But if Bob were eating Alice, and Alice were eating Bob, then reproduction wouldn’t be an issue. STD’s, maybe, but not reproduction.
ITR champion, once a population exceeds the environment’s carrying capacity, or the population density that the environment can sustain while providing adequate resources to all inhabitants without deteriorating the environment itself, all of the population is in danger of dying off due to insufficient resources. As a VERY simplified example, if each deer needs 10,000 blades of grass to survive and the field has a total of 100,000 blades of grass, once the deer population goes above 10 deer, all of the deer will be getting fewer blades of grass than they need to survive, and ALL of the deer will starve to death unless there are inequalities that allow some deer to eat more grass than others, in which case some of the deer will survive and the others will starve very quickly. The deer may also overgraze the field to the extent that the grass can no longer reseed itself quickly enough to grow back each year, leading to a barren field and no living deer.
The latter situation is fairly common on farms with pastured livestock who do not adapt the herd size to the quality and quantity of pasture available. Horses in particular are fairly damaging to landscapes because they only eat the grass, leaving the weeds to flourish. If you have too many horses in one grass paddock, there will soon be nothing left but weeds and dirt, and you must move the horses to another field and/or keep them in their stalls and feed them hay to allow the pasture to recover, or you will never regain that grazing land.