1. Population fluctuations and cycles
Populations tend to fluctuate through time
Localized populations fluctuate more than entire populations

2. What could be causing these fluctuations?

3. Northern Bobwhite populations
Timing of density measurement affects population size estimate and degree of fluctuation detected
Northern Bobwhite populations

4. Resilience
Rate at which population springs back to equilibrium after jumping above or dipping below it due to a “disturbance”
Body size and lifespan effects
Smaller, shorter-lived species show wider swings in population size. Why?
Smaller, shorter-lived animals show greater resilience. Why?

5. Oscillating populations appear to have regular cycles

6. Key hypotheses for why populations oscillate
Time lags
Predator-prey relationships
Links to environmental cycles
Example: El Niño and kelp forests
Life cycles of parasites
Example: Malaria-carrying mosquitos
Changes in gene frequencies
Behavior...

7. Snowshoe hare cycles: Evidence of decline is evident just prior to peak
High winter weight loss
Decreased juvenile growth rate
Decreased juvenile overwinter survival

8. What makes a population vulnerable to extinction?
Low density
Difficult to find mates (Queen conchs in Florida)
Vulnerable to hybridization (less choosy)
Isolation
Restricted gene flow/cut off from metapopulation
Why a problem?
Small size
Could lose a higher proportion of individuals in a disaster
Smaller subset of genotypes; additional genetic drift

9. Mass extinctions
In which geologic period, and approximately how many millions of year ago, did the largest extinction occur?
Permian (250 mya)
When did the dinosaur extinction occur?
Cretaceous (60 mya)
Was this extinction restricted to dinosaurs or to land?

10. Process of extinction Role of localized extinctions
Habitat destruction or other cause 
Marginalization of individuals ( survival/ reproduction)
Fragmentation of habitat and populations 
Continuous populations  metapopulations
Reduction of gene flow
Inbreeding and genetic drift 
Local population wiped out by disaster 
Many localized extinctions  Species itself is threatened or eliminated

11. Deterministic causes of extinction
Occurs due to force or change with continued effects
No escape for population or species over the long term
Asteroid theory of Cretaceous extinction: species unable to escape climate change
Overhunting of passenger pigeon and great auk

12. Great auk: What led to its extinction?
Initial distribution: widespread across the North Atlantic
Localized killing for food and bait  fragmentation
For-profit killing for fat and feathers  extinction of local populations
When rare, high-paid trade in skin and eggs…
Last pair and one egg taken in Iceland, 1844
Image courtesy of the Canadian Museum of Nature

13. Stochastic causes of extinction
Examples?
How might determinstic and stochastic causes work together to cause extinction
Heath hen example… (also in text)