1. 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

2. 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

3. 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.

4. 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.

5. 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. 

6. 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

7. 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

8. 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

9. 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

10. 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…

11. 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

12. 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

13. 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?

14. 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 ?

15. 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.

16. 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

17. 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?