Why are some animals not present at each stage?

Some animals are not present at each stage of an ecosystem or food chain due to factors such as habitat requirements, competition, and evolutionary adaptations. Certain species may thrive in specific environments or conditions that are not available at every stage. Additionally, changes in climate, availability of food sources, or human impact can lead to the absence of certain animals. Evolutionary pressures can also result in the specialization of some species, making them suited for only particular niches.