In the local paper they recently mentioned that a plan has been created that involves removing barred owls from all northern spotted owl habitat via lethal measures. The reasoning behind this planned seemed to be mostly presumption and conjecture. Wanting to know more about the impact barred owls are having on northern spotted owls and the reasoning behind the plan I wrote about this issue for my Environmental Issues class presentation. What I discovered is that there isn't much scientific evidence that barred owls are to blame for the northern spotted owl population declines. Loss of habitat leading up to the northern spotted owl's listing under the Endangered Species Act is the main culprit but there are all manner of other confounding factors. The paper follows.
Northern Spotted Owls vs. Barred Owls: Interspecific Complications, the Endangered Species Act and Management Implications
The Situation
One of the greatest conservation and land use debates, in the Pacific Northwest, in the last two decades has been the listing of the northern spotted owl (Strix occidentals caurina) as Threatened under the Endangered Species Act (ESA) of 1973. The 1990 listing of the northern spotted owl lead to a 90% decrease in logging on 24 million acres of federal lands, managed by both the US Department of Agriculture Forest Service (USFS) and the Bureau of Land Management (BLM), over the last twenty years. This decrease in timber harvest lead to an economic collapse in many parts of the Pacific Northwest, particularly Oregon, as private mills and timber companies had to close their doors.
The northern spotted owl is a small brown owl with white mottling or spots that hide it well in the dense canopies of late-successional forests, also favored by timber companies, that are found in their geographic range. Various studies conducted in the 1980s indicate that habitat is a limiting factor in the survivability of northern spotted owls and as such must be conserved in order to ensure the survival of the species. The 1990s-era closure of much of the late-successional forests of the Pacific Northwest ensured the habitat would be available for the expansion of northern spotted owl populations. Despite the availability of habitat northern spotted owl numbers continue to decrease by three times the rate of decrease noted at the time of listing.
Concurrent with the decrease in northern spotted owl numbers scientists have noted an increase in the numbers of barred owls found in the Pacific Northwest. The barred owl (Strix varia varia) is a cousin of the northern spotted owl and originally found east of the Great Plains, from Maine to Florida. There are many theories as to how and why the barred owl arrived in the west. These theories include an increase in the number of small stands of trees in the Great Plains, increasing summer temperatures creating a more ideal temperature range in the boreal forests of Canada, increased fire suppression efforts and changing forest dynamics due to timber harvests. Regardless of the reason that barred owl range expanded west their ability to live in diverse habitats, eat a diversity of prey species, their more aggressive nature and 20% larger size allows them to outcompete northern spotted owls for habitat and food.
In 2007 resource managers began proposing barred owl elimination programs in an effort to combat the interspecific competition happening between barred and northern spotted owls. These programs call for the shooting of hundreds of barred owls in the Pacific Northwest and have been met with significant opposition from the scientific community and the general public. Many scientists agree that controlled barred owl elimination studies could provide helpful in determining the overall impact that barred owls are having on northern spotted owl populations but a large scale culling would be time consuming, could be expensive and would have to be regularly maintained in order to have lasting impacts.
Background Information
· In 1973 the ESA was passed. This allowed for the designation of species as either endangered or threatened, defining endangered as a species “in danger of extinction throughout all or a significant portion of its range” and threatened species as “a species likely to become endangered.” Under Section 4 of the ESA allows for subspecies, such as the northern spotted owl, to be listed as a threatened or endangered “species,” allow the same level of protection as is given a species and requires both a Recovery Plan and Critical Habitat Designation to be created for all listed species (Stanford Environmental Law Society, 2001).
· The Interagency Scientific Committee to Address the Conservation of the Northern Spotted Owl (ISC) found that northern spotted owls select habitat with a relatively closed canopy, a variety of vegetation layers with a diversity of canopy species dominated by large overstory trees, large trees with broken tops or cavities, the presence of large snags, large logs and woody debris on the forest floor and openings within and beneath the canopy. These habitat attributes are typically found in mature late-successional forest but can be found in younger mid-successional forests that retain either several individual large trees or small stands of large trees from the original late-successional stand (Thomas, Forsman, Lint, et al, 1990).
· At the time of listing, all the studies of Spotted Owl habitat use concluded that owls select old forests or younger forests that have retained characteristics of old forests. Studies published since the ESA listing decision provide additional support that habitat selection is significantly nonrandom (Noon and Blakesley, 2006).
· Barred owls were first sighted west of the Rocky Mountains in 1912, in southern Alberta, Canada (Levy, 2004).
· Northern spotted owl populations have continued to decline since the 1990s, despite being listed as Threatened under the Endangered Species Act and habitat conservation efforts. Scientists are uncertain as to why this decline isn’t reversing.
· Weather and climate can impact northern spotted owl reproduction, recruitment, annual survival and population growth rates. Increasing drought conditions and increased precipitation during nesting season will likely negatively affect northern spotted owl populations (Glenn, Anthony and Forsman, 2010).
· Interspecific hybridization between barred owls and northern owls, creating offspring called “sparred owls” occurs in some instances. These instances often occur in areas that barred owls have recently invaded and are therefore less common than northern spotted owls or in areas where northern spotted owls have become rare and barred owls are common.
· There are many examples of multiple owl species coexisting in the same habitat. These species are typically from different genera and are successful in coexisting due to interspecific differences in behavior (Gutierrez, et al.; 2006).
· Owls exhibit reverse sexual dimorphism; females are larger than males.
· Barred owls from the Strix varia sartorii subspecies and spotted owls from the Strix occidentalis lucida subspecies have a sympatric relationship in Mexico. S. v. sartorii is the largest subspecies of barred owl and S. o. lucida is the smallest subspecies of spotted owl. The difference in mass of these species is a factor of two, a ratio that facilitates coexistence. The northern spotted owl S. o. caurina and the northern-most subspecies of barred owl S. v. varia differ in mass by a factor of 1.8, a number that statistically seems to support competition between species (Gutierrez, et al.; 2006).
· Genetic bottlenecks have been detected in many populations of northern spotted owls. These bottlenecks create situations of inbreeding depression that could decrease the rate of reproduction and an increase in population decline. A decline in genetic variability combined with competition with and exclusion by barred owls, climate change and habitat loss may be confounding factors in the efforts to save the northern spotted owl from extinction (Funk, et al.; 2009).
· From 1994-2003, northern spotted owl populations declined by 7.5% per year throughout northern ranges and 2% per year throughout their southern ranges (Rapp; 2008).
· Timber harvest on federal lands has occurred at only about 54% of the amount allowed under the Northwest Forest Plan (Rapp; 2008).
· Fire suppression in areas where fires are a normal part of the ecosystem, for example in the Klamath-Siskiyou bioregion, can put northern spotted owls at greater risk of habitat loss because of the build up of fuels.
· About 59% of spotted owl habitat occurs on federal lands (Rapp; 2008).
· According to the 10-year review of the Northwest Forest Plan the wood products industry employment has dropped by 70% since the implementation of the Northwest Forest Plan, due in part to the lower than expected number of boardfeet of timber harvested.
· In areas where barred owl numbers are the highest and time since invasion has been the longest spotted owl numbers have decreased by 20-25% (Wiens, Anthony and Forsman; 2011).
· The high cost of putting together a timber sale due to the required surveys (spotted owl and various other wildlife surveys) and potential lawsuits has contributed greatly to the greater reduction in logging that required by the Northwest Forest Plan and northern spotted owl Habitat Conservation Plan.
· A recovery plan for the northern spotted owl was drafted in 1992 but was never signed because of the creation of the Northwest Forest Plan in 1994. In 2008 the US Fish and Wildlife Service issued a recovery plan for the northern spotted owl. After pressure from scientists and a court challenge of the recovery plan the Service filed for a voluntary remand of the plan, filing a Revised Recovery Plan in 2010. The final Revised Recovery Plan was released June 30, 2011.
· The Revised Recovery Plan will likely increase timber harvest in spotted owl habitat as the USFWS recommends “ecological forestry” management that allows for thinning in late-successional ecosystems that northern spotted owls are found in (USFWS; 2011).
Literature Cited and Bibliography
Buchanan, J.B., R.J. Gutierrez, R.G. Anthony, T. Cullinan, L.V. Diller, E.D. Forsman, A.B. Franklin. 2007. A synopsis of suggested approaches to address potential competitive interactions between Barred Owls (Strix varia) and Spotted Owls (S. occidentalis). Biological Invasions 9:679-691.
Funk, W.C., E.D. Forsman, M. Johnson, T.D. Mullins and S.M. Haig. 2010. Evidence for recent population bottlenecks in northern spotted owls (Strix occidentalis caurina). Conservation Genetics 11:1013-1021.
Glenn, E.M., R.G. Anthony and E.D. Forsman. 2010. Population trends in northern spotted owls: Associations with climate in the Pacific Northwest. Biological Conservation 143:2543-2552.
Gutierrez, R.J, M. Cody, S. Courtney and A.B. Franklin. 2006. The invasion of barred owls and its potential effect on the spotted owl: a conservation conundrum.
Levy, S. 2004. Native Incursions: Avian Range Expansions imperil Threatened Species. Bioscience 54:94-98.
Rapp, V. 2008. Northwest Forest Plan— The First 10 Years (1994–2003):
First-Decade Results of the Northwest Forest Plan. USDA General Technical Report PNW-GTR-720. Pacific Northwest Experimental Station, US Forest Service. Portland, OR.
Stanford Environmental Law Society. 2001. The Endangered Species Act. Stanford University Press, Standford, California.
Thomas, J.W., E.D. Forsman, J.B. Lint, E.C. Meslow, B.B. Noon and J. Verner. 1990. A Conservation Strategy for the Northern Spotted Owl. USDA Forest Service, Bureau of Land Management, US Fish and Wildlife Service and National Parks Service. Portland, Oregon.
US Fish and Wildlife Service. 2011. Revised Recovery Plan for the Northern Spotted Owl (Strix occidentalis caurina). Region 1 US Fish and Wildlife Service. Portland, OR.
Wiens, J.D., R.G. Anthony and E.D. Forsman. 2011. Barred Owl Occupancy Surveys within the Range of the Northern Spotted Owl. Journal of Wildlife Management 75(3):531-538.
Related Environmental Issues
1. Climate Change
2. Fire management and fire regimes (fire suppression, altered regimes, etc)
3. Congressional efforts to eliminate or decrease the effectiveness or “power” of the Endangered Species Act and the implications of these efforts on northern spotted owls.
4. Unsustainable (non-ecologically sound) forestry management practices
5. Exponentially expanding human population/carrying capacity of humans
6. Clear-cutting on private land
7. Management of native species vs. endangered native species
There are a variety of actions being proposed by various groups, agencies or organizations. Many timber companies are claiming that barred owls have been to blame for the spotted owl decline all along and are advocating for increased timber harvests in mature or late-successional forests in the northern spotted owl’s geographic range. Some forest managers have suggested eliminating barred owls from spotted owl habitats, while scientists prefer the idea of experimental removal, as not enough is understood about interspecific interactions between barred owls and spotted owls. Still other scientists, conservationists and members of the general public advocate for no action under the premise that barred owls are a native species and that it is natural for shifts in species to occur.
Author’s Position
The majority of information present explores northern spotted owls and habitat use or influences of habitat. Barred owl interactions with northern spotted owl populations have become a popular topic of study in the last five years. Based on the available science more research needs to be done in terms of how barred owls are affecting northern spotted owl populations and what other factors are confounding recovery efforts. One research area should be climate/weather as climate change or shift occurs researchers, conservationists and land managers could see further decreases in northern spotted owl numbers. Another area of study that will be important is the specifics of interspecies interactions that barred owls and northern spotted owls experience. Small scale experimental control of barred owl populations in northern spotted owl habitat will provide researchers with the information they need to assess the threat from barred owls to spotted owl recovery. The actions that should not be taken are increasing timber harvesting (aside from some thinning of areas that are in need of thinning to promote forest health), large scale removal of barred owls from spotted owl habitat and the “no action” action. Drastic measures are almost always ill-advised, particularly in manners of wildlife or land management.
1 comments:
good stuff
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