Population Viability Analyses (PVA)

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Population Viability Analyses (PVA)

• Presentation 1
• Lesson 7

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PVA

• Most PVAs consider internal factors (sex ratio,
  number of births, survival, etc) and the effect
  of these internal factors on population
  persistence
• We will study the effect of external factors
  (habitat, disturbances) in a later class.

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Ideas of population persistence

• Human the hunter realized that habitat type is
  important for distribution and abundance of prey
• Humans also understood that habitat quality is
  important
• With the advent of agriculture and animal
  husbandry humans realized that sex ratios, age of
  individuals, and morphological characteristics
  were important for reproduction.

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Ideas of population persistence (cont.)

• Island biogeography showed us habitat quantity is
  fundamental to survival of a population (even
  with the best habitat without enough of it
  population would not survive)
• In the 1990s we realized that habitat pattern
  important (metapopulation theory)
• Not only must we have enough good habitat it must
  be arranged in an appropriate pattern
• E.g. northern spotted owl in NW US and Canada

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Review of metapopulation theory

• Source and sink dynamics
• Source surplus individuals produced migrate to
  areas available for them to live
• Sources have demographics that lead to population
  increase
• Source has habitat suitable for population
  increase
• Sink space available for migrants

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Source and sink dynamics

• Studies show that small of total population may
  be located in source habitat
• As little as10 of population could be source
• Provided new definition for key (or critical)
  habitat
• Key habitat defined by habitat specific
  reproductive success and survivorship not just
  population density

Pulliam, H.R. 1988. Sources, sinks and
population regulation. Am. Nat. 132652-661
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Northern Spotted Owl
NW US and B.C. Non-migratory Natural low
population Dense timber (old growth) Thomas et al
(1990) used PVA to devise conservation strategy
Ccourtesy of Eric Forsman, U.S. Forest Service
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Purpose of Population Viability Analyses (PVA)

• To study how habitat loss, environmental
  uncertainty, demographic stocasticity, and
  genetic factors determine extinction
  probabilities for individual species

PVA can be used to tell us how large a
population is required for the group to survive
for a know period of time, e.g. 95 probability
of extinction in 100 yrs.
Metapopulation theory provides us with the basis
to understand these relationships
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Application of PVA

• We can use PVA to model the probability of a
  population failing or prospering under a given
  set of circumstances
• Can be used to set size of population required to
  restore SAR or maintain species of interest.
• Can use to find features that make a population
  vulnerable

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Application of PVA

• This will provide us answers to management
  questions
• Where should we concentrate our efforts?
• Increasing births?
• Reducing deaths of young?
• Reducing deaths of old?
• Reducing deaths of males?
• Reducing deaths of females?

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Vulnerability

• Small populations are vulnerable because of
  chance events
• Chance operates at several levels
• When organisms die
• How many off spring they have
• If they find mates
• Effects of weather on food, shelter
• Effects on genetic makeup of population
• Catastrophes

Demographic stocasticity
Environmental stocasticity
Genetic stocasticity
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Chance and populations

• As general rule
• Genetic and demographic uncertainty important for
  viability of small populations (50 - 250 breeding
  individuals)
• Environmental uncertainty important for
  populations in the order of 1,000 to 10,000
  individuals
• Natural catastrophes important for all population
  sizes

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Vulnerability

• Chance events become more important as population
  becomes smaller
• Chance events can reinforce the negative effects
  of one another

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Minimum viable population (MVP)

• The smallest isolated population having a 99
  chance of surviving 100 years.
• Can change and years depending on objectives
• Once we have a MVP can multiply it by the home
  range figures to calculate minimum area needed to
  support MVP

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History of PVA

• First used by Shaffer (1983) to study grizzly
  bear in Yellowstone National Park
• Used detailed 12 yr data on grizzly population to
  construct population dynamics by tracking
  individual bears and incorporating effects of
  chance events

NPS Photo
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Grizzly bears

• If you wanted a 95 chance for grizzly bears to
  survive for 100 years you have to have enough
  habitat to allow 70-90 bears
• If you want a 99 chance or longer time of
  survival then more bears needed
• Study did not model genetic influences or chance
  natural catastrophes

NPS Photo
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History of PVA

• Today over 30 PVA studies
• Theoretical basis for population viability still
  developing
• Do not have models sophisticated enough
• Do not have life history description and data for
  most organisms

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Limits of PVA

• Only as reliable as inputs!
• Depends on realism of models logic!
• Need lots of demographic data (not usually
  available)
• Long term projections imply habitat stability and
  all else being equal (often not the case in the
  real world)

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Best bets for future

• Cannot do PVA for all SAR
• Identify spp indicative of natural system
  (indicator spp or keystone spp)
• PVA for these will provide area requirements for
  others that require the same system
• These indicators are likely to be top carnivores
  long-lived, slow reproduction, large body

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Best bets for conservation

• Create multiple populations single catastrophe
  cannot destroy spp
• Increase size of each population so that genetic,
  demographic and environmental factors less
  threatening (very difficult to do)

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PVA in harvest regulation

• Elements of PVA is used to determine minimum
  population before harvest is sustainable.
• Need information on population dynamics, life
  history, demographics to determine sustainable
  harvest levels.

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PVA and large populations
Photo NPS
Passenger Pigeon(Ectopistes migratorius)
Photo NPS
Photo Cornell U
What can PVA tell us about the sudden demise of
large populations?
Photo NPS
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PVA and large populations

• Populations do not exhibit constant population
  declines

of Individuals in population
A
B
Time
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PVA and large populations

• Under some circumstances populations will exhibit
  threshold responses dramatic population changes
  over short time period
• See Lande (1987) for hypothetical responses
• Thresholds are difficult to predict

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PVA and large populations

• Assume
• Bird population, juveniles migrate, if suitable
  habitat found they survive
• Habitat near agric land less suitable, less food,
  pesticides
• Low reproduction near agric land
• When agric land increases, reprod lower
• When reprod mortality, suitable habitat
  occupied declines rapidly

Threshold
suitable habitat occupied
landscape agricultural
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Modelling populations

• A simple population model
• Nt1 Nt B - M
• Nt1 Population tomorrow
• Nt Population today
• B Births
• M Mortality

Can put in one average value and come up with one
result
Try these values in the formula Nt
100 B50 M30 What is Nt1?
Ans 120
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• Questions?