Mass Effect 2:
The krogan homeworld boasts extreme temperatures, virulent diseases, and vicious, predatory fauna. Around 1900 BCE, the krogan discovered atomic power and promptly instigated many intraplanetary wars, sending Tuchanka into a nuclear winter. With most of their industrial base destroyed, the krogan entered a new dark age and warring tribal bands dominated. Populations remained low for the next 2,000 years.
First contact with the salarians made resurgence possible. Krogan brought to less hostile planets bred exponentially and returned to reconquer their home. They built vast underground shelters to shield themselves from surface radiation, which proved prescient during the Krogan Rebellions when many of them isolated themselves in a vain attempt to avoid the genophage. Convinced they could outbreed the genophage, they transmitted it into more than 90 percent of the sealed bunkers. Today, Tuchanka's population is sharply limited and while individual krogan are long-lived, the genophage ensures few replacements.
Mass Effect 3:
Tuchanka, the krogan homeworld, boasts extreme temperatures, virulent diseases, and predatory fauna. Around 1900 BCE, the krogan discovered atomic weapons and promptly sent their planet into a nuclear winter. The majority of the population retreated to underground bunkers, and krogan culture slipped into a dark age dominated by tribal clans.
In 80 CE, decades into the Rachni Wars, the Salarian Union made first contact with the primitive krogan and initiated a "cultural uplift" to shape them into a modern army capable of confronting the rachni. During this uplift, the salarians constructed the Shroud facility on Tuchanka to shield the planet from harmful forms of solar radiation. Later, during the Krogan Rebellions, reproductive rates were curtailed by the genophage, ensuring the krogan remained a species in decline--and Tuchanka a desolate wasteland.