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. This article addresses both the mechanisms of facilitation and the increasing information available concerning the impacts of facilitation on community ecology.
Categories
There are two basic categories of facilitative interactions:- Mutualism is an interaction between species that is beneficial to both. A familiar example of a mutualism is the relationship between flowering plants and their pollinators. The plant benefits from the spread of pollen between flowers, while the pollinator receives some form of nourishment, either from nectar or the pollen itself.
- Commensalism is an interaction in which one species benefits and the other species is unaffected. Epiphytes have a commensal relationship with their host plant because the epiphyte benefits in some way while the host plant is apparently unaffected.
Mechanisms
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.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, 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. 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 increased the fitness of marsh elder 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.
Refuge from predation
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 along the North Carolina, US, coastline. This crab species takes refuge in the branches of the compact ivory bush coral 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 and certain ants in Central America. The acacia provides nourishment and protection 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.Refuge from competition
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 on kelps can protect mussels from overgrowth by kelps competing for space in the subtidal zone of the Gulf of Maine, US.In most cases, facilitation and competition are inversely proportional.
Studies suggest that facilitation events in nature are rare compared to competition events and thus, competition is a greater driver for ecological processes.
Improved resource availability
Facilitation can increase access to limiting resources such as light, water, and nutrients for interacting species. For example, epiphytic plants often receive more direct sunlight in the canopies of their host plants than they would on the ground. Also, nurse plants increase the amount of water available to seedlings in dry habitats because of reduced evapotranspiration beneath the shade of nurse plant canopies. A special case concerns human facilitation of sap-feeding birds. Three African bird species regularly feed on the sap flowing from holes made by local wine tappers in oil‐palm trees Elaies guineensis in the Bijagós archipelago, Guinea‐Bissau.However, the most familiar examples of increased access to resources through facilitation are the mutualistic transfers of nutrients between symbiotic organisms. A symbiosis is a prolonged, close association between organisms, and some examples of mutualistic symbioses include:
; Gut flora:Associations between a host species and a microbe living in the host's digestive tract, wherein the host provides habitat and nourishment to the microbe in exchange for digestive services. For example, termites receive nourishment from cellulose digested by microbes inhabiting their gut.
; Lichens:Associations between fungi and algae, wherein the fungus receives nutrients from the alga, and the alga is protected from harsh conditions causing desiccation.
; Corals:Associations between reef-building corals and photosynthetic algae called zooxanthellae, wherein the zooxanthellae provide nutrition to the corals in the form of photosynthate, in exchange for nitrogen in coral waste products.
; Mycorrhizae:Associations between fungi and plant roots, wherein the fungus facilitates nutrient uptake by the plant in exchange for carbon in the form of sugars from the plant root. There is a parallel example in marine environments of sponges on the roots of mangroves, with a relationship analogous to that of mycorrhizae and terrestrial plants.