Ecological facilitation

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. Much of classic ecological theory (e.g., natural selection, niche separation, metapopulation dynamics) has focused on negative interactions such as predation and competition, but positive interactions (facilitation) are receiving increasing focus in ecological research. 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, USA. 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, USA, 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.

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