Entropy.
It's all about the consumption of limited resources. Entropy occurs because it is the nature of all energy in the universe. Plants compete for minerals, water, and light. The most immense source of energy. The Sun. Then, herbivorous consumers must ingest enough plant material to supply energy to fuel the act of foraging, fuel tissue regeneration, fuel growth, and fuel reproduction. Energy is lost to heat, motion, and more. So, these consumers must consume even more producers. If the population becomes to dense, relative to the amount of resources available, famine occurs.
Replace the producers with the consumers and the consumers with secondary consumers (predators) in the level to understand predators.
Really, though, it all boils down to entropy.