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Slide 1
Population Biology: PVA & Assessment Mon. Mar. 14
Slide 2
Red List Trends Albatross decrease: breeding sites, long-line
fishing, disease, & invasives.
Slide 3
Living Planet Index Developed in the WWF starting in the late
1990s. Uses population trend data base for about 10,000 studies on
about 2500 vertebrate species. A composite trend is developed from
these individual studies. Hope is that individual fluctuations will
cancel and reveal underlying population trends.
Slide 4
Magnitude of Threat in Different Habitat Types According to the
Millennium Ecosystem Assessment
Slide 5
Population Trends and the Living Planet Index Terrestrial,
Marine and Freshwater vertebrates, by taxon Terrestrial, Marine and
Freshwater vertebrates, by composite (LPI)
Slide 6
Extinction Factors
Slide 7
Factors that Put a Species in Peril Endangered Miami Blue
Butterfly (top) Larval rearing (below)
http://www.nps.gov/bisc/parknews/images/Miami-Blue-butterfly-larvae.jpg
http://www.floridastateparks.org/bahiahonda/images/visitors/BAH-MiamiBlueButterfly-MichelleWisniewski.jpg
Slide 8
Rarity: Population Size, Range, and Specialization
Slide 9
Extinction Vortex
Slide 10
Population Viability Analysis (PVA) What is the minimum viable
population (MVP)? The MVA is the smallest present population size
that has a certain probability of surviving to some point of time
in the future. Population viability analysis is a technique that
allows us to estimate the MVP. It takes into account initial
population size and various stochastic factors that affect
populations to predict the likelihood a population of a certain
size (the MVP) will persist to some point in the future.
Slide 11
Deterministic vs. Probabilistic (Stochastic) Modeling We have
already considered a number of deterministic models. The
exponential, logistic growth, and competition models were all
deterministic because they assumed that there was no variation in
their underlying parameters (b and d) from one year to the next.
Here is another deterministic model of population size: l t =
chance of surviving through time period b = birth rate per
individual surviving through the period Pop Size in future =
present members that survive + new "recruits"
Slide 12
Predicting Whether or not a Population Will Become Extinct If
conditions remain constant, and if we assume that small population
effects (what are these?) are minimal, any population will persist
indefinitely into the future. However, we cannot see the future.
The facts are (a) small population effects do exist and that (b)
environments do not remain the same. Note: regarding the
environment, changes can be viewed as randomly varying about some
mean, at least when looked at over a long time span. We have
already considered genetic effects.
Slide 13
How Do Stochastic Factors Enter into the Chance that a
Population Will Go Extinct?
Slide 14
Demographic Stochastic Processes Recall that demographic
factors relate to what we have called basic population parameters:
Population size Age structure (#s of individuals of different ages)
The sex ratio of reproductive individuals Age-specific natality and
mortality
Slide 15
Demographic Processes, continued We know that events can occur
with some regularity that cause: significant reduction in
population sizes and that may: differentially hit certain age
groups or sexes more than others
Slide 16
Environmental Stochasticity Year-to-year fluctuations in the
environment (continuously variable) Catastrophic environmental
events (discrete, severe events)
Slide 17
Genetic Stochasticity Variation in allele frequencies as a
result of: genetic drift bottlenecks These processes may lead to
lower H and P causing: short-term problems with inbreeding
depression lack of enough variation to handle a major environmental
change
Slide 18
PVA Use
Slide 19
How Does a PVA Work? -- part 1 Contrast with simple
deterministic model for predicting the size of a population in the
next generation. Recall: A deterministic model: takes the present
population size (N t ) and multiplies it by chance of surviving to
the next time period, l t, This gives the number that survive to
the next time period. multiplies the number of survivors (N t l t )
times the birth rate per survivor (b) to obtain the number of new
individuals that are born into the population. The sum of the
survivors and newly born individuals is the new population size, N
t+1
Slide 20
How Does a PVA Work? -- part 2 Thus, in the deterministic
model, we work with the entire population (all individuals in it)
at once (as a group) and by doing so we average out the stochastic
factors. By contrast, in a PVA we incorporate stochasticity from
the beginning. There are several ways to do this: Build a model
that includes the chance that each individual will survive
independently of each other by using probability distributions for
survival. We do the same for reproduction and other demographic
factors. Build alternative demographies representing different
environmental conditions and go between them at some probability.
Greater complexity can be added. e.g., spatial effects.
Slide 21
What Does a PVA Need? Must have accurate demographic and life
history data. These parameters must hold over the time of the
simulation.