Small Population Paradigm

Question 1

Small population characteristics

Answer 1

Abundance occupancy relationship (range size and pop size)
-tight relationship
Small pops have few in a small area
Increased extinction risk
Big horn sheep in America - populations of <15 went extinct

Question 2

Reasons why populations are vulnerable

Answer 2

1. Environmental stochasticity
2. Demographic stochasticity
3. Behavioural factors - Allee effects
4. Genetic Factors

Question 3

Environmental stochasticity

Answer 3

Chance events that disrupt populations
Any perturbation to the environment - especially climate, over large areas, so even large geographic ranges are at risk
Sudden deterioration that have an impact on pop growth rate
Plants more resistant as have dormant stage

Question 4

Environmental stochasticity examples

Answer 4

Volcanic eruption on Monterrant bird population
Direct effects = volcanic dust and habitat loss - density independent
Indirect = lower food (due to acid rain) - DD
Drought - can cause population fluctuations and extinctions
-Fringe-food lizard shows drought-induced population fluctuations

Question 5

Demographic stochasticity

Answer 5

Chance events that disrupt population growth
Can see risk of extinction when looking at probability of offspring surviving to reproduction (bigger pop - less chance of 0)
Non-linear relationship
Sex ratio problems - higher chance of having offspring with same sex if in small pop - common in birds

Question 6

Behavioural factors - Allee effects

Answer 6

Positive density dependence - higher survival or greater fecundity
Difficult to find mate in smaller populations (polar bears)
Competition for mates from similar species - hybridisation (in bird pops e.g.)
Comes a point where negative DD comes in again
Min viable population density = 1 (below driven to Ex by Allee effects, above -DD)

Question 7

Allee effect examples

Answer 7

Plants pollinated by insects - as pop decreases, plants become further apart & small bushes are less attractive (effects rainforest trees due to logging)
Common in social verts
-colonial nesting birds (lower predation etc.)
-pack hunters (wolves)
-flamingos won’t reproduce in small colonies

Question 8

Genetic factors

Answer 8

Smaller population = reduced GD
Homozygosity = how many diff alleles are there in each locus of the genome of individual
Allelic richness = no. of different alleles per locus in that population

Question 9

Effective population size

Answer 9

Ne = No. of individuals contributing to gene pool of next generation
-no selection can be occurring here
-ΔF is the expected drop in heterozygosity per gen
-it will fall the smaller Ne value - non-linear relationship
ΔF = 1 / 2Ne

Question 10

Why is effective pop size different to total?

Answer 10

1. Not all individuals mate - some contribute more than others
   - uneven sex ratio
   - non-monogamous systems - lekking spp or extra-pair copulations are occuring
2. Population size fluctuates and crashes - population bottle necks
3. Overlapping generations - especially in small mammals

Question 11

Genetic Drift

Answer 11

Leads to rapid loss of GD
Stochastic events that determine which alleles are passed to the next gen
Can be rescued by...
1. Mutations (rare)
2. Migrations - even if low is useful

Question 12

Why is low GD bad?

Answer 12

Deleterious recessive alleles are more likely as they tend to be recessive
Natural selection acts on GD - reduces GD reduces evolutionary potential
BUT - 50% of GD consists of deleterious alleles - so loss may get rid of them!

Question 13

Genetic factors example

Answer 13

NZ birds…
Small population due to bottle neck correlated with higher % hatching failure
NZ introduced birds, smaller starting populations correlated with lower hatching success than native pops of that spp
Meta-analysis of animal studies…
20% of explanation of variation in fitness and correlation between fitness and GD

Question 14

Are genetic factors really an issue?

Answer 14

Other extinction drivers act faster…
Species pairs (one threatened other not) - threatened in animals and plants had lower GD
Higher GD can cope with larger ranges of environmental conditions and keep playing ecosystem service roles
Aspen forests show greater primary productivity and energy flux with greater GD

Question 15

Combined factors…

Answer 15

Extinction vortex…
Multiple factors affecting the population following a sudden decrease
E.g. Heath hen - small island and hunted a lot, bottle neck reducing them to 70
-conservation management back up to 2,000
-went extinct still due to…
fire and cold winters
sex ratio bias & pop fluctuations (severe)
inbreeding - disease outbreaks

Question 16

Naturally occurring small populations?

Answer 16

Socorro island hawk - has 15-20 pairs for several 1000yrs
Make sure isn’t in trouble by:
-climate history of island
-monitor sex ratio of populations
-take DNA samples to ensure heterozygosity - compare with closely related spp or museum specimens
-investigate Allee effects by looking at different size colonies and breeding success