Ecological facilitation or probiosis describes species interactions that benefit at least one of the participants and cause harm to neither. [1] Facilitations can be categorized as mutualisms, in which both species benefit, or commensalisms, in which one species benefits and the other is unaffected. [1] [2] [3] [4] [5] 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. [1] [3] 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. [1] [3]
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. [1] [3] [4] [5] 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. [1] 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 ). [6] 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. [1] [3] [4] [5] 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. [7] 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. [8]
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. [3] In both terrestrial and marine environments, herbivory of palatable species is reduced when they occur with unpalatable species. [1] [3] [4] 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. [1] [3] 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. [9] 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. [2] 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. [2] 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. [10] 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. [9] 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. [11]
In most cases, facilitation and competition are inversely proportional. [12]
Studies suggest that facilitation events in nature are rare compared to competition events and thus, competition is a greater driver for ecological processes. [13]
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. [3] 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. [3] A special case concerns human facilitation of sap-feeding birds. Three African bird species (village weaver Ploceus cucullatus, common bulbul Pycnonotus barbatus, and mouse‐brown sunbird Anthreptes gabonicus) 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. [14]
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:
The movement by animals of items involved in plant reproduction is usually a mutualistic association. Pollinators may increase plant reproductive success by reducing pollen waste, increasing dispersal of pollen, and increasing the probability of sexual reproduction at low population density. [2] In return, the pollinator receives nourishment in the form of nectar or pollen. [2] Animals may also disperse the seed or fruit of plants, either by eating it (in which case they receive the benefit of nourishment) or by passive transport, such as seeds sticking to fur or feathers. [2] [4]
Although facilitation is often studied at the level of individual species interactions, the effects of facilitation are often observable at the scale of the community, including impacts to spatial structure, diversity, and invasibility.
Many facilitative interactions directly affect the distribution of species. As discussed above, transport of plant propagules by animal dispersers can increase colonization rates of more distant sites, which may impact the distribution and population dynamics of the plant species. [2] [4] [5] Facilitation most often affects distribution by simply making it possible for a species to occur in a site where some environmental stress would otherwise prohibit growth of that species. This is apparent in whole-community facilitation by a foundation species, such as sediment stabilization in cobble beach plant communities by smooth cordgrass. [6] A facilitating species may also help drive the progression from one ecosystem type to another, as mesquite apparently does in the grasslands of the Rio Grande Plains. [15] As a nitrogen-fixing tree, mesquite establishes more readily than other species on nutrient-poor soils, and following establishment, mesquite acts as a nurse plant for seedlings of other species. [3] Thus, mesquite facilitates the dynamic spatial shift from grassland to savanna to woodland across the habitat. [15]
Facilitation affects community diversity (defined in this context as the number of species in the community) by altering competitive interactions. For example, intertidal mussels increase total community species diversity by displacing competitive large sessile species such as seaweed and barnacles. [4] Although the mussels decrease diversity of primary space holders (i.e., large sessile species), a larger number of invertebrate species are associated with mussel beds than with other primary space holders, so total species diversity is higher when mussels are present. [4] The effect of facilitation on diversity could also be reversed, if the facilitation creates a competitive dominance that excludes more species than it permits. [1]
Facilitation, in certain cases, has evolutionary outcomes, increasing diversity in communities. [16]
Other mechanisms such as resource partitioning and sampling effect act in tandem with facilitation to increase biodiversity (observable evidence in plant communities). [17]
Facilitation of non-native species, either by native species or other non-native species, may increase the invasibility of a community, or the ease with which non-native species become established in a community. In an examination of 254 published studies of introduced species, 22 of 190 interactions studied between introduced species in the studies were facilitative. [18] It is worth noting that 128 of the 190 examined interactions were predator–prey relationships of a single plant-eating insect reported in a single study, which may have overemphasized the importance of negative interactions. Introduced plants are also facilitated by native pollinators, dispersers, and mycorrhizae. [19] Thus, positive interactions must be considered in any attempt to understand the invasibility of a community.
Symbiosis is any type of a close and long-term biological interaction between two biological organisms of different species, termed symbionts, be it mutualistic, commensalistic, or parasitic. In 1879, Heinrich Anton de Bary defined it as "the living together of unlike organisms". The term is sometimes used in the more restricted sense of a mutually beneficial interaction in which both symbionts contribute to each other's support.
A herbivore is an animal anatomically and physiologically adapted to eating plant material, for example foliage or marine algae, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthparts adapted to rasping or grinding. Horses and other herbivores have wide flat teeth that are adapted to grinding grass, tree bark, and other tough plant material.
Mutualism describes the ecological interaction between two or more species where each species has a net benefit. Mutualism is a common type of ecological interaction. Prominent examples include most vascular plants engaged in mutualistic interactions with mycorrhizae, flowering plants being pollinated by animals, vascular plants being dispersed by animals, and corals with zooxanthellae, among many others. Mutualism can be contrasted with interspecific competition, in which each species experiences reduced fitness, and exploitation, or parasitism, in which one species benefits at the expense of the other.
Commensalism is a long-term biological interaction (symbiosis) in which members of one species gain benefits while those of the other species neither benefit nor are harmed. This is in contrast with mutualism, in which both organisms benefit from each other; amensalism, where one is harmed while the other is unaffected; parasitism, where one is harmed and the other benefits, and parasitoidism, which is similar to parasitism but the parasitoid has a free-living state and instead of just harming its host, it eventually ends up killing it.
Pollination is the transfer of pollen from an anther of a plant to the stigma of a plant, later enabling fertilisation and the production of seeds, most often by an animal or by wind. Pollinating agents can be animals such as insects, birds, and bats; water; wind; and even plants themselves, when self-pollination occurs within a closed flower. Pollination often occurs within a species. When pollination occurs between species, it can produce hybrid offspring in nature and in plant breeding work.
In ecology, a biological interaction is the effect that a pair of organisms living together in a community have on each other. They can be either of the same species, or of different species. These effects may be short-term, or long-term, both often strongly influence the adaptation and evolution of the species involved. Biological interactions range from mutualism, beneficial to both partners, to competition, harmful to both partners. Interactions can be direct when physical contact is established or indirect, through intermediaries such as shared resources, territories, ecological services, metabolic waste, toxins or growth inhibitors. This type of relationship can be shown by net effect based on individual effects on both organisms arising out of relationship.
Myrmecophytes are plants that live in a mutualistic association with a colony of ants. There are over 100 different genera of myrmecophytes. These plants possess structural adaptations that provide ants with food and/or shelter. These specialized structures include domatia, food bodies, and extrafloral nectaries. In exchange for food and shelter, ants aid the myrmecophyte in pollination, seed dispersal, gathering of essential nutrients, and/or defense. Specifically, domatia adapted to ants may be called myrmecodomatia.
Intertidal ecology is the study of intertidal ecosystems, where organisms live between the low and high tide lines. At low tide, the intertidal is exposed whereas at high tide, the intertidal is underwater. Intertidal ecologists therefore study the interactions between intertidal organisms and their environment, as well as between different species of intertidal organisms within a particular intertidal community. The most important environmental and species interactions may vary based on the type of intertidal community being studied, the broadest of classifications being based on substrates—rocky shore and soft bottom communities.
Myrmecophily is the term applied to positive interspecies associations between ants and a variety of other organisms, such as plants, other arthropods, and fungi. Myrmecophily refers to mutualistic associations with ants, though in its more general use, the term may also refer to commensal or even parasitic interactions.
In ecology, a community is a group or association of populations of two or more different species occupying the same geographical area at the same time, also known as a biocoenosis, biotic community, biological community, ecological community, or life assemblage. The term community has a variety of uses. In its simplest form it refers to groups of organisms in a specific place or time, for example, "the fish community of Lake Ontario before industrialization".
Insect ecology is the scientific study of how insects, individually or as a community, interact with the surrounding environment or ecosystem.
Plant ecology is a subdiscipline of ecology that studies the distribution and abundance of plants, the effects of environmental factors upon the abundance of plants, and the interactions among plants and between plants and other organisms. Examples of these are the distribution of temperate deciduous forests in North America, the effects of drought or flooding upon plant survival, and competition among desert plants for water, or effects of herds of grazing animals upon the composition of grasslands.
Conservation is the maintenance of biological diversity. Conservation can focus on preserving diversity at genetic, species, community or whole ecosystem levels. This article will examine conservation at the species level, because mutualisms involve interactions between species. The ultimate goal of conservation at this level is to prevent the extinction of species. However, species conservation has the broader aim of maintaining the abundance and distribution of all species, not only those threatened with extinction. Determining the value of conserving particular species can be done through the use of evolutionary significant units, which essentially attempt to prioritise the conservation of the species which are rarest, fastest declining, and most distinct genotypically and phenotypically.
Tolerance is the ability of plants to mitigate the negative fitness effects caused by herbivory. It is one of the general plant defense strategies against herbivores, the other being resistance, which is the ability of plants to prevent damage. Plant defense strategies play important roles in the survival of plants as they are fed upon by many different types of herbivores, especially insects, which may impose negative fitness effects. Damage can occur in almost any part of the plants, including the roots, stems, leaves, flowers and seeds. In response to herbivory, plants have evolved a wide variety of defense mechanisms and although relatively less studied than resistance strategies, tolerance traits play a major role in plant defense.
A mycorrhizal network is an underground network found in forests and other plant communities, created by the hyphae of mycorrhizal fungi joining with plant roots. This network connects individual plants together. Mycorrhizal relationships are most commonly mutualistic, with both partners benefiting, but can be commensal or parasitic, and a single partnership may change between any of the three types of symbiosis at different times.
Flowering synchrony is the amount of overlap between flowering periods of plants in their mating season compared to what would be expected to occur randomly under given environmental conditions. A population which is flowering synchronously has more plants flowering at the same time than would be expected to occur randomly. A population which is flowering asynchronously has fewer plants flowering at the same time than would be expected randomly. Flowering synchrony can describe synchrony of flowering periods within a year, across years, and across species in a community. There are fitness benefits and disadvantages to synchronized flowering, and it is a widespread phenomenon across pollination syndromes.
Alien species, or species that are not native, invade habitats and alter ecosystems around the world. Invasive species are only considered invasive if they are able to survive and sustain themselves in their new environment. A habitat and the environment around it has natural flaws that make them vulnerable to invasive species. The level of vulnerability of a habitat to invasions from outside species is defined as its invasibility. One must be careful not to get this confused with invasiveness, which relates to the species itself and its ability to invade an ecosystem.
Nurse plants are plants that serve the ecological role of helping seedlings establish themselves and protecting young plants from harsh conditions. This effect is particularly well studied among plant communities in xeric environments.
A facilitation cascade is a sequence of ecological interactions that occur when a species benefits a second species that in turn has a positive effect on a third species. These facilitative interactions can take the form of amelioration of environmental stress and/or provision of refuge from predation. Autogenic ecosystem engineering species, structural species, habitat-forming species, and foundation species are associated with the most commonly recognized examples of facilitation cascades, sometimes referred to as a habitat cascades. Facilitation generally is a much broader concept that includes all forms of positive interactions including pollination, seed dispersal, and co-evolved commensalism and mutualistic relationships, such as between cnidarian hosts and symbiodinium in corals, and between algae and fungi in lichens. As such, facilitation cascades are widespread through all of the earth's major biomes with consistently positive effects on the abundance and biodiversity of associated organisms.
Plant-animal interactions are important pathways for the transfer of energy within ecosystems, where both advantageous and unfavorable interactions support ecosystem health. Plant-animal interactions can take on important ecological functions and manifest in a variety of combinations of favorable and unfavorable associations, for example predation, frugivory and herbivory, parasitism, and mutualism. Without mutualistic relationships, some plants may not be able to complete their life cycles, and the animals may starve due to resource deficiency.