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, diversity, and invasibility. [edit]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.[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.[11] 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.[11] [edit]Diversity Facilitation affects community diversity (defined in this context as the numbe

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] [edit]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 species, 22 of 190 interactions studied between introduced species in the studies were facilitative.[12] 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.[13] Thus, positive interactions must be considered in any attempt to understand the invasibility of a community.