Ecological facilitation or probiosis describes species interactions that benefit at least one of the participants and cause harm to neither. Facilitations can be categorized as mutualisms, in which both species benefit, or commensalisms, in which one species benefits and the other is unaffected. This article addresses both the mechanisms of facilitation and the increasing information available concerning the impacts of facilitation on community ecology.

There are two basic categories of facilitative interactions:

Strict categorization, however, is not possible for some complex species interactions. For example, seed germination and survival in harsh environments is often higher under so-called nurse plants than on open ground. A nurse plant is one with an established canopy, beneath which germination and survival are more likely due to increased shade, soil moisture, and nutrients. Thus, the relationship between seedlings and their nurse plants is commensal. However, as the seedlings grow into established plants, they are likely to compete with their former benefactors for resources.

The beneficial effects of species on one another are realized in various ways, including refuge from physical stress, predation, and competition, improved resource availability, and transport.

Facilitation may act by reducing the negative impacts of a stressful environment. As described above, nurse plants facilitate seed germination and survival by alleviating stressful environmental conditions. A similar interaction occurs between the red alga Chondrus crispus and the canopy-forming seaweed Fucus in intertidal sites of southern New England, US. The alga survives higher in the intertidal zone—where temperature and desiccation stresses are greater—only when the seaweed is present because the canopy of the seaweed offers protection from those stresses. The previous examples describe facilitation of individuals or of single species, but there are also instances of a single facilitator species mediating some community-wide stress, such as disturbance. An example of such "whole-community" facilitation is substrate stabilization of cobble beach plant communities in Rhode Island, US, by smooth cordgrass (Spartina alterniflora). Large beds of cordgrass buffer wave action, thus allowing the establishment and persistence of a community of less disturbance-tolerant annual and perennial plants below the high-water mark.

In general, facilitation is more likely to occur in physically stressful environments than in favorable environments, where competition may be the most important interaction among species. This can also occur in a single habitat containing a gradient from low to high stress. For example, along a New England, US, salt marsh tidal gradient, a presence of black needle rush (Juncus gerardii) increased the fitness of marsh elder (Iva annua) shrubs in lower elevations, where soil salinity was higher. The rush shaded the soil, which decreased evapotranspiration, and in turn decreased soil salinity. However, at higher elevations where soil salinity was lower, marsh elder fitness was decreased in the presence of the rush, due to increased competition for resources. Thus, the nature of species interactions may shift with environmental conditions. Facilitation has a greater effect on plant interactions under environmental stress than competition. Another example is the positive effects of facilitation on desert plants that face the effects of rising aridification. Shrubs are known to provide favourable abiotic conditions in these dry regions.

Another mechanism of facilitation is a reduced risk of being eaten. Nurse plants, for example, not only reduce abiotic stress, but may also physically impede herbivory of seedlings growing under them. In both terrestrial and marine environments, herbivory of palatable species is reduced when they occur with unpalatable species. These "associational refuges" may occur when unpalatable species physically shield the palatable species, or when herbivores are "confused" by the inhibitory cues of the unpalatable species. Herbivory can also reduce predation of the herbivore, as in the case of the red-ridged clinging crab (Mithrax forceps) along the North Carolina, US, coastline. This crab species takes refuge in the branches of the compact ivory bush coral (Oculina arbuscula) and feeds on seaweed in the vicinity of the coral. The reduced competition with seaweed enhances coral growth, which in turn provides more refuge for the crab. A similar case is that of the interaction between swollen-thorn acacia trees (Acacia spp.) and certain ants (Pseudomyrmex spp.) in Central America. The acacia provides nourishment and protection (inside hollow thorns) to the ant in return for defense against herbivores. In contrast, a different type of facilitation between ants and sap-feeding insects may increase plant predation. By consuming sap, plant pests such as aphids produce a sugar-rich waste product called honeydew, which is consumed by ants in exchange for protection of the sap-feeders against predation.

Another potential benefit of facilitation is insulation from competitive interactions. Like the now familiar example of nurse plants in harsh environments, nurse logs in a forest are sites of increased seed germination and seedling survival because the raised substrate of a log frees seedlings from competition with plants and mosses on the forest floor. The crab-coral interaction described above is also an example of refuge from competition, since the herbivory of crabs on seaweed reduces competition between coral and seaweed. Similarly, herbivory by sea urchins (Strongylocentrotus droebachiensis) on kelps (Laminaria spp.) can protect mussels (Modiolus modiolus) from overgrowth by kelps competing for space in the subtidal zone of the Gulf of Maine, US.