Lecture Outline: Population Viability Analysis

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Lecture Outline Population Viability Analysis

• Extinction rates
• Key threats
• Predicting extinction risk from species traits

• PVA
• Uses
• Types
• Limitations

• Extinction risk demography, genetics, or both?

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Recent extinctions of animal species
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Are current extinction rates elevated relative to
background rates?

• Most ecologists agree that current rates are
  relatively high and mainly due to human effects

• However, disagreement exists regarding the
  magnitude of the current extinction periodsome
  equate current period with historical mass
  extinctions others think predictions are
  overestimates.

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Problems with evaluating current and future
extinction rates

• We dont know how many species there are on earth
• (perhaps 6-20 million but only 1.5 million
  described)

• Extinctions of certain groups are well documented
  (mammals, birds) but others are not (insects,
  plants).

• Background levels are estimated from fossil
  record and have great deal of uncertainty
  associated with them.

• Future predictions might overestimate species
  loss because the species most susceptible to
  human impacts might be lost first.

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Extinction Risks for Vertebrates
No. threatened in 2008
Threatened
Described
Evaluated
1275
Fish
4
37
1905
30
30
Amphibians
31
5
423
Reptiles
12
1222
12
Birds
21
21
1141
Mammals
(Source IUCN Red List 2008)
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Causes of Endangerment for Vertebrates
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Species traits that might increase extinction risk

• Habitat specialist
• Habitat overlap with humans
• Sensitive to disturbance
• Limited dispersal ability
• Rarelow population density, restricted
  geographic range
• Low growth rate capacity (life-history
  constraints)
• Large body size
• Harvestable
• High trophic levels

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The demise of the passenger pigeon

• Probably the most abundant bird in North America.

• Continental population might have been 6 billion
  and represented 25-40 of
• all of the birds in North America.

• In 1866, a cloud of birds passed into southern
  Ontario that was a mile wide,
• 300 miles long, and took 14 hours to pass a
  single point.

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Predicting Extinction Risk in Declining Species

• Used complete phylogenies of contemporary
  carnivores and primates.
• Asked which traits are associated with high risk
  of extinction.
• Assessment of extinction risk from IUCN Red List.

(Purvis et al. 2000. Proc. Royal. Soc. London B
2671947-1952)
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• Most important predictors
• high trophic level
• low population density
• slow life history
• SMALL GEOGRAPHIC RANGE
• Key traits explained 50 of variation in
  extinction risk.
• Remaining variation explained by anthropogenic
  factors that affect risk independently from
  species biology.

(Purvis et al. 2000. Proc. Royal. Soc. London B
2671947-1952)
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Extinction Risk for Mammals
Current Risk
Predicted Risk
Latent Risk
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Hotspots of Latent Extinction Risk
(Cardillo et al. 2006. PNAS 1034157-4161)
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Population Viability Analysis (PVA)

• A quantitative assessment of a populations risk
  of extinction, quasi-extinction, or projected
  growth rate given current conditions or those
  expected due to proposed management.

• We already have conducted various sorts of PVA in
  lab with Ramas Ecolab
• Effect of TEDs on loggerhead sea turtles
• Habitat improvement for a spotted owl
  metapopulation
• Environmental stochasticity and helmeted
  honeyeater demography

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Potential Uses of PVA
from Morris and Doak. 2002. Quantitative
Conservation Biology.
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• Initial focus of PVA was to determine Minimum
  Viable Population (MVP) size.
• For instance, what is the population size of
  grizzly bears needed to be 95 certain that the
  population should remain extant for 100 years?

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Main Types of PVA

• Deterministic Single Population Models
• e.g., age-structured matrix model using only
  mean vital values

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IBM
Metapopulation (Detailed Multi-site)
Increasing Data Requirements
Stochastic Single Population
Deterministic Single Population
Increasing Realism
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Criticisms of PVA Are Models Reliable?
1. Poor data quality

• Data requirements for even deterministic model
  are not trivial
• Good estimates of means (and variances) of vital
  rates are difficult to obtain for endangered
  species
• Dispersal is especially tough to estimate with
  certainty

2. Form of density dependence unknown
5. Models not validated with field data
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Test of PVA1

• Extensive evaluation of PVA using data from 21
  long-term studies

• Surprisingly close match between model
  predictions and real outcomes.
• Actual population sizes fell within bounds
  predicted by stochastic simulations.

PVA is the best tool we have for estimating
extinction risk, and the alternatives are
subjective, less rigorous, and likely to provide
poorer predictions
1Brooks, BW et al. 2000. Predictive accuracy of
population viability analysis in conservation
biology. Nature 404385-387.
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Critique of Brook et al.1

• Argue that Brook et al.s conclusions were worded
  too strongly and a result of bias in studies
  included in the evaluation.

• Only used long-term studies with high-quality
  data and these conditions are the exception for
  populations of endangered species.

• Suggested that PVA will only be accurate for
  predicting extinction probability if data are
  extensive and reliable and if estimated vital
  rates are likely to apply into the future.

PVAs could be useful for comparing the
consequences of different management or
conservation strategiesHowever, we doubt the
general claim that they can be accurate in their
ability to predict the future status of wild
populations
Coulson et al. 2001. The use and abuse of
population viability analysis. TREE 16219-221.
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General Recommendations for using PVA
1. PVA should be treated as a model. Validity of
models should be tested with independent field
data and PVA adjusted accordingly.
2. Evaluate relative rather than absolute rates
of extinction or growth.
3. Do not focus on single value such as MVP
models are not accurate enough to make such
precise predictions.
4. Include uncertainty analysis in the broadest
sense (vital rate estimates, model structure and
assumptions).
5. Compare short-term and long-term projections.
All models are wrong but some are useful
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Extinction Demography or Genetics?

• Russell Lande (1988) proposed that theory and
  empirical evidence suggest that demography is
  usually of more immediate importance than
  population genetics in determining risk for wild
  populations.
• Suggested need to understand interactions between
  demographic and genetic factors.

• Spielman et al. (2004) argued that most species
  are not driven to extinction before genetic
  factors affect them adversely.
• Compared average heterozygosities in 170
  threatened taxa with those for taxonomically
  related but non-threatened taxa.
• Heterozygosity was on average 35 lower in
  threatened taxa.

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Extinction Vortex
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SMALL GROUP DISCUSSION
What should be the role of PVA in conservation of
threatened species?
Given the lack of demographic data for many
species, what other approaches should be used to
assess extinction risk (i.e., rules of thumb)?