Filtration of the enteric bacteria Escherichia coli by two filter-feeding bivalves,Venus verrucosa and Mytilus galloprovincialis

We investigated the filtration and utilization of the enteric bacteria Escherichia coli by two suspensionfeeding bivalves, Venus verrucosa (Linné), collected, during April 1990, in Port-Vendres harbour (France), and Mytilus galloprovincialis (Lamarck) collected from rearing units in the bay of Banyuls-sur-mer during June 1990. Because of a significant decrease in the concentration of culturable E. coli in filtered seawater, we used ¹⁴C glutamic acid to label the bacteria. Labelling efficiency was low (20%) compared to the 30% reported for the marine bacteria Lactobacillus sp. by Amouroux (1982). However, the labelling by this radioisotope was very stable, enabling its use to monitor filtration. Concentrations of culturable E. coli decreased more rapidly in the presence of M. galloprovincialis than in the presence of V. verrucosa. In both bivalve species, changes of radioactivity within the bivalve, particulate organic matter (POM), dissolved organic matter (DOM) and CO₂ compartments were similar and resulted from the interaction of several processes: filtration, biodeposition, and recycling. This interaction complicates the determination of the actual ingestion and assimilation rates, and necessitates the use of mathematical modelling.     

Nitrogen niches revealed through species and functional group removal in a boreal shrub community

Most theories attempting to explain the coexistence of species in local communities make fundamental assumptions regarding whether neighbors exhibit competitive, neutral, or positive resource-use interactions; however, few long-term data from naturally assembled plant communities exist to test these assumptions. We utilized a 13-year experiment consisting of factorial removal of three shrub species (Vaccinium myrtillus, V. vitis-idaea, and Empetrum hermaphroditum) and factorial removal of two functional groups (tree roots and feather mosses) to assess how neighbors affect N acquisition and growth of each of the three shrub species. The removal plots were established on each of 30 lake islands in northern Sweden that form a natural gradient of resource availability. We tested the hypotheses that: (1) the presence of functionally similar neighbors would reduce shrub N acquisition through competition for a shared N resource; (2) the removal of functional groups would affect shrub N acquisition by altering the breadth of their niches; and (3) soil fertility would influence the effects of neighbor removals. We found that the removal of functionally similar neighbors (i.e., other shrub species) usually resulted in higher biomass and biomass N, with the strength of these effects varying strongly with site fertility. Shrub species removals never resulted in altered stable N isotope ratios (delta(15)N), suggesting that the niche breadth of the three shrubs was unaffected by the presence of neighboring shrub species. In the functional group removal experiment, we found positive effects of feather moss removal on V. myrtillus biomass and biomass N, and negative effects on E. hermaphrotium N concentration and V. vitis-idaea biomass and biomass N. Tree root removal also caused a significant shift in foliar delta(15)N of V. myrtillus and altered the delta(15)N, biomass, and biomass N of E. hermaphroditum. Collectively, these results show that the resource acquisition and niche breadth of the three shrub species are often affected by neighbors, and further that both the identity of neighbors and site fertility strongly determine whether these interactions are positive, negative, or neutral. These findings have implications for understanding species coexistence and the reciprocal relationships between productivity and species diversity in this ecosystem.     

Filtration rate,using a new indirect technique,in thirteen species of suspension-feeding bivalves

A method for determining filtration rates in undisturbed suspension-feeding bivalves is described. Concentrations of particulate matter in the water collected in the inhalant (C _i) and exhalant (C _e) currents were estimated with an electronic particle counter. The clearance was calculated as , where Fl=flow rate through the tube collecting exhalant water. Only above critical levels of water flow (Fl) were clearances representative of filtration rates. At 10° to 13°C, the filtration rates (F, 1 h^-1) within one or two orders of magnitude of dry weight (w, g), in Cardium echinatum L., C. edule L., Mytilus edulis L., Modiolus modiolus (L.) and Arctica islandica (L.) followed the allometric equations: 4.22w ^0.62, 11.60w ^0.70, 7.45w ^0.66, 6.00w ^0.75 and 5.55w ^0.62, respectively. Five species of bivalves [Spisula subtruncata (da Costa), Hiatella striata (Fleuriau), Cultellus pellucidus (Pennant), Mya arenaria L. and Venerupis pullastra (Montagu)] filtered with the same rates as individuals of Cardium echinatum and A. islandica of equivalent soft weight. In Pecten furtivus and P. opercularis filtration rates were about twice the rates measured in individuals of Mytilus edulis of comparable body weight. The gill area in M. edulis increases with size at the same rate as the filtration rate.     

Predator identity and additive effects in a treehole community

Multiple predator species can interact as well as strongly affect lower trophic levels, resulting in complex, nonadditive effects on prey populations and community structure. Studies of aquatic systems have shown that interactive effects of predators on prey are not necessarily predictable from the direct effects of each species alone. To test for complex interactions, the individual and combined effects of a top and intermediate predator on larvae of native and invasive mosquito prey were examined in artificial analogues of water-filled treeholes. The combined effects of the two predators were accurately predicted from single predator treatments by a multiplicative risk model, indicating additivity. Overall survivorship of both prey species decreased greatly in the presence of the top predator Toxorhynchites rutilus. By itself, the intermediate predator Corethrella appendiculata increased survivorship of the native prey species Ochlerotatus triseriatus and decreased survivorship of the invasive prey species Aedes albopictus relative to treatments without predators. Intraguild predation did not occur until alternative prey numbers had been reduced by approximately one-half. Owing to changes in size structure accompanying its growth, T. rutilus consumed more prey as time progressed, whereas C. appendiculata consumed less. The intermediate predator, C. appendiculata, changed species composition by preferentially consuming A. albopictus, while the top predator, T. rutilus, reduced prey density, regardless of species. Although species interactions were in most cases predicted from pairwise interactions, risk reduction from predator interference occurred when C. appendiculata densities were increased and when the predators were similarly sized.     

Effects of environmental factors on radiocadmium uptake by four species of marine bivalves

Temperature, salinity, bottom-sediment type, and zinc concentration all influenced Cd uptake by 4 marine bivalves (Mya arenaria, Mytilus edulis, Mulinia lateralis and Nucula proxima) in short-term static assay systems using ¹⁰⁹Cd as a tracer. The experimental system consisted of aquaria containing 20 l of seawater maintained under controlled light and temperature conditions. The water contained either 5 or 20 g/l Cd and tracer. Distribution and kinetics of the metal were monitored in the water column and organisms. The results demonstrate that Cd uptake rates differed widely among the organisms tested. An increase in temperature increased Cd uptake rate by all test organisms. A decrease in salinity increased Cd uptake by all organisms tested. The presence of bottom sediment depresses Cd accumulation in some benthic animals. Zinc in concentrations of 0.5 mg/l substantially decreased Cd uptake by Mytilus edulis and Mulinia lateralis. It is suggested that all important species and environmental variables be considered when studying heavy-metal uptake by marine organisms or when establishing water-quality criteria.     

Context-dependent acceptance of non-nestmates in a primitively eusocial insect

The benefits of cooperation are essential in driving group formation. However, an individual can gain significant benefits by acting selfishly at a substantial cost to others in the group. Thus, group members must find a balance between accepting and rejecting potential new members. Here, I explore the factors that mediate acceptance of non-related individuals during the period of group establishment in the primitively eusocial wasp Mischocyttarus mexicanus. In this species, group composition changes during establishment, with non-related females (non-nestmates) sometimes accepted into a foreign colony. By experimentally introducing non-nestmates to newly established colonies, I test the hypothesis that acceptance threshold of nestmates towards non-nestmates shifts depending on the ecological context, as predicted by the Optimal Acceptance Threshold Model. I explored how non-nestmate age (young vs. old), stage of colony establishment (early vs. late), initial behavior of the non-nestmates (non-aggressive vs. aggressive), and the behavioral response by nestmates (non-aggressive vs. aggressive) affected the rates of acceptance. My results show an effect of both non-nestmate age and stage of colony development on non-nestmate acceptance. Young non-nestmates were more frequently accepted in early than in late colonies. Late colonies more frequently rejected both young and old non-nestmates, suggesting a cost of accepting potential usurpers into late colonies. Surprisingly, non-nestmate aggressive behavior did not have a direct effect on their acceptance, but it triggered an aggressive response from nestmates. These findings reveal a shift in the acceptance threshold, suggesting an effect of the social context and the specific needs of a colony on non-nestmate acceptance.     

Context-dependent effects of nestling growth trajectories on recruitment probability in the collared flycatcher

Nestling growth is known as an important determinant of fitness in altricial birds, but its evolutionary potential has been debated, and little is known about detailed patterns of current selection on growth. Relationships are often reported between nestling growth and attributes of nestlings and parents, but the interpretation of these depends on the advantages a given growth difference confers to the chicks. Increased growth may have positive, negative or context-dependent effects on offspring fitness, but these effects are largely unknown in natural populations. We measured growth trajectories of body mass in fostered broods of collared flycatchers (Ficedula albicollis) in 3 years of contrasting food conditions. We examined the growth of young and their recruitment probability to the breeding population in relation to year quality, hatching rank, sex, paternal age and paternal attractiveness. We also looked at the interactive effects of growth and intrinsic offspring attributes on recruitment probability. The predictors of nestling growth and those of recruitment did not agree. Moreover, the recruitment consequences of a given nestling growth rate were significantly influenced by nestling rank and paternal ornamentation. Differential recruitment effects of nestling growth in relation to parental traits and nestling attributes suggest that using growth as a generally applicable measure of nestling quality may not be justified. These findings also have implications for morphological evolution and the indicator value of sexual signals.     

Context-dependent effects of symbiosis: Zoanthidea colonization generally improves Demospongiae condition in native habitats

In order to reconcile existing data and hypotheses on the relationship outcomes of Caribbean Zoanthidea-Demospongiae symbioses, the context-dependent survival and growth (metrics of host sponge condition) were monitored for 8–12?months, with and without Zoanthidean symbionts, in native (coral reef) and novel (mangrove) habitats. Experiments repeated over space and time revealed significant positive effects of Zoanthidea colonization on metrics of host condition and indicated that the outcome of these symbioses can shift from mutualism to parasitism when transplanted to novel habitats. Although the precise mechanisms of the symbioses remain obscured and most of the species associations have not been examined, these experiments demonstrate an example where relationship outcomes are more conserved across their evolutionary history than host associations.     

Running to stand still: temperature effects on species richness, species turnover, and functional community dynamics

The information on temperature-mediated changes in biodiversity in local assemblages is scarce and mainly addresses the change in species richness. However, warming may have more consistent effects on species turnover than on the number of species. Moreover, very few studies extended the analysis of changes in biodiversity and species composition to questions of associated ecosystem functions such as primary production. Here, we synthesize 4 case studies employing microalgal microcosms within the Aquashift priority program to ask (1) do warming-related shifts in species richness correspond to changes in the rate of biomass production, (2) do similar relationships prevail for evenness, and (3) do warming-related shifts in species turnover stabilize or destabilize biomass production? Two of the four cases are previously unpublished, and for a third case, the link between diversity and functional consequences of temperature was not analyzed before. We found accelerated loss of species with warming in all cases. Biomass production was lower with lower species richness in most cases but increased with lower evenness. Most importantly, the relation between functional and compositional stability was different between cases: More rapid extinction resulted in more variable biomass in 2 cases conducted with a limited species pool, indicating that compositional destabilization relates to functional variability. By contrast, the only experiment with a large species pool (30 species) allowed previously rare species to become dominant in the community and showed more stable biomass at high turnover, indicating that compensatory dynamics (turnover) can promote functional stability. These 4 independent experiments highlight the need to consider both compositional and functional consequences of altered temperature regimes.     

Reproductive biology of a filter-feeding amphipod, Leptocheirus pinguis , with extended parental care

Leptocheirus pinguis (Stimpson, 1853) is a widely distributed, abundant, endobenthic amphipod that engages in extended parental care, i.e. females host their juveniles in their burrows for extended time periods. I examined reproduction and population biology of L. pinguis at mean low water (MLW) in muddy sediment in Lowes Cove, Maine, USA. Cores around individual burrows were taken monthly in 1994 and 1995, and four seasonal samples were taken at different tidal heights. During the major reproductive periods in spring/early summer and in the fall, females produced several consecutive broods and hosted growing offspring in their burrows. Juveniles remained in their mothers' burrows until they reached a length of 5 mm (approximately one-third adult size) or more. At the study site, the majority of amphipods in individual burrows were adult females. Following the main reproductive periods, subadult individuals were found in their own burrows, but densities did not increase following the reproductive period in spring/early summer 1994, probably because large numbers of L. pinguis emigrated via the water column between June and December. L. pinguis is an annual species. Many members of the cohort born in spring/early summer start reproducing in the fall, and survive until the following spring when they produce several broods. Members of the cohort born in the fall start reproducing the following spring and also produce several consecutive broods. Both the spring/early summer and fall cohorts die off after the major reproductive period in the following spring/early summer. High standing stocks of microphytobenthos occur in soft-bottoms at MLW, and I conclude that L. pinguis can engage in extended parental care there because its food is abundant year-round. The limited expandability and low stability of burrows in soft-bottoms at MLW do not permit long persistence of parent–offspring groups in L.␣pinguis. Received: 27 March 1997 / Accepted: 30 July 1997     

Functional identity and functional structure change through succession in a rocky intertidal marine herbivore assemblage

Despite the great interest in characterizing the functional structure and resilience of functional groups in natural communities, few studies have examined in which way the roles and relationships of coexisting species change during community succession, a fundamental and natural process that follows the release of new resources in terrestrial and aquatic ecosystems. Variation in algal traits that characterize different phases and stages of community succession on rocky shores are likely to influence the magnitude, direction of effects, and the level of redundancy and complementarity in the diverse assemblage of herbivores. Two separate field experiments were conducted to quantify per capita and population effects and the functional relationship (i.e., redundancy or complementarity) of four herbivore species found in central Chile during early and late algal succession. The first experiment examined grazer effects on the colonization and establishment of early-succession algal species. The second experiment examined effects on the late-successional, dominant corticated alga Mazzaella laminarioides. Complementary laboratory experiments with all species and under natural environmental conditions allowed us to further characterize the collective effects of these species. We found that, during early community succession, all herbivore species had similar effects on the ephemeral algae, ulvoids, but only during the phase of colonization. Once these algae were established, only a subset of the species was able to control their abundance. During late succession, only the keyhole limpet Fissurella crassa could control corticated Mazzaella. The functional relationships among these species changed dramatically from redundant effects on ephemeral algae during early colonization, to a more complementary role on established early-successional algae, to a dominant (i.e., keystone) effect on late succession. This study highlights that functional relationship within consumer assemblages can vary at different phases and times of community succession. Differentiation in herbivore roles emphasizes the need to evaluate consumer's impacts through different times of community succession, and through experimental manipulations to make even broad predictions about the resilience or vulnerability of diverse intertidal assemblages to human disturbances.     

Role of filter-feeding in the nutritional biology of a deep-sea mussel with methanotrophic symbionts

The ability of an undescribed deep-sea hydrocarbon-seep mussel which contains endosymbiotic methanotrophic bacteria to clear, ingest, and assimilate radiolabeled bacteria (Vibrio pelagicus andEscherichia coli) and algae (Dunaliella tertiolecta) was compared with that of the bay musselMytilus edulis. The seep mussel, collected in August 1987 from the Louisana Slope in the Gulf of Mexico, was slower to clear bacteria and algae thanM. edulis. The ingestion and assimilation of filtered bacteria and algae was established from the presence of radiolabel in mussel tissues and feces. The seep mussel was somewhat less efficient in assimilating radiolabeled components from bacteria and algae thanM. edulis. The dietary carbon maintenance-requirement of the seep mussel could potentially be met at environmental concentrations of greater than 10⁶ bacteria ml^–1. At lower concentrations of particulate organic matter, filter-feeding could be an important source of nitrogen and essential nutrients not supplied by the endosymbionts.     

Context-dependent genetic benefits of extra-pair mate choice in a socially monogamous passerine

Extra-pair paternity is common in socially monogamous passerines; however, despite considerable research attention, consistent differences in fitness between within-pair offspring (WPO) and extra-pair offspring (EPO) have not been demonstrated. Recent evidence indicates that differences between maternal half-siblings may depend on environmental conditions, but it is unclear whether the influence of paternal genetic contribution should be most apparent under comparatively poor or favourable conditions. We compared phenotypic characteristics of WPO and EPO in 30 mixed-paternity broods of the tree swallow (Tachycineta bicolor) in relation to experimentally increased nest temperature (n = 13 heated nests; 17 control nests) and natural abundance of haematophagous parasites (Protocalliphora spp.). This allowed us to test the hypothesis that genetic benefits of extra-pair mating are environment dependent. EPO grew their ninth primary feathers faster than WPO regardless of nest temperature or parasite load and had significantly longer ninth primary feathers at fledging when parasite abundance was low, and when they were positioned early in the hatching sequence relative to WPO. In contrast, WPO under similar conditions did not differ from EPO in any phenotypic trait measured. These results indicate that the fitness benefits of extra-pair mating are likely to be context dependent, and that genetic effects on some phenotypic traits may be more apparent when conditions are relatively favourable.     

茶树上游猎型蜘蛛功能团对景观低碳管理模式的反应

目前温室效应加剧、生物多样性减少以及农田环境污染加重等问题已成为关注的焦点。由于蜘蛛是环境变化的指示标志和农业生中的重要捕食性天敌。通过选择茶树上游猎型蜘蛛为研究对象,调查分析景观低碳管理模式茶园（JG）与习惯管理模式茶园（XG）对茶树上游猎型蜘蛛组成、结构以及群落数量特征的影响。结果表明：JG茶园茶树上游猎型蜘蛛种类较多,有8个科,18个种;XG茶园有6个科,13个种;优势种和丰盛种中,JG茶园个体数为375头、195头,比XG茶园分别提高1.16、0.33倍。JG与XG两类茶园的蜘蛛种群数量特征在年龄结构、性别比例方面存在显著差异（P＜0.001）;JG茶园雌蛛个体数为157头、雄蛛个体数为320头,是XG茶园的1.12和1.32倍;JG茶园成蛛与幼蛛个体数是XG茶园的1.25和0.87倍,成蛛比幼蛛高出46.32%,而XG茶园成蛛比幼蛛高出21.82%;JG茶园蜘蛛种群数量特征发生动态显著高于XG茶园（P＜0.05）。JG茶园蜘蛛多样性指数和丰富度指数比XG茶园增加16.46%和12.14%。主成分分析表明,第一、二主成分累计贡献率达50.47%, JG茶园位置明显接近光盔蛛科（Liocranidae）、圆颚蛛科（Corinnidae）、跳蛛科（Salticidae）和管巢蛛科（Clubionidae）等蜘蛛群落数量分布较多的方向偏移,而XG茶园则向盗蛛科（Pisauridae）和猫蛛科（Oxyopidae）等蜘蛛群落数量分布较少方向靠近。由此可知,茶园实行低碳管理,实行复合种植,有利于减少CO2气体排放,保护茶树上游猎型蜘蛛种类多样性,提高对害虫的自然控制能力,为茶园固碳增汇,茶叶产值提高打下了基础。     

Competitive exclusion within the predator community influences the distribution of a threatened prey species

While much effort has been made to quantify how landscape composition influences the distribution of species, the possibility that geographical differences in species interactions might affect species distributions has received less attention. Investigating a predator-prey setting in a boreal forest ecosystem, we empirically show that large-scale differences in the predator community structure and small-scale competitive exclusion among predators affect the local distribution of a threatened forest specialist more than does landscape composition. Consequently, even though the landscape parameters affecting Siberian flying squirrel (Pteromys volans) distribution (prey) did not differ between nest sites of the predators Northern Goshawks (Accipiter gentilis) and Ural Owls (Strix uralensis), flying squirrels were heterospecifically attracted by goshawks in a region where both predator species were present. No such effect was found in another region where Ural Owls were absent. These results provide evidence that differences in species interactions over large spatial scales may be a major force influencing the distribution and abundance patterns of species. On the basis of these findings, we suspect that subtle species interactions might be a central reason why landscape models constructed to predict species distributions often fail when applied to wider geographical scales.     

Measurement of filtration rates by infaunal bivalves in a recirculating flume

A flume system and protocol for measuring the filtration rate of infaunal bivalves is described. Assemblages of multi-sized clams, at natural densities and in normal filter-feeding positions, removed phytoplankton suspended in a unidirectional flow of water. The free-stream velocity and friction velocity of the flow, and bottom roughness height were similar to those in natural estuarine waters. Continuous variations in phytoplankton (Chroomonas salinay) cell density were used to measure the filtration rate of the suspension-feeding clam Potamocorbula amurensis for periods of 2 to 28 h. Filtration rates of P. amurensis varied from 100 to 580 liters (gd)^-1 over a free-stream velocity range of 9 to 25 cm s^-1. Phytoplankton loss rates were usually constant throughout the experiments. Our results suggest that suspension-feeding by infaunal bivalves is sensitive to flow velocity.