1. Population ecology Exponential growth occurs when resources are not limiting. Logistic growth occurs when resources become more and more limiting as population size increases.

2. Population growth Population ecology –Does population growth continue without limits? Number of resources usually prevent populations from growing exponentially Carrying capacity (K) = maximum number of individual that an environment can support –When population reaches carrying capacity »birth rate = death rate »population growth rate = 0

3. Population ecology –Logistic growth model Population growth rates decreases as population approaches its carrying capacity Population growth rate Per capita growth rate Population size Adjustment for limited resources

4. Population ecology Logistic growth produces S-shaped curve; population growth rate decreases as N approaches K Population size (N) Time (t) K

5. Population ecology –Examples of logistic growth

6. Population ecology When N is very small (imagine N is 1 and K is 1000)... is close to 1, so population grows exponentially Population size (N) Time (t) (1) –How does logistic growth model work?

7. Population ecology When N approaches K (imagine N is 500, 600,...900 and K is 1000)... Gets closer and closer to 0, so population growth slowly approaches 0 Population size (N) Time (t)

8. Population ecology When N equals K (imagine N is 1000 and K is 1000)... is 0, so population growth is 0 Population size (N) Time (t)

9. Population growth Population ecology –Density-dependent population regulation As populations near carrying capacity…population growth rate declines –Per capita birth rates decrease (fewer resources available for production of offspring) –Per capita death rates increase (fewer resources for survival, predators focus attention on common prey) »fewer resources available for production of offspring »fewer resources for survival »predators focus attention on common prey

10. Population growth Population ecology –Example of density- dependence As population size increases, fecundity decreases As population size increases, mortality increases

11. Population growth Population ecology –Density-dependent factors include: disease predators competition for resources Allee effect = population growth rate reduced at low population density –Difficulty finding mates could reduce birth rates at low population size –Not all density-dependent factors result in reduced population growth rates as population size increase

12. Population growth Population ecology –Sometimes population regulated by density- independent factors Birth rates decrease and death rates increase regardless of population size –Extremely cold winter –drought –fires

13. Type of population regulation may influence life- history traits: Population ecology –Species regulated by density-dependent factors are selected to be good competitors (populations are often close to carrying capacity) Such species invest heavily in survival But at a cost of reduced reproductive potential (i.e. a life history trade-off) Called K-selected species Examples: elephants, oak trees

14. Type of population regulation may influence life- history traits: Population ecology –Species regulated by density-independent factors are selected to be good reproducers (populations are often below carrying capacity) Such species invest heavily in reproductive output But at a cost of reduced survival (i.e. a life history trade-off) Called r-selected species Examples: cockroaches, birch trees

15. r- and K-selected life history traits (ends of continuum) Population ecology traitr-selectedK-selected – age at 1st reproductionearlylate – lifespanshortlong – Survivorshiplow (type III)high (type I) – Fecundityhigh low – Paritysemelparityiteroparity – Offspring sizesmalllarge – Parental carenonelots