The overall population of a species decreased. What could have occurred to make this happen?
Disease, Over-predation, Loss of habitat, loss of food source and Inbreeding.
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.
This next few link nicely in with the rabbits in Spain/Portugal, over predation can cause a species numbers to reduce, that being from Iberian Lynx in Portugal/Spain, which go for the healthier rabbits as with most cats, they prefer freshly killed, rather than carrion. The healthier rabbits would have most likely been more able to survive the disease, and develop immunity, however, the loss of them have caused the rabbit population to fall further.
This over predation can also be seen in New Zealand, where introduced species, like Opossum, Rats, Cats, Pigs, eat the many of the unadapted creatures there, such as the Kiwi, whom's flightless-ness has been it's demise.
Loss of habitat in Spain/Portugal, to agriculture, urbanization and just natural causes (such as forest fires) have caused rabbit populations, as well as lynx populations, to plummet as they find themselves more exposed to the elements, and in direct conflict with humans, though, it is more of the latter species than the other for this point.
The resultant loss of rabbits in Spain/Portugal, have also caused Lynx populations to crash also, there now being less than 500 Iberian lynxes left in the wild, this loss of food isn't new, many species are at threat, including those in the sea, where the coral is dying and can't rejuvenate fast enough, which causes a loss of food source for most of the species on the reef.
This loss of others of own species, can cause populations to decrease and become more susceptible to these reasons. Inbreeding is a common threat to most dwindling species, most notably in those of top predators, such as lynxes and tigers, this is why many are being captive bred and released, in hope they can increase the gene pool and variation in a species.