Behavioural interactions between ecosystem engineers may strongly influence community structure. We tested whether an invasive ecosystem engineer, the alga Caulerpa taxifolia, indirectly facilitated community diversity by modifying the behaviour of a native ecosystem engineer, the clam Anadara trapezia, in southeastern Australia. In this study, clams in Caulerpa-invaded sediments partially unburied themselves, extending >30% of their shell surface above the sediment, providing rare, hard substrata for colonization. Consequently, clams in Caulerpa had significantly higher diversity and abundance of epibiota compared with clams in unvegetated sediments. To isolate the role of clam burial depth from direct habitat influences or differential predation by habitat, we manipulated clam burial depth, predator exposure and habitat (Caulerpa or unvegetated) in an orthogonal experiment. Burial depth overwhelmingly influenced epibiont species richness and abundance, resulting in a behaviourally mediated facilitation cascade. That Caulerpa controls epibiont communities by altering Anadara burial depths illustrates that even subtle behavioural responses of one ecosystem engineer to another can drive extensive community-wide facilitation.

Caulerpa taxifolia is a fast-spreading invasive seaweed that threatens biodiversity in temperate Australian estuaries. To date, little is known about its effects on infauna. In the present study, we describe variation in demographic and life-history traits of the abundant infaunal bivalve, Anadara trapezia, in C. taxifolia and uninvaded habitats (seagrass and unvegetated sediments) at multiple sites across three estuaries in south-eastern New South Wales. Densities of A. trapezia were always lower in C. taxifolia than on unvegetated sediment, and lower in C. taxifolia than in seagrass at three out of four sites where they were compared. Dry tissue weight of A. trapezia was also lower in C. taxifolia than on unvegetated sediment at most sites, but was only lower in C. taxifolia than in seagrass at one of four sites. Populations were dominated by larger individuals (>45 mm length), but smaller individuals (35–45 mm length) were more common in C. taxifolia and seagrass. A. trapezia shell weight and morphology was variable and appeared weakly affected by invasion. Generally, our findings are consistent with the hypothesis that A. trapezia is negatively affected by C. taxifolia. However, C. taxifolia invasion appears complex and, at some places, its effects may not differ from those of native seagrass. There is a need for manipulative studies to understand the mechanisms underlying the effects of C. taxifolia on infauna.

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Summary 1.The global transfer of species by human vectors is continuing despite the use of managerial controls such as antifouling biocides and pesticide applications. The process of introduction now exposes species to novel conditions which may select for tolerance to a contaminant. Invader establishment success is influenced by both the supply of invasive propagules and disturbance. Therefore, it is important to understand whether tolerance to an anthropogenic disturbance, such as contamination, can change the parameters of supply in a way that inadvertently augments the invasion process. 2. To test whether the invasion process is influenced by a widespread contaminant, we investigated how recruitment of the invasive hull-fouling bryozoan Watersipora subtorquata is affected by exposure to copper-based antifouling paint. We quantified settlement patterns in control and copper environments and then assessed post-settlement survival and fitness components. 3. Copper significantly increased total recruitment success despite greater post-settlement mortality. Surviving recruits differed morphologically, with shorter ancestrulae and smaller colonies in high copper treatments. These results show a strong positive affiliation between larval W. subtorquata and high levels of copper although there are associated fitness costs. 4. Syntheses and applications. We found a direct positive effect of contamination on recruitment of a common invasive species. This process is likely to be relevant to other non-indigenous species (NIS) that exhibit a positive affiliation with metal contamination. Copper can potentially enhance success at multiple stages of the invasion process, including facilitating transport and establishment, by increasing the supply and retention of individuals into anthropogenically disturbed environments. Identification of tolerance to contamination as a species trait may also aid in predicting a species invasiveness and spread. Management of metal pollution through remediation and alternative copper-free antifouling techniques would help prevent the spread and establishment of many marine NIS.

The global spread of invasive species may be facilitated by adaptation to the practices that humans use to manage those species. For example, marine invertebrates that adapt to metal-based antifouling biocides on ship hulls may be more likely to be introduced to and establish in metal-polluted environments. We tested this idea by studying clonal variation in tolerance to, and ability to recover from, exposure to copper in a widespread invasive marine bryozoan, Watersipora subtorquata. We cloned colonies of this organism to independently test multiple environments in a genotype by environment design, and then created a genetic variance–covariance matrix. Genotypes were exposed to a gradient of copper concentrations and growth measured during exposure and after a recovery period. There was a significant genotype × environment interaction in growth during exposure and recovery. We found clonal variation in tolerance and ability to recover from exposure to copper, with growth during exposure apparently trading off against growth after exposure. A weak genetic correlation between growth during and after exposure further indicated that they are separate traits. Overall, the genetic variation within this population indicates that there is considerable potential for adaptation to copper, but this comes at a cost to growth in unpolluted environments.