But it followed from an observation by geologist Walter Alvarez in the late 1970s that there was an unusual concentration of the rare element iridium in sediments marking the end of the Cretaceous Period. Twenty years later, paleontologists are highly skeptical, although the theory remains popular in the media. Fossil-mammal expert David Archibald is impatient with any claims that don't jibe with the fossil record. Frogs and salamanders, which should have been most vulnerable to acid rain, survived unscathed. The updated difference, Archibald says, is that modern conservation biology explains how populations decline when patches of habitat shrink and fragment.
Today there are many examples of how introduced species are seriously disrupting ecosystems.
You're releasing new pests, predators, competitors. Since the plant fossil record does show a sudden extinction, plausibly caused by a reduction in light, Archibald favors a scenario that retains at least a supporting role for an impact in finishing off the dinosaurs after ten million years of decline.