Transcript Slide 1
EXTINCTION AND CONSERVATION BCB341: Conservation Biology 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 realisation 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 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. MECHANISMS OF EXTINCTION IN METAPOPULATIONS 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: Colonisationextinction 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 neutralise 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 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. 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 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. 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 selffertilisation 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 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. 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.