How to establish a reference period

Because data on the number of existing (extant) and extinct populations is needed, a reference baseline time period is needed for estimating the total number of populations. The monitoring framework of the GBF recommends using “…the period from 2011–2020, where data is available, as the reference period, unless otherwise indicated, for reporting and monitoring progress” (paragraph 2 of CBD, 2022b). The framework also notes that “baselines, conditions and periods used to express desirable states or levels of ambition in goals and targets should, where relevant, take into account historical trends”. For this reason, we recommend considering a baseline period for the Populations Maintained (PM) indicator that is before the Industrial Era. Since the exact period representing this varies by country and may depend on the species and what data are available, we suggest using a relatively broad baseline time period of 50–200 years ago for data retrieval, and allowing for more specific baselines to be defined by countries.

Thus, the date range for which the indicators are available is dependent on data availability at the national scale. Typically Nc for the Ne 500 indicator will be obtained from the past decade e.g. post 2010. Going forward it will be reported every 2 to 5 years, typically every 4 years, making it suited to the CBD reporting schedule. As the indicator is increasingly deployed, indicator calculation can be made in temporal windows, including through the use of older biodiversity observation data, reports and consultation with knowledge holders, likely extending indicator assessment at least back to the 1990s.

Note that, while it’s ideal for the time range of the Ne 500 and PM indicators to be same, this may not be possible depending on data availability.

Threshold of Data Recency

When reviewing the available data for calculating indicators, there may not be “recent” data available. Specifically, it may be necessary to decide how to assess species’ populations which have been known to exist, but which haven’t been visited recently, such that the current existence and size of that population are unknown. Here, we provide a framework to help handle these situations based on different example scenarios where the elapsed time since the population was last observed and any known trends in population size and/or threat status vary. Note that this framework is meant only to give general guidance to assessors, and indicator calculations will vary from species to species based on the available data. In all cases, assessors can consider completing multiple assessments to reflect uncertainty (see How to account for uncertainty). For instance, a species could have two assessments: one in which all populations that are “possibly extant” are reported as extant with last known values, and another separate assessment in which those populations are reported as extinct.

The precise years and thresholds below are examples, and should be adapted to unique scenarios, as is typical for baselines.

  1. Scenario 1: Known past population size trends

    • Example 1: Population estimated at 200 individuals in 1980 and 50 individuals in 2000; unvisited since.
    • Example 2: Population estimated at 200 individuals in 1980 and 150 individuals in 2000; unvisited since.
    • Recommendation: Assuming a known negative trend in population size which would result in zero individuals when extrapolated to the present, and no recent (≤10 years) visits, the population should very likely be considered extinct (Example 1). Otherwise, if the population size trend indicates a nonzero number of individuals when extrapolated to the present (Example 2), then the population should be considered extant.

  2. Scenario 2: Unknown population size trends but known threats

    • Example: Population estimated at 200 individuals in 1980; unvisited since. However, there are known threats (e.g. habitat loss, disease, poaching, etc.) to the species and/or the population in question which are ongoing, strongly suggesting population decline.

    • Recommendation: Based on threat severity, the organism’s likelihood of persistence given different traits (life span, growth habit, life history, etc.), and available expert opinion, the assessor should make their best judgement about extinction as follows:

      • Population visited and known as extant within the past 20 years: likely consider extant
      • Population not visited in more than 20 years: possibly consider extinct
      • Population not visited in more than 40 years: likely consider extinct. (In the example above, the population would be considered extinct.)

  3. Scenario 3: Unknown population size trends and unknown threats

    • Example: Population estimated at 200 individuals in 1980; unvisited since. No knowledge regarding threats.

    • Recommendation: Based on any available expert opinion, the assessor should make their best judgement about extinction as follows (note that these recommendations are similar to those for Scenario #2, but shift the degree of certainty because threats are not known):

      • Population visited and known as extant within the past 20 years: consider extant
      • Population not visited in more than 20 years: likely consider extant
      • Population not visited in more than 40 years: possibly consider extinct. (In the example above, the population would be considered extinct.)

Previous: How to define populations Next: Extinct and extant populations