Genetic population estimates

Project Details


The evaluation of conservation status and the Appropriate Assessment (Passende Beoordeling) give rise to a need for estimates of the population size of 107 species covered by the Habitats and Birds Directives, as well as for the monitoring of the status of these species. Reliable counts are very hard to obtain and/or very labour-intensive for many species. Techniques from population genetics in principle come in useful here: with a relatively small sample (25 to 50 individuals), an estimate can be made of the effective population size. Even for species for which, until recently, only relative numbers could be determined (for example, the catch per unit of effort in the case of fish monitoring), it is possible to make estimates of the effective population size on the basis of genetic samples.
The speed with which genetic diversity is lost is a function of the number of individuals participating in reproduction, and the variance in their number of offspring. Together, these factors determine the effective size of a population: the size of a theoretically ideal population that loses genetic diversity at the same rate as the population being studied. The effective population size is therefore a direct measure of the total population size and sustainability of populations’ survival (Frankham 1995; Frankham 2014 et al.). In version 3 of the Local Conservation Status, the effective size of populations and genetic connectivity between subpopulations are used as additional criteria for determining favourable conservation status, either directly or indirectly using other indicators such as the minimum land area of a favourable population and landscape configuration. This general principle is now also accepted by the Agency for Nature and Forest and civil society, meaning that genetic criteria can be used to evaluate populations of protected species; as a result, they form a basis for policy on conservation of Habitats Directive species and priority provincial species. Belgium is thus the first European Member State actually to include genetic criteria in the criteria for the implementation of the Habitats Directive, as required by the latest amendments to the directive. Meta-analyses show that on average across all species, the effective size (Ne) is smaller by a factor of 9 than the census size (Nc, the number of counted adult animals), so that the effective size can in principle be used as an indicator of the census size where such information is needed in addition to the effective size. This multiplication factor is not necessarily constant per species, and there is a need to determine more clearly how robust it is and on what other factors this relationship depends. One important point for consideration for the use of genetic population estimates is that the population size that is estimated is that of the population in an ecological and evolutionary sense: a more or less homogeneous group of interacting individuals. This does not necessarily correspond to what is regarded as the population in a zone or sub-zone of a special protection area (SPA) according to the legal framework. In many cases, especially with highly mobile species, the ecological population extends beyond a sub-zone of an SPA, which means that a genetic population estimate cannot be used to make any pronouncement at the level of the SPA sub-zone: at most, a pronouncement can be made at a higher geographical level. However, the European reporting system only requires an overall picture of the conservation status, rather than the local conservation status. Moreover, finding out whether gene exchange and therefore migration is taking place between (sub)populations is useful for estimating local trends properly and performing an appropriate assessment. A local decline in a species does not necessarily mean a decline in the total metapopulation, and vice versa. In addition to the use of genetic techniques to determine the (effective) population size, these techniques can also be used to determine population trends (in the ecological sense) without any prior knowledge. This is because genetic techniques make it possible to ascertain whether populations are (obviously) larger or smaller relative to a previous situation, and/or whether they are currently more or less genetically linked to other populations. For species or populations for which only sporadic historical data are available, such genetic analysis makes initial trend estimation possible, as well as the detection of negative trends before other indications are available.
Effective start/end date1/04/1430/06/19

Thematic List 2020

  • Protected nature
  • Wildlife management

Thematic list

  • Species and biotopes
  • Policy