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Society for Conservation Biology: 2002 Annual Meeting

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Society for Conservation Biology: 2002 Annual Meeting


Society for Conservation Biology 16th Annual Meeting July 14-July 19 2002
co-hosted by DICE and the British Ecological Society


Abstracts for Population Viability Analysis
Session Two

Thursday 18th July, 13.30 - 15.00, Grimond Lecture Theatre 3

Chair: Bruce Kendall




(BLOCK CAPITALS indicate the presenting author)


13.30 - 14.00
MACE, GEORGINA M., Institute of Zoology, Zoological Society of London, Regent’s Park, London, NW1 4RY, UK. (Georgina.Mace@ioz.ac.uk).

MAKING CHOICES IN SPECIES CONSERVATION - ASSESSING THREATS AND PRIORITIES

Choices among conservation actions may be made for species or sites, but even when sites are used, species are frequently involved in the assessments. Many attributes of species can be used to set priorities including threat intensity, biological, taxonomic and evolutionary uniqueness, logistical, economic, political and social issues. Starting with methods to assess relative threat level, a system is developed to examine the various factors that may be significant at local, national and global scales. This systematic review allows a clear distinction to be drawn between traits that are generally measurable and comparable across scales and circumstances (e.g. phylogenetic distinctiveness, recovery costs, extinction risk) to those that require value judgements and assessment within each particular context (e.g. economic value). Applying this formula to some model data sets indicates the value of systematic approaches for incorporating general principles into local realities to set transparent objectives.

14.00 - 14.15
SABO, JOHN L. and Leah R. Gerber. Department of Biology, Arizona State University, P.O. Box 871501, Tempe, Arizona 85287-1501, USA, <john.l.sabo@asu.edu>.

SPECIES INTERACTIONS AND POPULATION VIABILITY ANALYSIS

One of the most commonly used tools for assessing extinction risk is population viability analysis (PVA). PVAs allow practitioners to quantify risk in the face of environmental variability in demographic rates and population abundance. However, one critical assumption of many PVAs is that this variability is attributable entirely to random fluctuations in the environment. This assumption is clearly violated by interactions between focal (threatened) populations and other species (e.g., competitors, predators or disease agents). Species interactions alter the dynamics of threatened species in potentially non-random ways and thus may bias the results of simple PVA’s when ignored. We searched the literature for all published studies employing PVA to determine how often these studies included species interactions. Less than 10 % of published PVA studies considered the effects of species interactions in viability assessments for focal populations. Even fewer (< 5%) of these analyses included species interactions explicitly in population models. We use data from simulation models of interacting species to show how a variety of species interactions can bias predictions made from simple extinction risk models. Our results have important ramifications for risk assessment, especially for species threatened by the introductions of exotic competitors, predators or diseases.



14.15 - 14.30
KENDALL, BRUCE E., and Gordon A. Fox. Donald Bren School of Environmental Science and Management, University of California, Santa Barbara, CA 93106, USA, <kendall@bren.ucsb.edu> (BEK), and Department of Biology, University of South Florida, Tampa, FL 33620, USA (GAF).

INDIVIDUAL GROWTH RATE VARIABILITY INCREASES THE POPULATION GROWTH RATE

Individual variability is often ignored in population models; all individuals are usually assumed to have identical traits, with values set at the mean. In previous work we have shown that certain types of structured individual variation in birth and death rates can reduce the variance of population growth rate due to demographic stochasticity. In small populations this can substantially reduce the extinction risk. We have now turned our attention to variation in the individual growth rate. Using models, we show that in semelparous organisms, a population in which individuals differ in their times to maturity always has a higher population growth rate than a population in which all individuals mature at the same time, even though the mean time to maturity is the same. This result is independent of population size, and is true even if time to maturity is a purely random variable; the effect is increased if there is predictable structure in the variation. This result extends more generally to stage-structured populations, in which individuals vary in their probability of advancing to the next stage. Thus population viability models that ignore variation in individual growth rates will underestimate the population growth rate, and overestimate the risk of extinction.



14.30 - 14.45
ROBERT, ALEXANDRE, Denis Couvet and François Sarrazin. Université Pierre et Marie Curie, Laboratoire d'Ecologie, CNRS-UMR 7625, Bât.A, 7ème étage, 7 quai Saint Bernard, Case 237, F-75252 Paris Cedex 05, France, <arobert@snv.jussieu.fr> (AR; FS), CRBPO, Muséum National d'Histoire Naturelle, 55 rue Buffon, 75005 Paris, France (DC).

BOTTLENECKS IN LARGE POPULATIONS: THE EFFECT OF IMMIGRATION ON POPULATION VIABILITY

We model a large population that is subject to successive short bottlenecks, in order to investigate the impact of different extents of immigration on the change in genetic load and on viability. A first simple genetic model uncovers the opposite effects of immigration on fitness according to the type of deleterious mutations considered: immigration increases fitness if the genetic load is comprised of mildly deleterious mutations, while it decreases fitness if it is comprised of lethal mutations. When considering both types of mutations and adding explicit demographic considerations, in which bottlenecks are engendered by random catastrophes, the global impact of immigration on viability is dependent upon a balance between its opposite effects on the two components of the genetic load and on demographic stochasticity. In this context, immigration tends to increase the probability of extinction if it occurs preferentially when the density of population is high, while it decreases extinction when occurring preferentially towards low-density populations.



14.45 - 15.00
FOX, GORDON A., and Bruce Kendall. Department of Biology (SCA 110),
University of South Florida, 4202 E. Fowler Avenue, Tampa, FL 33620, USA,
<gfox@chuma1.cas.usf.edu> (GAF), and Donald Bren School of Environmental
Science and Management, University of California, Santa Barbara, Santa
Barbara, CA 93106, USA (BK).

ESTIMATING EXTINCTION RISK OF HETEROGENEOUS POPULATIONS

Treating individuals in populations as though they have identical demographic properties can seriously misestimate the importance of demographic stochasticity, and consequently, of extinction risk. Our recent work has shown that heterogeneity in demographic properties can either increase or decrease the variance in population growth rate, and therefore the risk of extinction. In particular, heterogeneity in survival rates always reduces the effect of demographic stochasticity (and extinction); heterogeneity in fecundity can either increase or decrease the extinction risk, depending on whether variance in fecundity is a convex or concave function of mean fecundity. Our previous work has relied on small-variance approximations to estimate the variance in population growth rate, and therefore to infer extinction risk. Here we
present robust results that allow us to estimate extinction risk of populations heterogeneous for demographic rates. Our approach relies on modelling change in the size of a heterogeneous population as a multitype branching process. Given estimates of the means and variances of demographic rates, our approach allows us to estimate extinction risk for any population.