Measuring temporal change

For the Ne 500 indicator, any population that goes extinct after the country’s baseline year (each country is directed by the CBD to choose a baseline, which defaults to 2010-2020 but which may be adjusted to country context) is assigned an Ne of 0 and are therefore below Ne 500. These populations must be retained in the calculation in order to avoid the perverse incentive to “raise” the indicator value through population extinction.

Temporal change can be calculated using multiple time points for population size and population persistence. Calculating temporal change in the indicator requires the use of the same set of species at all time points, similar to the Red List Index (Bubb et al 2009, “IUCN Red List index : guidance for national and regional use. Version 1.1”). **As a default guidance, all species used in the first time point should be included in the second. **

However, the country may wish to change or add to the species lists over time (e.g., owing to taxonomic revisions, additional data sources, etc.). In such cases, countries can do one of the following:

• Any species in which taxonomic revisions or data errors are identified to have impacted the indicator value, should be removed from both time points.
• Indicator values for any species affected by new knowledge or taxonomic changes can have their current and former indicator value retrospectively calculated. In other words, the entity being evaluated in the current time point can be re-evaluated for its previous time point using the most up to date guidance and data available.

In addition, it is anticipated that biodiversity monitoring capacity within countries will increase over time, and thus countries may wish to increase the number of species included in their indicator calculation, e.g. from 100 to 1000 species. In such cases, the species being newly evaluated can have retrospective indicator calculations made, assuming historical data is available. This highlights a broader opportunity, that such retrospective evaluation could extend indicator calculation into the past.

Temporal increases in the proportion of populations with Ne above 500 would indicate improvement in the maintenance of genetic diversity (on average slowing the rate of genetic erosion and eventually ‘bending the curve’ such that genetic diversity is restored via natural processes of mutation, migration, etc.). Decreases would indicate worsening status (accelerating rate of genetic erosion). Static values would indicate a stable state of the indicator (stable rate of genetic erosion - though not necessarily a halting of genetic erosion - it is only halted when Ne >500). The indicator is designed to be recalculated as new data are compiled, which in many species is a timescale of 2 to 5 years, thus the indicator would be calculated and reported on typically once every 4 years (fitting the timespan of CBD reporting).

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