1. 2.1.3 Populations Curves

2. Population Curves
Population curves describe a generalized response of populations.

3. MacArthur – Wilson Models
Two idealized categories for reproductive patterns but really it’s a continuum
r-selected & K-selected species depending on position on sigmoid population curve
r-selected species: (opportunists) reproduce early, many young few survive
Common after disturbance, but poor competitors
K-selected species: (competitors) reproduce late, few young most survive
Common in stable areas, strong competitors

4. Carrying capacity K
K species;
experience
K selection
Number of individuals
r species;
experience
r selection
Time

5. Little or no parental care and protection of offspring
r-Selected Species
cockroach
dandelion
Many small offspring
Little or no parental care and protection of offspring
Early reproductive age
Most offspring die before reaching reproductive age
Small adults
Adapted to unstable climate and environmental
conditions
High population growth rate (r)
Population size fluctuates wildly above and below
carrying capacity (K)
Generalist niche
Low ability to compete
Early successional species

6. Fewer, larger offspring High parental care and protection of offspring
K-Selected Species
elephant
saguaro
Fewer, larger offspring
High parental care and protection of offspring
Later reproductive age
Most offspring survive to reproductive age
Larger adults
Adapted to stable climate and environmental
conditions
Lower population growth rate (r)
Population size fairly stable and usually close
to carrying capacity (K)
Specialist niche
High ability to compete
Late successional species

7. r versus K Most organisms somewhere in the middle
Agriculture crops = r-selected
Livestock = K-selected
Reproductive patterns give temporary advantage
Resource availability determines ultimate population size

8. Survivorship curves Different life expectancies for different species
Survivorship curve: shows age structure of population
Late loss curve: K-selected species with few young cared for until reproductive age
Early loss curve: r-selected species many die early but high survivorship after certain age
Constant loss curve: intermediate steady mortality

9. Percentage surviving (log scale)
100 10 1
Age

10. Humans Impact Natural Populations
Fragmenting & degrading habitats
Simplifying natural ecosystems
Using or destroying world primary productivity which supports all consumers
Strengthening pest and disease populations
Eliminating predators
Introducing exotic species
Overharvesting renewable resources
Interfering with natural chemical cycling and energy flow

11. © 2004 Brooks/Cole – Thomson Learning
Disruption of energy flow through
food chains and webs
Disruption of biogeochemical
cycles
Lower species diversity
Habitat loss or degradation
Less complex food webs
Lower stability
Ecosystem collapse
© 2004 Brooks/Cole – Thomson Learning
Environmental Stress
Organism Level
Population Level
Ecosystem Level
Physiological changes
Psychological changes
Behavior changes
Fewer or no offspring
Genetic defects
Birth defects
Cancers
Death
Change in population size
Change in age structure
(old, young, and weak may die)
Survival of strains genetically
resistant to stress
Loss of genetic diversity
and adaptability
Extinction
Disruption of biogeochemical
cycles
Habitat loss & degradation
Lower species diversity
Less complex food webs
Lower stability
Ecosystem collapse