No abstract

Co-occurring foundation species can determine biological community structure via facilitation cascades. We examined the density dependencies of facilitation cascades, including how the density of a basal foundation species influences the density of secondary foundation species, and how the density of secondary foundation species influences community structure. The system in which we assessed density dependencies was a temperate mangrove forest in which pneumatophores trap the fucoid alga Hormosira banksii and provide substrate for the oyster, Saccostrea glomerata. The alga and oyster in turn determine benthic community structure. In the field, algal biomass was positively correlated with pneumatophore density. Oysters, by contrast, were highly over-dispersed and correlated with the presence/absence of pneumatophores. Epifaunal abundance and species richness were positively correlated with algal and oyster abundance, but their effects were independent. The positive effect of pneumatophore density on epifauna was primarily an indirect effect of trapping more algae. Pneumatophores did not directly influence invertebrate communities. Experiments revealed that, at very low pneumatophore densities, algal retention was insufficient to facilitate epifauna above that found on pneumatophores alone. At higher densities, however, increasing the density of pneumatophores increased algal retention, and the density and diversity of associated invertebrates. Shading by the mangrove canopy reduced algal biomass but did not modify the density-dependent nature of the cascade. Our results extend facilitation theory by showing that the density of both basal and secondary foundation species can be critical in triggering facilitation cascades. Our study also reveals that, where foundation species co-occur, multiple, independent cascades may arise from a single basal facilitator. These findings enhance our understanding of the role of density-dependent facilitation cascades in community assembly.

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.

Despite well-documented negative impacts of invasive species on native biota, evidence for the facilitation of native organisms, particularly by habitat-forming invasive species, is increasing. However, most of these studies are conducted at the population or community level, and we know little about the individual fitness consequences of recruitment to habitat-forming invasive species and, consequently, whether recruitment to these habitats is adaptive. We determined the consequences of recruitment to the invasive green alga Caulerpa taxifolia on the native soft-sediment bivalve Anadara trapezia and nearby unvegetated sediment. Initially, we documented the growth and survivorship of A. trapezia following a natural recruitment event, to which recruitment to C. taxifolia was very high. After 12 months, few clams remained in either habitat, and those that remained showed little growth. Experimental manipulations of recruits demonstrated that all performance measures (survivorship, growth and condition) were significantly reduced in C. taxifolia sediments compared to unvegetated sediments. Exploration of potential mechanisms responsible for the reduced performance in C. taxifolia sediments showed that water flow and water column dissolved oxygen (DO) were significantly reduced under the canopy of C. taxifolia and that sediment anoxia was significantly higher and sediment sulphides greater in C. taxifolia sediments. However, phytoplankton abundance (an indicator of food supply) was significantly higher in C. taxifolia sediments than in unvegetated ones. Our results demonstrate that recruitment of native species to habitat-forming invasive species can reduce growth, condition and survivorship and that studies conducted at the community level may lead to erroneous conclusions about the impacts of invaders and should include studies on life-history traits, particularly juveniles.

Increasing ocean temperatures are a threat to kelp forests in several regions of the world. In this study, we examined how changes in ocean temperature and associated nitrate concentrations driven by the strengthening of the East Australian Current (EAC) will influence the morphology, reproduction and development of the widespread kelp Ecklonia radiata in southeastern Australia. E. radiata morphology and reproduction were examined at sites in New South Wales (NSW) and Tasmania, where sea surface temperature differs by ~5°C, and a laboratory experiment was conducted to test the interactive effects of temperature and nutrients on E. radiata development. E. radiata size and amount of reproductive tissue were generally greater in the cooler waters of Tasmania compared to NSW. Importantly, one morphological trait (lamina length) was a strong predictor of the amount of reproductive tissue, suggesting that morphological changes in response to increased temperature may influence reproductive capacity in E. radiata. Growth of gametophytes was optimum between 15 and 22°C and decreased by > 50% above 22°C. Microscopic sporophytes were also largest between 15 and 22°C, but no sporophytes developed above 22°C, highlighting a potentially critical upper temperature threshold for E. radiata in Tasmania. Lower nitrate concentration had no effect on E. radiata gametophytes and sporophytes. Given forecast increases in ocean temperature of between 2 and 3°C in southeastern Australia by 2100, these findings suggest that E. radiata is likely to be affected by a strengthening EAC and highlight the susceptibility of the development and growth of early life-cycle stages to these changes.