Lecture 22: Community Biology I
November 28, 2011
Human Population Growth
Most K-selected species show logistic growth… what about us?
o K-selected species grow in a logistic pattern of growth
i.e. Exponential growth
o Humans can modify their environment for suitable growth
o No competitors, to compete for resources
CLCIKER QUESTIONS: Why is human population size (worldwide) still increasing?
ANS: Increased carry capacity, K; decrease death rate due to improvement in
medicine; increased birth rates.
Human Populations Vary in r
Population will generally increase by 2025
o Due to higher birth rates
o Lower death rates
r Reflects Age Structure
(Mexico) Large base range
o Big younger population
Higher birth rates in future
Increase in Population
(United States) ….
Increasing N, but Decreasing r
Globally, r=0.012 or 1.2% (per person per year)
o All-time high was 2.2% in the 1960’s
Demographic Transition Model
Over time, human societies get wealthier…
o Pre-industrial era, r was relatively low
o Transitional era, decrease in death rate (d). Value of r is at max
o Industrial era, death rates decrease, birth rates decrease, r decreases
o Postindustrial era, (i.e. Europe), birth rates reach to zero, death rates
decrease/ level off. r decreases
Population size isn’t everything… Community Ecology I
Community has multiple species
o Population includes only one specie
Why are populations distributed the way they are?
Some species limited by a single critical factor
Many species have multidimensional tolerance limits
Imagine this in n-dimensional Space
o Too hot on right, cold on left
o Middle is optimum (tolerance limit)