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A bioactive terrarium (or vivarium) is a terrarium for housing one or more terrestrial animals which also includes populations of small invertebrates and microorganisms to consume and break down the waste products of the primary species. This population of decomposers is colloquially referred to as the "cleanup crew" (abbr. "CUC"), and can include terrestrial isopods, springtails, earthworms, millipedes, and various beetles as determined by compatibility with the habitat of the primary species (e.g. tropical rainforest vs. desert). In a fully functional bioactive terrarium, all of the waste products will be broken down by the cleanup crew, eliminating the need for cage cleaning and preventing odors. While most bioactive terrariums are also naturalistic (with live plants, soil, lighting, and naturalistic decor), a terrarium may be bioactive without being naturalistic, and naturalistic without being bioactive).
Although any enclosure can be made bioactive by addition of a "cleanup crew" and appropriate substrate, such enclosures are almost always naturalistic terraria constructed of PVC, wood, glass and/or acrylic; bioactive enclosures in "rack" style caging is uncommon.
Bioactive enclosures require some form of particulate substrate to provide habitat for the cleanup crew. The choice of substrate is typically determined by the habitat of the primary species (e.g. jungle vs desert), and created by mixing a variety of components such as organic topsoil (free of pesticides and non-biological fertilizers), peat, coco fiber, sand, long-fiber sphagnum moss, cypress mulch, and orchid bark in varying proportions. In wet habitats, there is typically a layer of coarse rocks beneath the substrate to allow drainage, separated from the overlying substrate with a fine plastic mesh. Additionally, some bioactive terraria include leaflitter, which can serve as food and microhabitat for the cleanup crew.
Waste products of the primary species are consumed by a variety of smaller invertebrates, termed a "cleanup crew". These can include terrestrial isopods, springtails, earthworms, millipedes, and various beetles, with different species being preferred in different habitats - the cleanup crew for a tropical rainforest bioactive terrarium may rely primarily on springtails, isopods, and earthworms, while a desert habitat would use mostly beetles. If the primary species is insectivorous, they may consume insects in the cleanup crew, and thus the cleanup crew must have sufficient retreats to avoid being completely depopulated.
Additionally, bioactive terraria typically have a flourishing population of bacteria and other microorganisms which break down the wastes of the cleanup crew and primary species. Fungi may occur, and, if non-toxic, may provide food for the cleanup crew and primary species.