The Problem with Spotting Owls | ||
"It may sound strange, but we have no count of exactly how many
spotted owls are out there," said Joseph Lint, a wildlife biologist
at the Bureau of Land Management. For the past twenty years Lint and
other scientists have been keeping track of owls by first-hand
observation. Lint explained that biologists pick out an area of land,
go into the forest, and search for pairs of male and female owls. When
the owls are spotted, researchers release mice and wait for the birds to
snatch up a free meal, said Lint. The birds usually take the food back
to their nests and the biologists follow. If any young are found in the
nest, the scientists band them. The banded, spotted owls are then
tracked down several years later to see how many of them survived and if
they are having young of their own.
During the course of one of these surveys, the researchers typically
track down a few hundred owls over a range of tens of thousands of
acres. Lint said the scientists use these numbers to infer the survival
rate of the owls, the total acreage of owl habitat in the area, and the
structure of that habitat. Theyve found over the past ten years
that the number of owls they've monitored have been decreasing at a rate
of about 4 percent every 5 years. "So there isnt a sharp decrease in the
survival rates of the owls tagged. The population is probably
stabilizing," said Lint. |
Traditional methods of monitoring spotted owls are time and labor intensive. Individual birds must be caught and banded by hand. (Photograph courtesy U.S Fish and Wildlife Service) | |
While this mark and recapture method is the only one proven to work,
it would be very difficult to use it exclusively to monitor the
25-million acres specified in the federal plan. The costs to maintain
personnel and equipment would likely run to $3 million a year, and it
would require a very large number of personnel (Lint et al., 1999).
Instead, a model was needed that could accurately predict the health of
the owl population in the Pacific Northwest using a minimum of
groundwork.
"One of the first steps [in developing a predictive model] is to know the extent of spotted-owl vegetation and the range of the spotted owl," said Jim Alegria, the manager of the Interagency Vegetation Mapping Project for the Bureau of Land Management in Washington and Oregon. He said the BLM and USFS agreed that the solution to their problem lies in Earth-imaging satellites. Over the next few years they hope to have old-forest growth and owl-habitat maps for Oregon and Washington. The advantage of satellites is that they can map out an area of landscape in a fraction of the time it takes to chart the area on the ground. More importantly, advanced imaging satellites such as Landsat 5 and Landsat 7 render images that can be used to identify large expanses of vegetation on the Earth. The satellites' sensors can highlight where one type of vegetation begins and another ends (Townshend et al., 1993). Color Coordinating Satellites
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Burgundy colored regions on this map represent the historical range of the northen spotted owl in the United States (it also extends north into British Columbia). One hundred-fifty years of logging, agriculture, and urbanization have reduced the amount of old growth forest (potential spotted owl habitat) by 85-90%. |
Color Coordinating Satellites | ||
As can be seen through a prism, sunlight contains many different colors (wavelengths). When sunlight strikes
objects, certain wavelengths are absorbed and others are reflected or emitted.
The Landsat 5 and Landsat 7 satellites are sensitive to these different
types of light, including infrared wavelengths that cannot be seen with
the naked eye. Each satellite has several separate light detectors (photoreceptors)
on board, which measure the energy reflected or emitted by the Earth. One
light detector records only the blue light coming off the Earth (band 1).
Another observes all the yellow-green light (band 2), and still another
picks up on all the near-infrared light (band 4). The Landsat satellites
move in circular orbits very nearly from pole-to-pole around the Earth and
scan strip after strip of our revolving planet. Their data are beamed back
to the surface, where they can be processed into pictures.
To differentiate between types of vegetation and their attributes, scientists take an image of an area using the Landsat satellites light detectors. The researchers then compare the different colors of light reflecting off a patch of land. They mix and match these colors until they come up with the combinations that most distinguish the types of vegetation and vegetation characteristics such as the density of foliage. The combination is processed and made into an image that can be easily analyzed (Townshend et, al., 1993).
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The Thematic Mapper instrument aboard several of the Landsat satellites detects light in seven distinct bands. Red, green, and blue bands can be combined to create pictures as we would see them Different band combinations and comparisons reveal unique aspects of the Earth. (Image by Jesse Allen, GSFC Viualization Analysis Lab) | |
An example can be found in the way satellites are used to separate areas of conifer trees from hardwood trees. Researchers know that evergreen conifer trees absorb more infrared light (light to the right of red on the color spectrum) and reflect more red light than hardwood trees (Parkinson, 1997). This information provides them with a system to tell the two tree types apart. If the difference of red to infrared light recorded by the satellite (bands 3 and 4) is high, then they know a patch of trees viewed by the satellite is hardwood. If the difference is low, then the trees are probably conifer (Parkinson, 1997). The scientists can create a map by highlighting the variation between the two wavelengths. Searching for Spotted Owls from Outer Space
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Vegetation maps derived from satellite data can do more than show the presence of vegetation - they can also distinguish between conifers (left) and broadleaf trees (right). Since spotted owls prefer nesting in douglas fir and other conifers, it is necessary to determine vegetation type to map suitable habitat. (Image by Janine Savage, Global Change Master Directory, based on data from the Interagency Vegetation Mapping Project) |
Searching for Spotted Owls from Outer Space | ||
Discerning
the amount of potential owl habitat throughout the Pacific
Northwest is not as simple as gauging whether or not a group of trees is
conifer. "There are many layers and levels to this project,"
said Alegria. The researchers are now simply attempting to create a
reliable satellite vegetation map of the 25-million acres of land
specified by the FEMAT plan. When complete, the map should help
biologists identify areas of old-growth forest and potential owl
habitat.
Karin Fassnacht, a research forester at the U.S. Forest Service,
works with Alegria to develop these maps. She explained, the
scientists goal is to use Landsat satellites to map a number of
attributes of forest vegetation across the Pacific Northwest. These
attributes include the density of forest vegetation, the amount of
conifer trees, the amount of broadleaf plants, the average size of the
trees, and the complexity of the forests structure. Each of these
cover variables will appear on the finished Landsat maps in terms of
percentage, she said. |
This simple vegetation map (a portion of Washington State) only shows the amount of vegetation present. Clearcuts and barren mountaintops (shown as gray and brown) are easy to spot, but there is no way to distinguish conifers from deciduous trees or to determine the age of the forest. Click to see a 3D flythrough. (2.1MB) (Image and animation by Robert Simmon, based on North American Landscape Characterization data) | |
So far, the
scientists have found ways to combine the colors from the
Landsat images to locate four out of five of these forest attributes on
a given strip of land. "We are currently unable to map structure
at all [using the satellites]," said Fassnacht.
When the maps are fully developed, they may help scientists determine if the trees in a section of forest are old. High percentages of large conifer trees with greater structural complexity usually indicate old-growth forest in the Pacific Northwest. By looking for these attributes on the satellite map, researchers should be able to locate areas likely to contain old-growth forest. "Additional ground-based or photo-based measurements must then be made to determine if any old-growth exists," said Fassnacht. In the same manner, the maps may assist researchers in determining where owls and other animal species might exist. Those who know how the owls live could use the map, in part, to pick out those areas that have the percentage of conifer growth, structural complexity, and tree sizes required for the bird to survive Fassnacht said. These areas of potential habitat could then be verified on the ground. With enough effort, the scientists should be able to map out all the areas where northern spotted owls could possibly live in the 25-million acres. The same could be done for other threatened species in the old-growth forest, such as the marbled murrlet. "There are several ongoing projects aimed at mapping habitats," Fassnacht said. The BLM and USFS have just recently put out their first satellite-imaging map of the forests along the Coast Range, said Alegria. Though the map does not contain the structural complexity of the forests, it shows promise. Within the next few years the team hopes to have mapped the entire 25-million-acre plot of land specified by the federal plan. Not long after that, a map of all the potential owl habitats of the Pacific Northwest should be complete. |
Dark green areas on this map exhibit the characteristics of old growth forest, which are large trees, dense vegetation and a make-up of at least thirty percent conifer. The final feature - canopy complexity - may be measured by NASA's upcoming Vegetation Canopy Lidar mission. (full size map) (Image by Janine Savage, Global Change Master Directory, based on data from the Interagency Vegetation Mapping Project) |
Quadratic Mean Diameter | Searching for Spotted Owls from Outer Space | |