Have we really lost 52% of the world’s animals?

The methods used by the LPI should not be accepted without reservation

Turning a critical eye on the 2014 Living Planet Report.

WWF’s Living Planet Report (LPR) 2014 has been making headlines because of its alarming claim that population sizes of mammals, birds, reptiles, amphibians and fish have dropped by half since 1970. The report reached this stark (and widely shared) conclusion via the Living Planet Index (LPI) a “measure of the state of the world’s biological diversity based on population trends of vertebrate species from terrestrial, freshwater and marine habitats” developed by scientists at WWF and the Zoological Society of London (ZSL). The LPI was adopted by the Convention on Biological Diversity (CBD) as a progress indicator for its 2020 goal to “take effective and urgent action to halt the loss of biodiversity”, which sadly (but unsurprisingly) appears to be failing.

In the previous edition of the LPR published two years ago, the drop in vertebrate numbers was estimated to be 30%. Now the scientists behind the LPI claim to have improved the method, resulting in a much greater decrease (52%) than previously reported. But the methodology is still highly controversial.

The team estimated trends in 10,380 populations of 3,038 mammal, bird, reptile, amphibian and fish species using 2,337 data sources including published scientific literature, online databases, and grey literature. The data used in constructing the index are time series of either population size, density, abundance or a “proxy of abundance”, e.g. bird nest density when there were no bird counts available.

The collection and analyses of these data represent an enormous amount of work and the team responsible deserves praise for undertaking this huge project and for creating an urgent call to action for wildlife conservation. However, we need to bear in mind that this dramatic “halving” of the word’s vertebrates is a grotesque oversimplification of biodiversity loss. The diversity of data sources and types used, the variability in data quality, as well as the uncertainty behind many of the population trend estimates mean that the LPI is probably not very reliable.

Additionally, the 3,038 species included in the analyses represent only 4.8% of the world’s 62,839 described vertebrate species. (The report entirely omits invertebrates, which are often cornerstone species and vastly outnumber all vertebrate animals). Following criticism on the methodology of previous LPIs, this year the LPI team used the estimated number of species in different taxonomic groups and biogeographic areas to apply weightings to the data. This means that the population trend of a particular taxonomic group becomes more important if the group comprises a large number of species, whereas the population trend of a species-poor taxon is allocated considerably less weight. To illustrate this, let us consider fishes, which in the LPI analysis represent the largest proportion of vertebrate species in almost all biogeographic areas and therefore carry the most weight. My guess is that the fish species whose population trends are sufficiently documented to be included in the analysis are most often in serious decline, because well-studied species are usually those that are either overharvested or frequent victims of bycatch. Therefore, the negative fish trend contributed more to the final 52% figure than the decline of any other taxonomic group. Ironically, by trying to decrease error from taxonomic bias in available data, this method allows well-known species to drive the overall trend and does not deal with the problem of underrepresentation of less-studied species. Many of these less visible species, outside of human interest as food or pests, contribute substantially to overall biodiversity and ecosystem function.

Should we believe the shocking headlines? Have we really killed “half of the world’s animals”? Probably not. Conservationists hope that this type of dramatic statement will inspire action but the severity of the claim risks desensitising the public, achieving the opposite of its intended effect. Developing a clear picture of the degree of the threats humans pose to biodiversity is difficult, but imperfect knowledge is no excuse for negligence. We know for certain that we are driving species to extinction at an alarming rate and that this will have serious implications for the environment, economies, and human health. Is this knowledge really not sufficient to motivate urgent and meaningful conservation action?

Olivia Nater is a conservationist and biologist who is particularly fond of bees. Twitter @beeologist