1. Red List Criteria: Criterion A

2. Past, present or future population reduction
Time
Population Size
Criterion A is based on rate of population reduction in the past, present or future.

3. 3 generations or 10 years (whichever is the longer)
Time
Population Size
3 generations or 10 years (whichever is the longer)
All population declines in criterion A are measured over a time period of three generations or 10 years, whichever is the longer time period.
If the generation length is 2 years, three generations = 6 years. Therefore the population reduction should be measured over a 10-year time period.
If the generation length is 7 years, three generations = 21 years. You would thus measure the reduction over a 21-year period.
Therefore, to use criterion A, the first piece of information you need is: what is the generation length, or is it likely that three generations would be less than 10 years.
To use criterion A, we first need to know:
What is the generation length? Or is it likely that a three generation time period is less than 10 years?

4. Effects are reversible
Based on any of four criteria:
Timing of Reduction
Reduction Conditions
Past
Future
Causes Understood
Causes Stopped
Effects are reversible A1  A2 A3 * A4
Once you know the time period over which the reduction should be measured, you look at all the information available to indicate rate of decline over that time period. Criterion A is split into four subcriteria:
A1 is used for a reduction that took place over the past 10 years or 3 generations, where the causes of the reduction are understood AND have now ceased AND the reduction is reversible
A2 is used for a reduction that took place in the past 10 years or 3 generations, where the causes of the reduction may not be understood OR may not have ceased OR the reduction may not be reversible
A3 is used for a reduction that is expected to occur over the next 10 years or 3 generations in the future (measured up to a maximum of 100 years into the future). For A3, there should be some information available to support the future rate of decline used. Ideally a population decline model should be used based on current and likely future events.
A4 is used for a reduction over a period including some time in the past and some time in the future (also known as the shifting time window). Again, there is a time cap of 100 years on the future projection, so if you are assessing a very long-lived species, you are not required to project more than 100 years into the future.
For A4, there should be some past data on reduction rates available for part of the 3 generation/10 year time period. For the remaining part of the time period, the reduction rate is projected into the future based on information on past and current declines and threats and the likelihood of declines continuing.
* Up to a maximum of 100 years into the future

5. On the criteria summary sheet, criterion A appears as shown.
The criterion A1 thresholds for each category, and a summary of what the A1 requirements are appear here [in the red boxes shown on screen]. The thresholds for A1 are higher than those for A2, A3 and A4 because this criterion is designed mainly for commercial species that are under strict harvest management (but can be applied to other scenarios; e.g., one-off stochastic events). To use criterion A1:
The causes of the reduction must be known (e.g., deliberate exploitation, or a one-off stochastic event)
The causes of reduction must have stopped (e.g., exploitation has stopped or the current level of exploitation is no longer causing the population to decline, or the stochastic event has passed)
The reduction must be reversible (e.g., the population is showing signs of recovery)
If any of these requirements are not met, the taxon would likely be listed under A2.
The criteria A2, A3 and A4 thresholds for each category, and summaries of what the requirements are for these criteria, appear here [in the red boxes shown on the screen]. The thresholds are a lower than for A1 because the causes of the reduction are less clear (the lower thresholds are more precautionary than for A1).
For all of these subcriteria, the assessor must also state what the rate of decline is based on (direct observation, index of abundance, decline in range area or habitat quality, exploitation, or introduced species, disease, etc.). These letters a-e apply to all the subcriteria A1-A4.

6. EN A1abd Example: a whale Generation length ~ 31 years
Population decline 70-90% over past 3 generations. Populations appear to be recovering.
Population declines caused by commercial whaling:
Cause and effects of decline is fully understood
Commercial whaling has stopped
Pop decline appears to be reversible
Percentage decline based on:
Whale sightings and observations
Stock monitoring surveys and reports
Whaling statistics and records
Given uncertainty in the decline and current stability and population recovery, is listed as EN.
CR/EN A1
CR/EN A1a
Let’s look at an example: a whale species. The generation length for this species is about 31 years, so declines will be measured over a 93-year period. The population has declined by an estimated 70-90% over the past three generations, but is currently recovering. The population declines are due to commercial whaling, which has since stopped. The cause of the decline has been identified (commercial whaling) and its effects are fully understood, whaling has stopped completely, and the population reduction appears to be reversible (populations are now recovering). The species thus meets the thresholds for Endangered and the low end of Critically Endangered A1.
The percentage decline is based on whale sightings and observations, stock monitoring surveys and reports, and whaling statistics and records, so we would list subcriteria a (direct observation), b (index of abundance) and d (actual levels of exploitation).
Given the uncertainty in the percentage decline and the current stability and demonstrated recovery of the populations, the assessors felt is best to opt for the lower category of threat, as the species just barely met the threshold for CR. It is therefore listed as EN A1abd.
CR/EN A1b
CR/EN A1d
EN A1abd

7. Example: a tree
Generation length unknown; ~ 50 years in similar species
Endemic to dry forest, which have been intensively cut for agriculture over last century. Today only highly fragmented forest patches remain, and suffer:
Intense predation from deer and cattle
Uncontrolled fires
Estimated 95% habitat loss over past 150 years: given population densities & distribution, suspected population decline of at least 50%.
Habitat loss ongoing. Regeneration close to zero due to intensive grazing; seeds also consumed by butterfly larvae – suspected population decline of up to 80% over next 100 years.
Population decline rate based on:
Massive habitat loss
Introduced predators
EN A2
CR/EN A3
Here’s another example: A tree species has an unknown generation length, but similar species in the same genus have a generation length of around 50 years; experts believe this is a reasonable generation length for this species. It is endemic to dry forest, which have been reduced dramatically, both in size and quality, due to agricultural expansion over the past century. What remains today are highly fragmented patches that suffer intense predation from deer and cattle, and also suffer uncontrolled fires. Habitat reduction is estimated at 95% over last 150 years, and this degradation continues today. Based on population densities and the distribution of the species within its habitat, experts suspect a population reduction of at least 50% over the past 150 years as direct consequence of this habitat loss – the species thus meets EN A2, as the causes of the decline have not stopped and we don’t know if the decline is reversible. Regeneration of this species is close to zero because the seedlings are intensively grazed and a very high proportion of the seeds are consumed by butterfly larvae on some sites – experts thus suspect a population reduction of up to 80% over next century, as the population is made up of mostly adult trees that are not replaced when they die. Both past and future population decline rates are based on the effects of massive habitat loss and introduced predators, so we list subcriteria c and e.
Given that the future decline rate may be up to 80%, the low end of the CR threshold, experts decide that EN A3 is a more reasonable listing than CR A3. With the evidence of large declines over the last 150 years and very weak current regeneration abilities, it is likely that the species has declined by at least 50% over the past three generations and will likely decline by at least 50% over the next three generations, hence a listing as EN A2+3ce.
A2+3c
A2+3e
EN A2+3ce

8. Near Threatened (NT) example:
Should be close to meeting the VU thresholds or possibly meet some of the subcriteria
Population has declined by 40% in the last three generations
Causes of decline are understood, the causes have stopped, and the decline is reversible.
NT , nearly meeting VU A1
For the first example in this slide, there is evidence that the population has declined by 40% in the last three generations, but the causes of decline are understood, the causes have stopped, and the decline is reversible. Therefore A1 is the most suitable criterion to use for this species. Under A1, the VU threshold is >50%. Since the threshold is not met the species does not qualify as threatened. But the reduction (40%) is not far outside of the threshold, therefore a Near Threatened assessment is justified.
For the second example, there is evidence of a 20-25% reduction in the last three generations. This time the causes of decline are not understood and apparently are still happening (i.e., the population is still declining because some unknown cause). Since the three conditions for A1 (causes of reduction understood, stopped, and the reduction is reversible) are not fulfilled, and the reduction is in the past, criterion A2 is the most suitable for this species. The reduction doesn’t quite meet the VU threshold for A2 (>30%). But it is not far outside of the threshold, therefore a Near Threatened assessment is justified.
Population has declined by 20-25% in the last three generations
Causes of the decline are not understood and appear to be ongoing (i.e. the population is still declining)
NT, nearly meeting VU A2

9. Criterion A4 past & future: “shifting time window”
Present
10 years / 3 generations
10 years / 3 generations
10 years / 3 generations
For A4, the rate of reduction is measured over 3 generations or 10 years, based on some data from the past and expected reduction for some time in the future.
Criterion A4 is often called the “shifting time window” because the actual position of the 3-generation time period in relation to the assessment date may shift [as demonstrated in the animation on screen].
It is not possible to determine whether A4 is applicable only by looking at the qualitative pattern of the decline, or by calculating only past or only future reductions. If a taxon is listed under Criteria A2 and A3, it will usually also be listed under Criterion A4. But this isn’t always the case, and sometimes species will meet a higher category of threat for A4 than they do for A2 or A3.
To decide whether criterion A4 can be used, a “moving-window” reduction should be calculated.

10. Criterion A4 past & future: “shifting time window”
Past census data gathered every 2 yrs
Reduction rate over next 30 yrs
Estimated future populations
Year
Population size
1970
10,000
33%
2002
6,160
1972
38%
2004
5,680
1974
43%
2006
5,260
1976
47%
2008
4,900
1978
51%
2010
4,600
1980
54%
2012
1982
9,940
56%
2014
4,180
1984
9,820
57%
2016
4,060
1986
9,640
58%
2018
4,000
1988
9,400
2020
1990
9,100
2022
1992
8,740
2024
1994
8,320
52%
2026
1996
7,840
49%
2028
1998
7,300
45%
2030
2000
6,700
40%
2032
Present
Reliable data available
If a “moving-window” reduction cannot be calculated over the full time series:
To calculate a “moving-window” reduction, a time series of past population sizes and future projections should be created.
Then, 3-generation reductions should be calculated for all time frames that include at least one past time step and at least one future time step. The length of all those time frames (windows) must be 3-generations or 10 years (whichever is longer), but do not extend more than 100 years into the future (even for long-lived species).
Finally, find the maximum of these reductions [as shown by the red square when you click], which is the number to use in criterion A4. Whether a taxon is listed under A4 or not, of course, depends on whether it qualifies under any of the other criteria. See the User Guidelines for more details.
In many cases we have only limited reliable data on which to base our population decline. If not enough data is available to calculate a full “moving-window” reduction over all the possible time scales, then assessors will have to determine how confidently population declines can be projected into the past and future, and determine the rate of decline over the time period for which they have the most confidence. Ideally this will still involve using a population decline model.
Most confident 10 year / 3 generation time period

11. Points to remember:
3 generations? 10 years?
To use criterion A, an estimate of the generation length is needed
Population reduction may be a one-off event...
Points to remember:
To use criterion A, you must know or be able to estimate generation length, or if it is likely that three generations would be less than 10 years.
Remember that for criterion A, the population reduction may be a one- off event or it may be ongoing.
... Or it may be ongoing

12. Criterion A1 uses higher thresholds than A2, A3 and A4
Points to remember:
Criterion A1 uses higher thresholds than A2, A3 and A4
Points to remember:
Remember that sub-criterion A1 has higher thresholds than the other sub-criteria.
To use A1 the causes of population decline must be understood AND must have ceased AND the reduction is reversible.
(Note to Facilitators: These different rates reflect the understanding that taxa in which the causes of reduction are clearly reversible AND understood AND ceased are less at risk from extinction than those where the causes of reduction may not have ceased OR may not be understood OR may not be reversible.)

13. Points to remember:
Points to remember:
Because A3 is based on a population decline projected into the future, this cannot be based on “direct observation” (subcriterion a)…unless you are capable of predicting the future!
A3 = projected FUTURE reduction so cannot be based on direct observation (A3a)

14. Consider the pattern of population reduction
Points to remember:
Consider the pattern of population reduction
Points to remember:
Think about the different types of decline – it’s very common to have data for only a part of the 3- generation time period or to have population estimates from different time periods for different subpopulations, so we have to extrapolate back and forward in time to get the decline rate for the full time period. See the Guidelines for Using the IUCN Red List Categories and Criteria for more details.

15. Refer to User Guidelines and ‘CriterionA
Refer to User Guidelines and ‘CriterionA.xls’ for information and advice
Points to remember:
The User Guidelines provide a lot of information and advice to help you calculate reduction rates for criterion A, including information for assuming a pattern of decline, calculating moving- window reductions, estimating reductions for multiple subpopulations with different data availability, projecting population estimates backwards or forwards in time, issues specific to fisheries, and more. Utilize this resource!
See also the workbook file ‘CriterionA.xls for an example of how to calculate reduction for multiple populations, and an example of how to use criteria A2, A3 and A4.