Availability of microhabitats explains a widespread pattern and informs theory on ecological engineering of boulder reefs

The availability of suitable microhabitats has emerged as a key requirement for maximising species diversity gains from ecological engineering of coastal habitats. This includes intertidal boulder field habitat, which is threatened by increasing urbanisation. We examined faunal use of microhabitats offered by natural intertidal boulders on two continents, South Africa and Australia, and then used artificial boulders to test possible mechanisms driving the observed patterns. Gaps often occur between the undersides of boulders and the substratum on which they lie. Substrata underneath boulders are generally uneven, so gaps can occur anywhere along the boulder undersurface, but gaps are generally larger near the edges of naturally-occurring boulders. Boulder edges were found to provide a microhabitat that had greater densities of almost all macrofauna than closer to the centre of boulder undersides. To test the model that microhabitat use of boulder edges reflects their larger gap sizes (relative to boulder centres), artificial boulders were constructed with the underside surface either flat, or with a gap underneath mimicking the mean size of the gap normally found only under boulder edges (when their average shape is considered among numerous naturally-occurring boulders). Artificial boulders were deployed intertidally for seven weeks, and macrofaunal colonisation was compared (a) between flat boulders and those with gaps, and (b) between edges and centres. Five times more macrofauna colonised boulders with artificial gaps, while no effect of proximity to edges was found when the gap-width was controlled for. The size of gaps under boulders appears to be an important microhabitat feature that can explain a widespread distributional pattern in the diverse assemblages of boulder fields. Such gaps are clearly used by numerous species, including some that are rare or commercially important. Provision and augmentation of this microhabitat should be considered for any ecological engineering project that involves intertidal or subtidal boulders. (C) 2017 Elsevier B.V. All rights reserved.