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Estimating Abundance: Sightability Models. Visibility Bias. Virtually all counts from the air or ground are undercounts because can’t see all the animals due to vegetation cover or topographic irregularity

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Estimating Abundance: Sightability Models

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Estimating abundance sightability models l.jpg

Estimating Abundance: Sightability Models


Visibility bias l.jpg

Visibility Bias

  • Virtually all counts from the air or ground are undercounts because can’t see all the animals due to vegetation cover or topographic irregularity

  • Solutions utilize mark-resight methods, distance estimation (line transects), a correction factor or a sightability model


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Elk in Brushfield


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Elk in Light Timber


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Medium Timber


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Heavy Timber


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Sightability Model

  • Attempts to remove visibility bias by estimating a correction factor for each group of animals seen.

  • Adaptable to a variety of conditions.

  • Cost efficient, especially once model built

  • Only works if model is applicable and if visibility averages at least 33%.


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Developing a Sightability (or Visibility Bias) Model

  • Mark elk (deer, sheep, etc.) groups with radio-collars or have observers on ground keep track of individual groups when helicopter/plane passes over.

  • Fly aerial survey over the geographic area where the marked groups occur.

  • Determine which individual groups were seen and which groups were missed.


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Developing Sightability Models

  • Identify which factors such as group size, tree and shrub cover, snow cover, weather, observers, type of helicopter, etc. influenced whether a group was seen or missed.

  • Important: factors must be ones that will have the same effect each time a survey is conducted


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Developing Sightability Models

  • Keep some factors constant such as type of helicopter or fixed-wing, experience of observers, speed of flight, height above ground, etc.

  • Estimate the effects of the other important factors we can’t control such as group size, vegetation cover, etc. using logistic regression.


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Sightability Model:Analysis

  • Logistic regression is one of a number of statistical models that can be used to analyze the observations of groups seen and groups missed.

  • Prob(Seeing group) = em / 1 + em

  • where m = a + b1 X1 - b2 X2

  • e.g. X1 = group size, X2 = veg. cover


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Probability of Seeing Elk


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Factors Affecting Elk Sightability

  • Size of group

  • Percent vegetation cover around group

  • Percent snow cover

  • Secondary factors also statistically signif.:

    • Activity (moving vs. still)

    • Observer experience

    • Composition (Bull groups vs. others)

    • Type of helicopter or fixed-wing


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Sightability Model

  • Use the logistic regression model to calculate a the probability that each group is seen.

  • Estimate becomes???


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Simple Application

  • Suppose we see a group of 3 elk in an open forest with 40% cover of obscuring vegetation.

  • If our logistic regression model estimates that only ½ of groups of 3 in 40% cover are seen (p=0.5), then if we saw this one group of 3 animals, there was probably another group of 3 that we missed.


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Simple Application

  • So if we saw 3 there were actually 6 in the area.

  • How? Probability of detection = 0.5

  • True N = Nobs /Prob. of det. = 3 / 0.5 = 6


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Simple Application

  • If the next group we saw was a group of 2 animals in 80% cover and the model said that we only have a 20% chance of detecting such a group (p=0.2)

  • We would correct this group of 2 to represent 2/0.2 or 10 animals in the population.


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Simple Application

  • If the next group that we saw was a group of 7 elk standing in an open brushfield with only 15% obscuring cover

  • The sightability model might predict that such a group would have a 100% chance of being detected (p = 1.0).

  • What is the estimated true size of the group?


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Lochsa River Elk Herd

  • This sightability model was applied to the elk herd wintering on the Lochsa River in 1985.

  • Half of the winter range was flown obtaining a raw count of 2718 elk.

  • When the sightability model corrections were applied to the counts the corrected estimate was 4775 with 90% bound of 458.


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Lochsa Elk Herd


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Aerial Survey Program

  • All calculations easily performed

  • Variety of sightability models

  • Includes online users manual by Unsworth et al.

  • Available from Univ of Idaho’s Fish and Wildlife Dept. web site. Go to (http://www.uidaho.edu)


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