Exam 2 - Abundance estimation Flashcards
Population
A group of organisms occupying a defined area for a specified time
Abundance
Number of individual animals in the population
Density
Number of individuals per unit area
Relative density
Ratio of density
Census
A complete count of all animals in the population
Abundance or density estimate
An approximation of the population value based on sample data
Population closure
Absence of any added or subtracted individuals over some time period
- demographic
- geographic
Demographic
No birth or death over the time period
Geographic
No immigrants or emigrants over the time period
Open population
A population that is not closed over the sample period
Population index
Some statistic (usually not a count of animals) that is related to population size
Detection probability
The chance that an individual in a sampled area is counted during surveys
Issues in population estimation
- Detectability: we generally don’t see all animals in our research area (N=C/B)
- Sampling: we generally can’t apply the survey method to the entire area of interest (N=N/a)
(Combine the 2 issues to get one equation for population size estimators) (N=C/aB)
The accuracy of our population estimate is dependent on what?
a (alpha) & B (Beta)
The variation with a (alpha) is dependent on what?
The distribution of the animals (uniform=low, clumped=high)
The variation with B (beta) is dependent on what?
Our sampling methods. We want to either minimize it or get a good estimate of it.
Census methods
- Complete count of animals present
-Strip transect (fixed & estimated width) - Drive counts
- Territory mapping
- Aerial photography
- Population reconstruction
- Thermal scanners
- Incomplete counts
-Double sampling
-Marked subsample
-Multiple observers
-Distance sampling
-Capture/Removal
~Capture-Mark-Recapture
~Change in ratio
~ Change per unit effort
N
True population size
N’
Estimate for our sample area
C
Count
a
Proportion of total area sampled
B
Detection probability
Only under what circumstance do you attempt a complete count of animals present?
- Conspicuous species on open range
- Species in restricted habitat
- Small, enclosed area
Under a complete count of all animals present, N=C. Why?
Both a (alpha) and B (beta) are equal to 1 in this case.
What are drive counts mostly used for?
Ungulates and game birds
What are some limitations of drive counts?
- Assumes all animals present are seen and this is difficult to verify.
- Labor intensive
What is done during territory mapping? & what is the only species this technique is used on?
Map all breeding territories, boundaries determined by songbird songs.
Songbirds only.
What are some limitations of territory mapping?
- Limited application
- Birds only
- Breeding season/habitat are factors
- Counts only breeding males
- Polygamous systems
- Juveniles or floaters can bias data
- Time consuming
What is aerial photography?
Low altitude photography used to identify and count animals.
What is aerial photography mostly used for?
Ducks on migration stopovers
What are some limitations of aerial photography?
- Limited application
- High cost
- Uncertainty
How is population reconstruction done?
Records of only harvested animals are used to reconstruct a population where all harvest is recorded, in the past.
What are some limitations of population reconstruction?
- Assumes all dead animals are identified at time of death and aged accurately.
- Post-hoc
-Can’t get estimate for year X until all animals alive in
year X have died.
How are thermal scanners used?
Infrared detectors (FLIR) -aircraft or truck mounted
What are some limitations of thermal scanners?
- Detection rate varied from 31-89% of known population.
2. Expensive equipment & aircraft time
What is the problem with incomplete count methods?
Estimating B (beta) formally to correct our estimates
What is the primary count method for double sampling (waterfowl)? & what is done to calculate B (beta) for the primary count method?
- Aerial transect survey of breeding areas.
- Intensive ground counts on selected portions of transects.
What is used to find B (beta) in a marked subsample?
The presence of marked animals in the population; Lincoln-Peterson Esimator
What are some limitations of marked subsample?
- All marked animals are in the survey area at the time of the survey.
- Marked and unmarked animals are equally observable.
What is done using the multiple observers method?
Independent observers map the precise location of animals or other objects.
(Use the Lincoln-Peterson Estimator.)
What are some assumptions made using the multiple observer method?
- Mapping is perfect (ID animals observed by both)
2. Observations are independent
Distance sampling uses a statistical modeling to derive what?
“Detectability curve” or detection function.
program DISTANCE
What is distance sampling most commonly based on?
Random line transects through the area of interest.
What must be determined in distance sampling?
The perpendicular distance from transect line to animal
Distance sampling makes what assumptions?
- Lines are randomly placed
- All animals on the line are detected
- Animals are detected at original location
- Distances are correctly measured
- Observations are independent
What occurs during Capture-Mark-Recapture? How many events must there be?
- Animals are marked, then the proportion of marked animals in recapture is observed. (estimate original population by assuming proportion of marked animals in overall population is the same.)
- 2 or more
What are some assumptions made using Capture-Mark-Recapture?
- Populations is closed
- Relaxed in the Jolly-Seber Model
- All animals are equally likely to be captured
- Relaxed in some complex models (MARK program)
- Marks are not lost or overlooked
What are some new methods for CMR?
- Infrared-triggered cameras
-“captured” if photographed within the period - Genetic mark-recapture
-use noninvasive techniques (hair or feces) to get
tissue sample.
-Genetic finger printing to ID individuals
Both analyzed with MARK
When is the Change in Ratio method used?
Where there is removal biased toward one group of animals. (ex. males)
Ratio of males to females changes from pre- to post- harvest bc only males are removed
What are some assumptions made using the Change in Ratio method?
- Observed proportions of x-type and y-type animals are unbiased(detectability is equal)
- Population is closed except for removals
- Number of animals removed & their type is known
- The proportion of x-types in the removal differs from that in the population
What is the Catch Per Unit Effort about?
Several successive trapping events where animals are removed.
Can also mark them and consider all marked animals removed.
If less animals are available, what happens to the catch per unit effort?
Declines
What are some assumptions made using Catch Per Unit Effort?
- Population is closed.
- Complex models (program CAPTURE) relax this
- Catchability (=Detectability) is constant
- All removals are known
Measures of Relative Density do not do what?
Determine the population density but correlate with density.
What are measures of relative density used for?
To compare among time periods or sites
What are measures of relative density termed?
Population indices
What are some assumptions made by measures of relative density?
B (beta), though unknown, is constant & unbiased.
Examples of population indices
- Roadside counts
2. Call counts
How is the roadside count index to density determined?
Number of animals observed per survey mile on designated routes.
What is the potential bias with roadside counts?
If the roadside vegetation changes
What is the call count index to density?
The number of calls heard.
What are some well-known, long-term call count surveys?
- North American Breeding Bird Survey
- Began by Chandler Robbins
- Dove Survey
- Woodcock Singing Ground Survey
What are some types of animal sign that indices have been based on?
Tracks, nests or lodges, feces
The need to establish sign is proportional to what?
Animal abundance
Occupancy modeling estimates what, instead of abundance?
Probability of occupancy (occurrence)
Occupancy modeling is based on what principles?
- Detection probability is always <1 & is usually unknown.
- With unknown detection probability, survey results are unreliable.
- Multiple surveys allow us to estimate detection probability.
- Occupancy provides meaningful info
What program is associated with occupancy modeling?
PRESENCE
The 2 key parameters of occupancy modeling?
- Probability of occupancy
2. Probability of detection (p)
What are some occupancy assumptions?
- Population closure
- Detection probability is constant over time
- No false detections
What are some uses of occupancy modeling?
- Describe habitat characteristics associated with occupancy by a species.
- Quantitatively compare detection probability for various survey methods.
- Define species distributions.
