Download - Distance Sampling – Part 2
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Distance Sampling – Part 2
Transect line L
A
Point at whichobserver firstdetects object x
r OBJECT
FIELD BIOLOGY & METHODOLOGYFall 2015 Althoff
Lecture
11
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Transect line L
A
Point at whichobserver firstdetects object
OBJECT
x = perpendicular distance
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Transect line L
A
Point at whichobserver firstdetects object x = perpendicular distance
r OBJECT
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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) are measured accurately (ungrouped data) or objects are correctly counted in the proper distance interval (grouped data)
1
2
3
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Transect line L
Detection probability of 1
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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).
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Use of Histograms
• 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 _________________
•
By generating a histogram of the detections by distance intervals, we can gain insight into the following:
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Histogram – Expected number of detections in 8 distance classes___________________________
1 2 3 4 5 6 7 8
Distance (ft)
Freq
uenc
y (n
umbe
r of d
etec
tions
)
0
50
100
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Histogram – Expected number of detections in 8 distance classeswith tendency to detect ________ objects at ____________ distances
1 2 3 4 5 6 7 8
Distance (ft)
Freq
uenc
y (n
umbe
r of d
etec
tions
)
0
50
100
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Histogram – Expected number of detections in 8 distance classeswith tendency to detect fewer objects at greater distances
1 2 3 4 5 6 7 8
Distance (ft)
Freq
uenc
y (n
umbe
r of d
etec
tions
)
0
50
100
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Histogram – Expected number of detections in 8 distance classeswith tendency to detect fewer objects at greater distances
1 2 3 4 5 6 7 8
Distance (ft)
Freq
uenc
y (n
umbe
r of d
etec
tions
)
0
50
100
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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 1.126to estimate objects (individuals, nests, etc.) . Result =
___________________
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From distance data, a “___________________” is generated g(y)
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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.
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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
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Strip Transect Method
Point Count Method
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Dickcissels – Point count
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Dickcissels – Strip Transect
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Dickcissels
Strip TransectPoint Counts
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Grasshopper Sparrow – Point count
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Grasshopper Sparrow – Strip Transect
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Strip TransectPoint Counts
Grasshopper Sparrow
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Strip TransectPoint Counts
Brown-headed Cowbird
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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