Biodiversity is a difficult concept to grasp and generalise, because it is expressed at different levels of life: genes, species, habitats and ecosystems.
Understanding the value of biodiversity should enable efficient choices concerning conservation. Economists have thus sought to develop decision-making rules that include biological, ecological and economic criteria (amongst others) in order to guide the choices of decision-makers in a context of limited financial resources.
Classification approaches can produce contradictory results
Several methods have been developed to associate a value with biodiversity. Generally speaking, they aim to assign a diversity index to any finite series of elements. The researchers compared four types of indicators as a function of the classification they generated of two samples of eight elements: one sample comprised 2 tortoises, 4 butterflies and 2 jellyfishes, while the other contained 4 sardines and 4 cats. The first two families of indicators are widely employed in the life sciences; the others are mainly used in the fields of human and social sciences, and particularly in economics.
The cardinal criterion counts the number of species
This method consists simply in counting the number of different species present in each sample. Based on the number of species, it thus considers that sample 1 is more diversified than sample 2: it contains 3 species while the other only contains 2.
This method supposes that all species contribute equally to biodiversity, whatever their proportion in the sample: with an additional 10,000 cats, sample 2 still contains only two different species and thus remains less diversified than sample 1.
The efficient number of species takes account of the functional role of a species in biodiversity
Another method takes account of the relative abundance of a species and its role in the ecosystem. Depending on its functional role in the whole, a threshold value can be envisaged for each species, below which the health of the ecosystem may be disturbed. This approach gives rise to a family of indices that combine relative abundance and species richness, leading to an "efficient" number of species in the ecosystem.
Depending on the variants, this method classifies sample 1 as being more diversified than sample 2.
This method does not take account of any dissimilarities existing between the species; however, the closer the species, the more they will supply redundant genetic information.
Taking account of dissimilarities between species
The third approach focuses on dissimilarities between species. One option is based on the notion of the genetic distance between pairs of species, and evaluates the diversity of a sample using a procedure for the iterative pooling of distances. For each iteration it builds an ensemble to be evaluated as a function of the individual that will offer the greatest degree of dissimilarity. This approach requires extremely detailed biological knowledge (which is not always available) and considerable calculation power. Just a few months ago, the capacity of standard computers and methods was exceeded at more than 30 species. The INRA economists have now managed to increase this limit to 800 species.
A second option considers dissimilarities according to an order relation between pairs of species. Ordinal data are sufficient, such as "the dissimilarity between the sardine and the cat is greater than that between the tortoise and the butterfly". However, this method takes no account of the contribution of species to diversity when they offer fewer dissimilarities.
Unlike the two indicators referred to above, these two methods consider sample 2 to be more diversified than sample 1.
The final approach is based on an explicit arbitration between the attributes of species (such as "can fly", or "is a vertebrate", etc.). Each attribute is assigned a value (numerical), so that the decider can thus evaluate the attribute to which he attaches importance. The measurement is then obtained by adding together the values of the attributes expressed in the sample.
Thus sacrifice of the second sample would result in the disappearance of only those species that have the characteristic of being feline, while that of the first sample would be the loss of the only species that can fly and the only species representative of molluscs. This approach, which can a posteriori determine a rationale for any type of conservation, does not take account of the notion of relative species abundance.
All indicators are not equivalent and do not measure the "same thing". Nor is there any single and/or universal criterion for measurement that could be rendered equivalent under the different approaches.
Each method has its limitations. The theoretical bases for a particular method need to be reinforced. Problems of a practical nature, and notably acquisition or processing of the information necessary to calculate indicators (in particular, to measure dissimilarities between species) also constitute a challenge for research in both biology and economics.
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