Although it is difficult to estimate the consequences of a reduction in the number of species, t is evident that this decrease results in a continuous decline in the functionality of ecosystems (Lawton J.H. & Brown U.K., 1994 in Dallai R., 2003). Effectively, the various members of an ecosystem interact both among themselves and with the abiotic environment. The autotrophic producers: green plants on earth and bacteria and protists under water, and the heterotrophic consumers and decomposers are strongly associated and interdependent. All the interventions which alter the activities of one of these components and which exceed the limit of tolerance of the system affect negatively the functioning of the ecosystem as a whole. Without the primary producers there would be no storage of chemical energy in the form of biomass. Without the various trophic levels of decomposing organisms, the world would be submerged in humus, dead animals and organic waste. Failure to maintain the level of consumers would result in a lack of control over the number of low-rank consumers. There is thus a continuous stream of matter and energy which flows through the ecosystem, each of its components depending on controlling at the same time the other components. Thus we understand how the elimination of a single link of this chain can affect the final outcome of the transformation.

In nature the possible modifications of the environment affect the organisms comprising the ecosystem, and this is even more the case if their links are inflexible and specialized. If, however, there is sufficient variability between the organisms and if the links are flexible enough, then the ecosystem evolves, by originating a new combination more adapted to the new environmental conditions. A high biodiversity assures the ability of the ecosystems to adapt themselves to changeable conditions, by determining both the evolution and duration of the ecological equilibrium on which human beings depend.