POPULATION DISTRIBUTIONS
**Excellent Resource: Scott, J. M., P. J. Heglund, and M. L. Morrison et al. 2002. Predicting species occurrences: issues of accuracy and scale. Island Press, Washington D. C.
A. Keep in Mind Scale and Resolution
1. Hierarchy of groupings
Deme through Species
2. Forman’s (1964) distributional hierarchy
Different factors and combinations of factors are limiting at the various levels of aggregation
3. Resolution
Must be fine enough to separate groups
B. Presence/Absence vs. Density
1. Density = Abundance/Area. Used as a measure of the number of animals per unit area. For distribution maps, this can be translated into a probability of occurrence.
2. Presence/Absence - Dichotomous representation of distribution.
3. Does density = habitat quality?
Fretwell's Ideal Free Distribution - implies that there will be a higher density in higher quality habitats
Factors decoupling density and habitat quality (Van Horne 1983)
Environmental characteristics - seasonal habitat, temporal unpredictability, patchiness
Species characteristics - social dominance interactions, high reproductive capacity, generalist
C. Estimating Static Distributions
1. Drawing polygons on a map
Relies on empirical knowledge of specialists
Likelihood of occurrence is seldom specified
2. Presence/Absence "Grid"
Distribution is defined by all subunits where presence is confirmed
Reasons for blank areas are ambiguous
3. Habitat mapping
2 Phases
Occurrence-habitat relationship analysis: Assess populations preferred ranges of values for environmental variables
Model distribution: Extrapolate results from first phase to larger areas
Example: bull trout in south-central Idaho
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