Habitat‐complexity regulates the intensity of facilitation along an environmental stress gradient

Abstract

Positive interactions between foundation species and their associated species are expected to be influenced by the degree of environmental stress as well as trait variations of the species involved. However, there is scarce empirical evidence regarding how these two factors interact and shape the intensity of facilitation. To test how facilitation varies with stress, a colonization experiment using artificial algal units that varied in a functional trait (morphological complexity) was conducted at different intertidal height levels. The isolated effects of shore-height and host-complexity are as expected: the number of species and overall abundance of epifauna decreased with lower complexity host units and greater exposure to environmental stress. However, in contrast to the initially proposed model, based on the stress gradient hypothesis, the magnitude of positive interactions increased at lower tidal heights (those levels with, presumably, milder environmental conditions). An additional predation exclusion experiment ruled out the effects of predation pressure as a major factor driving the structure of mobile epifaunal assemblages in these intertidal habitats. Thus, further studies are needed to identify the factors that explain the patterns of facilitation intensity found in the present study.

Extreme heatwave drives topography‐dependent patterns of mortality in a bed‐forming intertidal barnacle, with implications for associated community structure

“…Algal cover may have been too low in our study to provide adequate thermal and desiccation protection (e.g., Hay, 1981). Perhaps what thermal protection algae did provide could not be sustained during the acute, prolonged thermal stress of the heat dome as has been found for other algal facilitations under high levels of environmental stress (Navarro‐Barranco et al, 2022; Scrosati, 2017). Furthermore, the exact composition of algal species differed between sites and was not quantified; different species, by virtue of their morphology, may vary in the extent to which they retain moisture and provide thermal protection to underlying organisms.…”

“…While habitat provision by biogenic species remains an important mechanism by which the environmental stress experienced by associated species can be mitigated (Jurgens et al, 2021; Jurgens & Gaylord, 2018), there may be an upper limit to the stress under which such facilitations can occur (Navarro‐Barranco et al, 2022; Scrosati, 2017). Extreme heatwaves like the 2021 Western North American Heat Dome will only occur more frequently through time (Perkins‐Kirkpatrick & Lewis, 2020), and events of this magnitude may prove lethal for foundation species or disrupt their ability to facilitate associated organisms.…”

Facilitation cascades across space: Monitoring estuarine foundation species from satellites to the microscope

“…However, the spatial scale and environmental context of facilitation effects in estuaries are poorly studied, despite their ecological importance. Moreover, it is unknown whether and how the detection and monitoring of foundation species can be improved using remote sensing methods, which can cover much larger areas, and thereby increase the ecological relevance, compared with traditional quadrat sampling (Bishop et al, 2012;Montie & Thomsen, 2023c;Navarro-Barranco et al, 2022). As such, monitoring the occurrence of the dominant foundation species may provide insights into the wider functioning of estuarine ecosystems, biodiversity, spatiotemporal variability, and ecological complexity, which is also likely to vary with estuarine properties such as tidal elevation, currents, salinity, and temperature.…”

Facilitation of animals is stronger during summer marine heatwaves and around morphologically complex foundation species

“…Furthermore, primary foundation species can provide living space to secondary foundation species such as epiphytes attached to aquatic plants and trees, mussels embedded among marsh grasses, or canopy‐forming seaweed overgrowing turf‐forming species (Altieri et al, 2007 ; Angelini et al, 2011 ; Gribben et al, 2019 ). When primary and secondary foundation species co‐occur, the amount and complexity of biogenically formed three‐dimensional living spaces often increase (compared to when the primary foundation species exist in isolation) and thereby augment the biodiversity of habitat‐associated animals (Navarro‐Barranco et al, 2022 ; Thomsen et al, 2022 ; Tokeshi & Arakaki, 2012 ). Today, facilitation of animals from co‐occurring foundation species has been documented across habitats, ecosystems, and bioregions (Angelini et al, 2011 ; Kazanidis et al, 2022 ; Thomsen et al 2018b ).…”

“…To address this research gap, we here provide a first test of how controlled and manipulated MHWs affect short‐term colonisation and abundances of epifauna onto co‐occurring mimics of seaweed turf (primary foundation species) and seaweed holdfast (secondary foundation species) attached to heated plates, under realistic open oceanic field conditions. Non‐living mimics of different morphological complexities were used to model morphological and structural effects from foundation species, similar to that done in many other studies that have tested ecological theories on epifaunal communities (Edgar & Klumpp, 2003 ; Kelaher, 2003 , 2005 ; Matias et al, 2011 ; Navarro‐Barranco et al, 2022 ). Indeed, mimics are often used to test ecological theories because the texture, morphology, isolation, and successional stage of the experimental units can be controlled and manipulated, and because structurally robust mimics can be transplanted to specific environments and collected again with minimal loss (Hauser et al, 2006 ; Myers & Southgate, 1980 ; Smith & Rule, 2002 ; Thomsen et al, 2022 ).…”