In North America, more than half the large mammals became extinct at the end of the Pleistocene era. Some theories posit human predation as the cause, however there is significant evidence that climate change and attendant effects on vegetation and migration patterns also had a major impact:
http://en.wikipedia.org/wiki/New_World_Pleistocene_ExtinctionsClimate change hypotheses
At the end of the 19th and beginning of the 20th centuries, when scientists first realized that there had been glacial and interglacial ages, and that they were somehow associated with the prevalence or disappearance of certain animals, they surmised that the termination of the Pleistocene ice age might be an explanation for the extinctions.
Increased temperature
The most obvious change associated with the termination of an ice age is the increase in temperature. Between 15,000 BP and 10,000 BP, a 6°C increase in global mean annual temperatures occurred. This was generally thought to be the cause of the extinctions.
According to this hypothesis, a temperature increase sufficient to melt the Wisconsin ice sheet could have placed enough thermal stress on cold-adapted mammals to cause them to die. Their heavy fur, which helps conserve body heat in the glacial cold, might have prevented the dumping of excess heat, causing the mammals to die of heat exhaustion. Large mammals, with their reduced surface area-to-volume ratio, would have fared worse than small mammals.
Shortcomings of the Temperature Hypothesis
More recent research has demonstrated that the annual mean temperature of the current interglacial that we have seen for the last 10,000 years is no higher than that of previous interglacials, so the same large mammals survived similar temperature increases. Therefore warmer temperature alone is not a sufficient explanation.<11><12><13><14><15><16>
In addition, numerous species such as mammoths survived in human-free refugia such as Wrangel Island<17> despite changes in climate. This is precisely the opposite of what would be expected if climate change were responsible. Under normal ecological assumptions island populations should be more vulnerable to extinction due to climate change because of small populations and an inability to migrate to more favorable climes.
Increased continentality affects vegetation in time or space
Other scientists have proposed that increasingly extreme weather — hotter summers and colder winters — referred to as "continentality", or related changes in rainfall caused the extinctions. The various hypotheses are outlined below.
Vegetation changes: geographic
It has been shown that vegetation changed from mixed woodland-parkland to separate prairie and woodland.<13><14><16> This may have affected the kinds of food available. If so, herbivores might not have found the plants with which they had evolved and thus would have fallen prey to the anti-herbivory toxins in the plants that remained available. Shorter growing seasons may have caused the extinction of large herbivores and the dwarfing of many others. In this case, as observed, bison and other large ruminants would have fared better than horses, elephants and other monogastrics, because ruminants are able to extract more nutrition from limited quantities of high-fiber food and better able to deal with anti-herbivory toxins.<18><19><20> So, in general, when vegetation becomes more specialized, herbivores with less diet flexibility may be less able to find the mix of vegetation they need to sustain life and reproduce within a given area.
Rainfall changes: time
Increased continentality resulted in reduced and less predictable rainfall limiting the availability of plants necessary for energy and nutrition.<21><22><23> Axelrod<24> and Slaughter<25> have suggested that this change in rainfall restricted the amount of time favorable for reproduction. This could disproportionately harm large animals, since they have longer, more inflexible mating periods, and so may have produced young at unfavorable seasons (i.e., when sufficient food, water, or shelter was unavailable because of shifts in the growing season. In contrast, small mammals, with their shorter life cycles, shorter reproductive cycles, and shorter gestation periods, could have adjusted to the increased unpredictability of the climate, both as individuals and as species which allowed them to synchronize their reproductive efforts with conditions favorable for offspring survival. If so, smaller mammals would have lost fewer offspring and would have been better able to repeat the reproductive effort when circumstances once more favored offspring survival.<26>
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