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Understanding Population Extinction Predictors: Demographic, Environmental, & Genetic Stochasticity

Explore factors like carrying capacity, growth rate, and stochastic events affecting population persistence. Learn to calculate SD, predict catastrophes, & genetic stochasticity impacts.

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Understanding Population Extinction Predictors: Demographic, Environmental, & Genetic Stochasticity

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  1. Announcements • HW Addendum for CONS670 • Reading assignment for BSCI363

  2. Mean r = 0, P(extinction) = ? Population Density (Ln) pop “a” pop “b” pop “e” pop “g” pop “c” pop “d” pop “h” pop “f” TIME

  3. General Predictors of Extinction Current population size + Population Growth - Population Size

  4. General Predictors of Extinction • Carrying capacity / population size. • Maximum growth rate. • Variation in growth rate • Demographic stochasticity • Environmental stochasticity • Genetic stochasticity

  5. Variation in B&D: Demographic Stochasticity • “Transparent” in VORTEX • Probabilistic nature of births and deaths, males and females • Function of • Birth and death rates • Fecundity = 0.34? • Sex ratio

  6. Variation in B&D: Demographic Stochasticity

  7. Variation in B&D: EV • Fecundity of adult spotted owls = 0.34 • In a “normal” year: 34% of adult females have 1 female offspring. • In a “bad” year, EV results in decreased r: e.g., births = 34% - “x” • In a “good” year, EV results in increased r: e.g., births = 34% + “x”

  8. Yearly Variation in Fecundity X= 34% frequency s.d. s.d. s.d. s.d. 14 24 34 44 54 % of females producing offspring ~68% ~95%

  9. Calculating S.D. from Data (> 5 yrs.)

  10. Calculating S.D. from Data (> 5 yrs.)

  11. Calculating S.D. From Data (Range) • Average fecundity = .34 (range .14 – .54) • Calculate S.D., based on years / data points • For N ~ 10, assume range defines +/- 1.5 SD. • For N ~ 25, assume range defines +/- 2SD • For N ~ 50, assume range defines +/- 2.25 SD • For N ~ 100, assume range defines +/- 2.5 SD • For N ~ 200, assume range defines +/- 2.75 SD • For N ~ 300, assume range defines +/- 3 SD

  12. “Last Ditch” Estimate of S.D. • Where mean value (e.g. fecundity) = 34% • “highly tolerant of EV” • let SD = 34%*.05 • “very vulnerable to EV” • let SD = 34%*.50 • “intermediate tolerance” • let SD = 34%*.25

  13. Variation in B&D: Catastrophes • Defined by VORTEX as episodic effects that occasionally depress survival or reproduction. • Types (up to 25, start with 1) • Independent causes of mass mortality. • Probability based on data (# per 100 years). • Loss due to catastrophe (= % surviving) • 0 = no survivors. • 1 = no effect.

  14. Catastrophes: Harbor Seals • Disease outbreaks in 1931, 1957, 1964, and 1980 • 1980: 445 seals out of ~10,000 died. • “Few” seals reproduce J. R. Geraci et al., Science215, 1129-1131 (1982).

  15. Disease outbreaks in 1931, 1957, 1964, and 1980 445 seals out of ~10,000 died. “Few” seals reproduce Probability of catastrophe: 26, 12, 14 years between outbreaks Average time between outbreaks = 17 years. 1 every 17 years or 6 every 100 years. Loss (e.g., % surviving) 9,555 / 10,000 ~ 95% Reproduction = ? Catastrophes: Harbor Seals J. R. Geraci et al., Science215, 1129-1131 (1982).

  16. Catastrophes: More Info • Mangel, M., and C. Tier. 1994. Four facts every conservation biologist should know about persistence. Ecology 75:607-614. • General background • Young, T. P. 1994. Natural die-offs of large mammals: implications for conservation. Conservation Biology 8:410-418. • Possible reference or starting point for term-paper • Access through JSTOR (www.jstor.org)

  17. Variation in B&D: Genetic Stochasticity Aa x Where a is deleterious Aa Homozygous recessive is lethal (Recessive Allele Model) Presence of “a” allele decreases fitness Reduced fitness = sum of lethal equivalents (Heterosis Model)

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