Agents of Temporal Changes of the Mosaic Structures

In the dryland of Zimbabwe, the land mosaic is strictly linked to many environmental as well as human factors. Biomass distribution is highly variable in space, and unpredictable according to the season (Scoone 1995).

The habitat structure at any point depends not only on the surrounding habitat but also on its history, magnitude, and the trajectory of changes at multiple scales. The hypothesis that animal distribution in a shrubsteppe habitat depends not only on current landscape but also on habitat change was tested by Knick and Rotenberry (2000) in 200,000 hectares of Southern Idaho. In this region the invasion of Bromus tectorum by overgrazing and failed agricultural homesteads has increased the risk of fire affecting the community of big sagebrush (Artemisia tridentata), winterfat (Kraschenninikova lanata), and shad-scale (Atriplex confertifolia). The frequency of fire has increased over the last years, fragmenting and reducing the shrub cover from 51 to 30% of the total area.

Birds in particular seem to be affected by these changes although they show a strong philopatry or site tenacity. The use of satellite images allows us to establish the increase of reflectance of about 33.8% of the scanned pixels as a consequence of wild fires in shrubsteppe.

Disturbance plays a fundamental role in shaping landscape distribution of vegetation as recently demonstrated in Mt. St. Helens 20 years after the eruption (Lawrence and Ripple 2000). The variables utilized in this study were: disturbance type, distance from crater, tephra thickness, blast exposure, distance from surviving forests, slope gradient, slope curvature, elevation, and aspect.

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