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California spotted owls prefer protected areas
To understand which characteristics of forests California spotted owls prefer, researchers determined it was important to take a look back in time.
As part of a study published in The Condor: Ornithological Applications, a research team compared how characteristics of four different sites in the Sierra Nevada affected site occupancy of California spotted owls (Strix occidentalis) in the areas from 1993-2011. Three of the study areas were national forests which were managed for logging and other activities — Lassen National Forest in the northern Sierra Nevada and southern Cascades, Eldorado and Tahoe National Forests in the central Sierra Nevada — while the fourth study area was in Sequoia and Kings Canyon National Parks, which is a protected area in the southern Sierra Nevada. All of the forests studied are primarily mixed-conifers.
“Most of the time when we do these retrospective studies, we are looking at areas managed in very similar ways,” said long-time TWS member Rocky Gutiérrez, a Gordon Gullion Chair Emeritus at the University of Minnesota and coauthor of the study. “Here, we have one that’s protected. And what we found was that the occupancy rates were higher on the protected areas while the rates showed decline on the three national forests.”
While Mexican spotted owls (Strix occidentalis lucida) and northern spotted owls (Strix occidentalis caurina) are listed as federally threatened species, the California spotted owl still has two listing petitions before the U.S. Fish and Wildlife Service. Gutiérrez hopes this research helps inform that decision.
As part of the study, which Gutiérrez says is the first to link the various characteristics of four study areas with trends in California spotted owl occupancy, the researchers looked at whether or not the birds were detected in these areas while simultaneously taking note of habitat characteristics such as canopy cover, climate covariates, logging and fire presence. In the national forests, some areas are available for logging and there were no large-scale fire restoration projects like there are in national parks.
Gutiérrez and his colleagues completed a statistical analysis in which they compared different variables with occupancy of owls in these study sites. They found that canopy cover was most closely correlated with higher occupancy. “Owls do better in higher canopy cover areas than they do in low canopy areas,” he said. Although the team didn’t examine whether the owls preferred high canopy cover formed by small or big trees as part of this study, there’s ongoing research to address that angle, which would help determine if there is flexibility in performing treatments such as logging to reduce the risk of forest fire. “If owls are flying only in high canopy cover in large trees, then doing forest restoration activities in forests that have high canopy cover but small trees would be less likely to impact owls,” Gutierrez said.
Since the U.S. Forest Service hadn’t documented when and how logging occurred in the past, Gutiérrez and his team used remote sensing data to analyze vegetation to determine when logging likely occurred. “From that we were unable to actually show any relationship between changes in territory occupancy based on what we inferred were changes in logging,” he said. “However, we have to be cautious.” Gutiérrez added that anytime researchers attempt to infer logging from a secondary source, there is a possibility that they can underestimate logging over time. “Although we couldn’t show any effects, it doesn’t mean it didn’t occur,” he said.
When looking at the effects of fire on owl occupancy, the team was able to analyze actual fire maps that showed the perimeter of fire and how much fire was located in particular areas. “It was a much easier characteristic to estimate,” he said. The researchers found that fire only appeared to negatively affect occupancy of owls on the national park study areas.
After analyzing the results of their study, Gutiérrez and his team concluded that there’s good evidence that the California spotted owls are declining as estimated by occupancy trends on the national forest study areas, but owl occupancy remained stable in the protected national parks.