Ecological facilitation
Encyclopedia
Facilitation 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 commensalism
s, 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.
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 plant
s 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.
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
, USA, 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 aphid
s produce a sugar-rich waste product called honeydew
, which is consumed by ants in exchange for protection of the sap-feeders against predation.
s 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 mussel
s (Modiolus modiolus) from overgrowth by kelps competing for space in the subtidal zone of the Gulf of Maine
, USA.
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, termite
s receive nourishment from cellulose
digested by microbes inhabiting their gut.
Lichen
s:Associations between fungi and algae
, wherein the fungus receives nutrients from the alga, and the alga is protected from harsh conditions causing desiccation
.
Coral
s:Associations between reef-building corals and photosynthetic algae called zooxanthella
e, wherein the zooxanthellae provide nutrition to the corals in the form of photosynthate, in exchange for nitrogen in coral waste products.
Mycorrhiza
e:Associations between fungi and plant roots, wherein the fungus facilitates nutrient uptake (particularly nitrogen) 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.
of pollen, and increasing the probability of sexual reproduction at low population density
. In return, the pollinator receives nourishment in the form of nectar or pollen. 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.
, and invasibility
.
apparently does in the grasslands of the Rio Grande Plains. 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. Thus, mesquite facilitates the dynamic spatial shift from grassland
to savanna
to woodland
across the habitat.
species such as seaweed
and barnacle
s. 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. The effect of facilitation on diversity could also be reversed, if the facilitation creates a competitive dominance that excludes more species than it permits.
, 22 of 190 interactions studied between introduced species in the studies were facilitative. 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, disperser
s, and mycorrhizae. Thus, positive interactions must be considered in any attempt to understand the invasibility of a community.
Biological interaction
Biological interactions are the effects organisms in a community have on one another. In the natural world no organism exists in absolute isolation, and thus every organism must interact with the environment and other organisms...
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 commensalism
Commensalism
In ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
s, in which one species benefits and the other is unaffected. Much of classic ecological theory (e.g., natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
, niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
separation, metapopulation
Metapopulation
A metapopulation consists of a group of spatially separated populations of the same species which interact at some level. The term metapopulation was coined by Richard Levins in 1970 to describe a model of population dynamics of insect pests in agricultural fields, but the idea has been most...
dynamics) has focused on negative interactions such as predation and competition
Competition (biology)
Competition is an interaction between organisms or species, in which the fitness of one is lowered by the presence of another. Limited supply of at least one resource used by both is required. Competition both within and between species is an important topic in ecology, especially community ecology...
, 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.
Categories
- A 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 pollenPollenPollen is a fine to coarse powder containing the microgametophytes of seed plants, which produce the male gametes . Pollen grains have a hard coat that protects the sperm cells during the process of their movement from the stamens to the pistil of flowering plants or from the male cone to the...
between flowers, while the pollinator receives some form of nourishment, either from nectar or the pollen itself. - A commensalismCommensalismIn ecology, commensalism is a class of relationship between two organisms where one organism benefits but the other is neutral...
is an interaction in which one species benefits and the other species is unaffected. EpiphyteEpiphyteAn epiphyte is a plant that grows upon another plant non-parasitically or sometimes upon some other object , derives its moisture and nutrients from the air and rain and sometimes from debris accumulating around it, and is found in the temperate zone and in the...
s (plants growing on other plants, usually trees) have a commensal relationship with their host plant because the epiphyte benefits in some way (e.g., by escaping competition with terrestrial plants or by gaining greater access to sunlight) while the host plant is apparently unaffected.
Strict categorization, however, is not possible for some complex species interactions. For example, seed germination
Germination
Germination is the process in which a plant or fungus emerges from a seed or spore, respectively, and begins growth. The most common example of germination is the sprouting of a seedling from a seed of an angiosperm or gymnosperm. However the growth of a sporeling from a spore, for example the...
and survival in harsh environments is often higher under so-called nurse plants
Nurse log
A nurse log is a fallen tree which, as it decays, provides ecological facilitation to seedlings. Broader definitions include providing shade or support to other plants. Some of the advantages a nurse log offers to a seedling are: water, moss thickness, leaf litter, mycorrhizae, disease protection,...
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.
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 FucusFucus
Fucus is a genus of brown algae found in the intertidal zones of rocky seashores almost throughout the world.-Description and life cycle:...
in intertidal sites of southern New England
New England
New England is a region in the northeastern corner of the United States consisting of the six states of Maine, New Hampshire, Vermont, Massachusetts, Rhode Island, and Connecticut...
, 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
Disturbance
In ecology, a disturbance is a temporary change in average environmental conditions that causes a pronounced change in an ecosystem. Outside disturbance forces often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass...
. An example of such “whole-community” facilitation is substrate stabilization of cobble beach plant communities in Rhode Island
Rhode Island
The state of Rhode Island and Providence Plantations, more commonly referred to as Rhode Island , is a state in the New England region of the United States. It is the smallest U.S. state by area...
, USA, by smooth cordgrass (Spartina alterniflora
Spartina alterniflora
Spartina alterniflora is a perennial deciduous grass which is found in intertidal wetlands, especially estuarine salt marshes. It grows 1-1.5 m tall, and has smooth, hollow stems which bear leaves up to 20-60 cm long and 1.5 cm wide at their base, which are sharply tapered and bend down...
). Large beds of cordgrass buffer wave action, thus allowing the establishment and persistence of a community of less disturbance-tolerant annual
Annual plant
An annual plant is a plant that usually germinates, flowers, and dies in a year or season. True annuals will only live longer than a year if they are prevented from setting seed...
and perennial plant
Perennial plant
A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...
s 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
Salinity
Salinity is the saltiness or dissolved salt content of a body of water. It is a general term used to describe the levels of different salts such as sodium chloride, magnesium and calcium sulfates, and bicarbonates...
was higher. The rush shaded the soil, which decreased evapotranspiration
Evapotranspiration
Evapotranspiration is a term used to describe the sum of evaporation and plant transpiration from the Earth's land surface to atmosphere. Evaporation accounts for the movement of water to the air from sources such as the soil, canopy interception, and waterbodies...
, 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.
Refuge from predation
Plant defense against herbivory
Plant defense against herbivory or host-plant resistance describes a range of adaptations evolved by plants which improve their survival and reproduction by reducing the impact of herbivores. Plants use several strategies to defend against damage caused by herbivores...
of seedlings growing under them. In both terrestrial and marine environments, herbivory
Herbivore
Herbivores are organisms that are anatomically and physiologically adapted to eat plant-based foods. Herbivory is a form of consumption in which an organism principally eats autotrophs such as plants, algae and photosynthesizing bacteria. More generally, organisms that feed on autotrophs in...
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
North Carolina
North Carolina is a state located in the southeastern United States. The state borders South Carolina and Georgia to the south, Tennessee to the west and Virginia to the north. North Carolina contains 100 counties. Its capital is Raleigh, and its largest city is Charlotte...
, USA, coastline. This crab species takes refuge in the branches of the compact Ivory Bush Coral
Ivory Bush Coral
The ivory bush coral is a U.S. National Marine Fisheries Service species of concern. Species of concern are those species about which the U.S...
(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
Acacia
Acacia is a genus of shrubs and trees belonging to the subfamily Mimosoideae of the family Fabaceae, first described in Africa by the Swedish botanist Carl Linnaeus in 1773. Many non-Australian species tend to be thorny, whereas the majority of Australian acacias are not...
spp.) and certain ants (Pseudomyrmex
Pseudomyrmex
Pseudomyrmex is a genus of stinging, wasp-like ants in the family Formicidae . They are large eyed, slender ants with an arboreal habitat. They are found exclusively in the New World in tropical and subtropical regions...
spp.) in Central America
Central America
Central America is the central geographic region of the Americas. It is the southernmost, isthmian portion of the North American continent, which connects with South America on the southeast. When considered part of the unified continental model, it is considered a subcontinent...
. 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 aphid
Aphid
Aphids, also known as plant lice and in Britain and the Commonwealth as greenflies, blackflies or whiteflies, are small sap sucking insects, and members of the superfamily Aphidoidea. Aphids are among the most destructive insect pests on cultivated plants in temperate regions...
s produce a sugar-rich waste product called honeydew
Honeydew (secretion)
Honeydew is a sugar-rich sticky liquid, secreted by aphids and some scale insects as they feed on plant sap. When their mouthpart penetrates the phloem, the sugary, high-pressure liquid is forced out of the gut's terminal opening. Honeydew is particularly common as a secretion in the Hemipteran...
, 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 logNurse log
A nurse log is a fallen tree which, as it decays, provides ecological facilitation to seedlings. Broader definitions include providing shade or support to other plants. Some of the advantages a nurse log offers to a seedling are: water, moss thickness, leaf litter, mycorrhizae, disease protection,...
s 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
Strongylocentrotus droebachiensis
Strongylocentrotus droebachiensis is commonly known as the green sea urchin because of its characteristic green color. It has the longest genus-species names in the animal kingdom...
) on kelps (Laminaria
Laminaria
Laminaria is a genus of 31 species of brown algae , all sharing the common name "kelp". This economically important genus is characterized by long, leathery laminae and relatively large size. Some species are referred to by the common name Devil's apron, due to their shape, or sea colander, due to...
spp.) can protect mussel
Mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...
s (Modiolus modiolus) from overgrowth by kelps competing for space in the subtidal zone of the Gulf of Maine
Gulf of Maine
The Gulf of Maine is a large gulf of the Atlantic Ocean on the east coast of North America.It is delineated by Cape Cod at the eastern tip of Massachusetts in the southwest and Cape Sable at the southern tip of Nova Scotia in the northeast. It includes the entire coastlines of the U.S...
, USA.
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. However, the most familiar examples of increased access to resources through facilitation are the mutualistic transfers of nutrients between symbiotic organisms. A symbiosisSymbiosis
Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...
is a prolonged, close association between organisms, and some examples of mutualistic symbioses include:
Gut flora
Gut flora
Gut flora consists of microorganisms that live in the digestive tracts of animals and is the largest reservoir of human flora. In this context, gut is synonymous with intestinal, and flora with microbiota and microflora....
:Associations between a host species and a microbe
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
living in the host’s digestive tract, wherein the host provides habitat and nourishment to the microbe in exchange for digestive services. For example, termite
Termite
Termites are a group of eusocial insects that, until recently, were classified at the taxonomic rank of order Isoptera , but are now accepted as the epifamily Termitoidae, of the cockroach order Blattodea...
s receive nourishment from cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
digested by microbes inhabiting their gut.
Lichen
Lichen
Lichens are composite organisms consisting of a symbiotic organism composed of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
s:Associations between fungi and algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...
, wherein the fungus receives nutrients from the alga, and the alga is protected from harsh conditions causing desiccation
Desiccation
Desiccation is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic substance that induces or sustains such a state in its local vicinity in a moderately sealed container.-Science:...
.
Coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s:Associations between reef-building corals and photosynthetic algae called zooxanthella
Zooxanthella
Zooxanthellae are flagellate protozoa that are golden-brown intracellular endosymbionts of various marine animals and protozoa, especially anthozoans such as the scleractinian corals and the tropical sea anemone, Aiptasia....
e, wherein the zooxanthellae provide nutrition to the corals in the form of photosynthate, in exchange for nitrogen in coral waste products.
Mycorrhiza
Mycorrhiza
A mycorrhiza is a symbiotic association between a fungus and the roots of a vascular plant....
e:Associations between fungi and plant roots, wherein the fungus facilitates nutrient uptake (particularly nitrogen) 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.
Transport
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 dispersalBiological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
of pollen, and increasing the probability of sexual reproduction at low population density
Population density
Population density is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans...
. In return, the pollinator receives nourishment in the form of nectar or pollen. 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.
Community Effects
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, diversityBiodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
, and invasibility
Invasive species
"Invasive species", or invasive exotics, is a nomenclature term and categorization phrase used for flora and fauna, and for specific restoration-preservation processes in native habitats, with several definitions....
.
Spatial structure
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. 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. A facilitating species may also help drive the progression from one ecosystem type to another, as mesquiteMesquite
Mesquite is a leguminous plant of the Prosopis genus found in northern Mexico through the Sonoran Desert and Chihuahuan Deserts, and up into the Southwestern United States as far north as southern Kansas, west to the Colorado Desert in California,and east to the eastern fifth of Texas, where...
apparently does in the grasslands of the Rio Grande Plains. 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. Thus, mesquite facilitates the dynamic spatial shift from grassland
Grassland
Grasslands are areas where the vegetation is dominated by grasses and other herbaceous plants . However, sedge and rush families can also be found. Grasslands occur naturally on all continents except Antarctica...
to savanna
Savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...
to woodland
Woodland
Ecologically, a woodland is a low-density forest forming open habitats with plenty of sunlight and limited shade. Woodlands may support an understory of shrubs and herbaceous plants including grasses. Woodland may form a transition to shrubland under drier conditions or during early stages of...
across the habitat.
Diversity
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 sessileSessility (limnology)
In limnology, sessility is that quality of an organism which rests unsupported directly on a base, either attached or unattached to a substrate. It is a characteristic of vegetation which is anchored to the benthic environment. There are two families of sessile rotifers: Flosculariidae and...
species such as seaweed
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...
and barnacle
Barnacle
A barnacle is a type of arthropod belonging to infraclass Cirripedia in the subphylum Crustacea, and is hence related to crabs and lobsters. Barnacles are exclusively marine, and tend to live in shallow and tidal waters, typically in erosive settings. They are sessile suspension feeders, and have...
s. Although the mussels decrease diversity of primary space holders (i.e., large sessile species), a larger number of invertebrate
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
species are associated with mussel beds than with other primary space holders, so total species diversity is higher when mussels are present. The effect of facilitation on diversity could also be reversed, if the facilitation creates a competitive dominance that excludes more species than it permits.
Invasibility
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 speciesIntroduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
, 22 of 190 interactions studied between introduced species in the studies were facilitative. 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, disperser
Biological dispersal
Biological dispersal refers to species movement away from an existing population or away from the parent organism. Through simply moving from one habitat patch to another, the dispersal of an individual has consequences not only for individual fitness, but also for population dynamics, population...
s, and mycorrhizae. Thus, positive interactions must be considered in any attempt to understand the invasibility of a community.