Extinction

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Extinction

• deterministic
• consequence of some progressive change in
  environment - addition of predator, loss of food
  source, degradation/loss of habitat
• stochastic
• results from normal, random changes more
  important for smaller populations

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Extinction

• higher probability per unit time for species
  with
• smaller range
• fewer subpopulations
• highly stochastic environment
• low genetic diversity?
• low migration among subpopulations

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Extinction

• primary species of concern tend to be large
  animals
• no clonal propagation
• long generation time
• small number of propagules
• low dispersal rates inability to recolonize or
  escape catastrophic events
• species in stable environments

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Extinction

• Probability of extinction increases as population
  size decreases
• Probability increases with length of time

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Causes of extinction

• Population extinctions occur due to
• intrinsic factors
• demographic stochasticity changes in sex ratio,
  reproduction, survival
• Allee effect - threshold density or N below
  which population goes to extinction
• due to social interactions, physical alterations
  of environment, probability of finding a mate,
  etc.

stanfordalumni.org
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Causes of extinction

• Population extinctions occur due to
• intrinsic factors
• demographic stochasticity
• genetic stochasticity - founder effect, genetic
  drift, inbreeding

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Causes of extinction

• Population extinctions occur due to
• intrinsic factors
• demographic stochasticity
• genetic stochasticity
• extrinsic factors
• environmental stochasticity
• variation in predators, pathogens, food supply
  (biotic)

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Causes of extinction

• Population extinctions occur due to
• intrinsic factors
• demographic stochasticity
• genetic stochasticity
• extrinsic factors
• environmental stochasticity
• catastrophe
• fires, floods, droughts (abiotic)

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N 50
and severe winter
1927, N 13, mostly males
1932
extrinsic factors intrinsic factors
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Most extinctions are due to multiple factors
interacting simultaneously For example causes
of fish extinctions in N. America physical
habitat alteration (73) introduced species
(68) chemical pollution (38) hybridization
(38) overharvest (15)
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Extinction vortices
A vortex
F vortex
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Extinction vortices

• D vortex (demographic discontinuity vortex)
• decrease in N and increase in Var(r) alters
  spatial distribution and patchiness of species
• Ne and therefore persistence of isolated
  populations decreases

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Extinction vortices

• R vortex (demographic, based on intrinsic rate of
  increase, r)
• chance decrease in N increases variance of the
  population growth rate Var(r)
• population becomes more vulnerable to
  environmental stochasticity

normal demographic fluctuations
decline in N small catastrophe
N
increased popn variance
Time
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Extinctions are forever -
ivory-billed woodpecker blue pike spoonhead
sculpin .....
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Minimum viable population (MVP)

• Survival probability of a population of a given
  size for a designated period of time

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Minimum viable population (MVP)

• Survival probability of a population of a given
  size for a designated period of time
• specific to individual species and location no
  universal MVP exists

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Minimum viable population (MVP)

• Key issues
• effect of chance events on
• population persistence - scientific
  issue
• time frame for conservation value
• probability level desired judgments

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Minimum viable population (MVP)

• Example red-cockaded woodpecker
• live in colonies (breeding pair plus helper
  offspring) requiring about 215 acres
• management plan across species range 500
  individuals needed in each of 15 populations

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Minimum viable population (MVP)

• Example red-cockaded woodpecker
• live in colonies (breeding pair plus helper
  offspring) requiring about 215 acres
• management plan across species range 500
  individuals needed in each of 15 populations
• S. Carolina deme had mean time to extinction of
  41.5 yrs, 72 probability of extinction within
  200 yrs
• annual addition of 3F 2M for 10 years doubled
  projected mean time to extinction probability of
  extinction in 200 yrs reduced to 4

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Population viability analysis (PVA)

• A process to determine the probability that a
  population of a given size will go extinct within
  a given number of years
• Used to identify strategies for conservation

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Population viability analysis - estimating MVPs

• experimentation
• powerful, species-specific tool
• ethical issues for endangered species
• takes too long conservation plans needed within
  a decade or two

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Population viability analysis - estimating MVPs

• simulation models
• allow estimation of extinction probability
  (run 1,000 simulations, tally number of
  extinction events)
• indicates which factors are most important in
  declines
• requires large amounts of data
• not generalizable - build anew for each species
• off-the-shelf programs available (VORTEX,
  GAPPS, RAMAS metapopulation)

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Population viability analysis - estimating MVPs

• simulation models
• Bay checkerspot butterfly

current popn. trajectory result of
simulations (only 3 shown) at current
mean rate of population growth (r 0.002)
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Puerto Rican Parrot (endangered)
Declined due to poaching (pet trade), predation,
cyclones, to N 13 birds
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Puerto Rican Parrot (endangered)
13-14 animals in wild captive popn started PVA
with 40 birds, 30 prob. of extinction in 100
yrs - primary risk catastrophe (hurricanes)
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Puerto Rican Parrot (endangered)
13-14 animals in wild captive popn started PVA
30 prob. of extinction in 100 yrs primary
risk catastrophe (hurricanes) Strategy
stockpile food establish 5 populations
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Puerto Rican Parrot (endangered)
13-14 animals in wild captive popn started PVA
30 prob. of extinction in 100 yrs primary
risk catastrophe (hurricanes) Strategy
stockpile food establish 5
populations Issues population fragmentation,
diseases from captive birds
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Florida panther declined due to habitat loss,
poaching, road kills evidence of inbreeding low
fertility, sperm abnormalities, cowlicks, kinked
tails, cardiac defects, undescended testicles,
high disease rate
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Florida panther PVA inputs population size
sex ratio age distribution of each sex age at
first reproduction maximum breeding age of
each sex breeding each year sex ratio at birth
mating strategy number of offspring per year
probability of survival at each age harvest
probability of catastrophic events estimated
carrying capacity
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Florida panther declined due to habitat loss,
poaching, road kills evidence of inbreeding low
fertility, sperm abnormalities, cowlicks, kinked
tails, cardiac defects, undescended testicles,
high disease rate 1989 PVA at N lt 50,
predicted decline of 6-10/year, extinction
in 25-40 yrs - possible earlier extinction due
to disease
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Florida panther Genetic studies indicated low
variability P H DNA H Florida 4.9 1.8 10.4 We
stern US 9.9 4.3 29.7 Other felids 8-21 3-8 45.9
Found to have hybridized with S. American
sub- species introgressed animals with higher P

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Florida panther Outbred with sub-species from
Texas - added 8 females in 1995 F1 hybrid
kittens do not have cowlinks or kinked
tails Texas genes are now 15-29 of total Road
kills lower due to addition of culverts PVA in
1999 extinction probability much
lower
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Population viability analysis

• Models work with N, probability, time
• what about habitat requirements?
• Convert population size needed for 95
  probability of persistence into area of habitat
  needed

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Population viability analysis

• The predicted area required to support a given
  minimum population size varies with
• the mammals feeding ecology (herbivore vs.
  carnivore)
• the mammals environment
• temperate vs. tropic (area required is larger for
  tropical species)
• high environmental variance requires more area

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Population viability analysis

• The predicted area required to support a given
  minimum population size varies with
• the mammals feeding ecology (herbivore vs.
  carnivore)
• the mammals environment
• temperate vs. tropic (area required is larger for
  tropical species)
• high environmental variance requires more area
• In general,
• larger mammals require smaller populations, but
  larger habitat areas
• carnivores require more habitat area than
  herbivores
• populations with more variation in growth rate
  must be larger to persist.
• densities tend to be lower in tropics (may be due
  to greater spp. diversity, leading to greater
  number intensity of biotic interactions)

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Model predictions about species persistence in
the parks of the world (Frankel Soulé 1981 -
Conservation and Evolution) For largest
mammalian carnivores (10-100 kg) 0-22 of parks
should permit persistence for 100 yrs no parks
expected to permit persistence for 1000 yrs For
largest mammalian herbivores (100-1000 kg)
4-100 of parks should permit persistence for
100 yrs 0-22 of parks should permit persistence
for 1000 yrs