Alistair G. B. Poore

The tropicalization of temperate marine ecosystems: climate-mediated changes in herbivory and community phase shifts

Climate-driven changes in biotic interactions can profoundly alter ecological communities, particularly when they impact foundation species. In marine systems, changes in herbivory and the consequent loss of dominant habitat forming species can result in dramatic community phase shifts, such as from coral to macroalgal dominance when tropical fish herbivory decreases, and from algal forests to ‘barrens’ when temperate urchin grazing increases. Here, we propose a novel phase-shift away from macroalgal dominance caused by tropical herbivores extending their range into temperate regions. We argue that this phase shift is facilitated by poleward-flowing boundary currents that are creating ocean warming hotspots around the globe, enabling the range expansion of tropical species and increasing their grazing rates in temperate areas. Overgrazing of temperate macroalgae by tropical herbivorous fishes has already occurred in Japan and the Mediterranean. Emerging evidence suggests similar phenomena are occurring in other temperate regions, with increasing occurrence of tropical fishes on temperate reefs.

Global patterns in the impact of marine herbivores on benthic primary producers

Despite the importance of consumers in structuring communities, and the widespread assumption that consumption is strongest at low latitudes, empirical tests for global scale patterns in the magnitude of consumer impacts are limited. In marine systems, the long tradition of experimentally excluding herbivores in their natural environments allows consumer impacts to be quantified on global scales using consistent methodology. We present a quantitative synthesis of 613 marine herbivore exclusion experiments to test the influence of consumer traits, producer traits and the environment on the strength of herbivore impacts on benthic producers. Across the globe, marine herbivores profoundly reduced producer abundance (by 68% on average), with strongest effects in rocky intertidal habitats and the weakest effects on habitats dominated by vascular plants. Unexpectedly, we found little or no influence of latitude or mean annual water temperature. Instead, herbivore impacts differed most consistently among producer taxonomic and morphological groups. Our results show that grazing impacts on plant abundance are better predicted by producer traits than by large-scale variation in habitat or mean temperature, and that there is a previously unrecognised degree of phylogenetic conservatism in producer susceptibility to consumption.

PREFERENCE–PERFORMANCE RELATIONSHIPS AND EFFECTS OF HOST PLANT CHOICE IN AN HERBIVOROUS MARINE AMPHIPOD

The fitness of herbivores that have limited mobility as juveniles or larvae may often depend on the host choice behavior of adults. For such herbivores, selection should favor adults that choose plants that maximize the performance of their offspring, resulting in positive correlations across host plants among adult preferences, offspring performance (growth, survival, etc.), and for herbivores that are restricted to living on host plants, population-level parameters such as abundance on different hosts. We tested this hypothesis for the marine, nest-building amphipod Peramphithoe parmerong, using a series of behavioral and performance assays and relevant field data. Adults displayed strong preferences among eight species of brown algae in habitat choice assays, with Sargassum linearifolium and S. vestitum highly preferred; Colpomenia peregrina and Padina crassa of lower preference; and Dictyopteris acrostichoides, Dictyota dichotoma, Dilophus marginata, and Zonaria diesingiana consistently avoided. Juvenile amphipods were relatively immobile and, thus, mostly constrained to the host alga selected by their mother. Differences in the growth and survival of juvenile amphipods raised on single-species diets were consistent with adult nest-building preferences among algae, with the best performance on the two high-preference species of Sargassum. Thus, adult preferences for host plants largely determined juvenile performance due to (1) restricted movement by juveniles and (2) differences among algal species in their effects on growth, survival, and onset of reproduction. In contrast to host plant preferences by adults, feeding rates on different algae were not as clearly correlated with juvenile performance. In particular, the low-preference C. peregrina was consumed at a high rate, but survivorship on this alga was relatively poor. Differences in abundances of P. parmerong on different host algae in the field were consistent with laboratory preferences and not related to relative algal abundance or epiphyte loads, supporting the hypothesis that there were population-level consequences of host plant choice in this species. With regard to mechanisms underlying host use, amphipod preferences were closely (negatively) correlated with the presence of nonpolar secondary metabolites. Nonpolar crude extracts from nonhost species, all of which contain secondary metabolites, affected amphipod host acceptance behavior, whereas extracts from host species, all of which lack such metabolites, did not. Overall, host plant use was unrelated to the nutritional value of the algal species, but one low-preference host, C. peregrina, was notably nutritionally poor. These observations, and the correlations among adult preferences, offspring performance, and field densities for P. parmerong, are consistent with intrinsic host plant qualities determining host plant range for this species. This contrasts with previous studies that emphasized the importance of extrinsic factors, particularly predation, in determining amphipod host use.

Adaptive Capacity of the Habitat Modifying Sea Urchin Centrostephanus rodgersii to Ocean Warming and Ocean Acidification: Performance of Early Embryos

Background Predicting effects of rapid climate change on populations depends on measuring the effects of climate stressors on performance, and potential for adaptation. Adaptation to stressful climatic conditions requires heritable genetic variance for stress tolerance present in populations. Methodology/Principal Findings We quantified genetic variation in tolerance of early development of the ecologically important sea urchin Centrostephanus rodgersii to near-future (2100) ocean conditions projected for the southeast Australian global change hot spot. Multiple dam-sire crosses were used to quantify the interactive effects of warming (+2–4°C) and acidification (−0.3−0.5 pH units) across twenty-seven family lines. Acidification, but not temperature, decreased the percentage of cleavage stage embryos. In contrast, temperature, but not acidification decreased the percentage of gastrulation. Cleavage success in response to both stressors was strongly affected by sire identity. Sire and dam identity significantly affected gastrulation and both interacted with temperature to determine developmental success. Positive genetic correlations for gastrulation indicated that genotypes that did well at lower pH also did well in higher temperatures. Conclusions/Significance Significant genotype (sire) by environment interactions for both stressors at gastrulation indicated the presence of heritable variation in thermal tolerance and the ability of embryos to respond to changing environments. The significant influence of dam may be due to maternal provisioning (maternal genotype or environment) and/or offspring genotype. It appears that early development in this ecologically important sea urchin is not constrained in adapting to the multiple stressors of ocean warming and acidification. The presence of tolerant genotypes indicates the potential to adapt to concurrent warming and acidification, contributing to the resilience of C. rodgersii in a changing ocean.

Direct and indirect effects of ocean acidification and warming on a marine plant–herbivore interaction

The impacts of climatic change on organisms depend on the interaction of multiple stressors and how these may affect the interactions among species. Consumer–prey relationships may be altered by changes to the abundance of either species, or by changes to the per capita interaction strength among species. To examine the effects of multiple stressors on a species interaction, we test the direct, interactive effects of ocean warming and lowered pH on an abundant marine herbivore (the amphipod Peramphithoe parmerong), and whether this herbivore is affected indirectly by these stressors altering the palatability of its algal food (Sargassum linearifolium). Both increased temperature and lowered pH independently reduced amphipod survival and growth, with the impacts of temperature outweighing those associated with reduced pH. Amphipods were further affected indirectly by changes to the palatability of their food source. The temperature and pH conditions in which algae were grown interacted to affect algal palatability, with acidified conditions only affecting feeding rates when algae were also grown at elevated temperatures. Feeding rates were largely unaffected by the conditions faced by the herbivore while feeding. These results indicate that, in addition to the direct effects on herbivore abundance, climatic stressors will affect the strength of plant–herbivore interactions by changes to the susceptibility of plant tissues to herbivory.

PHYLOGENETIC AND GEOGRAPHIC VARIATION IN HOST BREADTH AND COMPOSITION BY HERBIVOROUS AMPHIPODS IN THE FAMILY AMPITHOIDAE

Abstract Predicting the host range for herbivores has been a major aim of research into plant–herbivore interactions and an important model system for understanding the evolution of feeding specialization. Among many terrestrial insects, host range is strongly affected by herbivore phylogeny and long historical associations between particular herbivore and plant taxa. For small herbivores in marine environments, it is known that the evolution of host use is sculpted by several ecological factors (e.g., food quality, value as a refuge from predators, and abiotic forces), but the potential for phylogenetic constraints on host use remains largely unexplored. Here, we analyze reports of host use of herbivorous amphipods from the family Ampithoidae (102 amphipod species from 12 genera) to test the hypotheses that host breadth and composition vary among herbivore lineages, and to quantify the extent to which nonpolar secondary metabolites mediate these patterns. The family as a whole, and most individual species, are found on a wide variety of macroalgae and seagrasses. Despite this polyphagous host use, amphipod genera consistently differed in host range and composition. As an example, the genus Peramphithoe rarely use available macrophytes in the order Dictyotales (e.g., Dictyota) and as a consequence, display a more restricted host range than do other genera (e.g., Ampithoe, Cymadusa, or Exampithoe). The strong phylogenetic effect on host use was independent of the uneven distribution of host taxa among geographic regions. Algae that produced nonpolar secondary metabolites were colonized by higher numbers of amphipod species relative to chemically poor genera, consistent with the notion that secondary metabolites do not provide algae an escape from amphipod herbivory. In contrast to patterns described for some groups of phytophagous insects, marine amphipods that use chemically rich algae tended to have broader, not narrower, host ranges. This result suggests that an evolutionary advantage to metabolite tolerance in marine amphipods may be that it increases the availability of appropriate algal hosts (i.e., enlarges the resource base).

Ecosystem structure, function and composition in rangelands are negatively affected by livestock grazing

Reports of positive or neutral effects of grazing on plant species richness have prompted calls for livestock grazing to be used as a tool for managing land for conservation. Grazing effects, however, are likely to vary among different response variables, types, and intensity of grazing, and across abiotic conditions. We aimed to examine how grazing affects ecosystem structure, function, and composition. We compiled a database of 7615 records reporting an effect of grazing by sheep and cattle on 278 biotic and abiotic response variables for published studies across Australia. Using these data, we derived three ecosystem measures based on structure, function, and composition, which were compared against six contrasts of grazing pressure, ranging from low to heavy, two different herbivores (sheep, cattle), and across three different climatic zones. Grazing reduced structure (by 35%), function (24%), and composition (10%). Structure and function (but not composition) declined more when grazed by sheep and cattle together than sheep alone. Grazing reduced plant biomass (40%), animal richness (15%), and plant and animal abundance, and plant and litter cover (25%), but had no effect on plant richness nor soil function. The negative effects of grazing on plant biomass, plant cover, and soil function were more pronounced in drier environments. Grazing effects on plant and animal richness and composition were constant, or even declined, with increasing aridity. Our study represents a comprehensive continental assessment of the implications of grazing for managing Australian rangelands. Grazing effects were largely negative, even at very low levels of grazing. Overall, our results suggest that livestock grazing in Australia is unlikely to produce positive outcomes for ecosystem structure, function, and composition or even as a blanket conservation tool unless reduction in specific response variables is an explicit management objective.

Chemical defense in a marine alga: heritability and the potential for selection by herbivores

Herbivores have major impacts on many marine algae and are assumed to have been important forces driving the evolution of algal chemical defenses. However, an evolutionary change in chemical defenses in response to herbivory requires that there is both heritable variation for that trait and a response by herbivores to such variation. The subtidal red alga Delisea pulchra produces four main secondary metabolites (furanones) that vary in concentration at a variety of spatial and temporal scales. Here we determined the heritability of furanone concentrations and then, with a series of feeding experiments, determined how feeding by herbivores associated with D. pulchra varied as a function of nonpolar extract and furanone concentration. Total furanone concentration showed signif- icant broad-sense heritability (h 0.234), but broad-sense heritability of the four individual 2 C furanones varied in magnitude, ranging from 0.058 to 0.321. Compound 3, which is often the most abundant furanone, was the only one of the four furanones to show significant genetic variation. However, all four furanones were strongly genetically correlated, indi- cating that selection acting on any one of them will cause a corresponding change in the others. With the exception of the mesograzer gastropodPhasianotrochus eximius,herbivores generally consumed D. pulchra at lower rates compared to other common macroalgae. Macrograzers (sea urchins and large gastropods) were mostly deterred by nonpolar extract that contained levels of furanones that spanned the range of furanone variation in the field. The amphipod Ampithoe ngana was not deterred at the lowest extract concentration, and P. eximius was not deterred by any naturally occurring extract concentration. Compound 3 was deterrent to macrograzers at 3 and 5 mg/g dry mass, but was not deterrent at 1 mg/ g dry mass. Compound 3 was also not deterrent to A. ngana at any of those concentrations. For the most part D. pulchra in the field contains concentrations of furanones that are strongly deterrent to most herbivores. However, at some places and times, furanones do occur at lower concentrations that are not deterrent to herbivores. Given the heritable variation for furanones, when this overlap occurs there is the potential for selection by herbivores for higher concentrations of furanones in D. pulchra.

New Ampithoid Amphipods from Port Jackson, New South Wales, Australia (Crustacea : Amphipoda : Ampithoidae)

Seven species of ampithoid amphipods are reported from subtidal macroalgae living in Port Jackson, Sydney, Australia. In the genus Ampithoe the Indo–West Pacific species Ampithoe kava Myers, 1985, is recorded for the first time from Australia, and two new species, A. caddi, sp. nov. and A. ngana, sp. nov., are described. One new species ofCymadusa,C. munnu, sp. nov., is described. The genera Exampithoe (Melanesius), Peramphithoe and Plumithoe are recorded for the first time from Australian waters. The new species E. (M.) kutti, sp. nov. and Peramphithoe parmerong, sp. nov. are described, and Plumithoe quadrimana (Haswell, 1879b), comb. nov. is redescribed and a neotype is selected. New ecological and behavioural information is presented for these species. A new key and diagnoses for all known genera of Ampithoidae are presented. Paradusa Ruffo, 1969 is synonomised with Cymadusa Savigny, 1816.Exampithoe (Melanesius) gracilipes Ledoyer, 1984 is transferred to Exampithoe (Exampithoe) and Cymadusa uncinata Stout, 1912 and C. variata Sheard, 1936 are transferred to Paragrubia.

Contamination of marine biogenic habitats and effects upon associated epifauna

Habitat-forming organisms are frequently used as biomonitors in marine environments due to a widespread ability to accumulate toxic contaminants. Few studies, however, have considered the consequences of these accumulated contaminants on the abundant and diverse fauna associated with these habitats. In this review, we summarize research which has investigated the contamination of biogenic habitats (including seagrasses, macroalgae, ascidians, sponges and bivalve reefs) and the impact of this contamination on the habitat use, feeding behaviour and survival of associated epifauna. In many cases, ecological impacts upon epifauna are not simply predicted by levels of contamination in their habitat, but are determined by the foraging, feeding and reproductive behaviours of the inhabiting organisms. Thus, a thorough understanding of these ecological processes is essential in order to understand the effects of contaminants upon epifaunal communities. The scope of biomonitoring studies which assess the contamination of biogenic habitats should be expanded to include an assessment of potential effects upon associated epifauna. When combined with manipulative field experiments such an approach would greatly assist in our understanding of indirect effects of contaminants in these important benthic habitats.