We take up, another parameter of ecosystem balance. One factor that affects the stability or persistence of some ecosystems under small or moderate environmental stress is species diversity - the number of species and their relative abundance in a given ecosystem. High species diversity tends to increase long-term persistence of the ecosystem.
It is because with so many different species and the linkages between them, risk is spread more widely. An ecosystem having a good variety of species has more ways available to respond to most environmental stresses. For example, the loss or drastic reduction of one species in an ecosystem, with complex food web usually does not threaten the existence of others, because most consumers have several alternative food supplies. In contrast, the highly specialised agricultural ecosystem, planted with only one type of crop such as wheat or rice is highly vulnerable to destruction from a single plant disease or insects. Therefore, the essence of the whole discussion is that most balanced ecosystems contain many different species.