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Population BiologyPopulation BiologyAP Biology
Image taken without permission fron http://www.earthinstitute.columbia.edu/e-newsletter/2003/april03/SLElephantbyWater.jpg
How do you measure population size?
• Count all individuals
• Estimate population size by taking average density
How do you measure population size?
• Mark-Capture Method– Capture a set # of individuals (ex. 100) and tag them– Set them free– Return later and capture the same number of
individuals (100)– Determine how many in the second group are tagged
divide original # tagged by the number captured that are tagged the 2nd time
– Multiply this ratio by original # tagged (100) to determine actual population
Example problem• How many squirrels are living in McClellan
Ranch Park?– First time: 100 squirrels captured and tagged– Second time: 100 squirrels captured, 50 are
tagged– What is the actual squirrel population size?
Solution
• Original population tagged = 100
• Ratio = 100 total / 50 tagged
= 2 (this means for every squirrel you tagged there was another one you didn’t tag)
• 100 x 2 = 200
The actual population is 200 squirrels
One more practice problem Determine the number of seagulls at
Moonstone beach using the mark-capture method
• 150 seagulls captured and tagged at Moonstone Beach then set free
• 6 months later, another 150 are captured, this time 100 of them are already tagged.
• What is the actual seagull population size?
Solution
• Original population = 150
• Ratio = 150 total/ 100 tagged
= 1.5 (or 3/2)
• Actual population = 150 x 1.5
= 225 seagulls
Question…
• What are the limitations of the mark capture method? What assumptions have to be true for it to work? – Animals may learn to avoid trap the 2nd time– Assumption: All animals in population are
equally likely to be captured.
Survivorship curves• Tracks the number of
individuals alive in a group (cohort) at each age.
• Type I = low death rates in early and middle life
• Type II = constant death rate over life span
• Type III = high death rates in early life
Population growth
• Exponential growth– Characterized by rapid
growth in a very short period of time.
– Shows growth under ideal conditions (unlimited food, shelter, etc.)
Question…
• What are some factors what would limit the growth of a population?– Some factors include availability of food,
shelter, competition for resources
Population growth• Logistic growth
– A more realistic model of population growth
– A population will increase until it reaches carrying capacity
– Carrying capacity (K) = maximum number of individuals in a population an environment can support
Question…
• Can the carrying capacity change or is it a fixed number? – Carrying capacity is NOT a fixed value– it can
depending on the conditions present (ex. Availability of food)
Density Dependent Factors
• Density Dependent Factor – Affects population to varying degrees
depending on population density– When population density increases chance
of individual survival decreases – Ex. Predation, disease
Density Independent Factors
• Density independent factors – Will result in more deaths or fewer births in a
population regardless of population density – Ex. Weather (hurricanes, extreme cold, etc.)
• Most population growth is limited by both types of factors (not just one)
• Both types of factors can limit population growth
Reproductive Strategies• r strategists
– Reproduce very quickly
– Have lots of offspring but a short life expectancy
– Little or no parental care
– Very small at birth– Ex. mice, rabbits,
weeds
• K strategists– Prolonged
development– Have few young with
longer lifespan– Extensive parental
care– Larger at birth– Ex. Elephants,
tortoises, humans
Question…
• What connections do you see between certain reproductive strategies and survivorship curves? – Type I (ex. Humans) have extensive parental
care most live to full life expectancy (K strategy)
– Type III (ex. Bacteria) have large numbers of offspring with little parental care high death rates in youngest individuals (r strategy)
Population Fluctuations
• Population sizes fluctuate as a result of interactions with other populations
• Predator/Prey Interactions– The predator “lags” the
prey = changes in the predator population occur shortly after similar changes in prey population
GPS
• A method to track populations
• Ex. Monitoring the movements of grizzly bears to determine the effect of human activity on them
Images taken without permission from:www.colorado.edu/geography/ gcraft/notes/gps/gps.html