Dec. 1, 2000 When the Northern Spotted Owl (Strix occidentalis caurina) was federally listed as a threatened species in 1990, environmental groups working in the Pacific Northwest quickly embraced the animal as a mascot for the natural areas of the region.
Studies conducted early in the decade indicated that the birds had a strong affinity for old growth trees, and the mature forests were assumed to be equivalent to Spotted Owl habitat.
In the last decade, many acrimonious battles have been pitched over the fate of land which is simultaneously home to the owls and full of economically valuable timber. But beneath the din of these conflicts, little attention has been paid to other factors which may be influencing the species.
Results from a long-term study published this month by the Ecological Society of America provide insight into the population dynamics of these birds of prey and explore the role which climate and habitat quality play in determining the survival of this species.
The study, published in the peer-reviewed ESA journal Ecological Monographs (Vol. 70 no.4), was conducted by Alan B. Franklin and David Anderson of the US Geological Survey at Colorado State University, and R. J. Gutierrez and Kenneth P. Burnham of Humboldt State University (now at the University of Minnesota and the US Geological Survey of Colorado State University, respectively).
The team analyzed data from a ten year study in the forests of Northwestern California where owls were surveyed, their reproductive output recorded, and relative fitness estimated.
The study area, which is centered around Trinity River east of Eureka, is located upon land managed by the US Forest Service and the Bureau of Land Management. The area is considered to be one of the most complex and diverse in the western United States due to its unique blend of coniferous and hardwood trees. The area is logged, however, by timber companies, much like many other forests within the region.
A total of ninety owl sites were examined. Climate data were also collected, including precipitation amounts and temperatures on the study area. The territories defended by the birds in the study were recorded, and these territories were measured and tracked throughout the ten years in which data was gathered.
Cold, Wet Springs Decrease Survival Rates
Results showed that the annual survival of adult owls varied the least over time, whereas the rate at which they successfully produced and raised young varied the most. Climate explained most of the variation over time in life history traits; years with cold, wet springs resulted in a decreased survival rate for the owls.
The research team discovered that Spotted Owls are likely to be most effected by climate during the spring when they are reproducing. This is because Spotted Owls have critical energy demands during the breeding season, and precipitation may inhibit the birds' ability to hunt, thus decreasing the survival rates of both the adult and the immature owls during this period.
Data gathered by the team indicated, in fact, that extreme climate conditions during the early nesting period may exacerbate an energetic stress on an individual by decreasing the time it takes for an owl to starve to death.
The owls, the research team hypothesizes, may employ a "bet hedging" strategy; adults may survive during years with unfavorable climatic conditions even though the young do not. Thus, the owls which survive will live to reproduce again in other years when the climate is more favorable.
"Even if habitat conditions remain unchanged, Northern Spotted Owl populations may experience declines just due to bad weather," says Franklin.
Edge Important to the Species
The researchers also discovered that the habitat which is most conducive to high survival and successful reproductive output for the owls contained a mixture of old growth forest and other vegetation types. Interior old growth forest allowed for better bird survival, possibly providing them with an area in which to hide from predators, such as the Great Horned Owl (Bubo virginianus). Great Horned Owls primarily depend upon their sense of sight when hunting, and so the darker areas of the forest provide Spotted Owls with ample hiding places.
But an uneven edge between old growth forest and other vegetation types promoted higher reproduction rates and was also an important part of the Spotted Owls' territories. These edge areas can be abundant with dusky-footed woodrats, the primary prey species for Spotted Owls. Thus a mosaic of mature and old growth forest interspersed with other vegetation types may be very beneficial to the birds, at least in Northwestern California.
In the area of this study, this mosaic appeared to provide the highest quality habitat. In addition, those individuals living in these high quality habitats had a much better chance of surviving unfavorable weather. Those in poorer quality habitat showed a survival rate decrease of as much as 26 percent in cold, wet years. Good quality habitat buffered owls during the unfavorable weather periods much better than did poor quality habitat.
The researchers assert, however, that current logging practices do not generate the type of mosaic observed to be best for the owls. Logging clearcuts are currently conducted by making large, regularly shaped patches with clean edges.
Past forest disturbances, the researchers point out, were governed primarily by wildfires, not logging. Wildfires tend to leave smaller, irregularly shaped patches with uneven edges. Fire disturbance also leaves a variety of successional stages based upon the frequency of low, moderate and severe burns over time. In contrast, clear-cut logging leaves large, regularly shaped patches with clean edges.
The role of wildfire in these forests is still poorly understood, however, and the researchers point out that increased knowledge of this and other kinds of disturbances could be used to develop new kinds of management practices which might provide or encourage the development of high quality habitat for the owls.
The researchers further suggest that dispersed patches of different vegetation types may provide a stable prey resource to the owls which could help to buffer them against the effects of a variable climate.
Also, say the scientists, it would be far more likely that a future catastrophe in the form of long periods of poor weather would affect all of the owls simultaneously if habitat quality is decreased over a very large area.
"Our research suggests that excessive loss of key landscape habitat components, such as that which occurs when old growth and mature forests are heavily clearcut, can exacerbate the effects of an unfavorable climate," says Franklin. "Most conservation plans for the Northern Spotted Owl assume that their overall population will decline from habitat loss and then stabilize as habitat amount eventually stabilizes. We have learned, however, that as habitat quality and populations decrease in size, the effects of climate 'catastrophes' could be extremely detrimental to these animals."
Other factors besides habitat, says Franklin, need to be considered in order for conservation plans to succeed.
Founded in 1915, the Ecological Society of America (ESA) is a scientific, non-profit, organization with over 7000 members. Through ESA reports, journals, membership research, and expert testimony to Congress, ESA seeks to promote the responsible application of ecological data and principles to the solution of environmental problems. For more information about the Society and its activities, access ESA's web site at: http://esa.sdsc.edu
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