Problems of Small Populations

Rarity 

  • Some species are naturally rare

  • Restricted to uncommon type areas and conditions (specialists)

  • Limited to a small geographic range (endemics)

  • Occur at low population densities (large organisms)

  • Artificial rarity


Demographic Stochasticity

  • Variation in birth and death rates among individuals and across years

  • Ideal population: 

    • Increase until K → births=deaths

  • <50 individuals:

    • Population size fluctuates more 

    • Unequal sex ratio

  • Allee effect

    • Population size ←> population density ←> population growth rate 

    • Low population density = low population growth


Environmental Stochasticity

  • Random variation in biological and physical environment

  • Affects all individuals in a population

  • Catastrophes


MVP

  • Minimum viable population size

  • # individuals necessary to ensure long-term survival

  • 99% chance of surviving next 1000 years (or 100)

  • Population Viability Analysis


Vortex

  • Number of catastrophes: 0

  • Polygynous mating system

  • Age of 1st offspring for females: 4

  • Age of 1st offspring for males: 2

  • Max. age of reproduction: 10

  • Max. # of broods per year: 1

  • Max. # progeny per brood: 5

  • Sex ratio at birth - % male: 50

  • % adult females breeding: 20 + 5 %

  • # offspring per female per brood: 3

  • Mortality: 10 + 1% for all ages & sexes

  • % males in breeding pool: 50%

  1. Number of catastrophes: 0

|Polygynous mating system|

  1. Age of 1st  offspring for females: 4

Age of 1st offspring for males: 2

Max. age of reproduction: 10

Max. # of broods per year: 1

Max. # progeny per brood: 5

Sex ratio at birth - % male: 50

% adult females breeding: 20 + 5 %

# offspring per female per brood: 3 + 1

  1. Mortality: 10 + 1% for all ages & sexes

|% males in breeding pool: 50% |


Loss of Genetic Variability

  • Genetic drift- random changes in gene frequencies

  • Most pronounced in small populations

  • Always results in a loss of genetic diversity

Genetic Drift Simulation

  • Lions in Ngorongoro Crater, Tanzania

  • 60-75 individuals

  • 1962 – outbreak of biting flies

  • Pop reduced to 9 females & 1 male

  • Low genetic variability


Inbreeding Depression

  • Inbreeding: mating between close relatives

  • Inbreeding depression: decreased fitness in inbred offspring

Effective population size (Ne)

  • Number of individuals genetically contributing to the next generation

  • Often much lower than the census population size

  • Factors that affects Ne 

    • Unequal sex ratio

    • Variation in reproductive outout

    • Population size fluctuations

Extinction Vortex