Population Biology AP Biology Image taken without permission fron 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 2 nd 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 2 nd 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

More population calculations A population of 1,492 Baltimore Orioles is introduced to an area of Blackberry farm. Over the next year, the Orioles show a death rate of while the population drops to 1,134. What is the birth rate for this population?

More population calculations Number of deaths = death rate x starting population x 1492 = deaths Change in population = births – deaths  births = change in population + deaths  ( ) = = orioles born Birth rate = /1492 = (15.5%)