Denying the “undeniable reality”

In their paper “Holocene shifts in the assembly of plant and animal communities implicate human impacts”, Lyons et al report that that the proportion of species pairs that are aggregated (i.e. co-occur) rather than segregated began to decline in the mid-Holocene, coinciding with the spread of agriculture across North America. Exciting stuff (perhaps), picked up by at least a couple of dozen media outlets, that informs the debate about the start date of the Anthropocene.

In an accompanying News and Views, “Ecology: Different worlds”, Dietl writes that

Lyons et al detail a compelling case that this extraordinary situation is an undeniable reality for the rules that govern how plant and animal communities are structured.

An undeniable reality? That is a like a red rose to a goat: something that must be chewed. Contrast Dietl with R. A. Fisher

A scientific fact should be regarded as experimentally established only if a properly designed experiment rarely fails to give this [p = 0.05] level of significance.

No single research paper can ever be sensibly described as presenting an “undeniable reality”. It takes a large body of mutually supporting work to approach an “undeniable reality” (think evolution, greenhouse gases). Certainly, Lyons et al cannot be an undeniable reality: I have just submitted a comment to Nature that denies it.

In the comment, my coauthors (Joe Chipperfield, Hilary and John Birks) and I detail three critical problems with Lyons et al that invalidate their conclusions (we do not claim to have an exhaustive list). I’ve posted a preprint (permitted by Nature’s guidelines on posting preprints on personal blogs), and will eventually discuss some aspects in more depth than can be squeezed into a 1200 word comment.

Today I want to discuss the rest of Dietl’s News and Views, which is perhaps the only piece many non-specialists will read.

A small cadre of voices argues that a human-dominated present limits the use of the past as a key to unlocking the future. In this view, the world we live in today, and the immediate future that our grandchildren will inherit, has no analogue in the geological past. As a consequence, referencing ‘natural experiments’ in the distant past as a guide to predict what might happen, now or in the future, is a flawed strategy. Out is the use of uniformitarianism as a guiding principle, and in is a new kind of ‘post-normal’ science.

Dietl’s argument is that humans have changed the environment so much that it is without analogue in the geological past and hence information from the  past cannot be used to predict what will happen in the future. This is an extremely depressing argument for palaeoecologists: one of our main rationales is to understand how ecosystems worked in the  past so we can better understand present ecosystems and predict how they will behave in the future.

The lack of exact analogues in the past is obvious: there were no electric fences in the Silurian; no bee-killing neonicotinoids in the Jurassic. Welcome to a world without analogue where roses eat goats; tigers lettuce; and swarms of polar bears swing through tree-tops hunting root-sucking parrots.

Perhaps not.

No matter how much we trash our beautiful planet, predators will eat their prey, species will compete for resources, and pathogens will do their worst. A future remake of “Life on Earth” will feature a diminished cast, with a simpler and more repetitive script, but the themes will remain the same. Plus ça change, plus c’est la même chose.

Analogues from the past cannot be used uncritically. Nobody knows that better than palaeoecologists. For example, estimates of post-glacial tree migration rates cannot directly predict migrate rates under future warming because species now have to move through a fragmented landscapes. But they do provide some information.

Does the Palaeocene-Eocene thermal maximum tell us nothing about the impacts of huge releases of carbon into the atmosphere and their impacts through ocean acidication on marine biodiversity because of micro-plastics?

Can information from Amazonian forests inform research on Ugandan forest butterflies?

This is a general problem in ecology. We want to apply information learnt from one study system to another, perhaps remote, it time or space. Analogues are never perfect, it is our job as ecologists to find the general patterns amidst the peculiarities of each case. Just as it is the job of lawyers to distinguish the ratio decidendi from the obiter dictum in an 19th century legal case involving stream trains and telegraphs and apply it to a modern case.

Uniformitarianism is not dead – just as useful as it was yesterday, and the day before that.

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