Increasing Vulnerability in Small Populations Brook Milligan - - PowerPoint PPT Presentation

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Increasing Vulnerability in Small Populations

Brook Milligan

Department of Biology New Mexico State University Las Cruces, New Mexico 88003 brook@nmsu.edu

Fall 2009

Brook Milligan Increasing Vulnerability in Small Populations

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Designation versus protection

Recognition of species at risk

9 IUCN categories can lead to official designation, e.g., Federally-listed endangered species, which conveys legal status

Biological status

unchanged by designation may continue to deteriorate mitigation of detrimental factors required in addition to designation

Brook Milligan Increasing Vulnerability in Small Populations

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Core Concepts

Core concepts

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

Populations below a certain threshold exhibit much higher probability of extinction, while those above are much more likely to persist

Brook Milligan Increasing Vulnerability in Small Populations

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MVP: Bighorn Sheep

Brook Milligan Increasing Vulnerability in Small Populations

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MVP: Channel Island Birds

Brook Milligan Increasing Vulnerability in Small Populations

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MVP: Ipomopsis aggregata

Brook Milligan Increasing Vulnerability in Small Populations

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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 to below 100 1920s: increasingly sterile, male skewed sex ratio 1932: extinct

Brook Milligan Increasing Vulnerability in Small Populations

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General Use of MVP and MDA

Result: quantitative assessment of risk in face of uncertainty and 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 fluctuation environmental fluctuation loss of genetic variability

Brook Milligan Increasing Vulnerability in Small Populations

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MVP and Types of Stochasticity

Brook Milligan Increasing Vulnerability in Small Populations

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Demographic Stochasticity

Variation among individuals in survival or reproductive success “Arises from chance events in the survival and reproductive success of a finite number of individuals” (Shaffer, 1981) Measurable as the variance in fitness among individuals compared with the mean of the population

Brook Milligan Increasing Vulnerability in Small Populations

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Environmental Stochasticity and Natural Catastrophes

Temporal variation in survival or reproductive success affecting entire population Two types

environmental stochasticity

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

natural catastrophes

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

Brook Milligan Increasing Vulnerability in Small Populations

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Loss of Genetic Variability

Inbreeding: increase in common ancestry Heterozygosity: decline in variation Effective population size: Ne Inbreeding depression: decrease in fitness

Brook Milligan Increasing Vulnerability in Small Populations

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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)

Brook Milligan Increasing Vulnerability in Small Populations

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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)

Brook Milligan Increasing Vulnerability in Small Populations

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Loss of Heterozygosity

Brook Milligan Increasing Vulnerability in Small Populations

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Loss of Genetic Variability: Effective Population Size

Effective population size: Ne

unequal sex ratio Ne = 4NmNf Nm + Nf (6) variation in reproductive output population 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)

Brook Milligan Increasing Vulnerability in Small Populations

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Effective Population Size

Brook Milligan Increasing Vulnerability in Small Populations

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Loss of Genetic Variability: Inbreeding Depression

Inbreeding leads to greater homozygosity Increased homozygosity exposes more recessive deleterious alleles Greater expression of recessive deleterious alleles leads to reduction in fitness

Brook Milligan Increasing Vulnerability in Small Populations

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Inbreeding Depression

Brook Milligan Increasing Vulnerability in Small Populations

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Inbreeding Depression

Brook Milligan Increasing Vulnerability in Small Populations

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Detrimental Positive Feedback: Extinction Vortices

Any reduction in population size increases probability of further reductions

increases demographic stochasticity increases vulnerability to environmental stochasticity increases loss of genetic variation

Example: mutational meltdown

fixation of deleterious alleles reduction in population growth rate increased risk of fixation of deleterious alleles

Brook Milligan Increasing Vulnerability in Small Populations

References

Shaffer, M. L. 1981. Minimum population sizes for species

• conservation. BioScience, 31:131–134.

Brook Milligan Increasing Vulnerability in Small Populations