Paul E. Gribben

Behavioural interactions between ecosystem engineers control community species richness

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.

Density-dependent facilitation cascades determine epifaunal community structure in temperate Australian mangroves

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.

MORPHOMETRIC RELATIONSHIPS, GAMETOGENIC DEVELOPMENT AND SPAWNING OF THE GEODUCK CLAM PANOPEA GLOBOSA (BIVALVIA: HIATELLIDAE) IN THE CENTRAL GULF OF CALIFORNIA

Abstract This paper describes morphometric relationships, and the timing of gametogenic development and spawning for the geoduck clam Panopea globosa (Dall 1898) from a population in the east central Gulf of California. Clams were collected monthly for a year (October 2004 to October 2005), and were measured and weighed to obtain morphometric relationships. Standard histological analysis and measurements of oocyte diameters were used to describe the timing of gametogenic development and spawning. Most morphometric variables were significantly correlated, however the coefficients of determination were generally low (<0.5) indicating high variation in measured traits. Early gametogenic development was observed in late summer/autumn when SST was high (∼30°C). Development increased as water temperatures fell with ripe individuals observed in early winter (∼20°C). Spawning occurred between January and February (winter), when SST were at their coolest (∼18°C). Cytological characteristics of the gonad and averages of oocyte diameters for the different reproductive phases were similar to other geoduck species previously described. Overall sex ratios were equal. This study is the first ecological study to be conducted for P. globosa. Given the development of fisheries for P. globosa and the interest in developing aquaculture for this species, the data provide valuable information for fisheries managers and aquaculturalists and represent the basis for further research on this species.

Less inhibited with age? Larval age modifies responses to natural settlement inhibitors

Abstract As larvae of marine invertebrates age, their response to settlement cues can change. This change can have significant consequences to both the ecology of these organisms, and to their response to antifouling coatings. This study examines how larval age affects the settlement response of larvae to two naturally derived settlement inhibitors, non-polar extracts from the algae Delisea pulchra and Dilophus marginatus, the former of which contains compounds that are in commercial development as antifoulants. Two species of marine invertebrates with non-feeding larvae were investigated: the bryozoans Watersipora subtorquata and Bugula neritina. Larval age strongly affected larval settlement, with older larvae settling at much higher rates than younger larvae. Despite having strong, inhibitory effects on young larvae, the non-polar extracts did not inhibit the settlement of older larvae to the same degree for both species studied. The results show that the effects of ecologically realistic settlement inhibitors are highly dependent on larval age. Given that the age of settling larvae is likely to be variable in the field, such age specific variation in settlement response of larvae may have important consequences for host-epibiont interactions in natural communities.

A decline in the abundance and condition of a native bivalve associated with Caulerpa taxifolia invasion.

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.

Sublethal effects on reproduction in native fauna: are females more vulnerable to biological invasion?

Although invasive species are a major threat to survivorship of native species, we know little about their sublethal effects. In soft-sediment marine systems, mat-forming invasive species often have positive effects, facilitating recruitment and enhancing the diversity and abundance of native invertebrates. However, because mat-forming invasive species change the habitat in which they invade, and benthic invertebrates are sensitive to environmental disturbance, important sublethal effects on native species may exist. Using a model marine system we show that the widespread mat-forming invasive alga Caulerpa taxifolia (Vahl) C. Agardh has strong negative effects on the reproductive traits of a native bivalve Anadara trapezia (Deshayes, 1840) (e.g. timing of reproductive development and spawning, and follicle and gamete production) even though the invader has positive effects on recruitment. Moreover, gender specific responses occurred and indicated that females were more susceptible to invasion than males. Our results indicate that sublethal effects of an invasive species on reproductive traits will have severe consequences for fitness of the native species.

Sydney Harbour: a review of anthropogenic impacts on the biodiversity and ecosystem function of one of the world's largest natural harbours

Sydney Harbour is a hotspot for diversity. However, as with estuaries worldwide, its diversity and functioning faces increasing threats from urbanisation. This is the first synthesis of threats and impacts in Sydney Harbour. In total 200 studies were reviewed: 109 focussed on contamination, 58 on habitat modification, 11 addressed non-indigenous species (NIS) and eight investigated fisheries. Metal concentrations in sediments and seaweeds are among the highest recorded worldwide and organic contamination can also be high. Contamination is associated with increased abundances of opportunistic species, and changes in benthic community structure. The Harbour is also heavily invaded, but invaders’ ecological and economic impacts are poorly quantified. Communities within Sydney Harbour are significantly affected by extensive physical modification, with artificial structures supporting more NIS and lower diversity than their natural equivalents. We know little about the effects of fishing on the Harbour’s ecology, and although ocean warming along Sydney is among the fastest in the world, we know little about how the ecosystem will respond to warming. The interactive and cumulative effects of stressors on ecosystem functioning and services in the Harbour are largely unknown. Sustainable management of this iconic natural system requires that knowledge gaps are addressed and translated into coherent environmental plans.

Morphological traits and density of foundation species modulate a facilitation cascade in Australian mangroves.

Facilitation cascades are critical to the maintenance of biodiversity in a variety of habitats. Through a series of two experiments, we examined how the morphological traits and density of interacting foundation species influence the establishment and persistence of a facilitation cascade in temperate Australian mangrove forests. In this system, mangrove pneumatophores trap the free-living alga, Hormosira banksii, which, in turn, supports dense and diverse assemblages of epifaunal mollusks. The first experiment, which manipulated pneumatophore height and density, revealed that these two traits each had additive negative effects on the establishment, but additive positive effects on the persistence of the cascade. High densities of tall pneumatophores initially served as a physical barrier to algal colonization of pneumatophore plots, but over the longer-term enhanced the retention of algae. The increased algal biomass, in turn, facilitating epifaunal colonization. The second experiment demonstrated that the retention of algae by pneumatophores was influenced more by algal thallus length than vesicle diameter, and this effect occurred independent of pneumatophore height. Our study has extended facilitation theory by showing that the morphological traits and density of basal and intermediary facilitators influence both the establishment and persistence of facilitation cascades. Hence, attempts to use foundation species as a tool for restoration will require an understanding not only of the interactions among these, but also of the key traits that modify interrelationships.

Sydney Harbour: What we do and do not know about a highly diverse estuary

Sydney Harbour is a global hotspot for marine and estuarine diversity. Despite its social, economic and biological value, the available knowledge has not previously been reviewed or synthesised. We systematically reviewed the published literature and consulted experts to establish our current understanding of the Harbour’s natural systems, identify knowledge gaps, and compare Sydney Harbour to other major estuaries worldwide. Of the 110 studies in our review, 81 focussed on ecology or biology, six on the chemistry, 10 on geology and 11 on oceanography. Subtidal rocky reef habitats were the most studied, with a focus on habitat forming macroalgae. In total 586 fish species have been recorded from the Harbour, which is high relative to other major estuaries worldwide. There has been a lack of process studies, and an almost complete absence of substantial time series that constrains our capacity to identify trends, environmental thresholds or major drivers of biotic interactions. We also highlight a lack of knowledge on the ecological functioning of Sydney Harbour, including studies on microbial communities. A sound understanding of the complexity, connectivity and dynamics underlying ecosystem functioning will allow further advances in management for the Harbour and for similarly modified estuaries around the world.

Personality and plasticity: consistent responses within-, but not across-temperature situations in crabs

Recent research suggests that repeated assays of behaviour, conducted both within and across situations, could reveal important insights into two traditionally distinct areas of study: animal personality and individual differences in behavioural plasticity. However, at present relatively few studies present such data, and few consider how changing abiotic conditions affect behavioural plasticity. Individual differences in metabolic rate have been suggested as a proximate mechanism promoting personality, leading one to speculate that individual differences in metabolic sensitivity to temperature may affect behavioural responses in ectotherms. At present, only one study (out of two) has tested for and shown individual differences in behavioural responses to temperature. Here, we repeatedly assayed the behaviour of a marine crab across a narrow range of temperatures to test for individual differences in responses to temperature. We observed large inter-individual differences in behaviour that were consistent over time at a given temperature (evidence for personality), and individual differences in responses to temperature (evidence for plasticity). This study adds to the very scant literature on ectotherm behavioural sensitivity to temperature, and suggests the phenomenon might be widespread. We speculate about the role of metabolism as a proximate mechanism that might explain these individual differences in plasticity and make suggestions for future research to test this hypothesis.