There is no single universal biodiversity metric, but we don't need to measure everything either

There has never been a bigger need to quantify and monitor biodiversity. With a multitude of the world’s governments having committed to conserving 30% of lands and oceans by 2030 and increasing numbers of corporations grappling with the assessment of their nature-related dependencies and impacts, the search for the ultimate biodiversity metric that can appropriately represent all dimensions of biodiversity and allow tracking of nature-positive actions has been in full swing. However, it is now abundantly clear that there cannot be a single universal metric of biodiversity, akin to CO2 for climate, because biodiversity is far too complex and multidimensional a concept. So, how should we go about appropriately quantifying biodiversity in all its dimensions? Is the only solution to quantify and keep track all relevant biodiversity dimensions individually in all cases?

The global assessment of extinction risk in freshwater animals published in Nature today (Sayer et al. 2025, Nature) highlights how key efficiencies can be gained when faced with the challenge of quantifying and monitoring multiple biodiversity dimensions. Among its many crucial contributions, the study details how the global distribution of threatened freshwater animals relates to the distribution of previously assessed threatened terrestrial vertebrates. Contrary to previously published evidence, the study finds that knowing where threatened terrestrial vertebrates are represents a good indicator – or “surrogate” – of where threatened freshwater animals are as well. This conclusion is the latest example of surrogacy among biodiversity dimensions, with previous studies concluding that terrestrial vertebrate classes are good surrogates for each other when assessing the distribution of extinction threat globally (Cox et al. 2022, Nature) and that the diversity of species is a good surrogate for the diversity of ecological roles and evolutionary lineages (Rapacciuolo et al. 2019, Nature E&E).

This type of knowledge is crucial, because it pinpoints how data for one dimension of biodiversity – for example extinction risk in terrestrial animals – may be sufficient to understand variation in another dimension of biodiversity – extinction risk in freshwater animals – thus precluding the need to obtain data on both biodiversity dimensions individually. Think of it as the biodiversity equivalent of knowing the exchange rate among various monetary currencies: even though you may not know the exact price of an item in your own currency when traveling abroad, overall knowledge of exchange rates will allow you to understand the value of the item.

It is important to note, however, that there is no true substitute for exact knowledge of different biodiversity dimensions, because a portion of the variation is lost in the use of surrogates. In fact, all published evidence of surrogacy also highlights how the use of surrogates is likely to lead to ignoring edge cases, which often hold extremely high biodiversity value (e.g. range-restricted freshwater fishes are unlikely to be prioritized incidentally through co-occurrence with terrestrial vertebrates).

Zooming back out to the quest for key biodiversity metrics, analyses like the one published today in Nature suggest that understanding how different biodiversity dimensions are linked with each other across large scales may be just as important as pinpointing which individual components need to be reflected as biodiversity metrics for a particular application. Knowledge of interrelationships among biodiversity dimensions is likely to establish which data are unnecessary, introduce efficiencies partially compensating for the huge biodiversity financing gap, and ultimately make quantifying and tracking biodiversity more feasible.