Bee

Bees were a sapient species of insect from Melissa Majoria. Earth also had many of its own species of (non-sapient) bees, as well as a migrant population from Melissa Majoria. (DW: The Stolen Earth) The Doctor is rather fond of Bees. (DW: The Robots of Death)

Biology
It is aerodynamicaly imposible for Bumblebees to fly yet they do it. (DW: The Robots of Death)

History
By 2009, bees were disappearing on Earth, which Donna Noble noticed and mentioned to the Doctor on two separate occasions. He agreed that this was odd. (DW: Partners in Crime, Planet of the Ood)

Bees were also disappearing in the alternate reality where Donna had turned right instead of left. (DW: Turn Left)

When Earth was stolen by the New Dalek Empire, it became clear that this transport had for some time created a disturbance in the Tandoka Scale which the migrant bees used as carrier signals. The migrant Bees had sensed danger and had gone home. (DW: The Stolen Earth)

Behind the Scenes
This fall in bee populations is mirrored in real life, and possibly inspired by,Colony Collapse Disorder, for which the driving forces are still largely unknown.