Lobster predation on barren-forming sea urchins is more prevalent in habitats where small urchins are common: a multi-method diet analysis

ContextIn Tasmania, Australia, the government's response to range-extending, barren-forming longspined sea urchin (Centrostephanus rodgersii) includes rebuilding of southern rock lobster (Jasus edwardsii) stocks to increase predation. But lobster preference for native species and continued barren expansion challenges the control efficacy.AimsTo determine the impact of lobster predation on C. rodgersii in different habitats.MethodsMulti-method dietary analysis consisting of stomach contents, faecal DNA and stable isotopes was performed on 64 lobsters from four habitats varying in barren extent and density of urchins and lobsters.Key resultsC. rodgersii contributed to lobster diet in all barren habitats and was found in lobsters of every size class. Stable isotope and DNA analyses showed that C. rodgersii was more common in lobster diet than were targeted native species at incipient barren sites. Surprisingly, in extensive barrens C. rodgersii is less prominent in lobster diet.ConclusionsCombined with site-specific urchin population data, our findings indicated that lobster predation may be less effective at sites where most C. rodgersii individuals have reached a size refuge than at sites dominated by small urchins.ImplicationsLobster predation may provide a useful control for smaller C. rodgersii, but top-down predatory control may be constrained at sites dominated by urchins that exceed the size suitable for lobster predation. Using a diverse set of methods, we investigated lobster diet in Tasmania to see whether lobsters prey on, and, consequently, contribute to the control of a destructive, range-extending sea urchin. We found that lobsters do consume the urchins, but to a lesser extent in the habitat where suitably sized urchins are scarce. These findings highlight the complexity of using native predators for pest control.