L13 - Extirpation

Question 1

What is extinction?

Answer 1

Death of the last individual of the species

Question 2

How do we demonstrate extinction?

Answer 2

• If there are no longer individuals found where previously there had been, we can classify this species as extinct
• A “thorough search” is conducted to classify a species as extinct, but different agencies disagree as to what constitutes a “thorough search”

Question 3

What is the last stage before full extinction?

Answer 3

• Extinct in the wild
• A species can be extinct in its habitat, but still found in captivity
  
  • Ex: Tasmanian tiger, Passenger pigeon, California condor (1987)

Question 4

Why won’t species survive in captivity?

Answer 4

• Do not breed well
• Prone to disease
• Low genetic diversity

Question 5

What is functionally extinct?

Answer 5

• A species which still has members present in the environment
  
  • Population is greatly reduced compared to the ancestral population
  • Have decreased below their minimum viable population
  • Have obvious factors in the environment preventing populations from recovering
  • Are no longer performing their role in the ecosystem

Question 6

Define minimum viable population

Answer 6

• Genetic diversity may be too low for healthy breeding over the long term.
• Greater likelihood that “chance events” could wipe out your entire population

Question 7

What is an example species that is functionally extinct?

Answer 7

American Chestnut
- Once the most common nut tree in Eastern North America
- Many different animal species ate the nuts, making it a primary producer for forest ecosystems
- Early 1900s: fungal disease killed ~4B trees
- Today: few thousand trees remain in the wild but this species is no longer an important contributor to the ecosystem

Question 8

Why do species go extinct?

Answer 8

• Global extinction is a result of local extirpations

Question 9

What is extirpation?

Answer 9

• Local extinction of a population from a
  geographical range.
• Other populations of the species survive
  elsewhere.

Question 10

Why do populations get extirpated?

Answer 10

• Population dynamics: changes in population size (N) and composition over time
• Extirpation: N=0

Question 11

What determines the number of individuals (N) in a population over time?

Answer 11

1. Numbers in the population in previous time, Nt-1
2. Intrinsic rate of population increase, r
3. Nt= Nt-1*(1+r)
   Extirpation occurs when Nt=0
   ** Can also add “stochasticity”

Question 12

What are density-dependent factors?

Answer 12

• Factors affecting population numbers
• An environment only has enough resources (food, space…) to support a specific number of individuals in a population

Question 13

What is the carrying capacity K?

Answer 13

The number of individuals an environment can support before resources run out or the environment begins to degrade.

Question 14

Other factors that affect population numbers?

Answer 14

• Human impacts (harvesting, animal husbandry)
• Extreme events (natural catastrophes, spread and impact of disease)
• Pollution (chemicals, changes in environment)
• Land-use change (changing environments, loss of habitat)

Question 15

What are the consequences of low population size?

Answer 15

• Increased effect of stochasticity (random chance)
• Population processes (Allee effects)

Question 16

Allee effects: what are the possible side effects of a loss in genetic diversity?

Answer 16

• Greater susceptibility to disease
• Greater chance of deleterious alleles (detrimental to survival) becoming prominent in the population

Question 17

What are metapopulations?

Answer 17

Networks of spatially isolated populations, connected by some exchange of individuals (or pollen, gametes) over time.

Question 18

How does immigration affect population numbers?

Answer 18

• Allows for gene flow between isolated populations

Question 19

What is gene flow?

Answer 19

• The movement of alleles between two geographically separated populations
• Gene flow can be caused by a single individual (immigration/emigration) moving between populations

Question 20

Population viability: role of chance?

Answer 20

• In probability of natural catastrophes and population demography…
  
  • Stochasticity in life history parameters and in environmental conditions that influence survival and reproduction
• In metapopulation dynamics…
  
  • Degree of movement among adjacent populations and environmental conditions that influence emigration/immigration
• In effects of small population size
  
  • Detrimental influence on interactions among individuals (Allee effect)
  • Interbreeding depression
  • Genetic bottlenecks (loss of alleles)

Question 21

What are our past and current species numbers?

Answer 21

• ~ 5B species have evolved on Earth
• ~8.7 Million species currently
  
  • Extinction is the rule for species, not the exception

Question 22

What are the human mechanisms that have caused species extinctions?

Answer 22

• Direct predation by humans
  Secondary effects:
• Land burning
• Habitat change
• Introduced species
• Cascade effects

Question 23

Did humans cause extinctions?

Answer 23

• Lines of evidence: Overkill hypothesis
  
  • Extinctions fall at 10 000 BP, kill sites lead us to believe that humans caused extinctions of large mammals
  • But also
    
    • There are extinctions of species not caused by humans
    • Some vulnerable species survived
    • Extinctions occurred where there were no humans present
    • Kill sites are not that abundance and there is a lack of evidence in the fossil recor

Question 24

Climate caused extinctions?

Answer 24

• Last glaciation coincides with extinctions
• Changing vegetation, food source, competition
  
  • May explain impacts on species that weren’t hunted
    BUT
• No extinctions through dozens of past glacial advances and retreats
• extinctions may be too rapid