The make-up of the survivor populations might consist of groups that are more susceptible to attack by invaders. After the extinction, with much-reduced species and population numbers, the invading groups could get a foothold. It took North America several million years to come to the same state of stable ecology as the other world sites did in a much shorter time. Before this current work Dave had examined the background extinctions of mollusks before and after extinction events. Dave notes that mass extinctions only account for a small fraction of all the extinctions that have taken place over time.
Most extinctions take place during the quiet, uneventful times , i.e. background extinctions.
This says that even though mass extinctions account only for a small fraction of all the known extinctions, they provide manifestly different results from normal background extinctions. Mass extinction events act as catalysts for more radical changes. Thus, mass extinctions play a very important evolutionary role. Mass extinctions mediate the major evolutionary changes. The Cretaceous-Tertiary is most interesting to Dave because this has been studied globally and it is thought that extinction happened in lock step around the world.