Population Viability Analysis
IUCN RED LIST
Criterion
Critically
Endangered
Endangered
Threatened
Reduction in
population size
10 yrs 3
generations
>80%
>50%
>30%
Small range
<100 km2
<5000 km2
<20,000 km2
Very Small
population
<50
<250
<1000
Quantitative analysis
>50%
>20%
>10%
Probability
10 yrs 20 yrs
100 yrs
of extinction
Or 3 gen
or 5 gen
Population Viability Analysis (L14 and L15)
What is it?
How do the models work
Uses
Issues
Accuracy
More uses - case studies
Additional reading (FYI)
Papers for next week
Principles of Conservation Biology pp 433-35
Intro to Conservation Genetics Ch 20
Habitat loss
Exotic
species
Poaching
Pollution
Catastrophes
Small population
Management
options
Fluctuating
population size
Human impact
Environment
Inbreeding
Life
history
Disease
PVA models examine the effects of different life
histories, environmental and threat factors on the
population size and extinction risk of populations
BASIC APPROACH
Collect population data
size and number populations
birth and death rates
habitat capacities (K)
frequency and effect of threats
Put into PVA model package
VORTEX, GAPPS, RAMAS, ALEX
Predict extinction probabilities
BASIC MODEL- count-based PVA
One Pop’n
Breed
Immigrate
N
Death Emigrate
Supplement
Census
Harvest Truncate
at K
Run for 50-100 Generations
BASIC MODEL - count based PVA
PVA models are NOT deterministic
Models need following data
Variance in fecundity
Variance in survival
Variance in K
Form of density dependence
Frequency/magnitude of catastrophes
Covariance in demographic rates
Q. What does deterministic/variance/covariance mean?
BASIC EXAMPLE
Silvereyes on Heron Island
Model
Initial N=375
Productivity and survival (26 yrs of data)
Catastrophes - cyclones in Jan/April
- chance of event 7.5%
- effect on mortality +22%
K = 450 (± 10%)
Brook and Kikkawa 1998
Cyclone-Jan or
One Pop’n
April
Set K
Breed
Census
N
Death Truncate at K
Run for 100 years
MODEL OUTPUT
N
Year
Chance of extinction in 100 yrs = 15%
ADDING COMPLEXITY
1. Individuals are different
Eg 1 Elephants
only breed after 10+ years
productivity increases and then declines
have age specific mortality rates
Eg 2 Red cockaded woodpeckers
breed in groups
may float prior to joining a group
one male breeds, others help
Q. How would you alter the PVA model?
MORE COMPLEX MODEL
Breed
Immigrate
Supplement
N at each
AGE or
STAGE
Census
Each
St(age)
Death Emigrate
Harvest Truncate
at K
Individuals age or
change categories
Stage-structured PVAs
Additional Data Requirements
St(age)-specific productivity
St(age)-specific survival
Transition probabilities between
stages
ADDING COMPLEXITY
2. additional populations
---> multi-site PVAs
Simple - presence/absence
- use IFM to estimate extinction/
colonisation
- model viability of metapopulation
Complex - spatially-explicit individual based models
- track individuals in complex landscape as
they are born, move, die
- landscape can be “patches” or “real”
Multi-site PVAs
Additional Data Requirements
IFM - ???
Spatially explicit individual based-
Data needed – check metapopulation and corridor lectures
???
WHAT ARE PVA models
HOW DO THEY WORK
USES OF PVA
1. Assessing extinction risk
ISSUES WITH PVAs
ACCURACY of PVAs
ALTERNATIVE USES
1. Assessing the extinction risk of population
Grizzlies in Yellowstone National Park
Shaffer (1978,1983) and others since
Figure 55.11
1. Assessing the extinction risk of population
Grizzlies in Yellowstone National Park
Shaffer (1978,1983) and others since
Age-structured model (cubs, 1..4, 5…25)
Sex
(male, female)
Data - 12 yrs
Craighead, YNP
Environmental stochasticity -->productivity
Demographic stochasticity --->sex of cubs
K= 230; M + F + cubs
Craighead, YNP
1. Assessing the extinction risk of population
Grizzlies in Yellowstone National Park
Shaffer (1978,1983) and others since
Age-structured model
Sex
Data - 12 yrs
Environmental stochasticity
RESULTS Size
Probability of persisting 100yrs
10
20
30
40
50
0
0.36
0.74
0.94
0.98
AREA REQUIRED
Update
1975 - 136 bears in Yellowstone
2004 - 580 and rising
2005 - proposal to delist grizzlies from ESA
2007- population 500+
grizzlies delisted - no protection outside park
Issues with PVA models - 1
Population at risk are typically examined using densitydependent single species model.
Environmental factors are included by varying K
Interactions between populations and environment appear as
reduction in realized rate of increase as population size
approaches K
Issues with PVA models - 2
Data requirements are far greater than the amount of data
available for most species
Rubbish in ----> rubbish out
Issues with PVA models - 3
PVAs typically do not incorporate all genetic effects on
population viability
Q. What genetic processes could influence viability?
PVAs can incorporate inbreeding depression
BUT
how susceptible are populations?
what fitness components are affected?
how is inbreeding depression related to F?
is purging likely?
is inbreeding depression greater during catastrophes?
etc etc
HOW ACCURATE ARE PVAs?
Brooks et al. 2000
Used 21 long term (> 10 year) datsets
Used first half of data to set up models
Tested predictions from 5 PVA packages
= probability of population decline
with what actually happened afterwards
= did population decline
HOW ACCURATE ARE PVAs?
Brooks et al. 2000
HOW ACCURATE ARE PVAs?
Brooks et al. 2000
“PVA is a valid and sufficiently accurate
tool for categorizing and managing
endangered species”
Coulson et al. 2001 Response
The 21 studies selected were biased
the data available was of very high quality
most species were not endangered
PVAs will frequently be unreliable
because the data to estimate vital rates is limited
and vital rates of endangered spp will change
Puerto Rican parrot
Black footed ferret
What sort of PVA model is appropriate?
DISCUSS
What is gained from using PVAs?
Next MORE USES OF PVA
1. Assessing the extinction risk of
population
2. Comparing the relative risk in 2+
populations
3. Identify key life history stages to
protect
4. Determining minimum reserve size
5. Determining numbers to release
6. Setting harvest guidelines
7. Deciding how many (which) populations
are needed