increasing vulnerability in small...

References

Increasing Vulnerability in Small Populations

Brook Milligan

Department of BiologyNew Mexico State University

Las Cruces, New Mexico [email protected]

Fall 2009

Brook Milligan Increasing Vulnerability in Small Populations

mailto:[email protected]

References

Designation versus protection

Recognition of species at risk

9 IUCN categoriescan lead to official designation, e.g., Federally-listedendangered species, which conveys legal status

Biological status

unchanged by designationmay continue to deterioratemitigation of detrimental factors required in addition todesignation


References

Core Concepts

Core concepts

Minimum viable population size (MVP)Minimum dynamic area (MDA)

Populations below a certain threshold exhibit much higherprobability of extinction, while those above are much morelikely to persist


References

MVP: Bighorn Sheep


References

MVP: Channel Island Birds


References

MVP: Ipomopsis aggregata


References

Which Factors Cause Extinction?Heath hen: Tympanuchus cupido cupido

Once fairly common from New England to Virginia

Declined steadily with European settlement

1876: remained only on Martha’s Vineyard

1900: fewer than 100 survivors

1907: refuge on Martha’s Vineyard and predator control

1916: increase to over 800 birds

1916: fire destroyed most nests and habitat

1916 winter: high predation by goshawks (Accipiter gentilis)

1917: reduced population to 100–150 individuals

1920: 200 individuals, but disease reduced population tobelow 100

1920s: increasingly sterile, male skewed sex ratio

1932: extinct


References

General Use of MVP and MDA

Result: quantitative assessment of risk in face of uncertaintyand stochasticity as a function of population size

Analogous to flood prediction

Framework for evaluating alternative management options

Factors leading to population decline and extinction (Shaffer,1981)

demographic fluctuationenvironmental fluctuationloss of genetic variability


References

MVP and Types of Stochasticity


References

Demographic Stochasticity

Variation among individuals in survival or reproductive success

“Arises from chance events in the survival and reproductivesuccess of a finite number of individuals” (Shaffer, 1981)

Measurable as the variance in fitness among individualscompared with the mean of the population


References

Environmental Stochasticity and Natural Catastrophes

Temporal variation in survival or reproductive successaffecting entire population

Two typesenvironmental stochasticity

“due to temporal variation of habitat parameters and thepopulations of competitors, predators, parasites, and diseases”(Shaffer, 1981)measurable as the variance through time in the populationmean fitness

natural catastrophes

“such as floods, fires, droughts, etc., which may occur atrandom intervals through time” (Shaffer, 1981)measurable as the variance through time in mean acrosspopulations


References

Loss of Genetic Variability

Inbreeding: increase in common ancestry

Heterozygosity: decline in variation

Effective population size: Ne

Inbreeding depression: decrease in fitness


References

Loss of Genetic Variability: Inbreeding

Increase of homozygosity or identity by descent

F (t + 1) = F (t)(1) + (1− F (t))

(1

2Ne

)(1)

∆F (t) = (1− F (t))

(1

2Ne

)(2)


References

Loss of Genetic Variability: Heterozygosity

Loss of heterozygosity

H(t + 1) =

(1− 1

2Ne

)H(t)

= λH(t) (3)

∆H = H(t + 1)− H(t)

=

(1− 1

2Ne

)H(t)− H(t)

= − 1

2Ne(4)

H(t) =

(1− 1

2Ne

)t

H(0)

= λtH(0)

≈ H(0) exp

(− t

2Ne

)(5)


References

Loss of Heterozygosity


References

Loss of Genetic Variability: Effective Population Size

Effective population size: Ne

unequal sex ratio

Ne =4NmNf

Nm + Nf(6)

variation in reproductive outputpopulation fluctuation

1

Ne=

1

t

t∑i=1

1

Ni(7)

1

Ne=

1

5

(1

10+

1

20+

1

100+

1

20+

1

10

)=

1

5· 31

100

=1

16.1(8)


References

Effective Population Size


References

Loss of Genetic Variability: Inbreeding Depression

Inbreeding leads to greater homozygosity

Increased homozygosity exposes more recessive deleteriousalleles

Greater expression of recessive deleterious alleles leads toreduction in fitness


References

Inbreeding Depression


References

Detrimental Positive Feedback: Extinction Vortices

Any reduction in population size increases probability offurther reductions

increases demographic stochasticityincreases vulnerability to environmental stochasticityincreases loss of genetic variation

Example: mutational meltdown

fixation of deleterious allelesreduction in population growth rateincreased risk of fixation of deleterious alleles


References

Shaffer, M. L. 1981. Minimum population sizes for speciesconservation. BioScience, 31:131–134.


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