2_IntroductionDistanceSamplingPSHPL.pdf

8/10/2019 2_IntroductionDistanceSamplingPSHPL.pdf

1/18

2. Introduction to distance sampling

Quick introduction to distance sampling and surveydesign in preparation for data collection later today

Cover basic line transects and survey design


2/18

Strip transect sampling

0 20 40 60 80 100

0

10

20

30

40

50

0 20 40 60 80 100

0

10

20

30

40

50

0 20 40 60 80 100

0

10

20

30

40

50

0 20 40 60 80 100

0

10

20

30

40

50

Size of study area = 5000 (A)

Total transect length = 50x5 = 250 (L)

Width of half the strip = 1 (w)

Area searched = 2wL= 2x1x250 = 500 (a)

Number of animals counted = 36 (n)

wLa 2

a

nD

ADN D = density

Aa

nN

N = abundance


3/18

Strip transect sampling - variance

0 20 40 60 80 100

0

10

20

30

40

50

0 20 40 60 80 100

0

10

20

30

40

50

0 20 40 60 80 100

0

10

20

30

40

50

0 20 40 60 80 100

0

10

20

30

40

50

ni= number of animals seen

onith transect (n=ni)li= length ofi

th transect (L=li)

k= number of transects

var

SE

N

SECV

1var

1

2

k

L

n

l

nlL

n

k

i i

ii


4/18

The probability of detecting an animal in the strip is not 1

Collect data that allow that probability to be estimated

Line transect sampling

d

r= radial distance= angle

r

d= perpendicular distance = r sin()


5/18

Perpendicular distance

Frequen

cy

w0

Estimating probability of detection

If detection probability = 1in whole strip out to w,

histograms would be here

Animals assumed tohave been missed

Assume probabilityof detection is 1 at

zero distance

ap area under curve

area under rectangle


6/18

Line transect abundance estimate

Lw

nD

2

n= number of animals seen

L= length of transect

w= strip half width

Strip transect density (D) estimated as:

pa= average probability of detection

apLw

nD

2

Line transect density estimated as:


7/18

Strip width x average probability of detection (wxpa) is

often referred to as the effective strip half width oresw

Effective strip half width

0

5

10

15

20

25

30

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.4 2.6 2.8 3

Perpendicular distance (d)

Frequency

esw


8/18

Study area is sampled representatively Equal coverage probability

Animals do not move Randomly

In response to surveyor

All animals are detected on the transect line

i.e. probability of detection at zero perpendicular distance = 1 All measurements are accurate

Species identification

Distances (and angles)

Group size if relevant

Assumptions of line transect sampling


9/18

Animals are not distributed randomly in space

Sampling must be random/systematic

To achieve equal coverage probability

Obtaining a representative sample of data


10/18

Randomly spaced parallel lines Give equal coverage probability

Survey design


11/18

Completely random designs can be logistically moredifficult to implement

And may be more likely to lead to high variance

Survey design


12/18

Evenly spaced parallel lines often used in practice With a random starting point

Survey design

But time wasted in transit


13/18

Systematic zig-zag lines

With a random starting point

Appropriate for large areas

Survey design


14/18

Used for logistical reasons

And when density is known (expected) to vary

Analyse strata separately

Can reduce variance of total estimate

Stratification of survey area


15/18

Stratified design with zig-zag lines

Survey lines across density contours

Stratified survey design


16/18

Stratified design with zig-zag lines

Survey effort in proportion to expected density reduce variance even more

Stratified survey design


17/18

Define the area within which abundance is to be estimated

Search along pre-determined transect lines for animals Effectively searching a strip to either side of the transect

When encounter an animal (group of animals): Identify species

Determine group size

Measure perpendicular distance from animal/group to transect line

Continue searching along transect lines

When finished:

Number of animals encountered is known (n)

Length of total transect is known (L)

Use distance data to estimate average probability of detection (pa)

Line transect sampling in practice


18/18

Data organisation and analysis

n= number of animals seen

L= length of transectw = strip half width

pa= average probability of detectionapLw

nD

2

Organise sightings and effort data Analyse in program DISTANCE

2_IntroductionDistanceSamplingPSHPL.pdf

Documents