Wildfire regimes throughout western North America are changing and threatening the long-term persistence of native species. However, the degree and pace of those changes varies from one region to the next, as does a species’ ability to cope with those changes. In the frequent-fire forests (forests that burn every 10 – 20 years) of the Pacific Northwest, changing fire regimes are causing concern about the influence the changes may have on critically imperiled northern spotted owl populations (Franklin et al. 2021).

The northern spotted owl was listed as threatened under the Endangered Species Act in 1990 (U.S. Fish and Wildlife Service 1990) citing habitat loss due to logging as the main factor contributing to its decline. Since its listing, habitat loss due to logging on public lands has been greatly curtailed, but populations have continued to decline in large part due to the relatively recent invasion of nonnative barred owls into the range of the northern spotted owl, where they outcompete northern spotted owls for territories (Wiens et al. 2021). Remaining northern spotted owl population strongholds currently exist in these frequent-fire forests of southwestern Oregon and northwestern California; a region that is expected to see increasing large fire activity over the coming decades (Davis et al. 2017). Should uncharacteristically severe wildfires occur within this remaining population stronghold, the persistence of northern spotted owls could be put at risk.

Because northern spotted owls in this part of their range have evolved with frequent fire as a natural disturbance process, they have developed what ecologists refer to as, ‘disturbance-adapted traits’ to cope with, and persist through fire. However, if fires begin burning more frequently or severely than they did in the past, they could exceed the adaptive capacity of northern spotted owls and cause negative, population-wide impacts. By combining the two long-term datasets with concepts such as a native species’ adaptive capacity, the authors hoped to create a more ecologically informed assessment of changing fire regimes in this part of the owl’s geographic range.

Their recently published findings indicate that survival rates of spotted owls declined, and territory displacement increased as the extent and severity of wildfire within an owl’s territory increased. Importantly however, researchers noted that most of the fires that were used in their analysis burned with generally benign fire effects so that relatively few owls in their analysis were negatively affected by fire. Results suggest that although important negative effects of fire were observed for individuals, not enough individual owls were negatively affected to result in negative population-wide impacts.

“While our results may seem contradictory, we believe they actually reflect the historical mixed-severity fire regime of this region, which is characterized by relatively frequent, mostly lower severity fire with smaller patches of moderate and high severity fire intermixed, punctuated by occasionally more severe, weather-driven fires that would burn up multiple owl territories,” said Jeremy Rockweit, PhD candidate and study lead author.

Because spotted owls in this region have evolved with the mixed-severity fire regime, they likely have adaptations that allow them to persist in this frequent-fire environment provided that patterns of fire frequency and severity are similar to historical conditions under which the owls evolved. “Our results suggest that while we have observed an increase in fire activity over the past three decades in the fire-prone region of our study, this increase appears to have not exceeded the adaptive capacity of northern spotted owl populations at least through 2018, which was the last year of wildfire data we evaluated,” noted Rockweit.

However, the authors urge caution when interpreting their results. Although fire regimes in this region appear to currently be operating within the adaptive capacity of northern spotted owls, contemporary northern spotted owls displaced by fire face a whole suite of additional stressors such as habitat fragmentation and competition with barred owls that historically spotted owls did not have to contend with.

These potential ‘indirect’ effects exacerbated by fire could be severe, but the current study was not able to address those potential impacts. Furthermore, with wildfire activity predicted to continue increasing over the next several decades, it seems probable that the adaptive capacity of northern spotted owls will one day be exceeded. If and when this exceedance happens, it will add yet another stressor to the remaining populations of this critically imperiled species.

The ONB features articles from Cooperative Fish and Wildlife Research Units across the country. Working with key cooperators, including WMI, Units are leading exciting, new fish and wildlife research projects that we believe our readers will appreciate reading about. This article was written by Jeremy Rockweit, jeremy.rockweit@oregonstate.edu, PhD candidate, Oregon State University.