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Quantitative Assessment lectures 5-9 > Lecture 6 - Estimating Animal Abundance: Line Transects > Flashcards

Lecture 6 - Estimating Animal Abundance: Line Transects Flashcards

0
Q

What three things produce measures of relative density?

A

Counting error
Counting bias
Low precision

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1
Q

Why is relative density a problem with aerial surveys?

A

Because it is a consistent over or under estimation of density N(hat)

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2
Q

What is counting error?

A

Observer undercounts some samples and over counts on others. The two cancel each other

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3
Q

What is counting bias?

A

Most samples are consistently undercounted

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4
Q

When does counting bias increase?

A

As more demands are placed on the counter such as when increasing numbers of organisms are encountered/ must be counted quickly

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5
Q

How can the problem of low precision be reduced?

A

Increase precision by optimizing survey designs with high stratification and high replication

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6
Q

Why are transect samples used often?

A

They are the cheapest and simplest design of aerial surveys

Economical and less expensive

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7
Q

Describe transect sampling

A

Flight line are perpendicular to a baseline
Only one flight pass in a fixed wing plane per transect line
Can be done in flat open/unforested areas like the prairies

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8
Q

Describe quadrat sample designs

A
  • a grid superimposed on the census area
  • quadrants of equal size are randomly selected from the grid for surveillance
  • more lines parallel to base line as well as perpendicular
  • each quadrat is searched as required with multiple flight passes in a helicopter.
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9
Q

What are the advantage and disadvantages of using helicopters in aerial surveys?

A

Advantage is that they are maneuverable but disadvantage is that they are expensive

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10
Q

What do you do with small edge quadrats?

A

Omit them

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11
Q

What is an advantage and disadvantage of quadrat sampling?

A

Advantage is that it can be used in Any terrain
Disadvantage is that it can have serious edge effect from not being able to see the grid superimposed on the earth in the field

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12
Q

What are the main differences between quadrat and transect sampling?

A

Transect can only pass once and is only good for open terrain
Quadrat has more lines parallel to baseline but has serious edge effect

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13
Q

Define block sampling

A

Whole census area is stratified into blocks based on topographic features (large blocks avoided)
Blocks randomly selected for surveillance and searched as necessary (multiple passes)

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14
Q

What kind of features are used to define block sampling boundaries?

A

Well defined terrain boundaries such as watersheds, rivers/creeks/tributaries, ridges and height of land

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15
Q

What is an advantage of block sample design?

A

It is easy to know where the boundary lies

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16
Q

What is a difference between quadrat and block design?

A

Block is based on topographic boundaries and quadrat is a grid

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17
Q

If an area is heterogeneous, what is a goo way to divide it?

A

Based on areas that exhibit the most homogeneity

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18
Q

What design exhibits the most boundary effect?

A

Highest in quadrat an block due to difficulty in boundary location

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19
Q

What design exhibits the least boundary effect?

A

Transect because the width of the strip I predetermined by streamers

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20
Q

How is the width in a transect survey determined? By streamers or fiberglass rods trailing parallel to the airplane fuselage and attached to the wing struts

A

By streamers or fiberglass rods trailing parallel to the airplane fuselage and attached to the wing struts

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21
Q

What is the area on the ground that the streamers account for?

A

200m on either side of the aircraft

Counted by observers on each side of the plane

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22
Q

What is design in best to deal with sampling error?

A

Sample variance is lowest with transects when the sample size is held constant (reduced boundary effects compared to block and quadrat

23
Q

What technique has the most counting error?

A

Transect because there is only one pass whereas block and quadrat can have multiple passes

24
Q

What kind of terrain is best for transects?

A

Flat open landscapes because it is ineffective over rough or dense closed canopy vegetation

25
Q

What method(s) are suitable for all types of terrain?

A

Block and quadrat

26
Q

What technique has the lowest cost?

A

Transect because navigation is simple and straight

27
Q

What kind of sampling is necessary for the ratio method?

A

Sampling without replacement

28
Q

How do you calculate the average density?

A

Total animal counted (xi) divided by total area searched (zi) of transect

29
Q

How do you calculate the estimate of the total population?

A

The average density (R-hat) times the area of total census (Z)

30
Q

What is n in the estimation for aerial surveys

A

Total number of transects counted

31
Q

What is the variance in the ratio method?

A

Difference between number of animals counted (x and R) and the interaction of # animals per transect(x) multiplied by the area of each transect(z) = xz

32
Q

What happens to the variance when you add inappropriate transects/replicates (no animals)?

A

The variance increases

33
Q

What happens when the sum of xz is small?

A

Variance only decreases a small amount and remains high

34
Q

What happens when the sum of xz is large?

A

The variance decreases because there at appropriate transects

35
Q

How do you determine the perpendicular sighting distance in line transects?

A

Yi= ri sine theta of r (sighting angle from transect line

36
Q

Why do we take the inverse of the ri’s for the Haynes estimator of density?

A

Because summing the inverse distances of the ri’s emphasizes the shorter (more accurate) distances and reduces the influence of long distance (less accurate) and possibly outliers

37
Q

What is the z-test for with respect to the Haynes estimator?

A

It is a test of sighting/sampling ability and representation

38
Q

What are 5 assumptions of the Haynes estimator?

A

1) all organisms on the line are seen (detection of probability is 1)
2) animals have a fixed flushing distance (ri) at the initial sighting position, they do not move before detection, and are no animal is counted twice
3) distances and angles are measured exactly
4) sightings of animal are independent events
5) **critical assumption is that the average sighting angle is 32.7

39
Q

What is the null hypothesis for the angle and z-test? How to we determine whether to accept or reject?

A

H(0): the average sighting angle is no different from 32.7
Reject if z is greater than 1.96 or less than -1.96 at alpha =0.05
Then the average sighting angle is in fact different from 32.7

40
Q

What has happened if you fail the z-test?

A

All the shots are on the line(biased?) at 0 or almost behind you at 90 and you are not sampling correctly

41
Q

Why is the average sighting angle a critical assumption of the Haynes estimator?

A

Trying to avoid peripherals at 90 and sightings of 0 on the line. The angle assumption is an attempt to deal with extreme shootings

42
Q

When calculating the perpendicular in an aerial line transect, what is the equation?

A

bi= ai (altitude of plane) tan theta (declination angle to the sighting)

43
Q

What does it mean when Haynes assumes 100%?

A

Every individual is seen

44
Q

What is f(0)?

A

= the estimated probability density function for detection of an animal

45
Q

What is g(0)?

A

Probability of detection on the transect line

G(0)=1 if all animals on the line are seen

46
Q

What are the assumptions required to calculate the variance of the population estimate for aerial line transects?

A

The encounter rate where n (# of total sightings) divided by L (total transect length) and f, g, and s ( mean group size) are independent

47
Q

What is the area under the curve for probability detection functions of f?

A

Area is detectability

48
Q

What is the basic idea of the probability detection functions models?

A

The probability of detection of an animal falls with distance from the transect line and is equal to the total are under the curve
The detection curve should be known or if assumed justified

49
Q

What is the generalized exponential for probability detection functions?

A

Really good stability for probability of detection and then drops rapidly with distance from transect line about a third of the distance away on graph

50
Q

What is the Hayes and Buckland model for probability detection functions?

A

Probability detection falls rapidly with perpendicular distance from transect line from the start of the model. Looks like inverse graph.
As the animal is further away it means sight ability is low.

51
Q

What is the half normal model of probability detection functions?

A

Probability of detection falls gradually with perpendicular distance from transect line and then reache inflection point and becomes concave up and almost plateaus with greater distance from transect line

52
Q

Since it is not expected for 100% (g=1) of the animals to be seen on the transect lines, what should be determined?

A

A correction factor for g

53
Q

In the marsh deer example what was the detection function g?

A

An estimate of the difficulty of seeing an individual at the first approach of the aircraft.
When the animal was rapidly and easily seen it was considered visible Otherwise it was considered invisible or missed.

54
Q

Detection rate of a study tells us what?

A

What percentage of animals were easily seen and or were not.
Possibly needs to have a population number or know how many individuals by radio collar.

Quantitative Assessment lectures 5-9 (5 decks)

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