At the bottom are the plants that use sunlight to photosynthesize food resources. These are consumed by organisms that do not carry out photosynthesis. Along the way up the chain or through the web, there is combat for survival. An organism plays many roles. It may eat some, for example, while others are trying to eat it.
Predation is the name for one organism browsing to feed on another.
In continuous culture it is easy to see what happens in predation. The first analysis was by Lotka and Volterra who worked independently of each other. These same collisions that kill V result in growth of P. The K's are different rate constants. We neglect death by old age for both V and P because we can choose a high dilution rate where most will wash out before growing old.