Transcript The effective population size

Extinction and Conservation
Gwen Raitt
BCB 706:
Conservation
Biology
Available at http://planet.uwc.ac.za/nisl
Why is Extinction a Concern for Conservation
Biology?
 In Chapter 1, it was noted that conservation biology developed from
the growing awareness of the present (sixth) mass extinction.
 Four factors form the basis for this concern:

The unprecedented level of threats to biodiversity;

The escalation of the
threats to biodiversity;

The observation that the
threats to biodiversity are
synergistic;

The realization that what
harms biodiversity will harm
humanity.
 Conservation biology aims to prevent the extinction rate exceeding
the speciation rate – not to eradicate extinction.
Categorising Threats to
Biodiversity
 Threats to biodiversity fall into two
categories:

Systematic threats,

Chance threats.
 The effects of systematic threats usually include
increased vulnerability to chance threats.
Conservation Focus… Populations
 Extinction tends to bring specific species or other taxonomic units to
mind.
 While conservation of all levels of biodiversity is
important, the species is a pragmatic choice of
conservation unit but:

The threats that cause extinction act at the
population/metapopulation level,

Populations share an evolutionary future.
 Therefore, the population/metapopulation is the actual unit of species
conservation.
 Reducing the probability of chance extinctions for small populations
is an important part of conservation – both in situ and ex situ.
 It is most important to remember not to focus so intensively on small
population size that no action is taken to reduce the factors that
caused the original population decline.
Populations
 A population is a group of individuals of a given species
living in a specific geographic area at one time.
 Births and immigrations add to the population size while
deaths and emigrations reduce the population size.
 Population size and survival depend on:

The availability of resources

The amount of suitable habitat

Predation/parasitism

Disease

Social interactions
Mechanisms of Extinction in Single Populations
 Population extinction is certain if, in the long term, the mortality
and emigration rates are higher than the birth and immigration rate.
 Extinction mechanisms act by affecting the migration, mortality and
birth rates. The mechanisms may be grouped into three categories
for single populations:

Demographic uncertainty


Environmental uncertainty


Allee effects
‘Natural’ catastrophes
Loss of genetic diversity
 Mechanisms may interact, compounding the effect on the
population.
 Population size is critical to survival.
Metapopulations
 A metapopulation is made up of a number of spatially separated,
extinction-prone local populations (or subpopulations) that are
linked by migration. Other than the classical metapopulation, the
following types are recognized:

Mainland-island metapopulations

Source-sink metapopulations

Non-equilibrium metapopulations.
 Metapopulation survival
depends on:

Local population survival

Unoccupied suitable habitat
at suitable distances

Sufficient migration for
colonization of unoccupied
habitat to occur.
Mechanisms of Extinction in Metapopulations
 Extinction of a metapopulation is certain if the extinction rate of
local populations exceeds the rate at which new populations are
established.
 Local population extinction mechanisms are those of single
populations.
 The mechanisms acting at
the metapopulation level
may be grouped into two
categories:

Colonizationextinction
uncertainty

Regional uncertainty
Scientific Conservation Action in Response to
Population Decline
 The first step is understanding that a sustained population decline
signals a conservation problem. This means that longer term
population declines need to be identified and confirmed.
 The next step is to develop a basic understanding of the species’
ecology.
 Taking the ecological knowledge into consideration, all possible
causes of the decline should be listed.
 The level of each possible cause should be obtained in relation to
the present distribution of the species and its past distribution.
Should the results indicate that a particular cause is likely, a
hypothesis is created. This hypothesis must be tested by
experimentation.
 Once the cause(s) of a decline is identified, possible actions to
remove and neutralize it should be tested for effectiveness by
experimentation. All plans for action must involve monitoring.
Monitoring
 The status of a species can only be
determined by monitoring it.
 Monitoring is also necessary to judge
the effectiveness of conservation
actions.
 Monitoring may take three forms:

Inventories,

Surveys,

Demographic studies.
 The effectiveness of monitoring
depends on the scale at which it is
carried out.
 The information from monitoring may be used for population
viability analysis.
Population Viability Analysis
 Population viability analysis (PVA) is a risk assessment for
populations or species based on empirical data that estimates the
probability (risk) of extinction for a population of the specific
species for a selected time interval.
 Three approaches to PVA exist:

Pattern analysis of long term
studies,

Subjective assessment using
decision analysis based on expert knowledge,

Mathematical and/or statistical
modeling.
 The choice of approach depends on the quality and quantity of data
available. Long term data sets are not usually available for
endangered species.
Population Viability Analysis – Information Needed
 All the approaches to PVAs require information.
 The mathematical and statistical modeling used in population
viability analysis requires lots of detailed ecological information on
the growth and vital rates of the selected species to have any
degree of accuracy.
 For each species, information is required on the: morphology,
environment, distribution, biotic interactions, behaviour, population
demography, genetics and physiology.
 This information may be compiled from:

Published literature,

Unpublished literature,

Fieldwork,

The knowledge of experts,

The knowledge of locals.
Uses of Population Viability Analysis
 Population viability analysis (PVA) may be used to:

Estimate the extinction probability for a population;

Determine the minimum viable population;

Determine minimum reserve size;

Predict future population size;

Show the importance of recovery efforts;

Identify key stages of the life cycle on
which to focus recovery efforts;

Compare proposed management options
and develop action plans for recovery
efforts;

Evaluate existing recovery efforts;

Explore and evaluate the potential impacts of habitat loss or
the consequences of assumptions for small populations.
Minimum Viable Population
 The minimum viable population (MVP) may be defined as the lowest number of individuals needed
to ensure that a population has a selected probability of survival for a set time period without significant loss of evolutionary adaptability.
 Shaffer selected a 99% probability of survival for
1 000 years. These criteria are unrealistic.
 No MVP is applicable to all species.
 Three further points should be noted concerning an MVP: it is
applicable to a particular habitat in an ecological context; if it
includes genetic parameters, it is usually an estimate of the
effective population size not the actual population size needed
and the level (subpopulation/population, metapopulation or
species) at which the MVP is applied must be specified
 It may be beneficial to consider an MVP in terms of the area
needed to support it.
Effective Population Size
 The effective population size (Ne)
equals that of an ideal population that
is genetically influenced by random
genetic drift in the same measure as
the actual population (N).
 In an ideal population, mating is
random and the variation in individual
progeny (offspring) numbers is
random. For animals, a 1:1 sex ratio
exists and for plants, all individuals
reproduce sexually and are diploid and
bisexual, simultaneously producing
female and male gametes with a selffertilization rate of Ne-1.
 Effective population size is frequently
less than actual population size.
Factors Affecting Effective Population Size
 The effective population size is affected
by:

Unequal sex ratios,

Variation in reproductive output,

Population fluctuations,

Whether or not generations
overlap,

Age structure,

Dispersal,

The distribution of individuals,

Inbreeding.
Population Viability Analysis Using Modeling
 The use of models for population viability analysis (PVA) requires
caution and common sense.

A slight change in the parameters combined with a change in the
assumptions the model is based on may give very different
results.

The validity of a PVA depends on the model’s quality and
structure. Models may not include enough ecology to be reliable.

Computer programs do exactly what they have been told to do.
 The process of selecting a model needs to consider whether the
model assumptions are applicable in the population to be studied and
whether the data are adequate to provide reliable inputs into the
model.
 PVA software packages include INMAT and VORTEX.
 Scientific testing of models is necessary to determine reliability. The
use of PVAs does not replace monitoring.
Vulnerability to Extinction
 Conservation priority is based on
the level of threat of extinction
that a species faces.
 Some life history traits can be
used as a guide to the sensitivity
of species to habitat fragmentation and human disturbance.
 A single species may have several of these traits.
 Several of the categories in the
following slides may include common species.
 The following slides give a brief
overview of the identified categories of traits that make species
vulnerable to extinction.
Vulnerability to Extinction 2
 The following categories of species are vulnerable to extinction:

Species that only occur in threatened habitat types,

Species that are economically valuable to humans,

Species that do not have any/much experience of disturbance,

Species that have evolved in isolation within a limited
community without human contact,

Specialist species,

Species that depend on unreliable resources,

Species requiring large
home ranges,

Species that have declining populations.
Vulnerability to Extinction 3 - Rarity
Table 7.1: All the possible combinations of the three factors
(geographic range, habitat specificity and population size)
influencing species abundance modified slightly from Pullin (2002).
Geographic Range
Large
Small
Habitat specificity
Broad
Large population
size, dominant
somewhere
Locally
abundant in
several
habitats
over large
range1
Locally abundant
in a specific
habitat over
a large
range
Locally
abundant in
several
habitats but
geographica
lly restricted
Locally
abundant in
a specific
habitat but
geographica
lly restricted
Always sparse in
several
habitats
over a large
range
Always sparse in
a specific
habitat over
a large
range
Always sparse in
several
habitats and
geographically
restricted
Always sparse in
a specific
habitat and
geographically
restricted
Small population
size, not
dominant
1
Narrow
Broad
The only category that is not considered rare.
Narrow
Vulnerability to Extinction 4
 The following categories of species are vulnerable to extinction:

Short-lived species,

Species with a low adult survival rate,

Species with low genetic variability,

Species with a low intrinsic growth rate,

Species with very variable population size,

Species that lack long distance dispersal mechanisms,

Species that form aggregations, either
permanent or temporary,

Migratory species,

Large species,

Species feeding at a high trophic level.
Points to Ponder
 That which harms biodiversity will eventually harm humanity.
 No population survives forever.
 Monitoring is critical to identifying threatened populations/species.
 Population viability analysis is a conservation tool that needs to be
used with caution.
 Identifying and mitigating/removing (if possible) the causes of
population decline are as important as striving to protect the
reduced population from stochastic events as the reduced
population will not be able to increase substantially without the
mitigation of the original causes of decline.
 Conservation biology needs to focus some efforts
on reducing the ultimate cause of species population decline viz. human population expansion.
 Sharing information is central to achieving
changes in human attitudes and behaviour.
Chapter 1 What is conservation biology?
Links
to
Other
Chapters
Chapter 2 Threats to biological diversity 1: Habitat loss and
fragmentation
Chapter 3 Threats to biological diversity 2: Over exploitation
Chapter 4 Threats to biological diversity 3: Exotic species
Chapter 5 Threats to biological diversity 4: Environmental
factors
Chapter 6 When is a species endangered and where should
conservation efforts be placed?
Chapter 7 Extinction and conservation
Chapter 8 Conservation genetics
I hope that you found chapter 7 informative and that you will
enjoy chapter 8.