Refuge From Physical Stress
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.
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, USA, salt marsh tidal gradient, a presence of black needle rush (Juncus gerardi) increased the fitness of marsh elder (Iva) 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.
Read more about this topic: Ecological Facilitation, Mechanisms
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