Barred Owls reduce occupancy and breeding propensity of Northern Spotted Owl in a Washington old-growth forest

Abstract

Protected lands like national parks are important refuges for threatened and endangered species as environmental pressures on wildlife and their habitats increase. The Northern Spotted Owl (Strix occidentalis caurina), a species designated as threatened under the Endangered Species Act, occurs on public lands throughout the western United States including Mount Rainier National Park (MRNP), Washington. With virtually no history of timber harvest or large forest disturbance within MRNP boundaries since the park’s creation in 1899, MRNP provides an ideal place to evaluate potential impacts of climate change and invasive Barred Owls (Strix varia) on the Northern Spotted Owl. We used a multi-state, multi-season occupancy model to investigate how Northern Spotted Owl occupancy dynamics and breeding propensity are related to the presence of Barred Owls, local and regional weather, and habitat characteristics at MRNP from 1997 to 2016. Historical occupancy of Northern Spotted Owl breeding territories in MRNP has declined by 50% in the last 20 yr, and territory occupancy by breeding Northern Spotted Owls also decreased, reaching a low of 25% in 2016. Occupancy rates were higher on territories with steeper terrain and breeding rates were lower when Barred Owls were detected within historical territories. Our results also indicated that breeding propensity was higher when early nesting season temperatures during March and April were higher. In addition, the ability to detect breeding Northern Spotted Owls decreased when Barred Owls were present in the territory. Habitat variables from LiDAR were not correlated with Northern Spotted Owl occupancy dynamics, likely reflecting the dominance of old-growth forest in this protected park. This study illustrates the strong relationship between Barred Owls and Northern Spotted Owl demographics and breeding site selection in a landscape where habitat loss by timber harvest and fire has not occurred.

Range‐wide sources of variation in reproductive rates of northern spotted owls

“…Our estimates of state-specific occupancy transition probabilities strongly supported our prediction that sites not occupied by pairs the previous year were much less likely to be occupied by pairs in the current year, and sites at which successful reproduction occurred in the previous year were more likely to be occupied than previously occupied sites without successful reproduction. These findings are consistent with previous research on spotted owls (Lee & Bond, 2015;MacKenzie et al, 2009;Mangan et al, 2019) and other territorial raptor species (Le on- Ortega et al, 2017;Martínez et al, 2006;Wiens et al, 2018).…”

“…Habitat components, such as the availability of forest structure for nesting and roosting, are thought to influence nearly every vital rate of northern spotted owls: apparent survival (Dugger et al, 2005 , 2016 ; Franklin et al, 2000 ; Olson et al, 2004 ), local colonization and extinction rates (Dugger et al, 2016 ; Yackulic et al, 2019 ), and reproduction (Dugger et al, 2005 ; Franklin et al, 2000 ; Olson et al, 2004 ). Barred owls, native to forests of eastern North America, are now present across the range of the northern spotted owl and are causing significant, negative impacts to northern spotted owl site occupancy, breeding propensity (Diller et al, 2016 ; Dugger et al, 2016 ; Mangan et al, 2019 ; Wiens et al, 2014 ; Yackulic et al, 2014 , 2019 ), and breeding dispersal dynamics (Jenkins et al, 2019 , 2021 ). The use of site‐level cover type and barred owl covariates may help to explain some of the site‐level spatial drivers of successful reproduction that were not discernible under the previous meta‐analytic framework.…”

Spotted owls and forest fire: Comment

“…." In support of this conclusion, Lee (2018) cites studies suggesting that mixed-severity fire typically affects "a very small portion (0.02-0.50%) of spotted owl nesting and roosting habitat per year..." We agree that severe fire has not yet resulted in substantial declines in spotted owl populations at regional or subspecies scales and rather that recent declines (Conner et al 2016 have occurred for other reasons including competition with barred owls (Diller et al 2016, Mangan et al 2019, potentially the loss of large trees and oldforest habitat , and potentially shifts in prey communities (Hobart et al 2019). Certainly, severe fire has caused declines in spotted owl abundance at more local scales (e.g., 100s of km 2 ; Jones et al 2016) and resulted in an enduring loss of nesting and roosting habitat, but fire has not been an overriding driver of recently observed long-term spotted owl population declines.…”

Fifth Oregon climate assessment

“…Across the species' range, survival was higher in the year following a shift in the Pacific Decadal Oscillation from the cool to warm phase (Glenn et al 2011). In Mount Rainier National Park, Washington, which is cool and wet relative to much of the range of Northern Spotted Owls, territory occupancy was related positively to mean minimum temperatures during the early nesting period (March and April) (Mangan et al 2019) Accordingly, at that local extent, projected increases in spring temperatures may benefit the species.…”