How Much Does Biodiversity Really Matter?

When a professor at Oxford University told The Guardian last year, “Without biodiversity, there is no future for humanity,” that’s something worth paying attention to. It is the complex and myriad interactions between animal life and the earth’s physical environment that generally is credited with making the planet habitable for humans. Figuring out what those interactions are is what ecologists and environmental scientists do.

But what if the conceptual frameworks underpinning what is known as biodiversity-ecosystem functioning (BEF) are flawed? A new National Science Foundation-sponsored paper by Pradeep Pillai and Tarik C. Gouhier of the Marine Science Center at Northeastern University challenges the models on which those frameworks are built. Does that mean Americans can stop arguing over protection for the greater-sage grouse?

Pillai and Gouhier tackle the BEF relationship first. A positive relationship between biodiversity and ecosystem functioning, that is, the properties and processes in the environment, is typically determined by measuring the functioning in a community relative to neutral assumptions. They argue, however, “the assumption of neutrality is not appropriate to BEF studies because it is too strong or naïve, and tantamount to a straw man argument.”

The researchers go on to say that the positive BEF relationships are both “unsurprising and largely trivial” because the measurements “likely represents, at least in part,” an “artefact” of coexistence in the ecosystem.

A second challenge to most BEF research, according to Pillai and Gouhier, is a fundamental mathematical flaw in what is known as the Loreau-Hector (LH) statistical partitioning scheme, a method that has been used extensively experimentally to study biodiversity effects. The LH scheme was promulgated in 2001 to tease out the ecosystems’ selection effects (how a species with particular traits affects ecosystem functioning) and complementarity effects (how resource partitioning lead to increase total resource use).

Pillai and Gouhier demonstrate mathematically that the LH partitioning scheme is, in general, “incorrect.” In fact, it only holds true in one special and “unlikely” case.

What follows is a description of an alternative scheme the authors propose that they say, avoids the circularity of the neutrality measurement. Their scheme, unlike the current approaches to BEF, avoids the redundant effects of coexistence “that are likely artificially inflating the positive impact of biodiversity on ecosystem functioning.”

A detailed takedown of the LH partitioning scheme concludes that it is “meaningless” to talk of biodiversity effects on ecosystem functioning or on a selection or complementarity effect because these would be artefacts of an individual species and not biodiversity.

Does this analysis dismantle a quarter century of work on the “generally positive” BEF relationship? Pillai and Gouhier deny this, claiming that whether the measurement of the positive relationship is real is not the question. The issue is how meaningful are the measurements and whether inferences drawn from those measurements truly explain what’s going on out in the world.

And the sage grouse? Like northern spotted owls, grizzly bears and wolves, preserving habitat — a vibrant BEF, if you will — is critical to their survival. We have the metrics; now we need to make sure that we have the best interpretations of those metrics.

An abstract of the Pillai and Gouthier paper is available at the Ecological Society of America website. The full study is available for sale.