WLF 448: Fish & Wildlife Population Ecology 2004

LINE TRANSECT

•  The 2 most common distance-sampling methods for estimating abundance of wildlife populations are line transect and point counts (variable circular plot).

Underlying theory

• Not all individuals will be detected

• The probability of detecting an object decreases as its distance from the line increases.

Detection function

• Detection function: g(x) = Probability object observed given a distance (x) from the observer.

• The critical assumption permitting estimation from distance data is that all objects located directly on the line (distance = 0) are detected.

• The main problem in line-transect estimation involves developing an appropriate model for g(x).

There are a variety of models and associated estimators for g(x) that can be fit to the detection-distance data:

1. Exponential power series
2. Exponential polynomial
3. Negative polynomial
4. Negative exponential
5. Half-normal.
6. Uniform
7. Fourier series (Uniform with cosine expression)
8. Others...

Akaike's Information Criteria can be used to determine the "best" function (model) given the detection-distance data.

Calculating Density

Assumptions (in order from most to least critical):

1. Objects directly on the line will never be missed, i.e., g(0) = 1.

2. Points are fixed at the initial sighting position (i.e., no movement before or after sighting).

3. Distance and angles are measured accurately.

4. Sightings are independent events.

5. Shape criterion

Variable Circular Plots

Trapping Web (Bookhout 1994:243)

Data analyzed the same as variable circular plots where "detections" are captures.