1. Population Biology: PVA & Assessment Mon. Mar. 14

2. Red List Trends
Albatross decrease: breeding sites, long-line fishing, disease, & invasives.

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.

4. Magnitude of Threat in Different Habitat Types According to the Millennium Ecosystem Assessment

5. Population Trends and the Living Planet Index
Terrestrial, Marine and Freshwater vertebrates, by composite (LPI)
Terrestrial, Marine and Freshwater vertebrates, by taxon

6. Extinction Factors 6

7. Factors that Put a Species in Peril
Endangered Miami Blue Butterfly (top)
Larval rearing (below)

8. Rarity: Population Size, Range, and Specialization

9. Extinction Vortex 9

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. 10

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:
lt = 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"

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.

13. How Do Stochastic Factors Enter into the Chance that a Population Will Go Extinct?

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

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

16. Environmental Stochasticity
Year-to-year fluctuations in the environment (continuously variable)
Catastrophic environmental events
(discrete, severe events)
What is different about the two -- catastrophes are discrete not continuous events 16

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
Here we need to remember what we learned early in the course. Be sure that you remember what H and P are and what they indicate. 17

18. PVA Use

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 (Nt) and multiplies it by chance of surviving to the next time period, lt, This gives the number that survive to the next time period.
multiplies the number of survivors (Nt lt) 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, Nt+1

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.

21. What Does a PVA Need?
Must have accurate demographic and life history data.
These parameters must hold over the time of the simulation.

22. PVA Output