EXTINCTION AND EXTINCTION AND CONSERVATIONCONSERVATION

BCB341: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 realisation that what harms biodiversity will harm humanity.

Conservation biology aims to prevent the extinction rate exceeding the speciation rate – not to eradicate extinction.

WHY IS EXTINCTION A CONCERN FOR WHY IS EXTINCTION A CONCERN FOR CONSERVATION BIOLOGY?CONSERVATION BIOLOGY?

Threats to biodiversity fall into two categories:

Systematic threats,

Chance threats.

The effects of systematic threats usually include increased vulnerability to chance threats.

CATEGORISING THREATS TO CATEGORISING THREATS TO BIODIVERSITYBIODIVERSITY

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.

CONSERVATION FOCUS… POPULATIONSCONSERVATION FOCUS… 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

POPULATIONSPOPULATIONS

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

Allee effects

Environmental uncertainty

‘Natural’ catastrophes

Loss of genetic diversity

Mechanisms may interact, compounding the effect on the population.

Population size is critical to survival.

MECHANISMS OF EXTINCTION IN SINGLE MECHANISMS OF EXTINCTION IN SINGLE POPULATIONSPOPULATIONS

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 recognised:

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 colonisation of unoccupied habitat to occur.

METAPOPULATIONSMETAPOPULATIONS

Extinction of a metapopulation is certain if, in the long term, 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:

Colonisation-extinction uncertainty

Regional uncertainty

MECHANISMS OF EXTINCTION IN MECHANISMS OF EXTINCTION IN METAPOPULATIONSMETAPOPULATIONS

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 neutralise it should be tested for effectiveness by experimentation. All plans for action must involve monitoring.

SCIENTIFIC CONSERVATION ACTION IN RESPONSE SCIENTIFIC CONSERVATION ACTION IN RESPONSE TO POPULATION DECLINETO POPULATION DECLINE

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.

MONITORINGMONITORING

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 ex-pert 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 ANALYSISPOPULATION VIABILITY ANALYSIS

All the approaches to PVA require information.

The mathematical and statistical modeling used in PVA 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.

POPULATION VIABILITY ANALYSIS – INFORMATION POPULATION VIABILITY ANALYSIS – INFORMATION NEEDEDNEEDED

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.

USES OF POPULATION VIABILITY ANALYSISUSES OF POPULATION VIABILITY ANALYSIS

The minimum viable population (MVP) may be de-fined as the lowest number of individuals needed to ensure that a population has a selected probabi-lity of survival for a set time period without signifi-cant loss of evolutionary adaptability.

Shaffer selected a 99% probability of survival for 1000 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.

MINIMUM VIABLE POPULATIONMINIMUM VIABLE POPULATION

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 self-fertilisation rate of Ne

-1.

Effective population size is frequently less than actual population size.

EFFECTIVE POPULATION SIZEEFFECTIVE 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.

FACTORS AFFECTING EFFECTIVE POPULATION SIZEFACTORS AFFECTING EFFECTIVE POPULATION SIZE

The use of models for population viability analyses (PVAs) 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.

POPULATION VIABILITY ANALYSIS USING MODELINGPOPULATION VIABILITY ANALYSIS USING MODELING

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 fragmenta-tion and human disturbance.

A single species may have seve-ral of these traits.

Several of the categories in the following slides may include com-mon species.

The following slides give a brief overview of the identified catego-ries of traits that make species vulnerable to extinction.

VULNERABILITY TO EXTINCTIONVULNERABILITY TO EXTINCTION

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 un-reliable resources,

Species requiring large home ranges,

Species that have de-clining populations.

VULNERABILITY TO EXTINCTION 2VULNERABILITY TO EXTINCTION 2

VULNERABILITY TO EXTINCTION 3 - RARITYVULNERABILITY TO EXTINCTION 3 - RARITY

Geographic Range Large Small

Habitat specificity Broad Narrow Broad Narrow

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 geographically restricted

Locally abundant in a specific habitat but

geographically restricted

Small population size, not dominant

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 geographi-

cally restricted

Always sparse in a specific

habitat and geographi-

cally restricted

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).

1 The only category that is not considered rare.

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.

VULNERABILITY TO EXTINCTION 4VULNERABILITY TO EXTINCTION 4

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 popu-lation decline viz. human population expansion.

Sharing information is central to achieving changes in human attitudes and behaviour.

POINTS TO PONDERPOINTS TO PONDER