Population Biology AP Biology Image taken without permission

Population Biology AP Biology Image taken without permission fron http: //www. earthinstitute. columbia. edu/enewsletter/2003/april 03/SLElephantby. Water. 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 Mc. Clellan 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 0. 395 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 0. 395 x 1492 = 589. 34 deaths • Change in population = births – deaths births = change in population + deaths (1134 -1492)+589. 34= -358 + 589. 34 = 231. 34 orioles born • Birth rate = 231. 34/1492 = 0. 155 (15. 5%)