CORVALLIS, Ore. – A new analysis of northern spotted owls in the Pacific Northwest has found negative associations between population growth rates for northern spotted owls and both dry growing seasons and cold, wet winters and nesting seasons, raising concern that climate change may negatively affect their already fragile existence.
The study also found a link between lower population growth rates and the presence of barred owls, which are larger and more aggressive than spotted owls and can out-compete them for territory.
Results of the research have just been published in the journal Biological Conservation.
“Adult spotted owls have fairly high annual survival rates, while reproduction is much more variable,” said lead author Elizabeth “Betsy” Glenn, who conducted much of the research as part of her doctoral dissertation at Oregon State University. “We found that survival was more closely related to regional climate measures such as drought indices, while recruitment of new owls into the population was more often associated with local weather conditions.”
Cold, wet conditions in the spring can have “direct effects on the survival of fledgling owls,” Glenn said, while the availability of prey can affect both adult survival and reproductive success.
“Particularly dry summers may cause populations of northern flying squirrels, woodrats and other small mammals to decline, which ultimately affects survival, recruitment and population growth rates of owls,” Glenn pointed out.
Most climate change models suggest the Pacific Northwest will experience warmer, wetter winters and hotter, drier summers, according to Glenn, who is now working as a post-doctoral researcher at the University of Montana. These conditions were associated with lower population growth rates, survival, and recruitment in this study, raising concern that future climate conditions may be less favorable for spotted owls.
Other authors on the journal article were Robert Anthony, of OSU’s Department of Fisheries and Wildlife and the U.S. Geological Survey; and Eric Forsman, a U.S. Forest Service ecologist who has a courtesy appointment at OSU.
In their study, the scientists looked at the rate of population growth among spotted owls over a period of nearly 20 years at six Northwest locations: the Olympic Peninsula and Cle Elum in Washington; and the Oregon Coast Range, Tyee, the H.J. Andrews Forest, and the southern Cascade Mountains in Oregon. During the years of the study, from 1985 to 2005, there was a wide range of weather conditions that included stormy years, drought, and strong El Niño and La Niña events. Further, the six sites encompass different terrain and impacts of weather.
The amount of variation in population growth rates that was explained by weather and climate varied widely across all six areas, indicating that other factors – such as habitat – also are important, the authors point out. In addition to association with climate, the researchers found the presence of barred owls affected spotted owl population growth rates in four of the six locations.
Wrote the authors in their conclusion: “Given that natural resource managers cannot control climate variation and barred owls are likely to persist and increase in the range of the northern spotted owl, maintaining sufficient high quality habitat on the landscape remains the most important management strategy for the conservation of this subspecies.”