The IUCN critera and categories are well established and useful to enable us to compare within and between all groups of living beings, but for plants the criteria have proved to be of limited application as more often than not the information available regarding populations, generation time etc. are lacking, so we use herbarium specimens by proxy. Taylor, working in Brazilian Cacti (in Taylor & Zappi 2004) looked into Farjon & Page (1999), working in Gymnosperms, and attempted to give weights to different characteristics, such as Phylogenetic distinction (PD), Ecological Importance (EI), Genetic Diversity (GD). The formula used IUCN categories of threat x (PD scores + EI scores + GD scores). It was quite an interesting exercise, trying to refine further the IUCN categories, looking to prioritize species conservation. However, it is only as good as the data we have in hand!

It is very important to follow IUCN criteria in conservation, to be able to compare groups or regions etc.... However, they are not perfect. IUCN is aware of this, and they have specialist groups for different groups of organisms, which give suggestions how to adjust and apply to certain groups of beings. It can vary very much from group to group of organisms which criteria will be applicable and useful. However, if You know how to apply criteria, it should lead You (and all the others) to the same threat category with the same information set available on certain organism.

The main problem with UICN red lists are :

. They are scale dependent (through space and time)

. They don't take into account the breeding success of population

. Population health is ignored

For example Pink Flamingo was classified CR in France were the population (growing in all the Mediterranean basin) has to be controlled to avoid problems with riziculture !!!

IUCN is one of the biggest and most influential in the world of international nature conservation and I agree with you Sven that we should support and promote it. It is not politically correct to be overly critical however we have problems (as experssed by some following this quesiton). Any ideas on how to make the threat status evaluation system better?

The IUCN Red List Categories and Criteria are a quantitative, defensible, and widely applicable method for assessing the conservation status (i.e. risk of extinction) of all plant and animal life. The Red List is comprehensive and  incorporates data on species distribution, population size and trends, habitats and ecology (including life history and reproductive ecology), use and trade, and threats to both the species and their habitat. The Red List is crucial for informing conservation and environmental governance globally.  Attached are the Categories and Criteria and a link to other pertinent IUCN Red List resources. See also Rodrigues et al. 2006. 

http://www.iucnredlist.org/technical-documents/red-list-documents

Of course, opinions differ...that's biodiversity!

Kidding apart, I have been involved with IUCN threat categories since 1985, i.e. when they were just "expert-based", and, over the years, I have been/am with Caprinae SG, Deer SG, Bear SG and Reintroduction SG.

My (firm) opinion is that the first (1994, if I remember well) edition of IUCN "objective" criteria to classify status were... totally unrealistic and clearly created by people who did not have first hand experience of data collection in the field - to phrase it mildly.

Later on, in the following editions, these criteria had to be changed, slowly reverting to the stage when they were expert-based or nearly. The most amusing part is that the original data have always been "expert-based" all throughout, BUT rather complex elaborations were making them look very scientific and "objective"!

As I said, the 1994 criteria have become much more realistic with time and the present ones are not so bad. One may wonder why it has taken more than 20 years to realise that rarely you have detailed population info. for wild species, especially if they are rare, thus deserving protection...

One of the weakest features of IUCN criteria is probably that they have different reliability levels across species, e.g. you might assess rather precisely the distribution range or pop. size of a large mammal, but what about marine invertebrates or even large mammals of dense tropical forests?

Surely, trying to devise relatively objective criteria is a commendable effort. Yet, no refined GIS based or stochastic model will provide reliable results without reliable field data - which remain of vital importance to preserve a species. 

Cheers,

SANDRO