Announcements

• Corrected mistakes on slides from last lectures

• VORTEX Help– READ VORTEX survival guide!

• Leads you step by step through VORTEX.

– Get started this weekend so you can identify any problems before its too late!

General Predictors of ExtinctionP

opu

lati

on G

row

th+

-Population Size

Current population size

Deterministic Variation in B&D: Density

• Calculation of r from life table assumes density-independent model

Density-Dependence in Vortex

Pop

ulat

ion

Siz

e

Time0

XDD

DI: growth w/ ceilingK, maximum size

Where Are We Measuring “r”

Gro

wth

Rat

e

Density

“Intrinsic” r or r-max

K

0

“Realized” r

Are we measuring “r” at K?

Gro

wth

Rat

e

Density

r-max

K

0

DI Model

DD Model

DD in Vortex

Implication of DD vs. DI

• Assuming DI may bias the probability of persistence, depending on where r-realized is measured

• Unfortunately, these essential data are rarely available!

Rhino PVA

% B

reed

ing

Density33

45

40

Additional assignment for grad. students:Report to class next Wednesday.

DD in Vortex

• Only possible for % females breeding.

• “Work around” possible for DD in survivorship.

• If DD growth is expected for your population, specify that reproduction is DD, and follow instructions (see Tashi or myself for help).

The Allee Effect in Vortex

• Decide if an Allee effect is appropriate for your population.

• If so, specify that reproduction is DD, and follow instructions (see Tashi or myself for help).

Critiques of PVA

• Models require too much data– Models typically result in wide confidence

intervals.

• Models can not be “validated”– Focus instead on parameter estimation.

• Models are error prone

Model Output = Model Input

Parameter Estimates

Vortex

Model Predictions

Creativity and VORTEX

• Vortex assumes one reproductiveseason/year.

• If your species has 2 clutches/yr., then:

A Defense of PVA

Brook, B. W., J. J. O'Grady, A. P. Chapman, M. A. Burgman, H. R. Akçakaya, and R. Frankham. 2000. Predictive accuracy of population viability analysis in conservation biology. Nature 404:385-387.

Conceptual Utility of PVA

• It identifies the population, not land, as the critical unit for conservation purposes.

• The term “viability” stresses long term population persistence and emphasizes self-sustainability.

• The idea of “minimum” emphasizes that there may be a threshold below which a population is doomed to extinction.