Lecture 5 Flashcards
Which populations are most at risk?
Rare species - small geographic range or low population density
Because higher risk of extinction
Minimum viable population size
Minimum number of individuals needed in a population
Small isolated populations have reduced viability
Study on population viability of British birds
6-12 breeding pairs = 7.5 years to extinction
PHVA
Population and habitat viability analysis
Environmental risk and natural catastrophes
Both affect species irrespective of population size
General pattern on isolated island populations
Example of effect of natural catastrophe on species (1)
Rodrigues fruit bag
Cyclones devastate population irrespective of numbers
Solution may be increasing population size on Rodrigues or establishing a second population on nearby Diego Garcia
O’Brien et al., 2007
Example of effect of natural catastrophe on species (2)
Sisserou parrot (Amazona imperialis)
Dominican parrots live in one patch forest
After hurricane, shortage of fruits for some time
Parrot numbers decline for couple of years after hurricane
What is demographic stochasticity?
The result of chance independent events of individual mortality and reproduction, causing random fluctuations in population growth rate
How are small populations negatively affected by demographic stochasticity?
Small populations closer to extinction
Problem with small populations and reproductive rate
BIDE
Births
Immigration
Deaths
Emigration
Carrying capacity (K)
The natural limit set on populations by availability of resources
What affects carrying capacity?
Interspecific competition - one species affects resources available to another (varies between habitats)
Facilitation - when one species benefits from the other
What is irruptive growth?
Explosions and crashes in population numbers Regulated by resource availability Wide variation around carrying capacity Typical r-strategists Few species
What is logistic growth?
Sigmoidal curve
Growth rate regulated by intrinsic factors (density-dependent mortality, birth rates)
Typical K-strategists
Most species
K-strategist small populations
In small populations, individuals reproductive rates are high but numbers are low
If population is reduced, early recovery is slow
At half the carrying capacity there is a rapid escalation in numbers
Near carrying capacity the rate of population growth declines
What is the maximum sustainable yield?
Half carrying capacity
Inflection point K/2
Aims at a balance between too much and too little harvest to keep the population at an intermediate abundance with a maximum replacement rate
Example of problem with getting maximum sustainable yield wrong
Orange roughy fish found around seamounts discovered in 1990s
By 2008 only 10% of original stocks
This was due to using demographic factors from other fish to calculate MSY
Problems with calculating MSY for small populations
- Calculating population growth rates more difficult if wide variation in numbers
- Small populations suffer erratic size swings due to demographic stochasticity
Problems when applying MSY
- Very difficult to get accurate estimates of population size
- Carrying capacity changes; impossible to estimate
- Basic demographic data rare as varies between populations
- Difficult to get measures of other forms of mortality
- Social systems and mating strategies often unknown
- Pays to over-harvest long-lived species as they will be driven to extinction
What is the threshold response to habitat change over time?
Populations crash because response to perturbation not linear
Who wrote the paper showing the three basic responses of British mammals to habitat fragmentation?
Bright, 1993
What is the species pool of genetic diversity?
Total genetic variation of species partitioned into within versus between population diversity
Species pool of genetic diversity in 3 levels
- Variation within individuals
- Variation within populations
- Variations between populations
Species pool of genetic diversity level 1: variation within individuals
- Heritable genetic variation (basis for evolution)
- Species should be heterozygous
Species pool of genetic diversity level 2: variation within populations
- Gene pool
- Type of alleles and frequency they occur across the population
Species pool of genetic diversity level 3: variation between population
- Rare for a single species to be one large panmictic population
- Allows for individual populations to adapt to local circumstances
Is heterozygosity important?
Not much evidence but likely that it is
Correlations between fitness and heterozygosity are weak
Variation in natural populations
Don’t know how heterozygosity is translated into fitness
In what species is heterozygosity important?
Common toad
Less abnormalities with increasing heterozygosity
In what species is heterozygosity less important?
South African cheetahs
Low genetic diversity
Rule of thumb with heterozygosity
Larger populations have higher heterozygosity
Smaller populations lose heterozygosity over time
How is genetic diversity lost?
- Founder effects
- Demographic bottleneck
- Genetic drift
- Inbreeding
How is genetic diversity lost - founder effect
- Only a few individuals establish a new population
- New population biased
- Not representative of the whole population
- Lower genetic diversity
How is genetic diversity lost - demographic bottleneck
- Occurs when temporary reduction in population size
- Relatively few animals go through bottleneck so a lot of genetic variability is lost
How is genetic diversity lost - genetic drift
- Change in frequency of an existing allele in a population due to random sampling of organisms
- The rate of allele loss increases with disparity of adult sex ratio, and bias in mating success (e.g. polygynous species)
How is genetic diversity lost - inbreeding
- When individuals mate with close relatives
- Increases homozygosity
- Progeny can inherit deleterious traits
General results of inbreeding on offspring
- Reduced fecundity
- Offspring smaller
- Less likely to survive
- Increased physical deformities
How to help maintain genetic diversity?
- Understanding the genetic problems that small populations face
- Encourage heterosis (outbreeding/producing hybrids) as offspring have increased vigour and are fitter than both parents
Effective population size
Number of breeding individuals in population (not all animals breed)
Rate of genetic loss taking into account effective population size
N(e) is the effective population size
The rate of genetic loss is 1/2N(e)
What is the 50/500 rule?
Minimum effective population of 50 will limit deleterious effects of inbreeding
Longer-term minimum effective population size 500 will maintain genetic variation for adaptive evolution
What two species recovered from very low population sizes?
Arabian oryx
Przewalski’s horse
To allow for non-breeders, what is the alteration to the 50/500 rule for different animals?
100/200 for birds
1000/2000 for mammals
