Impacts of a non-native gastropod with a limited distribution; less conspicuous invaders matter too

Most studies focus on fast-spreading invasive species with high impacts, whilst invasive species with restricted distributions tend to be neglected in the ecological literature. The New Zealand gastropod, Zeacumantus subcarinatus, introduced into the Sydney region during the 1920-1930s, typically occurs in rock pools in the mid-to high intertidal. Surveys conducted in 2004 indicated Z. subcarinatus had a restricted distribution, but little is known about whether it is spreading, the environmental conditions that may promote or inhibit its spread, or its interactions with native species. Surveys conducted in 2012 and 2015 showed Z. subcarinatus maintained a similar distribution to that described in 2004. However, rock pools at sites where Z. subcarinatus was present or absent differed in key characteristics. Rock pools at sites lacking Z. subcarinatus had higher cover of macroalgae, reduced cover of bare substratum and higher densities of native gastropods. At invaded sites, there was a significant negative relationship between Z. subcarinatus and native gastropod densities. Experimental manipulation of Z. subcarinatus confirmed negative effects on the colonization of rock pools by the native gastropod Bembicium nanum, possibly via competition for resources. Impacts on B. nanum were discernable at low and high densities of Z. subcarinatus. By contrast, Z. subcarinatus responded positively to density manipulations of B. nanum-possibly higher densities of B. nanum indicate higher resource availability. We have shown that invasive species that have restricted distributions can have significant effects on native fauna and that they potentially deserve more research and management consideration than they currently receive.