Jacquez (1995) explored the overlap of respiratory illness and environmental ozone in southern Ontario. Exposure to high ozone can cause acute respiratory distress leading to pulmonary edema or even emphysema. Jacquez asked whether zones of rapid change in environmental ozone induced concomitant zones of rapid change in respiratory health. Ozone boundaries appeared to coincide with boundaries in hospital respiratory admissions; however, the overlap statistics were not significant. Most likely other factors were involved that may have obscured the relationship between ozone and respiratory health.
Fortin et al. (1996) used boundary overlap to assess the relationships between edaphic factors (soil types and moisture) and vegetation boundaries. They found that vegetation boundaries based on species stem density and species presence/absence overlapped boundaries in edaphic factors, but vegetation boundaries based on species diversity and richness did not. This pattern suggests a hierarchy of effects, with edaphic factors predicting species presence but not plant community structure.
To determine how much the variable examined influences boundary delineation, Fortin (1997) evaluated overlap among vegetation boundaries calculated from different data sets. She found that density, percent coverage, and presence/absence for trees, shrubs, and trees and shrubs together significantly overlapped. While most variables concurred, the tree-only and the shrub-only data did not. Thus, overlap analysis can be used to identify variables that covary and those that do not. Determining the degree of overlap between boundaries of interest would be useful for study design and ground truthing remotely sensed boundaries.
Hall and Maruca (in preparation) compared vegetation boundaries, obtained from spatially constrained agglomerative clustering, with difference boundaries (via irregular wombling) calculated from songbird abundance data in a 45 ha section of swampland in the northern Lower Peninsula of Michigan. They found that bird abundance boundaries were significantly associated with vegetation boundaries, but not vice versa. Upon investigating the composition of the 8 vegetation clusters, they found that the variable most likely driving the boundaries was the density of coniferous trees, a potentially important factor influencing the selection of nesting and foraging areas. The authors suggest that this approach may aid in the development of monitoring and recovery plans for threatened bird species that use mosaic landscapes, such as the four songbird species of conservation concern included in this study.
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