Return of the lamingtons - boom-and-bust populations of the barrens forming sea urchin Tripneustes australiae on Lord Howe Island Marine Park

Abstract

Sea urchins in the genus Tripneustes have fast growth and boom-and-bust population dynamics. Tripneustes australiae is endemic to southeast Australia and northwest New Zealand and is currently undergoing a population increase (‘boom’ phase) in Lord Howe Island Marine Park (LHIMP). Grazing pressure in population outbreaks of this species creates barrens-type habitat. We used Reef Life Survey data to document the density of T. australiae spanning 18 years (2006-2024). Two outbreaks were evident, the first started in 2006 followed by a marked decline and the second started in 2022. The Admiralty Islands appear to be a recruitment hot spot for this species. The size of T. australiae in 2024 (most < 5 cm test diameter) indicated recent recruitment. Most individuals were found at 6-12 m depth and were juveniles as confirmed by the absence of gonads. Over 18 years there was no difference in the densities of T. australiae in ‘no take’ sanctuary zones and habitat protection zones of LHIMP with the exception of the recent outbreak in 2024. The drivers of the boom-and-bust population dynamics of T. australiae are likely related to inherent demographic traits of the genus associated with fast growth to reproductive maturity and a short life span. Recruitment anomalies of T. australiae to LHIMP may be related to climate warming and altered current connectivity from the Australian continent. The population dynamics of and ecological impacts T. australiae are important to understand in the high conservation value Lord Howe Island World Heritage Area ecosystems