Restoring marine ecosystems: spatial reef configuration triggers taxon‐specific responses among early colonizers

1. The longstanding debate in conservation biology on the importance of single large or several small (SLOSS) habitats for preserving biodiversity remains highly relevant, given the ongoing degradation and loss of natural habitats worldwide. Restoration efforts are often constrained by limited resources, and insights from SLOSS studies therefore have important implications if restoration efforts can be optimized by manipulating the spatial configuration of restored habitats.

Yet, the relevance of SLOSS for habitat restoration remains largely unexplored. 2. Here, we report the effects of spatial reef configuration on early colonization of marine organisms after restoring boulder reef habitats. Reefs were restored in single large (SL) and several small (SS) designs in the western Baltic Sea, where century-long boulder extraction has severely degraded large reef areas and likely exacerbated regional declines in commercially important gadoids (Gadidae spp.).

We sampled the field sites using remote underwater video systems in a before-after control-impact (BACI) design and obtained probabilistic inferences on restoration and SLOSS effects from Bayesian hierarchical models. 3. Probabilities of a positive restoration effect were high (>95%) for gadoids, labrids and demersal gobies, moderate (60-75%) for species richness and sand gobies, and low (<5%) for flatfish abundance.

Notably, gadoid abundance increased 60-fold and 129-fold on average at SL and SS, respectively. The species composition at restored reefs deviated from control sites, mainly driven by large-bodied piscivores. 4. Spatial reef configuration had the strongest effect on small-bodied mesopredators, including gobies, which were more abundant at SS and driving distinct species assemblages between the reef designs.

In addition to providing suitable conditions for reef species, results suggest that SS can also benefit soft-bottom taxa, possibly through a dispersed predator-mediated effect relative to SL. 5. Synthesis and applications. This study demonstrates that boulder reef restoration can strongly promote the abundance of exploited gadoids and is therefore a promising management tool to support top-down controls by predatory fishes in degraded marine systems.

The higher abundance of mesopredators at reefs with a ‘several small’ configuration suggests that the ‘Single Large or Several Small’ (SLOSS) dilemma could have long-term implications for trophic structure and resilience of restored habitats, and should therefore become an important facet within restoration strategies.