What determines population? Why can’t we have 1,000,000 students here at Wilson K-8? Space Teachers Supplies Student Behavior These resources are our limiting factors Limiting factors limit the number of organisms that can live somewhere

Carrying Capacity Key Point #1: An environment can only support as many organisms as there is available food, water, and free space Carrying capacity = maximum number of organisms that can live somewhere, based on the food, water, and free space there Every species has a different carrying capacity. Food, water, and free space are the LIMITING FACTORS that determine carrying capacity

Imagine Your Fridge Your fridge is like all of the resources (food, water, free space) in an ecosystem. What if we want to throw a party? There’s only so much food in there, and it will only last so long until you re-fill it. Of course, we can’t just re-fill it all the time. Some of us have to wait until that next paycheck before we can buy groceries.

Imagine Your Fridge We can keep inviting people, as long as there’s enough food in the fridge. Each friend eats a different amount of food, just like each species has a different carrying capacity But with each new guest, there’s less to go around.

Imagine Your Fridge The fridge won’t replenish magically, and I don’t have the money to keep putting food in the fridge forever. So too many guests means that… So too many animals means that… Someone goes hungry… Not enough food/water/free space… And leaves the party.  And organisms die. 

Carrying Capacity Key Point #2: We can read a carrying capacity graph to predict changes in population size Time Population size Graph line = Population size at a specific time Dotted line = Carrying Capacity A = x-axis = time C = y-axis = population size B = graph shows population size at any given time D = carrying capacity

Carrying Capacity Time Population size Remember from math class, exponential increase = constantly doubling. (Makes a curved line on a graph.) Analogy: each person at the party invites a new friend. Each one of them invites another new friend. Etc. When a population is BELOW its carrying capacity, it will INCREASE in size Birth rate exceeds death rates

Carrying Capacity Time Population size Analogy: We run out of food at the party, and people start to leave. Remember over the last few days, when one population in a food web died, so another population that ate it decreased in size? That’s because the carrying capacity decreased! But if it increases too much and rises ABOVE its carrying capacity, it will DECREASE in size Death rate exceeds birth rate

Carrying Capacity Time Population size Remember from math class, exponential increase = constantly doubling. (Makes a curved line on a graph.) Analogy: each person at the party invites a new friend. Each one of them invites another new friend. Etc. When a population is BELOW its carrying capacity, it will INCREASE in size Birth rate exceeds death rates

Carrying Capacity Time Population size Analogy: Once there’s enough food again, more people will come. But we’ll probably be a bit more cautious, and not invite people as quickly. So if we go over carrying capacity, it won’t be by as much. This happens over and over… but the increases and decreases get smaller and smaller…

Carrying Capacity Time Population size Remember from yesterday: stable = does not change This is the concept that my students last year had the most trouble with. Really harp on this. Until eventually, the population size BECOMES STABLE AT THE CARRYING CAPACITY Birth rate = death rate

determined by Carrying Capacity Limiting Factors such as above it Food Water Population decreases below it Space Population increases WORD BANK: water, carrying capacity, pop. size increases, limiting factors, food, pop. size decreases, space

What are three factors that limit deer population size? Why does population size decrease in the highlighted portion of the graph? At which point does birth rate exceed death rate the most? What is the carrying capacity of deer on WallaWalla Island? At which point do deer exceed their carrying capacity?

In what three years does population size most exceed carrying capacity? Why does the population decrease after 1850? Why does population size rise again after each big drop? Identify three periods of time where birth rate exceeds death rate What is the relationship between population size and carrying capacity between 1800-1840?

Think…Pair…Share CHOOSE ONE, PART A: CHOOSE ONE, PART B: Lions hunt and eat wildebeest. If a hunter enters the grassland and kills all the lions there, what will happen to wildebeest population size? Explain why. Bald eagles eat vertebrate fish. If humans pollute the water and many fish die, what will happen to bald eagle population size? Explain why. Bears eat salmon. If a disease causes massive amounts of salmon to die, what will likely happen to the bears? Explain why. If a population has exceeded its carrying capacity, what will happen to it? Explain why.

In Summary… An environment can only support as many organisms as there is available food, water, and free space Carrying capacity = maximum number of organisms that can live somewhere based on these limiting factors We can read a carrying capacity graph to predict changes in population size Below carrying capacity = increase Above carrying capacity = decease Over time, population stabilizes at carrying capacity

Last example What is the carrying capacity of rabbits in this ecosystem? When is the carrying capacity for rabbits first reached? Why does rabbit population size increase in the highlighted section of the graph?