A terrarium is a miniature ecosystem which is a complex system of relationships between organisms and resources of different types. The predominant organisms in your terrarium ecosystem are plants and the micro-organisms in the soil. The major resources are water, sunlight, and soil. These all work together to form a miniature little system that can sustain itself for long periods of time. If you have a good balance of plants, soil, nutrients, and water in your terrarium you will be able to seal it up and watch it grow and thrive for very long periods of time - sometimes even years.