Gut feeling for herd condition?
Index of abundance?
Valid, consistent, repeatable estimates of abundance and condition?
critical assumption is that every animal is observed and counted.
1. Delineate area
2. Apply survey-sampling methods
3. Test assumptions and remove bias if necessary.
1. Correction Factor (ratio)
- Use depends upon assumption of a constant factor under highly variable conditions (e.g., cover type in survey units, use of cover by the animals, and size of groups often varies).
2. Mark-Recapture
- Well established statistical basis
- Questionable assumptions in most cases
- Costly in time and resources
3. Sightability Models
- Adaptable to a variety of conditions
- Cost efficient
- Not applicable if visibility is low
1. Fly aerial survey.
2. Mark elk (deer, sheep, etc.) groups with radio-collars.
3. Determine which groups were seen and which groups missed.
- depends on group size, tree and shrub cover, snow cover, observers, type of helicopter, etc.
4. Keep some factors constant (e.g, type of aircraft used, timing of survey, etc.)
5. Develop a sightability model for other factors.
- Use logistic regression
- p = eu / 1 + eu, where u = a + b1X1 + b2X2 + ...biXi
6. Use sightability model to correct for factors we cannot control.
1. Correct each group detected based on its probability of detection (visibility).
2. Sum all corrected groups in a sample unit for an unbiased estimate of actual number of animals present.
1. Calculate means, ratios, proportions, etc. according to survey design.
2. Calculate variances and confidence intervals.
- Sources of variation: sampling variance, sightability variance, model variance.
Revised: 25 August 2011