Each organism plays a role in its ecosystem--what it eats, where it lives, what organisms it hosts. This is called its niche.
Ask a number of students to describe their own niche. What are their eating habits? Are they aware that they are predators, even if in a remote sense? List facets of the niches on the chalkboard, and have other students add to the list for a human's niche . Include such factors as the amount of carbon dioxide each emits when driving vehicles and their interactions with other organisms, including people. If you have students who like to fish or hunt, they can be considered predators too.
This is a fun exercise that helps students more fully appreciate the concept of a niche and, hopefully, impresses upon them the fact that they are not separate from the ecosystem of which they are a part.
If you could ever choose just one film to show your students during a course in biology, choose Amazon: Land of the Flooded Forest by National Geographic Educational Videos. This film shows the phenomenal coevolutionary relationships that developed betwee n the river fishes and the trees, with some fish jumping out of the water to pick fruit, and the fruit and seed having to pass through the digestive tract of the fishes to enable germination. Fish reproduction is tied to seasonal flooding, when the fish a re able to literally swim among the treetops. The population dynamics of a number of fish species are discussed. The argument that biodiversity promotes ecosystem stability is clearly demonstrated. Students are mesmerized by this video--it is highly recom mended. And they get a glimpse of an ecosystem that is rapidly being destroyed (how and why are also discussed in the film).