The few species that seem to disappear in the uppermost Maastrichtian may constitute background extinction
pattern, or they could also be interpreted as the remaining Signor-Lipps effect.
According to David Jablonski at the University of Chicago, adaptive traits that enhance survival and diversification of species during times of background extinction tend to have little in common with those traits that increase the chances of survival during mass extinctions.
Currently evolutionary theory is based almost exclusively on pattersn of background extinctions, Jablonski says; scientists have assumed that mass extinctions simply accelerate or emphasize trends of background extinctions so that the same kinds of organisms are wiped out by a mass extinction, only in much greater numbers.
The team purposefully used a lower estimate for the values to see if even the lowest difference between current and background extinction
rates affirmed an advancing mass extinction.
At current levels of destruction, only 5 percent of tropical forests will remain in protected areas within 50 years, says the report, causing the rate of species loss to reach three or four orders of magnitude higher than the natural background extinction
rate of about one species each year.
To show the effect of introducing background extinction
into the process, we summarize the results of one typical simulation study in figure 5.
"Both background extinctions
, which represent most extinctions in the history of life, and mass extinctions tend to be clumped into particular evolutionary lineages," Jablonski said.
So too does the author find Gould's accent on the caprice of mass extinctions "very plausible," along with its implied, attendant claim that such calamities are qualitatively distinct from run-of-the-mill background extinctions
. Recently, some have suggested that certain so-called mass events had actually resulted from routine climate change, competition, or merely localized bad luck.