Distance Sampling – Part I

Ecological Methodology LEC-02 Althoff

Transect line L

A

Point at whichobserver firstdetects object x

r OBJECT

Distance Sampling (DS):In the beginning……

• Earliest development of theory and application was by David R. Anderson (Colorado State U.) in the 1960s, Kenneth P. Burnham (Colorado State U.) in the 1970s, and J.L. Laake (National Marine Mammal Laboratory) in the 1970s.

• S.T. Buckland (University of St. Andrews) started making contributions in the early 1980s

DS: Estimate Density or Abundance of Biological Populations

Two components: ______ and ______________Must have some understanding of theory to

properly design protocols for application in the field

Authors of “Distance Sampling” contend that DS provides a way to obtain _______ estimates of density of objects under fairly _______ _________________

DS is an extension of plot or quadrat sampling

Plot & Quadrat Sampling

• Assume with this type of sampling that ___ ______ within sample plot are counted

• Often use line or point ___________ to accomplish

• Most common use of point transects and line transects has been in the assessment and monitoring of ______ ____________….

Point Intercept Line InterceptQuadrats

…see next slides

Point-Intercept• Establish starting point at

random• May pick random direction, go

with contour, or parallel lines• Straight line common• 50- 200 m in length, 100 m

typical• Observations made every 1 m

typical• At each point, record either the

canopy species or the basal species or both typically for plants

Point-Intercept

• Measures distance (length) of plant coverage along line

• Or, measure GAP between plants (estimate of bare ground)

• Gives estimate of coverage….improvement over point-intercept method

Line-Intercept

Point-Intercept

• Quadrats every 5 m • 0.25 m2 frame for

forb frequency, and presence/absence

• 0.1 m2 frame set inside 0.25 m2 frame for grass (graminoid) frequency

• Compared results to point-intercept method

Line-Intercept Quadrats

Prosser, C.W., K.M. Skinner, and K.K. Sedivec. 2003. Comparison of 2 techniques for monitoring vegetation on military lands. Journal of Range Management 56:446-544.

DS….Objects of “interest”

• Typically vertebrate animals—including those that typically occur in clusters (i.e., flocks, schools, etc.). Can include _______________ animals and _______

• Sometimes the surveyed objects can be ________________—including bird nests, burrows, dung piles (fecal pats), or dead animals

The range of applications has included….

• Songbirds (both strip transect and point counts) (we will examine data sets)

• Seabirds• Gamebirds• Terrestrial mammals• Marine mammals• Fish• Reptiles• Plants (we will explore/attempt in the field)

Basic Measurements using Line Transect Surveys

• Know the length of the transect (L)• If detect “object” _____________ to transect

line, then record the sighting distance (x) from the line to the object

• If detect “object” ahead or behind you, then record the sighting distance (r) along with the sighting angle () to allow analysis of perpendicular distance (x), calculated as x = r sin (). Key to note the distance and angle at the ___________________!!!

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 ___________ (or point) are always detected (i.e., they are detected with probability 1, or g(0) =1)

• Objects are detected at their _____________, prior to any movement in response to the observer

• Distances (and angles where relevant) are measured ____________ (ungrouped data) or objects are correctly counted in the proper distance interval (grouped data)

1

2

3

Transect line L

1ASSUMPTION

Transect line L

Point at whichobserver firstdetects object

r x

2ASSUMPTION

OBJECT location when _____ detects you

OBJECT location when ____ detect it

Transect line L

Point at whichobserver firstdetects object

r

3

ASSUMPTION if measuring r and

Transect line L

OBJECT

x = perpendicular distance

3ASSUMPTION if measuring x

Some add a 4th major assumption for 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)

• One must be able to identify the “object of interest” ________

1

2

3

4

Some “tidbits”….relative to the _____ major assumption

• In __________ communities (or where two species are difficult to easily distinguish by appearance or vocalization), this assumption may be compromised. Ex. Songbirds

• If estimating male vs. female and there is no sexual dimorphism, then one might have to rely on behavioral cues for proper ID, this assumption may be compromised…or simply not be possible to use for estimating densities by sex

• Marine mammals that often occur in _______ ________ make it difficult to get species ID correct, therefore assumption may be difficult to meet

More on assumption 2 • If ____ movements of object (i.e., animal) relative to

speed of the observer, then simulations have indicated relatively small problems are encountered in the analysis and subsequent estimates of density

• If “responsive” (i.e., alarm response to either _____ _____ or _______ the observer) movements, then serious problems result in violation of theory of DS

• For point transects (aka point counts), __________ movement of animals is always problematic because the observer is stationary…mainly because a moving animal is more likely to be detected when close to the point.

2

Overall considerations relative to these 3 major assumptions of DS

• Careful attention must be given to _________ and ___________ to maximize likelihood of meeting these 3 assumptions

• Equally important, ___________ should have adequate a) ___________

b) ___________

• According to Buckland et al. (2001) all of these assumptions can be relaxed under “certain” circumstances.

• ______________ can sometimes cause difficulty !!!

(think asymmetry of detection capability)