Ecklonia radiata is one of the most widespread kelps globally, dominating temperate reefs throughout much of Australasia and southeastern Africa. Throughout much of its range, it is the only laminarian kelp and hence plays a key role in facilitating biodiversity and driving food webs, and it underpins immense ecological and socioeconomic values. This review synthesises the growing literature on E. radiata from its phylogeny and distribution through to its biology, ecology and recent changes.

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.

Habitat forming ‘ecosystem engineers’ such as kelp species create complex habitats that support biodiverse and productive communities. Studies of the resilience and stability of ecosystem engineers have typically focussed on the role of external factors such as disturbance. However, their population dynamics are also likely to be influenced by internal processes, such that the environmental modifications caused by engineer species feedback to affect their own demography (e.g. recruitment, survivorship). In numerous regions globally, kelp forests are declining and experiencing reductions in patch size and kelp density. To explore how resilience and stability of kelp habitats is influenced by this habitat degradation, we created an array of patch reefs of various sizes and supporting adult Ecklonia radiata kelp transplanted at different densities. This enabled testing of how sub-canopy abiotic conditions change with reductions in patch size and adult kelp density, and how this influenced demographic processes of microscopic and macroscopic juvenile kelp. We found that ecosystem engineering by adult E. radiata modified the environment to reduce sub-canopy water flow, sedimentation, and irradiance. However, the capacity of adult kelp canopy to engineer abiotic change was dependent on patch size, and to a lesser extent, kelp density. Reductions in patch size and kelp density also impaired the recruitment, growth and survivorship of microscopic and macroscopic juvenile E. radiata, and even after the provisioning of established juveniles, demographic processes were impaired in the absence of sufficient adult kelp. These results are consistent with the hypothesis that ecosystem engineering by adult E. radiata facilitates development of juvenile conspecifics. Habitat degradation seems to impair the ability of E. radiata to engineer abiotic change, causing breakdown of positive intraspecific feedback and collapse of demographic functions, and overall, leading to reductions in ecosystem stability and resilience well before local extirpation.