G. A. Skilleter

Variability at different spatial scales between a subtidal assemblage exposed to the discharge of sewage and two control assemblages

Abstract Environmental disturbances can alter the variability of assemblages of organisms in impacted sites compared to control sites. It has therefore been proposed that increased variability might be an important feature of stressed populations. Increased variability may be due to changes in the population structure of individual species or changes in the suite of species. In this study, spatial variances of shallow subtidal assemblages of organisms inhabiting vertical cliff-faces were compared among two control locations and one location that had for many years been exposed to the discharge of sewage. These assemblages covered nearly all available space on the substratum and consisted primarily of encrusting and foliose macro-algae and numerous filterfeeding animals, such as ascidians, sponges and bryozoans. Mean differences in abundances between these locations were investigated using Beyond BACI designs. In addition, these locations were used to examine the model that assemblages are more variable in disturbed than undisturbed environments and to try to distinguish differences in variability due to differences in the population structure of individual species from that due to changes in species composition. The assemblages were sampled at two spatial scales at each of three depths in each location. There were significant differences between the polluted location and one or other of the control locations in the mean abundances of some organisms, the variances of certain species (or recognisable types) at each spatial scale and for multivariate measures of species composition. There was, however, no evidence to support the prediction of increased variability in the apparently polluted location compared to the control locations. Importantly, for many measures of abundance and variability, the control locations were as different from each other as they were from the polluted location, suggesting that the latter fell within the range of natural subtidal assemblages separated by these spatial scales. These findings emphasise the need to include more than one control location in any study of a potential environmental impact, so that any effects of that impact can be distinguished from the range of natural variability.

Effects of habitat modification in mangroves on the structure of mollusc and crab assemblages.

The abundance and species richness of mollusc and crab assemblages were examined in a subtropical mangrove forest in Moreton Bay, Queensland, Australia, which has been disturbed and damaged by the construction of a wooden boardwalk and a path. Sections of the forest immediately adjacent to the boardwalk and path were compared with reference areas to determine whether changes to the small-scale structural complexity within the forest affected the benthic fauna. The disturbed area was characterised by having 65-80% fewer pneumatophores, significantly fewer species and individuals of molluscs, but significantly more species and individuals of crabs than the reference areas. The abundance of mangrove pneumatophores and the attached epiphytic algae were manipulated at two sites to determine whether observed differences in these features could account for the differences in the assemblage of molluscs in the disturbed area of the forest compared with reference areas. Five experimental treatments were used: undisturbed controls, pneumatophore removals (abundance reduced by ca. 65%), epiphytic algal removals (algae removed from ca. 65% of pneumatophores), pneumatophore disturbance controls and algal disturbance controls. The experimental reduction of the abundance of mangrove pneumatophores and the associated epiphytic algae led to significant declines (by as much as 83%) in the number of molluscs utilising the substratum in the modified plots. There was no significant difference in the abundance of molluscs in the pneumatophore and algal removal plots suggesting any effect was primarily related to removal of the epiphytic algae from the surface of the pneumatophores. The responses by the biota to the changes in the physical environment demonstrate that even relatively small-scale modifications to the physical structure of subtropical mangrove forests can lead to significant effects on the diversity and abundance of macrobenthic organisms in these habitats. Such modifications have the potential to cause cascading effects at higher trophic levels with a deterioration in the value of these habitats as nursery and feeding grounds. Future efforts at conservation of these estuarine environments must focus on the prevention or reduction of modifications to the physical structure and integrity of the system, rather than just on the prevention of loss of entire patches of habitat

Differences in benthic fauna and sediment among mangrove (Avicennia marina var. australasica) stands of different ages in New Zealand

Management of coastal environments requires understanding of ecological relationships among different habitats and their biotas. Changes in abundance and distribution of mangroves, like those of other coastal habitats, have generally been interpreted in terms of changes in biodiversity or fisheries resources within individual stands. In several parts of their range, anthropogenically increased inputs of sediment to estuaries have led to the spread of mangroves. There is, however, little information on the relative ecological properties, or conservational values, of stands of different ages. The faunal, floral and sedimentological properties of mangrove (Avicennia marina var. australasica) stands of two different ages in New Zealand has been compared. Older (>60 years) and younger (3-12 years) stands showed clear separation on the basis of environmental characteristics and benthic macrofauna. Numbers of faunal taxa were generally larger at younger sites, and numbers of individuals of several taxa were also larger at these sites. The total number of individuals was not different between the two age-classes, largely due to the presence of large numbers of the surface-living gastropod Potamopyrgus antipodarum at the older sites. It is hypothesized that as mangrove stands mature, the focus of faunal diversity may shift from the benthos to animals living on the mangrove plants themselves, such as insects and spiders, though these were not included in the present study. Differences in the faunas were coincident with differences in the nature of the sediment. Sediments in older stands were more compacted and contained more organic matter and leaf litter. Measurement of leaf chemistry suggested that mangrove plants in the younger stands were able to take up more N and P than those in the older stands

Effects of patch-size on the structure of assemblages in rock pools

Abstract Rock pools represent patches or islands of habitat different from the surrounding shore. They contain diverse assemblages of invertebrates and algae. The shape, size, height on the shore and exposure to waves are considered important influences on the abundances of organisms. In studies of natural pools, these factors are confounded because pools are rarely comparable; they differ in size, position on the shore and history of colonization. To solve this, experimental pools of different diameters (15, 30 and 50 cm) and depths (5, 15 and 30 cm) were drilled in sandstone rocky shores in Botany Bay, New South Wales, Australia. Replicate pools of each size were made in each of 24 randomly-chosen sites, six sites representing each combination of two heights (mid and low) on the shore and sheltered versus wave-exposed habitats. The influence of diameter of the pool is examined here, using data from a total of 360 experimental pools. Despite a 2–3-fold increase in surface-area of pools with increased diameter, there was no difference in densities of most taxa examined (gastropods, a starfish, sessile invertebrates, algae). An exception was foliose algae, the cover of which was generally greater low on the shore and in 50 cm than 30 cm diameter pools. There were also consistently more species in any stratum (i.e., depth in a pool) in the wider pools. There were approximately 25–50% more species in the widest than the narrowest pools, consistently from season to season and year to year. There were, however, significantly fewer species per unit area of a given stratum in pools of 50 versus 30 versus 15 cm diameter. These results require further investigation in relation to different theories to explain species-area relationships. Multivariate analyses revealed no influence of diameter on the structure of assemblages of organisms in pools of different diameter. This was true for different strata in pools and at mid- or low-shore positions on exposed or sheltered shores. Diameter of pools thus had remarkably little influence on the organisms; those effects found were consistent. Experimental pools were shown to be effective for testing hypotheses about proposed influences on the ecology of inhabitants. The findings about diameter of pools greatly simplify analyses of influences of depth and physical features of the surrounding habitat.

Mapping intertidal habitats and an evaluation of their conservation status in Queensland, Australia

The goal of biodiversity conservation has been described as the conservation of diversity at three levels: ecosystem, species and genetic diversity. Developing a representative system of marine protected areas (MPAs) is considered an effective way to achieve this goal in the marine environment. In the absence of detailed information relating to biological distributions there has been increasing use of biodiversity surrogates to determine MPA priorities at regional levels. The development of biodiversity surrogates at fine scales (i.e. habitats) will have an increasingly important role in the identification of sites that will contribute to a representative system of MPAs. This is because it will increase the likelihood that the system will adequately achieve biodiversity objectives by ensuring protection of a greater range of habitats and species. This article provides an explanation of an intertidal shoreline habitat surrogate used to describe 24,216km of Queenslands coastline. The protective status of intertidal habitats was evaluated to assist with designing a representative system of intertidal MPAs

Critical research needs for managing coral reef marine protected areas: Perspectives of academics and managers

Marine protected areas (MPAs) are a primary policy instrument for managing and protecting coral reefs. Successful MPAs ultimately depend on knowledge-based decision making, where scientific research is integrated into management actions. Fourteen coral reef MPA managers and sixteen academics from eleven research, state and federal government institutions each outlined at least five pertinent research needs for improving the management of MPAs situated in Australian coral reefs. From this list of 173 key questions, we asked members of each group to rank questions in order of urgency, redundancy and importance, which allowed us to explore the extent of perceptional mismatch and overlap among the two groups. Our results suggest the mismatch among MPA managers and academics is small, with no significant difference among the groups in terms of their respective research interests, or the type of questions they pose. However, managers prioritised spatial management and monitoring as research themes, whilst academics identified climate change, resilience, spatial management, fishing and connectivity as the most important topics. Ranking of the posed questions by the two groups was also similar, although managers were less confident about the achievability of the posed research questions and whether questions represented a knowledge gap. We conclude that improved collaboration and knowledge transfer among management and academic groups can be used to achieve similar objectives and enhance the knowledge-based management of MPAs.

Validation of rapid assessment of damage in urban mangrove forests and relationships with molluscan assemblages

Six mangrove forests in the urban region of Sydney, Australia, were examined to determine whether there were relationships between levels of human-induced damage and the structure of molluscan assemblages, the first stage in the validation of a technique for rapid assessment of the impacts of anthropogenic disturbance in urban mangroves. All six mangrove forests showed some damage but the amount varied considerably among and within each forest. Significant correlations between the levels of damage and abundance of different taxa, and changes in the structure of the molluscan assemblage along gradients of damage, suggest that indices based on measures of damage have the potential to provide a means for the rapid, wide scale evaluation of mangroves affected by impacts associated with human activities in urban areas.

Aspects of the biology of the yabby Trypea australiensis (Dana) (Decapoda: Thalassinidea) and the potential of burrow counts as an indirect measure of population density

The thalassinidean shrimp Trypea australiensis(the yabby) commonly occurs on intertidal sandflats and subtidal regions of sheltered embayments and estuaries along the east coast of Australia and is harvested commercially and recreationally for use as bait by anglers. The potential for counts of burrow openings to provide a reliable indirect estimate of the abundance of yabbies was examined on intertidal sandflats on North Stradbroke Island (Queensland, Australia). The relationship between the number of burrow openings and the abundance of yabbies was generally poor and also varied significantly through time, casting doubt on previous estimates of abundance for this species based on unvalidated hole counts. Spatial and temporal variation in population density, the size at maturity and the reproductive period of the yabby were also assessed. Except for an initial peak in abundance as a result of recruitment, the density of yabbies was constant throughout the study but considerably less than that estimated from a previous study in the same area. Ovigerous females were recorded at 3 mm carapace length (CL) which is smaller than previously recorded for this species and thalassinideans in general. Small ovigerous females were found throughout the study, including the summer months, which is unusual for thalassinideans in the intertidal zone. It was hypothesised that in the intertidal zone, small female yabbies may be able to balance the metabolic demands of reproduction and respiration at higher temperatures than can larger females allowing them to reproduce in the warmer months.

Convergent antifouling activities of structurally distinct bioactive compounds synthesized within two sympatric Haliclona demosponges.

A wide range of sessile and sedentary marine invertebrates synthesize secondary metabolites that have potential as industrial antifoulants. These antifoulants tend to differ in structure, even between closely related species. Here, we determine if structurally divergent secondary metabolites produced within two sympatric haliclonid demosponges have similar effects on the larvae of a wide range of benthic competitors and potential fouling metazoans (ascidians, molluscs, bryozoans, polychaetes, and sponges). The sponges Haliclona sp. 628 and sp. 1031 synthesize the tetracyclic alkaloid, haliclonacyclamine A (HA), and the long chain alkyl amino alcohol, halaminol A (LA), respectively. Despite structural differences, HA and LA have identical effects on phylogenetically disparate ascidian larvae, inducing rapid larval settlement but preventing subsequent metamorphosis at precisely the same stage. HA and LA also have similar effects on sponge, polychaete, gastropod and bryozoan larvae, inhibiting both settlement and metamorphosis. Despite having identical roles in preventing fouling and colonisation, HA and LA differentially affect the physiology of cultured HeLa human cells, indicating they have different molecular targets. From these data, we infer that the secondary metabolites within marine sponges may emerge by varying evolutionary and biosynthetic trajectories that converge on specific ecological roles.

Intra- and inter-specific competition for food in infaunal coral reef gastropods

Deposit-feeding gastropods have mostly been studied in the temperate northern hemisphere where these animals are typically found in fine muddy sediments, presumably as a response to the amount of food available. At One Tree Reef, in the southern Great Barrier Reef, deposit-feeding cerithiid gastropods, Rhinoclavis aspera (Linnaeus, 1758), R. fasciata (Bruguiere, 1792) and R. vertagus (Linnaeus, 1758), are most abundant in an atypical environment, the coarse sandy sediments of a back-reef sandflat. Densities in the finer sediments of the lagoon are several orders of magnitude smaller than on the sandflat. In this study, these three species were enclosed at a range of naturally occurring densities in fine and coarse sediments to investigate their competitive interactions and the influence of sediment on their growth. The type of substratum in experimental enclosures influenced the growth of each species. Growth was greatest in the fine sediments from the lagoon, despite the fact that recruitment of each species had been shown to be greatest into the coarse sediments of the sandflat. Adult R. aspera significantly reduced the growth of juveniles of the three species, but adults of R. fasciata and R. vertagus had no significant effect on the growth of juveniles of R. aspera. These results are probably a function of the large disparity in the densities of the three species: R. aspera occur in densities an order of magnitude greater than the other two species and naturally occurring densities were used in the experiments. Competition for food or other expendable resources was distinguished from direct interference competition for space by including in our experiments treatments with dead shells occupying the same volume as live adults, but not utilizing expendable resources. Direct competition for space between adults and juveniles of R. aspera is unlikely to be important as a factor affecting the growth of juvenile R. aspera. Dead shells had no significant effect on the growth of juveniles in coarse or fine sediment. Direct competition for space may be more important for R. fasciata and R. vertagus because, in some cases, the presence of dead shells reduced the growth of juveniles. These experiments demonstrate how complex competitive interactions among Rhinoclavis spp. are modified by the type of sediment and vary with the particular combination of competitors. Research in rocky intertidal systems has similarly shown how the physical environment may modify the nature of biological interactions such as competition and predation. Our results are consistent with findings from temperate systems in that growth of Rhinoclavis spp. was limited by the availability of food or a related resource such as the availability of suitably sized particles of sediment for ingestion. This system differs, however, because deposit-feeding snails occur in their greatest densities in a habitat where they have the slowest rates of growth. Further work is needed to explain these patterns of distribution and abundance and why they differ from those typical of studies in temperate systems in the northern hemisphere.