A Plant's Guide to Surviving the Chicxulub Impact

What comes to your mind when you hear the word ‘‘extinction?’’ Perhaps you imagine a critically endangered Giant Panda gnawing on bamboo or a tiger crouching in the brush amidst the seemly endless degradation of their habitats. Or perhaps you imagine herds of dinosaurs and other creatures that we only know from fossils that are millions of years old. Odds are pretty good that an image of a plant wasn’t the first (or the second, third, or fourth) thing to pop into your head. And yet, there are about as many plant species as animals species that are considered to be critically endangered by extinction (and there would probably be many more if the data were available). And what about the Chicxulub impact, the massive asteroid that collided with the Earth about 66 million years ago and almost certainly instigated the mass extinction that drove the non-avian dinosaurs and about 75% of the species on the planet extinct? Well, it turns out that this impact, at the boundary between the Cretaceous (K) and Paleogene (Pg) periods, has been implicated in the extinction of approximately half of all plant species that existed in North America at the time. There are two roads to extinction. First, a species can go extinct simply because of bad ‘‘luck.’’ Perhaps a species experiences a few years of poor reproduction and is then subject to a freak storm that causes high mortality of the few individuals left. As an analogy, think of gambling at the slot machines, which involves no skill or predictability in winning or losing. You walk to a machine with a handful of coins and, more often than not, you walk away a few minutes later empty-handed. The second road to extinction happens when a species’ traits are poorly matched to its environment and that species’ ability to replace itself through reproduction is outpaced by its mortality rate. A sudden shift in climate, for example, can create a hostile environment for species that were once well suited to a particular location, and they must either migrate towards more favorable conditions or face extinction. To go back to the gambling analogy, a poker player whose skills are poorly matched to others at the table will rapidly go broke despite the random likelihood of being dealt a good hand on occasion. So, what happened after the Chicxulub impact on what is now the Yucatán Peninsula 66 million years ago? Were the few winners that survived the mass extinction event just lucky? Or did they have traits that somehow made them better suited to survive the decades-long impact winter that followed? And if the latter, which traits were favored or disfavored? Ecologists studying contemporary extinctions have been asking these sorts of questions for decades and have devised a number of statistical tools to disentangle the two roads towards extinction. However, similar approaches have not been so forthcoming in paleoecology, in which data are often incomplete. In this issue of PLOS Biology, Blonder and colleagues overcome this barrier by combining modern ecological approaches with data from a diverse and exceptionally well-preserved set of fossilized plant leaves from North Dakota (United States) that spanned a 2.2 million-year period bracketing the K–Pg mass extinction event (Figure 1). The primary goal of Blonder and colleagues’ work was to determine whether plant species that went extinct during the K–Pg mass extinction event were