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Population Biology Population Biology AP Biology Image taken without permission fron http://www.earthinstitute.columbia.edu/e-newsletter/2 003/april03/SLElephantbyWater.jpg

Population Biology AP Biology Image taken without permission fron newsletter/2003/april03/SLElephantbyWater.jpg

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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