What causes competition in an eco system?

Increasing members in the higher trophic level causes competition in an ecosystem.

Explanation:

For example, if there is a shortage of producers, the primary consumers will face a higher competition for food. Thus, many will die. Death of primary consumers will increase the competition in secondary consumers as they are left with less food options. This will go on till the tertiary consumer, and the competition will continue until each trophic level is balanced.