Predators are those animals which hunt prey. Prey is the animals which is "hunted".
When the prey population is less (a decrease in prey population), There will not be more animals for "predators" to hunt and so, the competition of living will become more. Decrease in preys population would lead to a decrease in predator's population because many predators will die of hunger.
Another name for Circulatory system is Transport system. It brings a transport liquid into close contact with all cells in the body. In the case of Human, it is blood.