In the US, we’re often concerned about invasive and range-extending species entering ecosystems through waterways (zebra mussels, hydrilla), across our southern border (fire ants, nine-banded armadillos), or as escapees from gardens and yards (Chinese tallow, Nandina). However, one owl species-the Barred Owl, Strix varia-has expanded its range over the last 50 years eastward through Canada, and then across the border, into the western US. Many of us think of owls as charismatic and fascinating bird species, beneficial because they prey on rodent pests, but the invasion of the Barred Owl into the forests of the Pacific Northwest threatens the survival of a close relative, the Spotted Owl (Strix occidentalis). The Northern and California Spotted Owl subspecies have been the subject of a great deal of controversy and antagonism between environmentalists, government agencies, loggers, and the timber industry, and loss of the old growth forest habitats required for the reproductive success of these birds has severely reduced their numbers. And as Ishak and colleagues report in a recent PLoS ONE article (DOI: 10.1371/journal.pone.0002304), the invasive Barred Owl affects Spotted Owls through aggressive competition for food and nesting materials, and possibly through introduction of haemosporidian parasites.
Barred Owl, Strix varia
Image source: Wikipedia
Infectious diseases, such as those caused by the West Nile Virus and H5N1 avian influenza virus, are increasing in prevalence and distribution throughout the world, and in many cases are pertinent to conservation biology, as well as to human health. Woodworth and colleagues (2005) examined the infection rates for introduced vector-borne malaria in a Hawaiian species, the Amakihi (Hemignathus virens), and found that 55 to 83% of these birds were seropositive for Plasmodium relictum. The majority of infected birds succumb to the anemia and weight loss caused by the disease, and those individuals that survive remain infective to mosquitoes throughout their lifespan. In addition to Plasmodium, there are two other haemosporidian blood parasite genera, spread to birds by insect vectors: Leucocytozoon, and Haemoproteus. As for human hosts, blood parasites can have devastating effects on the fitness of avian hosts, and thus to compare infection susceptibility for the endangered Spotted Owl, Ishak and colleagues surveyed the species and distributions of haemosporidians in blood samples from North American, European, and African Owls.
Samples were obtained from 317 individuals of Strigidae species and 225 individuals of Tytonidae species, and the blood parasite species were identified using both sequence (mitochondrial cytochrome b gene) and morphological criteria. Sixty two percent of Strigidae family owls were infected with at least one blood parasite species, whereas only 24% of Tytonidae family owls showed signs of infection. The California Spotted Owl subspecies was more likely than the Northern Spotted Owl to be infected with blood parasites, and both subspecies of Spotted Owls were much more likely to have blood parasites than were the sympatric western Barred Owls. The cytochrome b sequence data allowed the researchers to identify two clades of Leucocytozoon, and the Strigidae family were found to be more susceptible to infection by both clades, than were Tytonidae (Barn Owls). The figure below shows the species diversity of Leucocytozoon lineages present in the blood of Great-horned, Spotted, Western Screech, and Barn Owls.
Similar analyses for Plasmodium and Haemoproteus species parasites revealed the first reported case of Plasmodium infection in a Spotted Owl, which may have originated from any one of several California owl species known to harbor this haemosporidian. Contrary to expectations, the researchers found that Spotted Owls and Barred Owls, though they are closely related and share similar habitats and life histories, exhibit markedly different rates of infection and susceptibilities to multiple parasite species. Three different explanations were proposed in the discussion: 1) Barred Owls utilize a wider range of habitats, perhaps resulting in different vector contacts, 2) the West Coast habitat of the Spotted Owl has fewer Plasmodium parasites than the historic Barred Owl range, and 3) Barred Owls have a better immune response to the parasites. In support of the third hypothesis, the investigators pointed out that the Spotted Owl population has recently experienced a loss of genetic variability, and these birds may have poorer immune health overall as a result. Another concern raised by this study is that the high lineage diversity for Leucocytozoon species in Spotted Owls indicates low host specificity, and thus increased risk of infection by novel parasites that may be more virulent. Future research should be directed at characterizing the immune health status of endangered Spotted Owl subspecies, and at determining the role of immigrant Barred Owls, if any, in the spread of blood parasites.
Woodworth, B.L., Atkinson, C.T., LaPointe, D.A., et al. (2005) Host population persistence in the face of introduced vector-borne diseases: Hawaii amakihi and avian malaria. Proc. Natl. Acad. Sci. USA 102(5), 1531-1536.
Ishak, H.D., Dumbacher, J.P., Anderson, N.L., Keane, J.J., ValkiÅ«nas, G., Haig, S.M., Tell, L.A., Sehgal, R.N., Baylis, M. (2008). Blood Parasites in Owls with Conservation Implications for the Spotted Owl (Strix occidentalis). PLoS ONE, 3(5), e2304. DOI: 10.1371/journal.pone.0002304