Final answer:
A terrarium desert ecosystem is a model representing a simplified version of the actual ecosystem, useful for research and educational purposes, though it does not capture the full complexity of the natural environment.
Step-by-step explanation:
When you build a small desert ecosystem in a terrarium, you are constructing a model of an ecosystem. A model is a representation or simulation of a system with similar structures, processes, and interactions that occur in the natural world. By selecting organisms that interact with each other and the environment, such as a cactus, a lizard, and various insects, you can simulate the desert ecosystem in a controlled manner.
The terrarium would act as a microcosm, providing us with insights on ecological interactions and processes on a smaller scale. This can be useful for educational purposes, as a research tool to study interactions within this desert community, or to monitor the effects of environmental changes on the ecosystem. However, it's important to note that this microcosm won't perfectly represent the complex dynamics of an actual desert habitat, as it lacks the scale, diversity, and uncontrollable variables found in the wild.