Distance Sampling – Part 2

Distance Sampling – Part 2

Transect line L

A

Point at whichobserver firstdetects object x

r OBJECT

FIELD BIOLOGY & METHODOLOGYFall 2015 Althoff

Lecture

11

Transect line L

A

Point at whichobserver firstdetects object

OBJECT

x = perpendicular distance

Transect line L

A

Point at whichobserver firstdetects object x = perpendicular distance

r OBJECT

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

Transect line L

Detection probability of 1

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).

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:

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

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

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

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

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 =

___________________

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

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.

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

Strip Transect Method

Point Count Method

Dickcissels – Point count

Dickcissels – Strip Transect

Dickcissels

Strip TransectPoint Counts

Grasshopper Sparrow – Point count

Grasshopper Sparrow – Strip Transect

Strip TransectPoint Counts

Grasshopper Sparrow

Strip TransectPoint Counts

Brown-headed Cowbird

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