Guest blogger Patrick Forber writes...
Peering back into the deep past is hard work—really, really hard work. Any sort of systematic scientific inquiry is hard, but reconstructing the history of geological, evolutionary, and paleontological change presents particularly acute challenges. Complex processes unfold at varying speeds across space and time. Mountains erode, rivers carve out canyons, species evolve, diversify, and go extinct. Traces of these changes are so frequently obliterated and those that remain are elusive, hard to track down and even harder to interpret. Yet even so, our reach into the past extends, and the resolution of our historical explanations increases. Here I want to approach the idea of extinction in order to take a closer look at one strand of inquiry in paleoecology that seeks to understand how changes in climate affect extinction patterns.
Extinction has a peculiar pull on our imaginations, perhaps due to its finality. The Darwinian struggle for existences continually claims casualties over the history of life. From the mass extinctions that restructure life on earth to the quiet demise of an isolated lineage in one corner of the globe, species die out and disappear. Yet many of us learn about extinction from the most spectacular of them all: the Cretaceous-Paleogene (K-Pg) extinction that claimed the dinosaurs and set the stage for the mammalian radiation (yet, as mass extinctions go, the Permian “great dying” deserves the top honor). While dinosaurs exert a special influence on popular culture, if the success of Jurassic World is any measure, they may, as Lukas Rieppel argues in his forthcoming book, be inextricably bound up in the economic history of the United States.
Culture and economics aside, attempts to reconstruct the extinction event itself sparked an excellent episode of scientific controversy. The proposal of the sensational hypothesis that an asteroid impact caused the K-Pg extinction set off a debate that raises a host of philosophical concerns about the nature of evidence and how historical reconstruction should proceed. The discovery of an impact crater and other telltale impact traces seemed to decide the issue, but the dust has yet to settle: innovative research published just this year suggests that the impact may have led to worldwide conflagrations, and it was the soot created by these, rather than the ejecta from the impact, that set off rapid global climate change which in turn caused the mass extinction. And so science moves forward—reevaluating our claims about the past in light of new techniques and evidence.
Whether by impact, fire, or volcano, climate change doomed the dinosaurs. This was extinction on a grand scale. What about more fine-grained climatic changes? Do they cause extinctions as well? How do these climate changes affect biotic interactions in ecological communities—the interplay of predation, symbiosis, and so on—in ways that lead some species to go extinct (either globally or locally) and others to flourish?
One way to get a handle on the effects of climate change on ecology is to look at co-occurrence in the fossil record. Co-occurrence can potentially act as a proxy for inferring biotic interactions in the deep past. It’s not perfect, for mere co-occurrence does not entail that two species are interacting. The ideal case provides supporting evidence to reconstruct the past food web, and when combined with other forms of evidence can show that two species interacted rather than just shared the neighborhood. One pattern that emerges is that climate changes tend to spell trouble for ecological specialists. Generalists tend to take over after extinction events and this community structure can persist for hundreds of thousands of years. While this is a rather coarse-grained result, there are ways to refine the research by developing strategies to reconstruct more precise patterns of co-occurrence, and building more sophisticated models to integrate the data.
Why dive into the complexities of paleoecological reconstruction? By looking back we can provide evidence for models that can help us move forward. For example, we can use these models to predict how climatic change may affect biological communities in the future—what is the nature and extent of extinction, and how do different processes biotic interactions amplify or ameliorate extinction? Regularities, principles we sometimes call laws of nature, extend both into the past and the future. As Neil Young put it, “rust never sleeps.” Forecasting is about what’s to come. We need models, regularities, and laws to forecast in effective ways. One way we can test our models is by making retrospective predictions about the past, then checking to see if the model’s predictions have been borne out by the available paleoecological data. To be clear, this strategy does not amount to mere accommodation when we would prefer novel prediction. Instead, it provides a way to synthesize various strands of support into a more general model that can account for the forces that shape and change ecological communities. If the fossil data provide suitable support, we can use the model to forecast ecological changes that should accompany future climate change. If not, then we have some evidence that we are in uncharted waters. In a time of profound and accelerating global climate change, this is something that would be good to know moving forward.
Acknowledgement
Thanks very much to Jessica Blois for taking the time to share her research with me. For more about the Blois lab: https://jessicablois.com