What are the most common causes of extinction?

The plight of the saiga antelope shows that a combination of factors can drive an animal to the brink of extinction.

Extinctions crop up over the millennia with disturbing frequency; even mass extinction events pepper the history of the planet every 65 million years or so. But when it comes to the causes of these phenomena (whether it's a sea-level shift, an asteroid strike, a volcano eruption or a nearby supernova), scientists have a hard time settling on just one cause for one event.

Take the extinction of many species of megafauna near the onset of the Holocene (the geologic period that we still live in today). Scientists have different theories for why it happened. Some experts believe a wild climate shift caused radical habitat alterations. Others pin the problem on human intervention: Maybe human advancements led to overhunting and habitat destruction. Or perhaps the problem was that the bipedal interlopers (and any animals they carted around the world with them) unwittingly acted as pathogen vectors, carrying new diseases to animals without pre-existing immunities.

Often a combination of factors is suspected of triggering certain extinction events. Take the recent plummet in the world's population of saiga antelope. Native to different regions of the former Soviet Union and once numbering more than a million, this endearingly gawky looking little antelope species was poached to the brink of extinction after the dissolution of the crumbling empire in 1991. As of 2010, only about 81,000 remain. But at the start of summer 2010 -- and in the span of just two weeks -- that number dropped dramatically to 67,000 animals [source: Platt].

The party to blame that time wasn't poachers but a parasite that causes pasteurellosis. The infection's associated bacterium is just fine as gut flora in healthy antelopes, but for an animal with a suppressed immune system, it's a big problem. The antelope struck down by the infection were likely malnourished since the region they lived in had just gone through a colder-than-average winter and a warmer-than-average spring, seasonal upsets that likely affected food supplies.