Population Viability Analyses (PVA)Presentation 1

Lesson 7

PVAMost 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.

Ideas of population persistenceHuman the hunter realized that habitat type is important for distribution and abundance of preyHumans also understood that habitat quality is importantWith the advent of agriculture and animal husbandry humans realized that sex ratios, age of individuals, and morphological characteristics were important for reproduction.

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 patternE.g.: northern spotted owl in NW US and Canada

Review of metapopulation theorySource and sink dynamicsSource = surplus individuals produced migrate to areas available for them to liveSources have demographics that lead to population increaseSource has habitat suitable for population increaseSink = space available for migrants

Source and sink dynamicsStudies show that small % of total population may be located in source habitatAs little as10 % of population could be source*Provided new definition for key (or critical) habitatKey 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

Northern Spotted OwlCcourtesy of Eric Forsman, U.S. Forest Service

NW US and B.C.Non-migratoryNatural low populationDense timber (old growth)Thomas et al (1990) used PVA to devise conservation strategy

Purpose of Population Viability Analyses (PVA)To study how habitat loss, environmental uncertainty, demographic stocasticity, and genetic factors determine extinction probabilities for individual speciesMetapopulation theory provides us with the basis to understand these relationships 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.

Application of PVAWe can use PVA to model the probability of a population failing or prospering under a given set of circumstancesCan 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

Application of PVAThis 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?

VulnerabilitySmall populations are vulnerable because of chance eventsChance operates at several levels:When organisms dieHow many off spring they haveIf they find matesEffects of weather on food, shelterEffects on genetic makeup of populationCatastrophesDemographic stocasticityEnvironmental stocasticityGenetic stocasticity

Chance and populationsAs 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 individualsNatural catastrophes important for all population sizes

VulnerabilityChance events become more important as population becomes smallerChance events can reinforce the negative effects of one another

Minimum viable population (MVP)The smallest isolated population having a 99% chance of surviving 100 years.Can change % and years depending on objectivesOnce we have a MVP can multiply it by the home range figures to calculate minimum area needed to support MVP

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

NPS Photo

Grizzly bearsIf 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 catastrophesNPS Photo

History of PVAToday over 30 PVA studiesTheoretical basis for population viability still developingDo not have models sophisticated enoughDo not have life history description and data for most organisms

Limits of PVAOnly 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)

Best bets for futureCannot do PVA for all SARIdentify spp indicative of natural system (indicator spp or keystone spp)PVA for these will provide area requirements for others that require the same systemThese indicators are likely to be top carnivores: long-lived, slow reproduction, large body

Best bets for conservationCreate multiple populations: single catastrophe cannot destroy sppIncrease size of each population so that genetic, demographic and environmental factors less threatening (very difficult to do)

PVA in harvest regulationElements 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.

PVA and large populationsPassenger Pigeon(Ectopistes migratorius) Photo: Cornell UPhoto: NPSPhoto: NPSPhoto: NPSWhat can PVA tell us about the sudden demise of large populations?

PVA and large populationsPopulations do not exhibit constant population declines# ofIndividuals in populationTimeAB

PVA and large populationsUnder some circumstances populations will exhibit threshold responses = dramatic population changes over short time periodSee Lande (1987) for hypothetical responsesThresholds are difficult to predict

PVA and large populations

% landscape agricultural% suitable habitat occupiedAssumeBird population, juveniles migrate, if suitable habitat found they surviveHabitat near agric land less suitable, less food, pesticidesLow reproduction near agric landWhen % agric land increases, reprod lowerWhen reprod = mortality, suitable habitat occupied declines rapidlyThreshold

Modelling populationsA simple population modelNt+1= Nt + B - M Nt+1 = Population tomorrow Nt = Population todayB = BirthsM = Mortality

Can put in one average value and come up with one result:Try these values in the formula:Nt =100B=50M=30What is Nt+1? Ans: 120

Questions?

Thomas, J. W., E.D. Forsman, J.B. Lint, E.C. Meslow, B.R. Noon and J. Verner. 1990. A conservation strategy for the northern spotted owl. U.S. Government Printing Office, Washington D.C. 1990-791-171/20026Shaffer, M.L., 1983. Determining minimum viable population sizes for the grizzly bear. International Conference on Bear Research and Management 5:133-139.Passenger pigeon: estimated population 2-3 billion, hunted for meat and killed for feeding on crops. Last died in Cincinnati Zoological Gardens Sept 1914Lande, R. 1987. Extinction thresholds in demographic models of territorial populations. Am. Nat.130:624-635.