All food chains begin with energy from the Sun.
A tree is a producer. It uses energy from the sun to grow acorns. They fall to the ground.
A squirrel is a consumer. It eats the acorns.
A hawk is a consumer and a predator. It hunts the squirrel.
Mushrooms are decomposers. When the hawk dies, the mushrooms break down the hawk's body and return the nutrients to the soil.