Transcript Estimating_Population

Lab 2: Estimating Population Sizes
• Changes in population sizes are an indicator for important changes in
community structure. Such changes may take years to become evident due to
the long lifespan and slow reproduction in some seabird species.
• A change in population trends might trigger further research to investigate the
causes of change, and management might formulate “species action plans” to
maintain a healthy seabird community.
Today’s Objectives
• Become familiar with methods of
random sampling
• Apply sampling methods to estimate
patterns of dispersion
• Understand the importance of sample
size and replication and how it can
affect results
What is a population?
biological definition:
A group of individuals of the same
species in a given area
Why might we sample
population size?
• Endangered/threatened
species efforts
• Monitor invasive species
• Effects of pathogen
• Understand impacts of
different species on
community structure
Cryphonectria parasitica
Gasterosteus aculeatus
Pacifastacus leniusculus
Iris lacustris
Castanea dentata
Apis mellifera
Estimating population size
• Typically not feasible to sample all
individuals (= actual population size)
– Not enough resources
– Scarce individuals
• So, we estimate population sizes
Estimating population sizes
can be difficult
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Very small organisms
Rare species
Aquatic and arboreal
Nocturnal
Distribution pattern (random, even, or
clumped)
Methods Available
• Census (rarely feasible)
• Sampling:
Mobile organisms
- Mark Recapture
Non mobile (mostly)
- Quadrats
- Point Center Quarter (PCQ)
The Quadrat
• Randomize start point
• Mark first point
• Count the number of
individuals within a
quadrat
*Try to be consistent
with first point as
corner
Point Center Quarter (PCQ)
• Randomize start point
(center)
• Split to 4 Quadrants
• Measure distance to
nearest individual in
each quadrant
Problems with quadrat and
PCQ
• Can be affected by patchiness of the
populations.
Sample Size & Replication
• Sample size
– Total # of observations
• Replication
– Repetition of an experimental treatment or
sampling
– One way to increase sample size
• Why is it important to get a large sample size?
– Improves precision of
mean estimate
Standard Error (SE):
SE = s / √n
Today’s Lab
• Estimate population size & dispersion of
trees in Sanford Woodlot
• Sample populations using Quadrat & PCQ
methods
• Return to class to analyze the data
• Materials to take with you:
– Random # table (pg 190)
– Lab activity
– Data sheet
Mark-Recapture!
Mark-Recapture
• M = Total marked in first sample
• n = Number in second sample
• R = Number of recaptures
• marked individuals in second sample
• N = Total population
• what we are estimating
N
n
=
M R
Solving for N
N = nM
R
Mark-Recapture: Assumptions
Why would failure to meet assumptions
affect population estimate?
• All individuals in the population have an equal
and independent chance of being captured.
• The ratio of marked to unmarked individuals
does not change between the 1st and 2nd
samples. (no birth, death, em/immigration)
• Marked individuals distribute themselves
homogeneously with respect to unmarked
individuals.
N = nM
R
Data Analysis
• Each group calculates ‘estimation in a
hectare’ for Quadrat & PCQ method
• Quadrat
– Count the number of individuals (x)
– Find Quadrat area (Q) in m2
– (x / Qm2) * (10,000m2 / 1 ha) = # indiv./ha
• PCQ
– Take mean of four distance measurements
in meter (d)
– 10,000m2 / d2 = # indiv./ha
Data Analysis
• Each group calculations ‘estimation in a
hectare’ for Quadrat & PCQ method
• For PCQ, calculate descriptive stats:
– Mean, variance, standard deviation, standard
error
• For point-to-plant data, calculate coefficient
of aggregation (A)
Data Analysis
• Turn in sheet when finished. One per group.
• After all groups are done, we’ll discuss
group projects.
• Review proposal feedback while you wait
for other groups to finish.
• Quiz (Surprise!)
– 10 min
– Turn over when finished
• After quiz, write responses on one-third
sheet:
– How is your group communicating about the
project?
– Anything you would change about your or
others’ contributions to the project?
• When everyone’s finished, we will discuss
group projects
Group Projects – Time to Start!
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Greenhouse: see e-mails
25 Nat Sci Lab: come during office hours
Spring break
Review proposals
Discuss with me any questions about methods
Check out any needed materials
You can continue to check out/return materials
during office hours
• Next week = spring break! Enjoy!