Facilitation cascade
[6] A typical example of facilitation cascades in a tropical coastal ecosystem[7] The term facilitation cascade was coined by Altieri, Silliman, and Bertness during a study on New England cobblestone beaches to explain the chain of positive interactions that allow a diverse community to exist in a habitat that is otherwise characterized by substrate instability, elevated temperatures, and desiccation stress.
The significance of facilitation cascades is often apparent through direct observation, however, experimental manipulations with mimics offer strong evidence for the magnitude of interaction importance.
For examples, using artificial mimics as replacements for primary and secondary foundation species allows for isolation of specific mechanisms that are hypothesized to underlie the cascading effects of facilitation on local ecosystem dynamics.
[12] A classic example of facilitation cascades in the marine environment is the relationship between mangroves, seagrasses, and stony corals that are adjacent to one another in a seascape.
These foundation species exchange resources and benefit each other by buffering against sedimentation and nutrient inputs from the terrestrial side, and reducing wave energy from the open ocean.
[18][19] In turn, the bivalves act as secondary habitat formers, facilitating epifaunal organisms by providing them with substrate to attach and settle.
On the cobblestone beaches of New England, cordgrasse ameliorates physical stress for the establishment of ribbed mussels which further facilitating increased diversity within the intertidal ecosystem as secondary foundation species.
This enhances growth of marsh grasses which act as secondary foundation species, facilitating invertebrates including bivalves, insects and birds.
[28][29] The seaweed subsequently provides habitats and supports the high diversity of small epiphytes, invertebrates, and fish in an otherwise bare soft sediment system.
[37][38] The example is notable because mistletoes can be parasitic and have a negative effect on their tree hosts, which is a reminder that the direction and strength of interactions associated with facilitation cascades can be context-dependent.
[3][39] This example is significant due to the chemical signals sent from secondary foundation species to attract the diversity of inhabitant snail to the cascade habitat.
However, only a few studies appear to have documented freshwater facilitation cascades, and it remains to be determined whether this is a function of the ecosystem structure or simply a reflection of historic research perspectives.
[40] First are nested configurations in which the two foundation species are found intermixed or with one facilitator living on another, as in a mangrove pneumatophore providing a surface for oyster colonization.
[22] For highly mobile beneficiary species, such as those with more distant ontogenetic habitat shifts, large foraging ranges, or the capability of long-distance migrations, the reach of the facilitation cascade may be quite extensive.
[3] Within a facilitation cascade, primary and secondary foundation can increase organismal survival, species richness, niche diversity, and habitat complexity, in turn enhancing biodiversity.
For example, a seismic uplift in New Zealand associated with the Kaikōura 2016 earthquake caused immediate mortality of both primary and secondary foundation seaweeds followed by cascading destruction of invertebrate biodiversity.
[57] These foundation species had not recovered by 2021, and large-scale natural disasters could potentially have legacies on facilitation cascades over decades to centuries as a function of recovery rates of habitat forming organisms.
[58] Mutualistic relationships and positive interactions that form the basis of facilitation cascades can ameliorate the impact of increased physical stresses such as drought, temperature extremes, and inundation time associated with climate change.
For example, harvest of trees can reduce the abundance and diversity of epiphytes that provide shelter and other resources of beneficiary insect communities.
[41] Second, while many of the best examples of facilitation cascades in applied contexts come from foundation species or ecosystem engineers that are conspicuous habitat dominants, practitioners should keep in mind that facilitators in a cascade can also include smaller and/or mobile organisms, such as Pollinators that have a positive effect on the reproductive success of habitat-forming vegetation, or mutualists such as Symbiodinium in corals and mycorrhizal fungi in terrestrial plants.
Third, facilitation cascades commonly incorporate multiple Trophic levels and/or disparate taxonomic and functional groups, and so restoration projects (or investigations for that matter) need to take a community-wide approach to their design.
[68] Finally, the overall importance of facilitation cascades is likely to increase with climate change as associated stressors such as elevated temperature and modified precipitation regimes intensify.
[69] Facilitation cascades may suddenly be apparent or important where they were previously undetected, and practitioners may become increasingly dependent on such ecological tools as adaptable and resilient components in their projects.
