we found strong support for the Enemy Release Hypothesis. Northern spotted owls had higher parasite diversity and probability of infection than sympatric barred owls, offering some support for the Parasite Spillback and Dilution Effect Hypotheses. Overall, this study demonstrates the complexity of host-parasite relationships and highlights some of the ways in which species’ range expansions may alter such relationships among both invasive and native hosts.

Included in

Life Sciences Commons