A number of authors who have investigated Late Quaternary extinctions of North America and elsewhere doubt that any of the single causes that have been identified to date are uniquely responsible. Instead, they argue that the losses probably occurred through the interaction of a combination of factors, the nature of which might have varied from place to place. For example, although many authors have noted that overkill may have seriously reduced the population size of several species of ungulates that Paleo-Indians may have hunted in large numbers, it is extremely unlikely that humans actually hunted out every last individual. Whatever the overarching cause of such losses might have been, the coup de grâce was likely to have been fairly ordinary. Population numbers may have been brought so low that recovery through natural increase (birth rate greater than death rate) became impossible. Or, in some cases, human depredations may have interfered with migration routes or mating seasons to such an extent that, once again, population size was brought below viability.
Such arguments can be backed up by modern experience with several endangered species that seem to be sliding toward ever-lower numbers and eventual oblivion no matter what the rescue efforts that are being made on their behalf.