There are different factors that may encourage substance abuse. Those factors can include stress of studies, trying to fit in because usually everyone at college parties do such kind of things, trying to get over something such as heartbreaks and even some substances help to become more hyper active and help with college errands.
However, there are also some reasons that might help a college student to stay away from such things. Those factors can include health reasons, parents pressure, college rules and even self-restrain
Most generally speaking, the arrows in the food webs/chains represent the flow of energy.
Most typically, the energy will flow from the Sun to the plants, and then those plants are eaten and when they are eaten the energy flows from the plants to whatever animal ate them.
You take responsibility in protecting yourself from substance abuse whether that means being distant from someone who abuses substances, or not giving into peer pressure, or even just avoiding it all together and telling yourself and others no.