The past is a foreign country: they do things differently there.
L. P. Hartley
Wooly mammoths. Saber-toothed cats. Giant sloths. You don’t need to think very long before some differences between modern ecology and the ecology of the past become apparent. Altered nutrient cycles. Introduced species. Altered soundscapes.
One of the important rationales for studying palaeoecology is that knowledge of past ecosystems can help understand modern ecosystems and make predictions about future ecosystems on our warming planet. This skill might be limited if, as Lyons et al suggested in Nature last week, the rules that govern how plant and animal communities are structured have been altered by the pervasive influence of humans on Earth’s ecosystems.
Lyons et al compile many datasets of species presence and absence for modern and fossil data over the last 300 million years and test whether pairs of species tend to aggregate together, segregate or co-occur at random.
In the modern datasets, most species pairs randomly co-occur. Species with non-random patterns tend to segregate. Lyons et al find the converse pattern in fossil data: pairs of species tend to aggregate. The switch from aggregation to segregation occurred about 6000 years ago, coincident with the expansion of agriculture.
It would be interesting to consider how the relatively limited impact of mid-Holocene agriculture could have such profound consequences. Could the processes, for example habitation fragmentation, suggested by Lyons et al change the rules of species assembly.
We could test the results of Lyons et al with alternative data, for example the European pollen database which is analogous to the North American pollen database used by Lyons et al.
But first we need, as Lyons et al do, to consider the possibility that the results are an artefact of some property of the data. Lyons et al first show that the result is not dependent on the modern data: a decline in aggregated species pairs is seen even if only the fossil data are analysed.
The Holocene decline in the proportion of aggregated species pairs is driven by a small number of 20th century “fossil” data sets. Quote marks on “fossil” as delving into the extensive supplementary material shows that only one of the 2oth century data sets actually comprises fossils. Regardless, I’m not convinced this decline is particularly robust.
Lyons et al test whether there is a relationship between the proportion of aggregated pairs and the spatial or temporal grain and extent of the datasets. They find no significant patterns, but the relationship with spatial extent is suggestive (p = 0.066). Lyons et al only test for these patterns in the fossil data sets, so the modern-fossil contrast is missing. The obvious thing to do is to add the modern data to this analysis, but that would take a lot of digging.
Lyons et al is interesting and provocative, but I am inclined to think that the result is probably an artefact. This paper will be the subject of our next palaeoecology journal club meeting; I’ll update this post afterwards.