Positive species interactions such as facilitation are important for enabling species to persist, especially in stressful conditions, and the nature and strength of facilitation varies along physical and biological gradients. Expansion of coastal infrastructure is creating hotspots of invasive species which can spillover into natural habitats, but the role of positive species interactions associated with biological invasions remains understudied. Theory suggests that stronger biotic pressure in natural habitats inhibits invasion success. In space-limited marine systems, sessile organisms can overcome this limiting resource by settling as an epibiont on a substrate organism - basibiont. Using a series of spatially extensive surveys, we explored the role of invasive and native basibionts in providing habitat for other invasive and native epibionts, and tested whether environmental context (i.e. if the receiving habitat was natural or artificial), altered ecological outcomes. Overall, provision of space by basibionts was more important for invasive epibionts than for native epibionts but was dependent on the environmental context. Invasive basibionts facilitated invasive epibionts in natural habitats, and appeared to be more important for native epibionts in artificial habitats respectively. Native basibionts facilitated invasive, but not native epibionts in both natural and artificial habitats. These results advance our understanding of facilitation and highlight the idiosyncratic nature of biofouling and epibiosis, and the potentially important influence of environmental context. The degree to which native habitat-forming species vs. invasive habitat-forming species either do or do not facilitate other native or non-native species is a rich area for investigation. Experimental work is required to disentangle the processes underpinning these patterns.

Study locations

This study was carried out at six intertidal locations across 1.54º longitude (~114 km) of coast in SW Britain between March and July 2018. At each location, both natural and artificial habitats were present in close proximity to one another and thus, characterised by similar environmental conditions. Artificial habitats comprised vertical seawalls (constructed of smooth natural rock and >20 years old) and adjacent (<20 m) natural habitats were characterised by flat smooth vertical/near-vertical bedrock.

Surveys comparing invasive and native taxa between natural and artificial habitats

Preliminary observations indicated that two basibiont species were suitable for this study: the native limpet Patella vulgata (Patella herein), and the invasive oyster Magallana gigas (Magallana herein). These two species were selected because they co-occur at the same tidal height in both natural and artificial habitats, have relatively large shells for the attachment of barnacle epibionts, and have been observed to support mixed epibiotic barnacle communities on their shells (Firth, pers. obs.). Whilst, the congeneric limpet P. depressa does occur in both natural and artificial habitats, it is much smaller and is often devoid of epibionts (Firth, pers. obs.). Throughout this study, the term ‘substrate/substrata’ refers to the substrate to which an organism is directly attached (i.e. rock, Patella, Magallana), and ‘habitat’ refers to whether the substrate is located in a natural (i.e. rocky shore) or artificial (i.e. seawall) environment.

In the first instance, we used surveys at each location to quantify the prevalence of all taxa (i.e. barnacles, Magallana, and Patella) on natural rocky shore and on artificial seawall habitats. The abundance of Magallana and Patella on rock substrata was quantified using 15 haphazardly-placed quadrats (50 × 50 cm) on flat vertical/near-vertical surfaces within a 10m² area at mid-shore elevation. Concurrently, a 3 × 3 cm photo-quadrat was taken within the larger quadrat, then imported into image analysis software (Fiji Cell Counter Tool, Schindelin et al. 2012) and used to quantify the abundance and relative proportion of invasive Austrominius and native barnacles living directly on the rock substrata (not as epibionts). In all surveys described below, native barnacles comprised S. balanoides, C. montagui and C. stellatus and were identified to species to assign them as native barnacles, but their identity was not considered further as we were interested in the relative differences between native and invasive groups rather than the species themselves. As such, we refer to the native barnacle community as simply 'native barnacles' herein ignoring species identity.

Comparison of facilitation between basibiont species and habitat types

To investigate the relative importance of positive interactions of native and invasive basibionts in natural and artificial habitats, photographs of all Patella and Magallana individuals encountered within quadrats were taken, alongside a photograph of the adjacent rock substrata. Photo-quadrats (3 × 3 cm) were placed over the section of the shell with barnacle epibionts. The abundance and relative proportions of invasive and native barnacles within photo-quadrats were counted using the same image analysis process as above.