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Distance Sampling – Part 2

FIELD BIOLOGY & METHODOLOGY Fall 2013 Althoff. Lecture 11. Distance Sampling – Part 2. Transect line L. Point at which observer first detects object. x = perpendicular distance. OBJECT. A. Transect line L. Point at which observer first detects object.

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Distance Sampling – Part 2

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  1. FIELD BIOLOGY & METHODOLOGYFall 2013 Althoff Lecture 11 Distance Sampling – Part 2

  2. Transect line L Point at which observer first detects object x = perpendicular distance OBJECT A

  3. Transect line L Point at which observer first detects object x = perpendicular distance q r OBJECT A

  4. Brings us to 3 major assumptions of DS • Objects directly on the line (or point) are always detected (i.e., they are detected with probability 1, or g(0) =1) • Objects are detected at their initial location, prior to any movement in response to the observer • Distances (and angles where relevant) aremeasured accurately (ungrouped data) or objects are correctly counted in the proper distance interval (grouped data) 1 2 3

  5. Transect line L Detection probability of 1

  6. Processing & Examining Distance Data • Assuming one has obtained “accurate” estimates of distances to detected objects (i.e., bird, mammal, frog, nest, dung pile, etc.), then one have a raw data file • The raw data file will include “___________”. It is generally assumed that not all objects of interest were “detected”. Therefore, examining the data, by _________________ is important to see the “pattern” of detections relative to the line (or the point if point counts).

  7. Use of Histograms • By generating a histogram of the detections by distance intervals, we can gain insight into the following: • If ______ objects of interest were detected • _____________________ starts to occur away from the observer(s) that objects are less likely to be detected or not detected at all • Where _________________________ might be affecting detections…and eventually affecting the resulting _________________

  8. Histogram – Expected number of detections in 8 distance classes___________________________ 100 Frequency (number of detections) 50 0 1 2 3 4 5 6 7 8 Distance (ft)

  9. Histogram – Expected number of detections in 8 distance classeswith tendency to detect ________ objects at ____________ distances 100 Frequency (number of detections) 50 0 1 2 3 4 5 6 7 8 Distance (ft)

  10. Histogram – Expected number of detections in 8 distance classeswith tendency to detect fewer objects at greater distances 100 Frequency (number of detections) 50 0 1 2 3 4 5 6 7 8 Distance (ft)

  11. Histogram – Expected number of detections in 8 distance classeswith tendency to detect fewer objects at greater distances 100 Frequency (number of detections) 50 0 1 2 3 4 5 6 7 8 Distance (ft)

  12. Correction Factor • Because ___________________would be detected in the ‘width’ of the area sampled, an adjustment is made to account for that • It is estimated from the ________________ • Example: If 62 detections in “area” sampled, then multiple, in this case, 62 x 1.126 to estimate objects (individuals, nests, etc.) . Result = ___________________

  13. From distance data, a “___________________” is generated g(y)

  14. Detection function • ___ = the ____________________ an object, given that it is at distance y from the random line or point = pr { detection| distance y} • y is the perpendicular distance x for line transects or the sighting (radial) distance r for point transects.

  15. Detection function…con’t • Use ____________________ to calculate the detection function • _____ from sampling effort to sampling effort • _____ most likely from species to species • _____ most likely from geographic area to geographic area • …in other words, _____ likely to get identical detection functions from one effort to the next

  16. Strip Transect Method Point Count Method

  17. Dickcissels – Point count

  18. Dickcissels – Strip Transect

  19. Dickcissels Point Counts Strip Transect

  20. Grasshopper Sparrow – Point count

  21. Grasshopper Sparrow – Strip Transect

  22. Grasshopper Sparrow Point Counts Strip Transect

  23. Brown-headed Cowbird Point Counts Strip Transect

  24. In summary...and/or recommendations. • Number of detections usually are a function of ________ from the line or point …usually _____ the further the object(s) are from the line or point • _______________ is used to generate a detection function • The detection function can be used to “______” counts to give popn estimate—more later • Generally need __________________________ to determine the detection function with any degree of statistical confidence

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