Population Viability Analysis

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Population Viability Analysis
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Applying knowledge about populations to specific
issues

• Population Viability Analysis (PVA)
• Identify threats to a species
• Evaluate the likelihood that a species will
  persist for a given time into the future
• Related to the concept of the minimum viable
  population (MVP)-the minimum of individuals
  that ensures a populations persistence

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PVA is used for the management of rare and
threatened species

• PVA is used to improve the species chances of
  survival
• Planning research and data collection
• Are populations particularly sensitive to
  demographic parameters guide research
• Assessing vulnerability
• Used to rank species, set priorities for research
• Ranking management options
• Compare different options such as
  reintroductions, captive breeding, prescribed
  fires, habitat rehabilitation, etc

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Previous use of these parameters

• Assessing sensitivity of populations to changes
  in demographic parameters
• MuskOxen, 2 conservation measures on sea turtles
  (TED vs beach protection), and the optimum
  reserve design for a spotted owl metapopulation.

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Extinction

• There are different types of extinctions
• Extinction in geological time
• Mass extinctions
• 5 major events, loss of up to 95 of all species
• Length of time a species persists
• Skewed to the right, modal value is around 2
  million years
• Median time to extinction-1 species per year

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Current extinction rates

• Most modern extinctions are in the following
  groups
• Mammals (sea cows, sloths)
• Birds (dodo, passenger pigeon, ground dwellers)
• Terrestrial snails
• Many other groups are not well known
  taxonomically (insects, tropical plants)
• Probably many undocumented extinctions

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Extinctions on islands

• The majority of modern extinctions have occurred
  on islands (75 of all animals)
• Island species typically have smaller populations
  than mainlands
• Often had not evolved flight (in birds) or fear
  of predators due to lack of natural predators

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Comparing extinction rates

• Background (ie, natural) extinction rate ranges
  from 1 to 5 species/year
• Evolution, competition, stochasticity
• Modern rate is around 5-100 species/year
  depending on taxa
• Current rate may actually be much higher due to
  incomplete taxonomic knowledge of most of the
  worlds species

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Causes of extinction

• In small populations, stochastic processes in
  demography and genetics coupled with extensive
  environmental variation will most likely lead to
  local population extinction
• Humans a major negative factor in causing
  extinctions, even for species with large
  populations

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Human impacts on species

• Habitat destruction and fragmentation
• Less habitat available, lower K
• Fragmentation may inhibit dispersal, edge effect
• Overexploitation (overharvesting)
• May reduce population to low levels, allee
  effects,
• Pollution
• May impact birth and death rates, decreased birth
  rates and higher death rates

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Human Impacts (Continued)

• Introduction of exotic species
• Better competitors due to lack of parasites,
  predators, herbivores, pathogens
• Global climate change
• Overwhelms ability of species to migrate to
  suitable climates, increases death rates,
  decreases birth rates

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Characteristics of endangered species

• Species that are locally rare, geographically
  restricted, or occur within a narrow niche
• Species that are highly variable in space and
  time (could be lost due to chance)
• Species with slow growth rates may take long time
  to recover from population reductions (elephant,
  rhino)

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Species characteristics continued

• Habitat overlap-species that live in areas
  preferred by people (high, dry, fertile ground)
• Harvesting-species that are used as a commodity
  (food, fiber, medicine)
• Home range requirements-animals with large home
  ranges usually occur at low densities
• Limited adaptability and resilience-low dispersal
  capacity, limited reproductive capacity, or
  narrow and inflexible habitat requirements

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Building a model of extinction risk

• Models will produce a quantitative assessment of
  the extinction risk
• Must use these models in conjunction with
  economics, politics, etc to decide what to do.

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Classifying Threats-IUCN

• Extinct-Dodo bird
• Extinct in the Wild-California Condor (being
  reintroduced)
• Critically endangered-high risk of extinction in
  the immediate future
• Endangered-high risk of extinction in the near
  future
• Vulnerable-High risk of extinction in the far
  future
• Lower risk-Evaluated, doesnt fit above
  categories
• Data deficient-cannot say based on existing data
• Not evaluated-no data to assess

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Risk-Based Criteria for assessment

• Critical-50 probability of extinction within 5
  years or 2 generations, whichever is longer
• Endangered-20 probability of extinction within
  20 years or 10 generations, whichever is longer
• Vulnerable-10 probability of extinction within
  100 years
• Key is the time to extinction and the probability
  of extinction

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Components of PVA

• Identifying the question and estimation of
  parameters
• Modeling, risk assessment, and sensitivity
  analysis
• Cost-benefit analysis
• Implementation, monitoring, and evaluation

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Limits of PVA

• Distinguishing between deterministic and
  stochastic processes
• The small population vs. the large but declining
  population paradigm
• Model is not the end in itself
• Must be incorporated in larger program of study
• Data availability and data precision