“Requisite Variety” describes the need for a system to have as many counter-moves as it will have threats from environmental “perturbations” in order to be able to cope with them. A system needs to have at least as much variety as its environment, or else the environment has the potential to get “one-up” on the system and pose a threat or challenge to which it cannot effectively respond. If it lacks this requisite variety it may fail to respond to perturbations or feedback appropriately and enter a positive feedback “death spiral”.
Variety and diversity of environmental perturbations are like exercise for a system; they strengthen it and stimulate its health, rather than distract or disrupt it. Provided, of course, the system has adequate intrinsic requisite variety to cope with the perturbation.
A “one trick pony” Type 1 System such as a ping-pong ball in a big bowl has no variety in the responses it can make. Each time the ping-pong ball is lifted to the top of the bowl and subjected to gravity, every time it bounces against the bottom or side of the bowl it can only give up some energy and bounce a little lower. Without the requisite variety in its responses, it can only end up static at the bottom of the bowl and is, therefore, a severely limited system.
At the other extreme, a conscious living being has a large range of responses to ensure its survival in the face to the huge diversity of challenges and threats it faces.
Requisite Variety is the sine qua non basis of autopoiesis and self-organization.
In biotic systems, Requisite Variety is largely defined as Bio-Diversity. Therefore, a reduction in Bio-Diversity constitutes a reduction in Requisite Variety. If it is reduced so much that the “requisite” or necessary variety is not present, then the qualities of self-organization which impart self-recovery and restorative capacity are impaired or lost.
In the case of a Waste Water Treatment works this results in a loss of effective capacity and performance.
In the case of a dam, lake, reservoir or lagoon this manifests as eutrophication with a hyper-dominance of algae and toxic cyanobacteria at the expensive of bio-diversity.
Understanding this fact alone makes it obvious why dosing huge numbers of a few selected alien strains of so-called “effective” bacteria into such compromised situations is the exact opposite of what is required and will do nothing to improve the situation.