Patterns of demography for rocky-shore, intertidal invertebrates approaching their geographical range limits: tests of the abundant-centre hypothesis in south-eastern Australia

The abundant-centre hypothesis predicts that species\u27 abundances peak at the centre of their geographical ranges and decline gradually towards their range limits. We tested predictions of this hypothesis for three rocky-shore, intertidal invertebrates with planktonic larvae (the whelk, Morula marginalba, the snail, Afrolittorina pyramidalis, and the barnacle, Tesseropora rosea) by quantifying their patterns of abundance and size, and inferring pulses of recruitment from size-frequency distributions, at multiple spatial scales spanning a 600-km region in south-eastern Australia and encompassing roughly the southern third of their geographical ranges. At the regional scale, abundances for all species were, as predicted, dramatically lower at their range limits. This decline was not gradual, however, because there were large variations in abundance at smaller spatial scales, and abrupt declines at the south-eastern corner of Australia. Size did not change towards the range limit for any species, but size-frequency distributions suggested a decline in the frequency of recruitment events at the range limit for T. rosea. We conclude that the abundant-centre hypothesis is not an appropriate model for abundance distributions of benthic marine invertebrates with planktonic larvae, because of the vagaries of dispersal and recruitment interacting with complex current patterns along non-uniform coastlines

Public perceptions of mangrove forests matter for their conservation

Abstract not available

Manipulating thermal stress on rocky shores to predict patterns of recruitment of marine invertebrates under a changing climate

For rocky intertidal organisms, temperature is often considered the most influential factor governing early survival and growth. Nevertheless, our review of the literature revealed that few studies have manipulated temperatures in the field to test for effects on these critical early life history processes. Here, we present the results from a novel manipulation of substratum temperature using settlement plates of different colour (black, grey and white) and infrared measurements of temperature to test hypotheses that temperature influences the early survival and growth of recent settlers of the intertidal barnacle Tesseropora rosea. Mean surface temperatures of black and grey plates were as great as 5.8°C (on average 2.2°C) and 4.8°C (on average 1.6°C) hotter than white plates across the sampling period, respectively. Cooler, white plates had significantly greater settlement and early growth than hotter, black plates, but differences in plate temperature did not significantly influence early survival or recruitment, though patterns were consistent with thermal variability. Comparisons between grey coloured natural rock and plates indicate that grey plates thermally mimic natural rock. Nevertheless, on average, more than twice as many larvae settled on plates than on natural rock, but early post-settlement survival on natural rock was double that on plates, suggesting that this artificial surface may not adequately capture the natural variability in early life history processes. Regardless, our simple and repeatable thermal manipulation represents a useful tool for experimentally investigating the effects of temperature on recruitment processes and simulating future temperature variability associated with climate change

Differences in architecture between native and non-indigenous macroalgae influence associations with epifauna

Non-indigenous invaders may play ecologically similar roles to native species, and this may be reflected in the abundance, richness and composition of associated species assemblages. We investigated whether associations of epifauna with their macroalgal hosts differed between the non-indigenous Codium fragile ssp. fragile and native, congeneric C. fragile on three rocky shores in southeast Australia. Of the 38 taxa we recorded, 13 were unique to the native Codium and four to non-indigenous individuals. Holdfasts of non-indigenous Codium had double the taxon richness of epifauna compared to native holdfasts, and epifaunal abundances showed a similar but non-significant difference. Patterns of abundance and richness of epifaunal taxa on thalli of native and non-indigenous Codium varied depending on whether these measures were expressed per individual alga, thallus area or number of branches. The composition of epifaunal assemblages between native and non-indigenous Codium were significantly different, but differences among rocky shores were as great as those between macroalgal species. On all shores, two taxa, the gastropod Alaba opiniosa and gammarid amphipods, contributed most to compositional differences between native and non-indigenous Codium, and their abundances were influenced by branch number and associated epiphyte load. Host choice experiments manipulating the complexity and subspecies of Codium revealed that amphipods were more strongly influenced by branch number adjusted for epiphyte load than the identity of Codium. Our results highlight the importance of habitat features, such as structural complexity and associated epiphyte load, in determining whether native and non-indigenous species provide functionally equivalent habitats for associated assemblages

Supply-side biogeography: Geographic patterns of settlement and early mortality for a barnacle approaching its range limit

Species range limits are often associated with reduced adult densities, and this may reflect the failure of a particular life-history stage. For benthic marine invertebrates, settlement is a time of great mortality that strongly influences adult population structure, at least at local spatial scales. In south-eastern Australia we determined that adult abundance of the intertidal barnacle Tesseropora rosea declines over a 450 km region of rocky shore from the middle to the southern limit of its range, and we tested the hypothesis that this biogeographic pattern reflects variations in the production, settlement, or early post-settlement mortality of larvae or adult mortality. Sampling at 2 sites on 11 rocky shores in this region over 2 yr revealed that none of the life-history stages or demographic processes displayed a latitudinal gradient or a clear decline towards the south, and settlement and adult mortality were highly variable among locations. Indeed local variation in early lifehistory processes and adult mortality appears to dictate regional variability and observed latitudinal patterns of adult abundance of T. rosea, but longer term studies spanning at least a decade may determine if storage from one strong year in recruitment can set patterns of adult abundance

Temperature variability at the larval scale affects early survival and growth of an intertidal barnacle

For rocky intertidal invertebrates, the transition from pelagic larva to benthic settler represents a critical life-history stage characterised by high mortality. This mortality has been attributed to biotic factors such as predation or individual larval quality, as well as to abiotic factors such as thermal or desiccation stresses. Surprisingly little is known about how temperature varies at very fine spatial scales relevant to newly settled larvae. We used infrared (IR) imagery to determine (1) whether in situ rocky substrates during aerial exposure exhibit repeatable fine-scale (1 mm) temperature variation at the larval scale, and (2) whether the presence of adult conspecifics ameliorates effects of substratum temperature and promotes early growth and survival of settlers. We tracked the settlement and early survival of larvae to determine whether fine-scale variation in temperature influences early life history processes of the intertidal barnacle Tesseropora rosea. Larval settlement did not vary with fine-scale variation in rock temperature, but early post-settlement growth and survival were both inversely related to temperature. Furthermore, we found that rock temperatures decreased significantly with increasing proximity to adult T. rosea and that larvae that settled within 15 mm of adults survived better than those that settled within 16 to 30 mm, highlighting positive effects of gregarious settlement. This is partially explained by conspecific adults shading rock and reducing rock temperatures. We present the first use of IR technology to test for variation in rock temperature at a scale relevant to individual larvae, demonstrating that such fine-scale variation in thermal stress impacts the early-life history stages of a benthic marine invertebrate

Effects of stand size on pollination in temperate populations of the mangrove Avicennia marina

Populations of the mangrove Avicennia marina in the Sydney region exist as stands of varying size, reflecting both natural and anthropogenic fragmentation. We hypothesised that, as observed in many terrestrial forests, small stands (plants) would experience lower pollinator densities and altered pollinator behaviour and visitation and, in consequence, would display reduced pollen deposition as compared with large stands (\u3e10,000 plants). Nevertheless, we recognise that such predictions may be overly simplistic because within this region A. marina attracts a diversity of flower visitors, but its only significant pollinator is the exotic honeybee Apis mellifera. Moreover, it is unclear how readily A. mellifera moves among groups of plants within different mangrove stands of varying sizes separated either by water or urban habitat matrix. Our detailed surveys within pairs of large and small stands in two locations support the predictions that pollinator density and pollen deposition are reduced or altered within small stands. Within small stands honeybee abundance and pollen deposition were on average reduced significantly by 84 and 61 %, respectively. Moreover, within small stands there was a non-significant 12 % increase in the mean time that honeybees spent foraging on individual plants and hence potentially depositing self pollen. Taken together, our data indicate that fragmentation affects the performance of A. mellifera as a pollinator of A. marina and reduce pollinator abundance, leading to pollen limitation in small as compared to large stands, which may negatively affect reproductive output