Population Viability Analyses (PVA)

1. Population Viability Analyses (PVA) Presentation 1 Lesson 7

2. PVA • Most PVA’s 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.

3. 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.

4. 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

5. 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

6. 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. 132:652-661

7. 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

8. 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

9. 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

10. 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?

11. 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

12. 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

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

14. 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

15. 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

16. 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

17. 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

18. Limits of PVA • Only as reliable as inputs! • Depends on realism of model’s 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)

19. 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

20. 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)

21. 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.

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

23. PVA and large populations • Populations do not exhibit constant population declines # of Individuals in population A B Time

24. 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

25. 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

26. Modelling populations • A simple population model Nt+1= Nt + B - M Nt+1 = 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 B=50 M=30 What is Nt+1? Ans: 120

27. Questions?