I will focus on the example of an ecosystem with fish, kelp and sharks. This is a typical predator - prey ecosystem. The agents are kelp, fish, and sharks. The environment is the waterbody. The interactions are that the fish and shark move randomly through the environment, while the kelp is stationary. The fish eat kelp, and the sharks eat fish. Fish and sharks gain energy by eating, and lose energy by moving around. They can reproduce if they have enough energy, and they die if their energy goes to 0 or if they are eaten.
I chose this example because it ties in with something in the news in the past few years - the invasive species of Asian Carp in the Great Lakes. This could be modeled by having Asian Carp as an additional agent in the model. Their danger to the ecosystem is that Asian Carp eat a lot of plankton (this could be modeled as the kelp) and thereby deprivie native fish of their food source. The eating and reproductive traits of Asian Carp could be modeled and studied with our complex adaptive system models.