EPHM7110 Chap.10 Extinction Risk and the IUCN Red List
Extinction Risk and the IUCN Red List
A rate, and a background to measure it against
Extinction means no member of a species remains alive anywhere in the world, and the course says when it happens: once conditions shift quicker than a species can track them. It can occur naturally, and the natural pace has a published figure, one species extinction per million species per hundred years.
Every claim about a crisis is a claim about how far the current rate sits above that background. The course reports the current position as caused by human activities, at more than a thousand times the natural rate, with a species lost every twenty minutes and an estimate that between ten and fifty per cent of species will be lost over the next twenty to fifty years.
The last of those is a projection and should be presented as one.
Functional extinction is the one a survey can miss
Beside full extinction sits functional extinction, where only a few individuals are left and the odds of successful reproduction are slim. A species can appear on a site list, be photographed, and still have no future, which is exactly what a presence and absence survey is blind to.
Whenever a report says a species is still present, the question to ask next is whether it is still breeding.
Nine categories, of which two describe the evidence
The Red List is described as the best known worldwide ranking system, holding nine categories assigned on criteria including rate of decline, population size, geographic distribution and fragmentation, with assessments carried out by experts and used to guide national and international policy.
Seven of the nine are statements about extinction risk. The other two say that the evidence is insufficient or that no assessment has been made, and confusing the two kinds is the commonest error in citing the list.
The insufficient evidence category is not a mild one, because it includes many species that are hard to observe and many of those may actually be threatened.
Five criteria, and a search for the one that bites
Criterion A is population size reduction measured over the longer of ten years or three generations.
Criterion B is geographic range, as extent of occurrence or area of occupancy, and additionally requires at least two of three further conditions. Criterion C is small population size together with decline. Criterion D is a very small population or a very restricted distribution. Criterion E is a quantitative analysis expressed as a probability of extinction.
A taxon is threatened if it meets any one of them, so an assessment is a search for the criterion that bites hardest rather than an average of all five.
What this chapter covers
- 01
Extinction defined, and the published background rate
- 02
The current rate, the twenty minute figure and the projection to be attributed
- 03
Functional extinction, and why presence is not persistence
- 04
The five mass extinctions and their dates
- 05
The small population loop and why it outlives the original threat
- 06
The share of assessed species threatened in six groups
- 07
Nine categories, and the two that describe evidence rather than risk
- 08
Criteria A to E and their published thresholds
Assess one freshwater fish against the criteria
- 3Assess the decline under criterion A and state the branch used.
- 4Assess the range and the count under criteria B and C.
- 2State the category and what it does not settle.
Key terms
- Background Rate
- The natural pace of extinction, given as one species per million species per hundred years.
- Functional Extinction
- A state in which so few individuals remain that successful reproduction is unlikely.
- Extinction Vortex
- The reinforcing cycle in which small population size produces effects that reduce the population further.
- Genetic Drift
- Random change in the genetic make up of a small population, unrelated to which traits are advantageous.
- Red List
- The international ranking of species by extinction risk, assigned by expert assessment against published criteria.
- Data Deficient
- A Red List category recording that information on abundance or distribution is insufficient to assess risk.
- Extent Of Occurrence
- The area contained within the outer limits of a species' known distribution, used in the range criterion.
- Area Of Occupancy
- The area within a species' range that it actually occupies, used in the range criterion.
- Biodiversity Hotspot
- A region holding exceptional concentrations of species and facing exceptional loss of habitat.
Extinction Risk and the IUCN Red List FAQ
Why is the insufficient evidence category not a reassuring one?
Because it reports a gap in our knowledge rather than a finding about the animal. The species placed there are often the ones that are hard to find, hard to tell apart or live somewhere rarely surveyed, and those are precisely the characteristics that go with small ranges and small numbers.
Summarising a fauna as mostly secure with a few of these has described the assessed part of the fauna, which is a smaller claim than it sounds.
Does removing the original threat rescue a very small population?
Not on its own. Once numbers are low, breeding between close relatives and the random loss of variation start operating independently of whatever caused the decline, and each of them reduces reproduction and raises mortality, which shrinks the group again.
Stopping the first cause is necessary and may not be sufficient, which is the argument for intervening while a population is still large and for linking separated groups so that they breed as one.
A site holds only species of least concern. Is clearing it therefore harmless?
Two things block that inference. The ranking is global while the clearance is local, so a widespread animal can still be present in its last district population here, and the site may hold organisms nobody has ever assessed. The ranking also speaks only to extinction risk, and says nothing about the services the ground provides or the community it supports, which is the whole of the rest of a planning case.
Why is the any one criterion rule so important to get right?
Because it changes the arithmetic of an assessment. Each criterion is a separate route to a listing, and a species that only just misses on decline can sit well inside the threshold on population size, so the correct result is the highest category any single route supports. Blending the five, or reporting the one that looks most typical, reliably produces a lower category than the evidence allows.
Exam move
Take three species you can find figures for and try to place each one yourself against the criteria, writing which criterion you used and which threshold decided it. Then compare your result with the published listing. Where you differ, the difference will almost always be a condition attached to a criterion that you skipped.
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