Demographic response of northern spotted owls to barred owl removal

Abstract

Federally listed as threatened in 1990 primarily because of habitat loss, the northern spotted owl (Strix occidentalis caurina) has continued to decline despite conservation efforts resulting in forested habitat being reserved throughout its range. Recently, there is growing evidence the congeneric invasive barred owl (Strix varia) may be responsible for the continued decline primarily by excluding spotted owls from their preferred habitat. We used a long-term demographic study for spotted owls in coastal northern California as the basis for a pilot barred owl removal experiment. Our demography study used capture-recapture, reproductive output, and territory occupancy data collected from 1990 to 2013 to evaluate trends in vital rates and populations. We used a classic before-after-control-impact (BACI) experimental design to investigate the demographic response of northern spotted owls to the lethal removal of barred owls. According to the best 2-species dynamic occupancy model, there was no evidence of differences in barred or northern spotted owl occupancy prior to the initiation of the treatment (barred owl removal). After treatment, barred owl occupancy was lower in the treated relative to the untreated areas and spotted owl occupancy was higher relative to the untreated areas. Barred owl removal decreased spotted owl territory extinction rates but did not affect territory colonization rates. As a result, spotted owl occupancy increased in the treated area and continued to decline in the untreated areas. Prior to and after barred owl removal, there was no evidence that average fecundity differed on the 2 study areas. However, the greater number of occupied spotted owl sites on the treated areas resulted in greater productivity in the treated areas based on empirical counts of fledged young. Prior to removal, survival was declining at a rate of approximately 0.2% per year for treated and untreated areas. Following treatment, estimated survival was 0.859 for the treated areas and 0.822 for the untreated areas. Derived estimates of population change on both study areas showed the same general decline before removal with an estimated slope of -0.0036 per year. Following removal, the rate of population change on the treated areas increased to an average of 1.029 but decreased to an average of 0.870 on the untreated areas. The results from this first experiment demonstrated that lethal removal of barred owls allowed the recovery of northern spotted owl populations in the treated portions of our study area. If additional federally funded barred owl removal experiments provide similar results, this could be the foundation for development of a long-term conservation strategy for northern spotted owls. Ó 2016 The Wildlife Society.

Barred owl habitat selection in west coast forests

“…Barred owls probably also displaced spotted owls there, but those timberlands have a lower percentage of historical spotted owl sites currently occupied by barred owls than in RNSP. A barred owl removal program was initiated in 2009 in parts of that area (GDRC , Diller et al ), but such removals did not occur in the areas that we studied. We did not learn the history of barred and spotted owl occupancy on the BLM's Headwaters Reserve, but it contained 3 pairs of northern spotted owls and 4–5 pairs of barred owls during the study (D. C. Anthon, personal communication).…”

Review of the effects of barred owls on spotted owls

“…Clearly, there is significant evidence that behavioral dominance and competition from barred owls negatively affects spotted owl population viability where the 2 species co‐occur. As further evidence of the negative effects of competition, after removal of barred owls in northern California, spotted owl occupancy increased in treated areas while occupancy continued to decline in untreated areas (Diller et al ). The preponderance of evidence suggests that without mitigative efforts, barred owls will eventually drive northern spotted owls to extinction throughout most of their range.…”

“…They determined that it was a relatively quick, effective, and low‐cost program. Following removal, spotted owl occupancy made a slow recovery in areas where barred owls were removed compared to untreated areas where spotted owl occupancy declined (Diller et al ). Spotted owl fecundity rates did not change in the treated areas; however, the greater number of pairs in treated areas resulted in greater population‐level productivity.…”

“…The rapid expansion of barred owls across the Pacific Northwest and, more recently, through the Sierra Nevada has motivated conservationists, researchers, and managers to assemble the local and regional management plans necessary to mitigate the harmful effects of barred owls on spotted owls. With the start of an experimental 5‐year plan to lethally remove barred owls from demographic study areas (Diller et al , ; Wiens et al , Wiens et al, ), there is evidence that removals can increase population growth of spotted owls in at least some areas. As noted earlier, evidence also suggests that quality habitat may buffer spotted owls from the negative influences of barred owls (Dugger et al ) and that spotted owls may have slight differences in use of forests and terrain, such as denser understory and steeper terrains, to carve out refugia (Jenkins et al ).…”

Exposure to rodenticides in Northern Spotted and Barred Owls on remote forest lands in northwestern California: evidence of food web contamination

“…However, over the course of several decades, the Barred Owl (BO; Strix varia) has expanded from its historical range in eastern North America to its current range including the Western United States (Gutiérrez et al 2007). Barred Owls now overlap ESA critical habitat designated for NSO and competes with NSO for natural resources that facilitate successful breeding, feeding, and sheltering for the species (United States Fish and Wildlife Service 2011, Wiens et al 2014, Diller et al 2016, Dugger et al 2016). …”