Causes and Measures of Population Viability 2008

Causes and Measures of Population Viability 2008 - Causes...

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Causes and Measures of Population Viability ESM 211 Jan. 9, 2008
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Why is a population declining?
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Factors influencing viability Mean growth rate Variability in growth rate Environmental stochasticity Demographic stochasticity Spatial variability Sampling variation Density dependence Genetics
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Mean growth rate Annual population growth rate 0< λ <1: decline λ >1: growth Instantaneous growth rate r <0: decline r >0: growth Balance of births ( bN ) and deaths ( dN ) 1 1 ( ) log t t r N N b d N rN t r e λ + = = + - = = =
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Growth rate variability: why it matters Average long-term growth rate of population given by geometric mean of annual growth rate ( λ G ) If there is variability, this is less than the arithmetic mean ( λ A ) Example: λ alternates between 1.1 and 0.9 λ A = 1 Does population size change after 2 years?
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Variation and the long-term population growth rate 0.6 0.7 0.8 0.9 1 1.1 0 0.2 0.4 0.6 0.8 1 Standard deviation of λ Stochastic population growth rate G λ A = 1.05 λ A = 0.95
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Environmental stochasticity Temporal variation in vital rates driven by changes in environment that are inherently unpredictable Affects all individuals simultaneously Variation in λ is independent of population size May be correlated across space and time
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NDVI and reindeer calf body mass Pettorelli et al. (2005) Biology Letters 1:24
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Demographic stochasticity Temporal variation in population growth driven by chance variation in actual fates of individuals An individual either lives or dies Offspring come in integer numbers Effects on each individual independent Variation in λ declines with population size
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Causes and Measures of Population Viability 2008 - Causes...

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