Exceeding the tipping point of range-extension meltdown

Coincident with recent ocean warming there has been a dramatic and ongoing increase in the occurrence of warmer-water marine species in eastern Tasmania. While the ecological impact of these ‘range-extending’ species remains largely ambiguous, the extension of the sea urchin Centrostephanus rodgersii is leading to dramatic and persistent ecological change as urchin abundance exceeds the critical tipping-point of kelp bed overgrazing. Here I show how this collapsed urchin ‘barren’ reef state is subsequently triggering wholesale ‘tropicalization’ of reef fish communities in north eastern Tasmania; as revealed by comparing standardised underwater visual census of reef fishes in kelp beds and barrens sampled during the winters of 2002 and 2014. Over this period, overwhelming establishment of ‘warm-affinity’ reef fishes has occurred with the presence of only a single such species in 2002 increasing to a total of 13 in 2014, with 12 fishes establishing on barrens yet only 2 within kelp beds. Increases in the abundance of warm-affinity fishes also show similar trends and the overall consequences for ecological function are explored. Derived from the timing of standardised ‘first-sightings’ across eastern Tasmania over the past 17 years, rates of range-extension for reef fishes also match that of the urchin itself. In combination with prior research demonstrating the impact of ecological overfishing of sea urchin predators, the current findings redouble the importance of rebuilding local resilience to not only reduce risk of collapse for kelp beds and associated native biodiversity, but also to resist a range-extension ‘meltdown’ under a rapidly warming ocean.