Question

A disease resulting in the deaths of one third of a dense population of bats in a cave would be a

Answer 1

I think it would be white nose syndrome.. because up to 6.7 million bats have died from white nose syndrome since 2006. And it typically will kill from 70 to 90 percent of the bats in a cave.

Answer 2

A disease resulting in the deaths of one third of a dense population of bats in a cave would be a density-dependent limiting factor.

The density dependent factor are those factors whose affects on the growth or the size of the population can vary with the population density. Different sorts of density dependent limiting factors exist, for example: disease, migration, predation, availability of food. The last one mention is the main factor  that could result in death, for instance in the death of a population of bats.