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

Abstract

Foundation species create biogenic habitats, modify environmental conditions, augment biodiversity, and control animal community structures. In recent decades, marine heatwaves (MHWs) have affected the ecology of foundation species worldwide, and perhaps also their associated animal communities. However, no realistic field experiment has tested how MHWs affect animals that live in and around these foundation species. We therefore tested, in a four‐factorial field experiment, if colonisation by small mobile marine animals (epifauna) onto plates with attached single versus co‐occurring foundation species of different morphological complexities, were affected by 3–5°C heating (that mirrored a recent extreme MHW in the study area) and if the heating effect on the epifauna varied within and between seasons. For this experiment mimics of turf seaweed represented the single foundation species and holdfasts of seven common canopy‐forming seaweed represented the co‐occurring foundation species with different morphological complexities. We found that the taxonomic richness and total abundance of epifauna, dominated by copepods, generally were higher on heated plates with complex seaweed holdfasts in warmer summer trials. Furthermore, several interactions between test‐factors were significant, e.g., epifaunal abundances, were, across taxonomic groups, generally higher in warmer than colder summer trials. These results suggest that, in temperate ecosystems, small, mobile, short‐lived, and fast‐growing marine epifauna can be facilitated by warmer oceans and morphologically complex foundation species, implying that future MHWs may increase secondary production and trophic transfers between primary producers and fish. Future studies should test whether these results can be scaled to other ecological species‐interactions, across latitudes and biogeographical regions, and if similar results are found after longer MHWs or within live foundation species under real MHW conditions.

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

“…Specifically, we found that Ulva attached to Austrovenus enhanced the abundance and richness of invertebrates, but also that the strength of facilitation was stronger in warmer seasons and with higher amount of Ulva , supporting previous research with other foundation species (Ba‐Akdah et al, 2016; Gunnill, 1983; Montie & Thomsen, 2023c; Navarro‐Mayoral et al, 2021; Taylor, 1997; Thomsen et al, 2015, 2022). Perhaps invertebrates were more abundant in warmer summer months because their recruitment and growth typically are faster in this season (Edgar, 1991), as high food availability, reproduction, and metabolism often correlate positively with higher summer temperatures (Ba‐Akdah et al, 2016; Gunnill, 1983; Montie & Thomsen, 2023a, 2023c; Navarro‐Mayoral et al, 2021; Taylor, 1997). Furthermore, invertebrates were likely more common on larger Ulva fronds because large foundation species provide more biogenic habitat and therefore more surface area for more shelter, refugia, and food (Thomsen et al, 2022).…”