The influence of parasites in ecosystems, especially on biodiversity, is discussed. Various examples illustrate the role that parasites play in the outcome of interspecific competition, in the success of invading species, and in the separation of emerging species. Parasites can be stabilizers or destabilizers, depending on factors such as susceptibility of hosts and size of the ecosystem. Parasites play a major role each time 'something' disturbs living beings at the populational and/or specific level, as they do at the individual level. Keywords: biodiversity, hybrid zones, outcome of competition, parasites, stability.