What must a healthy ecosystem contain in order to function properly

A healthy ecosystem must contain:

Plants. A plant has nutrients and energy that make it possible for herbivores to eat.

Primary consumers. These consumers will feed off of the plants to maintain a stable environment. They will also provide food and energy to secondary consumers

Secondary consumers. They will maintain the the population of the primary consumers. Once they die, they provide food and habitat for decomposes and scavengers

Habitat. The ecosystem must contain shelter for the creatures in the forest.

Stable temperatures. This is for the creatures who require specific amounts of heat and cold to survive. For example snakes would thrive in a swamp, but would die in the mountains.

There are probably more attributes, but here's a healthy list :) Hope it helps!