Lec 22: Community Ecology
Human population growth
• Most K-selected species show logistic growth… what about us?
• No evidence that human population growth has been leveling off
• Growth seems to be exponential
• Humans have technology, this allows us to manipulate the
environment and adjust the carrying capacity
• Ex. Agriculture
• Humans also don’t have any predators
CLICKER: why is human population size worldwide still increasing?
A) we are r-selected, not k-selected
B) We have succeddes in increasing the carrying capacity, K
C) We have succeedes in increasing per capita birth rates, b
D) we have succeeded in decreasing the per capita death rates, d.
Human populations vary in r
- There is geographic variation
- Different continents have different values for r
- Red bar is projected populations for the year 2025. Certain continents (asia,
africa) are projected to increase whereas, (eurpoe) population size tends to
r reflects age structure
- Difference in whether a population will increase or not is majorly due to age
- Graph on the left (united states) seems like they will have a stable population
because they have an equal age distribution \ - Graph on the right (mexico)- the population will increase because there are
many prereproductive individuals.
Increasing N, but decreasing r
• Globally, r = 0.012 or 1.2% (per person per year)
- Population will increase as long as r is above 0.
- r is decreasing; however, the population is still growing because r is >0 (still
- When population size decreases (aprox 2050) the harmful effects on the
environment (global warming, pollution) will decrease Demographic transition model
• Over time, human societies get wealthier…
- This is drawn from observation of birth and death rates in western europe.
- Preindustrial- there is high death rates, high birth rate because the
population is low early on- Low values of r
- Transitional (stage 2)- decrease in death rates, happens before population
responds by decreasing birth rates- High values of r- High rates of growth
- Industrial- People choose to family plan (decrease birth rate)
- Post Industrial- Birth rate falls below death rate
- Extreme poverty can keep populations stuck in transitional phase; however,
most countries will make it to industrial and post industrial stage.
Population size isn’t everything!
- Lower population puts less strain on the environment. All other things being
equal, not all humans have an equal use of resourses. Not everyone has the
same size carbon footprint.
- U.S person has a footprint 50x bigger then someone in Guyana.
- Species interact with eachother (ecological community)- the assemblage of
populations that interact in a given place.
- Communities are composed of multiple species
Why are populations distributed the way they are? • Some species limited by a single critical factor or resource (ex. Temp, soil
moisture, the amount of salt in the water, food supply)
• Many species have multidimensional tolerance limits. A particular species has
more then one resource it requires
Imagine this in n-dimensional space
@ optimal temperature, species are abundant
@ upper and lower limit, species are infrequent as a result of physiological stress
@ low/ high extreme temp