Negative and positive effects of an invasive gastropod on seagrass performance and biodiversity

Estuarine seagrass meadows are ecologically and socio-economically valuable habitats, but face pressures from nutrient runoffs, drift algal accumulations, and introduced species. In the Swan River estuary (Western Australia), the introduced snail Batillaria australis has proliferated since initial observations in 1954. Here, we quantified the distribution and effects of B. australis on associated benthic communities. Field surveys revealed extremely high densities of B. australis in Halophila ovalis seagrass beds, increasing from 424 ± 28 m² in 2006 to 1212 ± 122 m² in 2011 but then decreasing to 824 ± 98 m² in 2023. Importantly, B. australis shells provided a ubiquitous and novel attachment substrate for the seaweed Gracilaria comosa. A factorial field experiment, testing for effects of B. australis, G. comosa, and nutrients, showed that G. comosa significantly reduced cover and biomass of H. ovalis. Furthermore, a second field experiment showed that G. comosa reduced H. ovalis density and biomass (although not significantly), dramatically altered benthic community structure, and increased the abundances of both mobile and sessile species in H. ovalis beds and adjacent sand flats. We suggest that the negative effect of G. comosa on seagrass, but positive effects on biodiversity, is indirectly controlled by B. australis through a habitat cascade, where the basal habitat-former B. australis provides substrate for the intermediate habitat-former G. comosa, which subsequently provides food and more habitat for many other species. Together, these findings highlight complex ecological roles of invasive species and underscore the need to consider both their positive and negative effects when developing estuarine conservation and management strategies.