Species use restricted ecological niches, and the niches of all species are segregated, often with much overlap, by the use of different habitats, different geographic areas and seasons, and different food resources, to mention only a few of the many niche dimensions. The causes of niche restriction and segregation are important problems in evolutionary ecology. Background Two ecological paradigms deal with the problem. The first paradigm predominates in what may be called “classical” ecology. It assumes that niche space is largely saturated with individuals and species, leading to strong competition. Niches are restricted because “neighbouring” species, i.e., species with similar ecological characteristics such as similar habitats or food preferences, prevent expansion into other niches or even narrow niches down. This continual struggle for existence is an important assumption of natural selection introduced by Darwin as an explanation for evolution. The other paradigm assumes that niche space is to a large degree vacant, i.e., that there are many vacant niches. It is based on many empirical studies and theoretical investigations especially of Kauffman 1993. Causes of vacant niches may be evolutionary contingencies or brief or long-lasting environmental disturbances. Both paradigms agree that species are never “universal” in the sense that they occupy all possible niches; they are always specialized, although the degree of specialization varies. For example, there is no universal parasite which infects all host species and microhabitats within or on them. However, the degree of host specificity varies strongly. Thus, Toxoplasma (Protista) infects numerous vertebrates including humans, Enterobius vermicularis infects only humans. The following mechanisms for niche restriction and segregation have been proposed: Niche restriction Species must be specialized in order to survive. They may survive for a while in less optimal habitats under favourable conditions, but they will be extinguished when conditions become less favourable, for example due to changed weather conditions (this aspect was especially emphasized by Price 1983). Niches remain narrow or become narrower as the result of natural selection in order to enhance the chances of mating. This “mating theory of niche restriction” is supported by the observation that niches of asexual stages are often wider than those of sexually mature stages; that niches become narrower at the time of mating; and that microhabitats of sessile species and of species with small population sizes often are narrower than those of non-sessile species and of species with large population sizes. Niche segregation The random selection of