**5. Conclusions**

Rock-rubble groynes in urban environs have provided researchers and practitioners with a conservation dilemma. On the one hand, they can provide much-needed habitat for intertidal species that are decreasing in abundance, while on the other hand, by providing such habitat they may help non-indigenous species expand their ranges. Our results sugges<sup>t</sup> that the rock surfaces of rock-rubble groynes do provide potential benefit for species for which this habitat is suitable, including species that are of conservation concern. We found similar diversity on rock surface habitats between structure types and found higher abundances of sessile animals on groynes. The rock pool habitat represents a key di fference between the groynes and the natural rocky shores, with rock pools on groynes being smaller, fewer and less complex than on natural rocky shores. We identified clear communities of algae and endangered fish species (Figure 4) in pools, suggesting that rock pool creation can result in similar ecological communities on groynes, albeit at lower abundances. We also found very few non-native species on groynes, and those species were also found on corresponding natural rocky shores, o ffering no support for the notion that groynes pose a threat in terms of biological invasions

(at least in the UK). While structure age and equilibrium in the ecosystem must be considered, the results of this study sugges<sup>t</sup> that rock-rubble groynes have the potential to support under-pressure intertidal macro-organisms, particularly in urban environments, where the trend to replace natural shoreline with artificial structures is continuing.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-445X/9/5/131/s1: Supplementary Information 1: Details of the study sites. Supplementary Information 2: R Code. Supplementary Information 3: Species Data. Supplementary Information 4: Species name and code. Supplementary Information 5: Extra Analysis.

**Author Contributions:** Conceptualization, P.H. and R.F.; methodology, P.H. and R.F.; data curation, P.H.; formal analysis, P.H. and R.F.; writing—original draft preparation, P.H. and R.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Acknowledgments:** We would like to thank the reviewers and editors for their comments and suggestions. We would also like to thank our field assistants, Julia, Eddie, and Michael.

**Conflicts of Interest:** The authors declare no conflict of interest.
