Love thy neighbour? Depends on the neighbours and outside threats: diversity and patch dynamics of iconic encrusting species in (former?) cave and archway refugiaClimate Change Refugia

Chris Battershill¹, David Schiel²

¹University of Waikato, New Zealand - ²University of Canterbury, New Zealand

The Poor Knights Islands in northeaster New Zealand, one of the world’s most spectacular diving locations, is renowned for its great diversity of encrusting species associated with an extensive array of vertical reef walls, caves, and archways. This underwater topography is well-suited for examining encrusting species distributions and patch size dynamics across a variety of biophysical regimes. Over decades of occasional sampling, distinct patterns in species assemblages and their patch sizes are clearly evident and attuned to prevailing physical conditions, further influenced by competition for space. These dynamics appeared to be long-lasting in terms of basic pattern where, regardless of phyla, patch size reflected relative position on reef walls, with respect to depth, or distance into the reef wall system. Patch size also exhibited similar changes regardless of species/phyla, with respect to wave shock, light and currents; that is, at any one position on a reef wall, most species were of comparable patch size. These encrusting communities have a high degree of endemism, with many species (especially of sponges) still not described taxonomically. However, a recent and significant threat is an outbreak and expansion of the diadematid urchin Centrostephanus rodgersi, which has devastated kelp communities in eastern Tasmania. At the Poor Knights, it grazes extensively on encrusting species, both on walls and beneath kelp canopies. The rise in ocean temperatures and repetitive marine heat waves pose real threats to such iconic assemblages.