Population

Viability Analysis -

General Principles

and Applications

By: Barry Noon, Fred

Sampson and Nels Johnson

Department of Fishery &

Wildlife Biology

Colorado State University

The eighth in a series of 13,

Session 3

Background

Why Population Viability Analysis is

Important to Ecological Stewardship

Ecosystem approaches to natural

resources management are often

thought to represent a shift away

from single species management.

The ecosystem management paradigm, however, does not exclude

research and management for individual

species population viability. Rather,

ecosystem management reinforces the

need to view individual species within

the context of the ecosystems that sustain them. Population viability analysis

is a valuable and sometimes indispensable tool in understanding the role of

species in ecosystem processes.

Population viability analysis (PVA) is

the study of all factors that may cause a

species to go extinct. Population viability analysis is important to ecological

stewardship for several reasons. First,

PVA is a relatively rigorous way to identify cause and effect relationships in

conservation biology. PVA has accelerated our understanding of basic ecological

relationships and underlying patterns

in the distribution and abundance of

living things. Second, PVA can help to

monitor the health and integrity of

ecosystems. For example, PVA for certain “keystone species” — organisms,

such as salmon, that interact with a

range of other species and vital ecosystem processes — may help to identify

key factors or trends that affect the

integrity of the ecosystem. Third, by

focusing on the most limiting factors

to a species’ survival, PVA can help to

identify the most effective management

actions to protect endangered species.

Without well-executed PVA’s, mistaken

assumptions in species recovery plans

can lead to the waste of precious time,

money, and public support.

Discussion

What is Population Viability Analysis?

The principle motivation behind the

development of population viability

analysis is to assess the threats to a

species’ survival, and to intervene

before population declines become

inevitable. It is a structured, systematic, and comprehensive examination of

the interacting factors that place a

population or species at risk. To estimate the likelihood that a population

will persist for some chosen time into

the future (often a hundred years),

PVA requires the careful evaluation of

data, such as reproduction rates,

genetic characteristics, and geographic range requirements, generally by

the use of mathematical analyses and

computer simulations.

Key Findings

Population viability analysis requires a

sophisticated understanding of the biology of the species in question. Still,

there are several basic considerations

that can help the researcher and the

natural resource manager to better

understand the role of PVA in ecological stewardship.

Many factors affect the persistence of

a species — not all are of equal importance.The most important factors, such

as habitat loss, are called “deterministic”

because they make populations of a

species isolated and rare in the first

place. Secondary factors, such as

severe weather events or predation, may

contribute to the demise of a species

but are rarely the ultimate cause.

Over the short term, deterministic factors should get most of our attention.

Secondary factors, such as demographic

stochasticity, are usually symptoms of

deterministic factors. PVA can help to

identify which deterministic factors

have the most impact on a species.

Simple statistics, such as population

size and habitat area are not enough to

understand the ability of a species to

persist. It is crucial to have a sense of

spatial and temporal relationships

between habitat and demographic

factors. PVA — whether quantitative

or qualitative — should never be

undertaken without a fundamental

understanding of a species’ ecology.

Poorly executed PVAs can lead to misleading conclusions, inappropriate

management prescriptions, hasten

rather than avert a species extinction,

or lead to costly and unnecessary

actions for a species not in jeopardy.

There is no single “best” approach for

conducting a PVA. The format for a

given PVA is dictated by data availability, the degree to which the species

ecology and life history are understood, knowledge of risk factors, and

management goals.

To be useful PVAs should be directly

linked to risk factors that are under the

control of management. Population viability analysis is useful only to the extent

it helps to solve management problems.

The results of a PVA should inform

managers on what actions can be taken

to reduce threats to a species persistence.

When should PVA be used?

PVA should be used when a species is

believed to be declining. Species listed

under the Endangered Species Act

(ESA) in the United States are obvious

candidates, and a new listing may trigger local demand for PVA by resource

managers. PVA is often used to help

determine whether a species should be

considered for listing (and conceivably

for de-listing).

A “coarse-filter” screen may be the

best way to select species for PVA. The

best approach to selecting species for

analysis may be to simply screen all

species within the planning area for

viability concerns. A set of relatively

RESOURCE NOTES

NO. 30 DATE 07/26/00

Range

Wild Horse andBurro Program
simple criteria developed for the

World Conservation Union (IUCN)

Species Survival Commission can be

used to determine which species are

the best candidates for PVA. These criteria include population size, number,

isolation, and habitat area trends.

Other factors could include specific

life history traits (e.g., reproductive

rates), habitat specificity, and trends in

abundance and distribution.

PVA may also be a proactive tool.

For example, multi-species PVA is

being used to help prioritize habitat

restoration efforts and design monitoring systems in the Pacific Northwest.

PVA could also be useful in making

decisions about how to control invasive

exotic species.

What are the basic 

requirements?

The most basic requirement is to

know what data are needed. This will

vary depending on the species, area of

analysis, and the management goal.

Data on population size and dynamics, habitat area and dynamics, and

risk factors are essential. PVA models

can help to explicitly identify what

information is needed.

The next step is to find data. For some

species, data may already have been

collected by other researchers. For

many species, some or most data will

not exist. Information on population

status, in particular, is key but available for only a limited number of

species. For either quantitative or qualitative methods, some data will likely

have to be collected for the first time.

Basic awareness of population dynamics, genetics, and spatial and temporal

dimensions of population change are

essential. There is no substitute for

basic knowledge of population biology, genetics, ecosystem processes, and

conservation biology. This knowledge

is a prerequisite for conducting a PVA

even as software programs (e.g., VORTEX, RAMAS, ALEX, etc.) make

PVA more accessible.

PVA efforts should involve collaboration between researchers with both

field and theoretical knowledge about

a species.

PVA is not a casual undertaking.

Managers and researchers should have

an appreciation of the resources needed

to conduct rigorous analysis.

Limitations of PVA

PVAs often depends on incomplete

knowledge. We do not have basic population and life history information on

most species. For many species where

data and knowledge are limited, it may

not be appropriate to use rigorous PVA

methodologies since the risk of misleading results is high. Qualitative forms of

assessment, expert opinion workshops

for example, may be alternatives.

PVA does not predict the fate of a

species. PVA is a probabilistic rather

than a predictive tool. It focuses on

the factors that are most likely to limit

the persistence of a species over time.

Conclusion

Population viability analysis can be a

valuable and practical tool in natural

resource management. It is not practical or possible to conduct a PVA for

most species in a given management

situation. In general, it is advisable to

focus on species that are known or

thought to be vulnerable to extinction

or extirpation. A variety of existing categories can be used to identify species

that may be good candidates for PVA.

These include species currently listed

under the ESA as threatened or

endangered, species ranked as G1, N1,

or S1 in The Nature Conservancy/

Natural Heritage system, restricted

range or especially endemic species,

species that are exhibiting chronic or

precipitous population declines, including those not currently ranked as

threatened or endangered.

There are a range of methods that

analyze the risks faced by a species

over time. Six steps provide a basic

framework — regardless of the method

— for analyzing population viability in

the ecosystem management context:

1. Select species of concern

2. Describe population status

3. Describe risk factors

4. Identify suitable habitat amount,

distribution and trends

5. Describe relationship of population

dynamics to habitat dynamics

6. Assess likelihood of species 

persistence.

Most examples of PVAs found in the

published literature are atypical —

that is they are for the relatively few

species (<5%) for which we have good

data. It is important to recognize that

data for most species is sparse. This

requires either a concerted focus to

collect missing data (often a costly and

time consuming proposition) or the

use of more qualitative approaches.

Managers rarely have all the information needed to conduct fully quantitative PVAs. Managers must then

rely on simpler techniques that are

less rigorous and possibly less reliable. Alternative approaches to quantitative assessments include the use of

expert opinions, habitat inventories,

and basic information on current population status. With all methods,

extensive documentation and use of a

logical process are key to credibility.

Additional Source 

of Information

Noon, B.R., R.H. Lamberson, M.S.

Boyce, and L.L. Irwin. 1999.

Population viability analysis: A primer

on its principal technical concepts. In:

Ecological Stewardship: A Common

Reference for Ecosystem Management.

K. Johnson et al. (eds). Elsevier

Science Ltd., London

Contact 

Barry R. Noon, PhD.

Department of Fishery 

& Wildlife Biology

240 J.V.K. Wager

Colorado State University

Fort Collins, CO 80523-1474

phone 970-491-7905

fax 970-491-5091

e-mail: brnoon@cnr.colostate.edu Range

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