Population fluctuations and cycles

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
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)