Systematic culling controls a climate driven, habitat modifying invader

Long-term shifts in environmental conditions driven by climate change are predicted to persistently modify the distribution of a multitude of species. These range shifts can have significant effects on the functioning of ecological communities. Ocean warming along the southeast coast of Australia has seen a polewards shift in the distribution of the long-spined sea urchin Centrostephanus rodgersii. Barren areas formed by the destructive overgrazing of kelp beds by invading C. rodgersii are associated with a dramatic loss of habitat, species diversity and productivity. The ongoing range expansion of C. rodgersii will further increase sea urchin densities along the south eastern coast of Tasmania, where 'incipient' barrens have started to appear. As restoration of sea urchin barrens over a large-scale is costly and hard to achieve, effective management needs to focus on preventing further formation of extensive barrens. This study examines whether systematic culling of C. rodgersii in spatially discrete areas is effective to control their abundance. We show that systematic culling was highly effective in significantly reducing the mean density and aggregation of urchins in a discrete area which persisted for at least 12 months. Notably, there was no significant difference in the reduction of urchin density and aggregation when plots were culled twice rather than once. There was however an edge effect indicating a slow incursion of urchins back into the treatment plots. We also show that divers follow a classic prey-predator interaction with culling efficiency increasing with urchin density. Ultimately our results demonstrate that systematic culling is an effective method for controlling urchin abundance in discrete areas. The high cost and logistical constraints limit the application of a culling programme over large areas of reef. Culling, therefore, needs to be considered along with other management measures to control the effects of overgrazing by urchins over large scales.