DOI: 10.5061/dryad.05qfttf21
title: Data from: Do positive interactions between marine invaders increase likelihood of invasion into natural and artificial habitats?

description: 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.
description: <b><i>Study locations</i></b>
 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 &gt;20 years old) and adjacent
 (&lt;20 m) natural habitats were characterised by flat smooth
 vertical/near-vertical bedrock.
 <b><i>Surveys comparing invasive and native taxa
 between natural and artificial habitats</i></b>
 Preliminary observations indicated that two basibiont species
 were suitable for this study: the native limpet <i>Patella
 vulgata</i> (<i>Patella</i> herein), and the invasive
 oyster <i>Magallana gigas </i>(<i>Magallana
 </i>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 <i>P.
 depressa</i> 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,
 <i>Patella, Magallana</i>), 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, <i>Magallana</i>, and <i>Patella</i>)
 on natural rocky shore and on artificial seawall habitats. The abundance
 of <i>Magallana</i> and <i>Patella</i> on rock
 substrata was quantified using 15 haphazardly-placed quadrats (50 × 50 cm)
 on flat vertical/near-vertical surfaces within a
 10m<sup>2</sup> 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 <i>Austrominius</i> and native barnacles living
 directly on the rock substrata (not as epibionts). In all surveys
 described below, native barnacles comprised <i>S.
 balanoides</i>, <i>C. montagui</i> and <i>C.
 stellatus</i> 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. <b><i>Comparison of
 facilitation between basibiont species and habitat types
 </i></b> To investigate the relative
 importance of positive interactions of native and invasive basibionts in
 natural and artificial habitats, photographs of all
 <i>Patella</i> and <i>Magallana</i> 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.