The overall population of a species decreased. what could have occurred to make this happen?

Answer- Disease, Over-predation, Loss of habitat, loss of food source and Inbreeding.

Explanation- Disease is usually the first most common reason a species's population decreases. This is usually due to the species having a low immunity to the disease, and usually something new has appeared or occurred in the habitat which has brought the disease to come in contact with the species in question, such as the rabbits in Portugal/Spain, which have been hit by disease recently, due to the fact they've got low immunity to it.