, these four factors are collectively known as 'The Evil Quartet' and are the primary causes of biodiversity loss . Habitat loss and fragmentation is considered the most important factor driving extinction . Over-exploitation occurs when human need turns to greed, leading to the exhaustion of resources . Alien species invasions involve non-native species becoming invasive and causing the decline of indigenous ones , and co-extinctions occur when the extinction of one species leads to the loss of its obligatory associated species .