Limited habitat and conservation value of a young artificial reef
Loss and degradation of natural habitats and their biodiversity may, arguably, be mitigated or compensated through the creation of human-engineered habitats: the underlying conservation tenet is that these artificial habitats compensate for diminished diversity caused by human impacts at local or regional scales. This approach is widely used in the sea by purposefully scuttling ships to create artificial reefs, but its performance as a conservation tool is seldom critically examined in these situations. Here we test if the diversity of sessile invertebrate assemblages on a large, but young (3 years), artificial reef, created by sinking a 133 m long battle ship off Eastern Australia, can mimic that of nearby natural reefs. We use this system as a model to test whether this artificial reef can form compensatory habitat of comparable quality and levels of biodiversity. Our assessment is based on the abundance, species richness, and species composition of sessile invertebrate assemblages, including corals. Despite some signs that temporal trajectories of ecological metrics, such as cover, began to approach natural conditions after 3 years, the ecological structure of sessile invertebrate assemblages on this young wreck remained fundamentally different from those on nearby natural reefs. In particular, large, long-lived corals were abundant on natural rocky reefs, but were rare and covered little area on the young wreck. These data demonstrate that when trajectories to community convergence with natural habitats are prolonged, as may be the case here, any compensatory effects of artificial habitats will have a considerable time lag. Such lags have implications for appraising the conservation value of wrecks and artificial reefs, and they emphasize the need to explicitly acknowledge temporal dynamics when using artificial habitats as complementary conservation tools to augment larger conservation efforts on natural systems.