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.

Slide 22

PVA Output