Invasive seaweed enhances recruitment of a native bivalve: roles of refuge from predation and the habitat choice of recruits

Abstract

Invasive species may have a range of negative effects on native species in the region invaded. The invasive green alga Caulerpa taxifolia has invaded several temperate regions worldwide and now occurs in 9 estuaries in temperate eastern Australia. Despite the threat posed by C. taxifolia, virtually nothing is known of its effects on native estuarine infauna. In the present study, we investigated the distribution and abundance, habitat choice and predation of recruits (post-set juveniles) of the native Sydney cockle Anadara trapezia at 2 sites invaded by C. taxifolia in Lake Conjola, New South Wales, Australia. Recruitment of A. trapezia was significantly higher in C. taxifolia (both with sparse [30%] and with dense [100%] cover) than in Zostera capricorni and bare sediment. Up to 680 recruits m -2 were observed in C. taxifolia, with the highest recruit densities occurring at intermediate C. taxifolia densities. However, in habitat choice experiments, recruits showed no preference for C. taxifolia over the seagrasses Z. capricorni and Halophila ovalis, but a strong preference for adult A. trapezia over all macrophytes when A. trapezia were included as treatments in experiments. Field data showed that adult A. trapezia in bare sediments had very few recruits attached to them (< 0.07 recruits adult -1 ) compared to adults found within C. taxifolia (0.95 ± 0.25 and 1.23 ± 0.31 recruits adult -1 at the 2 sites). Given that recruits displayed a strong preference for adults, but were rare on adults living in bare sediments, we tested whether C. taxifolia provided a refuge from predation. Recruits on adults placed into a bare sediment habitat were all consumed by predatory fish within 15 min, whereas none of the recruits on adults in an adjacent C. taxifolia habitat were eaten. Our results show that recruits do not reject C. taxifolia as a recruitment substratum despite showing preference for adult conspecifics, and that C. taxifolia may, by providing recruits with a refuge from predation enhance the recruitment of A. trapezia compared to uninvaded sediment. Although the long-term consequences of this enhanced recruitment are unknown, our study demonstrates that, contrary to commonly held views, the effects of C. taxifolia are not always negative.

Larval settlement preference of a native bivalve: the influence of an invasive alga versus native substrata

“…seagrass, infaunal bivalves, fish and epibiota on hard substrata) are largely negative (Bellan-Santini et al 1996, Levi & Francour 2004, Longepierre et al 2005, Gribben & Wright 2006b, York et al 2006, Wright et al 2007, C. taxifolia has positive effects on the recruitment of Anadara trapezia (Gribben & Wright 2006a), epibionts on the shells of adult A. trapezia (Gribben et al 2009b) and epifauna in general (McKinnon et al 2009). In 2005, high abundances of post-settlement juveniles (recruits) of A. trapezia were observed in C. taxifolia (Gribben & Wright 2006a) compared to unvegetated sediments, adult A. trapezia and the native seagrass Zostera capricorni in Lake Conjola, New South Wales. Although C. taxifolia offered a refuge from predation by fish (Gribben & Wright 2006a), recruit abundance may also reflect preferential settlement of A. trapezia larvae onto C. taxifolia.…”

“…Provision of a refuge from predation and added habitat complexity are important mechanisms driving facilitation by habitat-forming species (Crooks 1998, 2002, Gribben & Wright 2006a. However, active habitat selection by larvae may also play a role in the facilitation of native fauna by habitat-forming invasive species.…”

“…A. trapezia are generally estuarine, occurring from the intertidal to shallow subtidal (0 to 2 m water depth) in sheltered mud, sand and seagrass habitats. A. trapezia are epifaunal as recruits and infaunal as adults (Gribben & Wright 2006a, Wright et al 2007.…”

“…Marine soft-sediments with invasive habitat-forming mussels (Crooks 1998) and macrophytes (Posey 1988, Neira et al 2005, Gribben & Wright 2006a often have higher invertebrate recruitment, species diversity and abundance compared to nearby unvegetated sediments. Provision of a refuge from predation and added habitat complexity are important mechanisms driving facilitation by habitat-forming species (Crooks 1998, 2002, Gribben & Wright 2006a.…”

Science, biodiversity and Australian management of marine ecosystems

“…Benthic communities contain organisms with a diversity of life strategies and spans and so they may be far more responsive to environmental changes than others. Some advancement has been made in understanding the impact of introduced species in nearshore shallow water environments (Galluci et al, in press;Gribben and Wright, 2006;Gribben et al, 2009) but we know far less about the impacts of introduced species in deeper water. Deeper water benthic communities are almost completely unstudied and even benthic communities closer inshore are poorly studied especially with regard to natural pertubations over time (Stephenson et al, 1970;Hutchings and Jacoby, 1994).…”