Trophic structure of coastal Antarctic food webs associated with changes in sea ice and food supply

Predicting the dynamics of ecosystems requires an understanding of how trophic interactions respond to environmental change. In Antarctic marine ecosystems, food web dynamics are inextricably linked to sea ice conditions that affect the nature and magnitude of primary food sources available to higher trophic levels. Recent attention on the changing sea ice conditions in polar seas highlights the need to better understand how marine food webs respond to changes in such broad-scale environmental drivers. This study investigated the importance of sea ice and advected primary food sources to the structure of benthic food webs in coastal Antarctica. We compared the isotopic composition of several seafloor taxa (including primary producers and invertebrates with a variety of feeding modes) that are widely distributed in the Antarctic. We assessed shifts in the trophic role of numerically dominant benthic omnivores at five coastal Ross Sea locations. These locations vary in primary productivity and food availability, due to their different levels of sea ice cover, and proximity to polynyas and advected primary production. The delta15N signatures and isotope mixing model results for the bivalves Laternula elliptica and Adamussium colbecki and the urchin Sterechinus neumeyeri indicate a shift from consumption of a higher proportion of detritus at locations with more permanent sea ice in the south to more freshly produced algal material associated with proximity to ice-free water in the north and east. The detrital pathways utilized by many benthic species may act to dampen the impacts of large seasonal fluctuations in the availability of primary production. The limiting relationship between sea ice distribution and in situ primary productivity emphasizes the role of connectivity and spatial subsidies of organic matter in fueling the food web. Our results begin to provide a basis for predicting how benthic ecosystems will respond to changes in sea ice persistence and extent along environmental gradients in the high Antarctic.     

Responses of echinoid larvae to food patches of different algal densities

High densities of larvae have been found in areas of high primary production, but it remains unclear whether this is the result of hydrodynamics or of larval aggregative behaviour in the presence of food. In this study, we examined changes in the vertical distribution and swimming patterns of four-armed larvae of the sea-urchin Echinometra lucunter (Linnaeus) around food patches of a range of microalgal densities. We reared larvae in the laboratory in a high or low concentration of either single (Isochrysis galbana) or mixed (I. galbana, Dunaliella tertiolecta, Thalassiosira weissflogii) microalgal species. In Plexiglas cylinders, we experimentally constructed haloclines in which the salinity of the bottom water-layer was 33‰ and that of the top water-layer was 24‰. In a thin layer in the middle of the halocline, we inserted a food patch that consisted of 0, 2500, 5000 or 10 000 T. weissflogii cells ml⁻¹. The presence of a food patch had a pronounced effect on the vertical distribution of larvae. This effect depended upon the algal density of the food patch and varied with dietary conditioning. The number of larvae that were above or within the patch decreased with increasing algal density, and was greater if larvae were reared in low-ration or single-species diets than in high-ration or mixed-species diets. Tracking of individual vertical swimming paths showed that within a few minutes, larvae swam into the patches of low algal density, and to positions just below the patches of the two higher algal densities, and remained there until the end of the experimental period. The greater number of algal cells in the digestive tracts of larvae from treatments with a food patch than in those without a patch confirmed that larvae were feeding on the microalgal cells of the patch. To our knowledge, this is the first study to experimentally show an aggregative behavioural response of invertebrate larvae to a food patch. Such a response may reduce the probability of food limitation and therefore enhance larval survival. Received: 14 February 1997 / Accepted: 24 September 1997     

Contribution of heterotrophic material to diet and energy budget of Antarctic krill, Euphausia superba

A novel approach was used to estimate the heterotrophic carbon component in the diet of the Antarctic krill Euphausia superba. Over 200 specimens from seven samples collected in the Lazarev Sea (January 1993 and 1995), at the Antarctic Polar Front (January 1993), and around South Georgia (February/March 1994) were dissected, and the total carbon content of their stomachs was estimated with a CHN-analyser. Gut-pigment contents were also measured by the gut-fluorescence technique in specimens collected at the same time, and the equivalent amount of their gut carbon was then subtracted from the total organic carbon content of guts from the same samples. The remaining carbon was assumed to originate entirely from heterotrophic food sources. This heterotrophic component accounted for a substantial proportion of the total food consumed by Antarctic krill, ranging from 17.4 to 98.9% of the mass of the gut contents (mean = 78.8% ± 21.2 SD). The results make an important contribution to the elucidation of the energy budget of krill and its daily carbon ration. With a few exceptions, previous estimates were largely calculated from a solely autotrophic carbon source, and were unable to account for the metabolic requirements of E. superba. Krill plays an important role in Antarctic food webs, as it often constitutes ≃50% of the total biomass of the zooplankton, and produces fast-sinking, dense faecal pellets which are important in the vertical transport of organic carbon from the euphotic layer to the deep ocean. High consumption rates of smaller heterotrophic organisms by krill suggest that this large microphage may be more important than previously believed in re-packaging micro- and mesozooplankton into a longer-lasting and more easily sequestered carbon pool. Received: 26 October 1998 / Accepted: 14 October 1999     

Rules of supply and demand regulate recruitment to food in an ant society

The process by which ant scouts move a group of nestmates toward a newly discovered food site is called recruitment. In this paper, I report on the interactions between scouts and nestmates that result in a graded recruitment response to graded food quality in the fire ant, Solenopsis invicta. Twelve experimental groups composed of 100 fire ant workers and 50 fire ant larvae were established (three experimental groups per colony × four stock colonies). Each experimental group was placed in a shallow, artificial nest with a glass cover. After a 48-h period of food deprivation, experimental groups were exposed to one of three concentrations of sugar water. Behavioral interactions between scouts and nestmates in each group were videotaped at 10× magnification for 20 min. Detailed behavioral data on a total of 120 scouts (10 scouts per experimental group) and ~1,000 nestmates (~90 nestmates per experimental group) were transcribed from the videotapes using standard play and frame-by-frame techniques. Throughout the recruitment process, scouts employed six discrete behaviors to inform nestmates of the location and quality of a food site. Scouts laid incoming trails, waggled their heads, increased walking tempo, stroked nestmates with their antennae, advertised with a brief food display, and led groups of nestmates to the food site by laying outgoing trails. In turn, nestmates assessed the food sample with antennae, then responded to or resisted recruitment based on the quality of food advertised, their employment status and their level of hunger. In summary, recruitment was an emergent property based on competent supply and demand decisions made face-to-face inside the nest rather than on the trail or at the food site.Communicated by J. Heinze     

Responses of nymphs of desert locust,Schistocerca gregaria to volatiles of plants used as rearing diet

Summary Gregarious nymphs of the desert locust,Schistocerca gregaria (Forsk.) (Orthoptera: Acrididae) were more attracted to volatiles from mechanically damaged food plants used for rearing than to either the undamaged or damaged food plants not used as diet in Y-tube olfactometer assays. Comparative analysis of the volatile emissions from plants used for rearing and food plants not used for rearing,e.g. Sorghum bicolor, Pennisetum clandestinum, Schouwia thebaica, wheat (Triticum sp., var. Nyangumi),Zygophyllum simplex, Heliotropium undulatum andTribulus terrestris was carried out by GC, GC-EAD and GC-MS. Significant quantitative and qualitative differences were found in the volatile emissions and olfactory responses of nymphs in GC-EAD assays. Up to 33 compounds were identified in volatiles of the plants of which 9 evoked EAGs. EAG-active components included common green leaf compounds (E)-2-pentenal, (E)-2-hexenal, 4-methyl-3-pentenal, (E)-3-hexenyl acetate, (Z)-3-hexenyl acetate, (Z)-2-hexenyl acetate, (Z)-3-hexen-1-ol and (Z)-2-hexen-1-ol. (Z)-3-Hexenyl butyrate and (Z)-3-hexenyl isovalerate were detected in stimulatory amounts only in the volatiles ofS. thebaica. (E, Z)-2,6-Nonadienal was detected as a component in the volatiles ofT. terrestris and was highly stimulatory. In EAG assays with seven common green leaf volatiles, (Z)-3-hexenyl acetate was most stimulatory while hexanal was the least. No significant differences were recorded between antennal responses of males and females to the tested compounds. These results are discussed with regard to current hypotheses on host plant recognition through detection of their airborne volatiles and the learning behaviour by nymphs ofS. gregaria.     

Verification of lipofuscin-based crustacean ageing: seasonality of lipofuscin accumulation in the stomatopod Oratosquilla oratoria in relation to water temperature

To verify the lipofuscin-based age estimation of crustaceans of unknown age from wild populations, we conducted a modal analysis of lipofuscin-concentration histograms in the stomatopod Oratosquilla oratoria seasonally collected from Tokyo Bay, Japan. Confocal microscopy and subsequent image analysis were used to quantify lipofuscin concentrations in the protocerebral bridge cell mass in the brain. Conventional body length frequency analysis failed to detect modes corresponding to ≥1 year old. In contrast, four or five distinct modes were found in the lipofuscin-concentration histograms, in which modes could be readily traced across months, suggesting that each mode represented an independent age group. The lipofuscin accumulation rate fluctuated seasonally: it was high in summer (June–July) and low in winter (December–January). Cross-correlation analysis showed that the highest instantaneous lipofuscin accumulation rate was achieved in early summer before maximum bottom temperatures were reached and when the temperature was increasing most rapidly. Our findings support the usefulness of lipofuscin concentration analysis in age estimation in wild crustacean populations.     

Female adaptation to developmental plasticity in male calling behavior

The katydid Neoconocephalus triops exhibits in North America substantial developmental plasticity of male mating calls. The AM rate of the summer calls is significantly faster than that of the winter calls at the same temperature. In the tropics, where N. triops originated, males express only the fast summer-call phenotype. We tested two alternative hypotheses: (1) call plasticity in the population from North America evolved in response to selection by female preference after N. triops colonized North America, or (2) call plasticity evolved before N. triops expanded into North America and its expression in the novel environment led to adaptive change of female preferences. First, we tested whether call plasticity was present in tropical populations of N. triops. Tropical males expressed the winter-call phenotype when reared under winter conditions, indicating that call plasticity did not evolve in response to temperate climates. Second, we compared female preferences among temperate and tropical populations. We found that the temperature dependence of preferred AM rate was significantly steeper in temperate N. triops than in a tropical population of N. triops. Third, we compared temperature dependence of female preference of the N. triops populations to three Neoconocephalus species without call plasticity. Only temperate N. triops had significantly steeper temperature dependence than the other species. This steeper temperature dependence matched female preference to the fast summer call at high temperatures and to the slow winter call at low temperatures in temperate populations. These results support the hypothesis that female preference changed in N. triops in North America to compensate for the plasticity of male calls.     

Stable macrofauna community structure despite fluctuating food supply in subtidal soft sediments of Oslofjord,Norway

At a locality at 32-m depth in Oslofjord, Norway, temperature varied from 4.8° to 9.2°C and salinity varied from 31.2 to 33.3 S over a two-year period. There was a peak in chlorophyll a, and C and N in April–June and a smaller peak in November in the sediment. Bacterial numbers showed maxima in July–August and November–December. The macrobenthic fauna was typical of a species-rich and undisturbed boreal community of silt-clay sediments. The community was predominantly composed of surface and subsurface deposit feeders. Over the twoyear period there was little variation in numerical abundance or biomass of the species despite the variation in food input. The lack of seasonality shown by the fauna probably relates to the lack of variability of the physical environment. The mechanism by which this control is achieved, however, is not known. There are large predators/disturbers in the community such as the polychaetes Lumbrineris fragilis, Glycera rouxii, G. alba, Nephthys spp. and the echinoid Brissopsis lyrifera, which probably play an important role in structuring the community.     

Variation in foraging effort by lactating Antarctic fur seals: response to simulated increased foraging costs

Seasonally breeding predators, which are limited in the time available for provisioning young at a central location, and by the fasting abilities of the young, are likely to maximize energy delivery to the young by maximizing the rate of energy delivery averaged over the whole period of investment. Reduction in food availability or increased foraging costs will alter the optimal behavior of individuals. This study examined the behavioral adaptations of a diving predator, the Antarctic fur seal, to increased foraging costs during lactation. One group of mothers (n=5, treatment) was fitted with additional drag to increase the cost of transport in comparison with a control group (n=8). At the scales of the individual dives, the treatment group made more shorter, shallower (< 30 m) dives. Compensation for slower swimming speeds was achieved by diving at a steeper angle. Overall, diving behavior conformed to several specific theoretical predictions but there were also departures from theory, particularly concerning swimming speed during diving. Diving behavior appears to be adjusted to maximize the proportion of time spent at the bottom of dives. At the scale of diving bouts, no difference was observed between the treatment and control groups in terms of the frequency and duration of bouts and there was also no difference between the two groups in terms of the proportion of time spent diving. At the scale of complete foraging cycles, time taken to return to the pup was significantly longer in the treatment group but there was no difference in the rate of delivery of energy (measured from pup growth rate) to the pups in each group. Since mothers in the treatment group did not use significantly more body reserves, we conclude that behavioral adjustments at the scale of individual dives allowed mothers in the treatment group to compensate for the additional foraging costs. Pup growth rate appears to be less sensitive to the foraging conditions experienced by mothers than foraging trip duration. Received: 14 June 1996 / Accepted after revision: 16 November 1996     

Niche complementarity due to plasticity in resource use: plant partitioning of chemical N forms

Niche complementarity, in which coexisting species use different forms of a resource, has been widely invoked to explain some of the most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function. However, classical models assume resource specialization in the form of distinct niches, which does not obviously apply to the broadly overlapping resource use in plant communities. Here we utilize an experimental framework based on competition theory to test whether plants partition resources via classical niche differentiation or via plasticity in resource use. We explore two alternatives: niche preemption, in which individuals respond to a superior competitor by switching to an alternative, less-used resource, and dominant plasticity, in which superior competitors exhibit high resource use plasticity and shift resource use depending on the competitive environment. We determined competitive ability by measuring growth responses with and without neighbors over a growing season and then used 15N tracer techniques to measure uptake of different nitrogen (N) forms in a field setting. We show that four alpine plant species of differing competitive abilities have statistically indistinguishable uptake patterns (nitrate > ammonium > glycine) in their fundamental niche (without competitors) but differ in whether they shift these uptake patterns in their realized niche (with competitors). Competitively superior species increased their uptake of the most available N form, ammonium, when in competition with the rarer, competitively inferior species. In contrast, the competitively inferior species did not alter its N uptake pattern in competition. The existence of plasticity in resource use among the dominant species provides a mechanism that helps to explain the manner by which plant species with broadly overlapping resource use might coexist.     

Morphological adaptation of a planktonic diatom to growth in Antarctic sea ice

Chaetoceros dichaeta Ehrenberg is one of the most important planktonic diatom species in the Southern Ocean, making a significant contribution to the total biomass in the region. Our observations on both field and culture material have revealed the existence of a specialized form of C. dichaeta adapted to living in sea ice. This sea ice form differs from the planktonic form by the shape and orientation of the setae and the aperture length between sibling cells. Thus, the diameter of the chain is equivalent to the apical axes of the cells and is accompanied by a two order of magnitude decrease in minimal space requirement. Here, we report for the first time on the extraordinary overwintering strategy of a planktonic diatom in sea ice facilitated by its rapid morphological adaptation to changing environmental conditions. This morphological plasticity enables it to thrive in the confined space of the sea ice brine matrix and retain its numerical dominance in recurrent growing seasons and has likely evolved to optimally exploit the dynamic ecosystem of the seasonally ice-covered seas of the Southern Ocean.     

Metachronal swimming in Antarctic krill: gait kinematics and system design

Metachronal swimming, in which adjacent appendages stroke in sequence, is widespread among crustaceans inhabiting the transitional flow realm in which both viscosity and inertia effects are important. However, the design and operation of this propulsion system in response to various hydrodynamic, energetic, and behavioral needs have not been well investigated. We examine free-swimming Antarctic krill (Euphausia superba) as a model species and identify three distinct behavioral swimming gaits. The pleopod kinematics of these gaits, hovering, fast-forward swimming, and upside-down swimming, are quantified via image analysis of high-speed video. Pleopod stroke amplitude and frequency were found to vary significantly among these swimming modes. In order to increase swimming speed, krill were found first to increase stroke amplitude and secondarily to increase beat frequency. The kinematics of these distinct swimming modes provide insight as we consider multi-appendage metachronal swimming from a design standpoint. The ratio of the distance between adjacent appendage bases and appendage length is identified as a key parameter in metachrony, the value of which is constrained to a narrow range for a wide variety of species.     

Induced responses to competition and herbivory: natural selection on multi-trait phenotypic plasticity

Herbivory and competition are two of the most common biotic stressors for plants. When occurring simultaneously, responses to one interaction can constrain the induction of responses to the other interaction due to resource limitation and other interactive effects. Thus, to maximize fitness when interacting with competitors and herbivores, plants are likely to express particular combinations of plastic responses. This study reports the interactive effects of herbivory and competition on responses induced in Tithonia tubaeformis plants and describes how natural selection acts on particular plastic responses and on their different combinations. Competition induced a stem elongation response, expressed through an increase in height and mean internode length, together with a decrease in basal diameter. Interestingly, realized resistance increased in both competition and herbivory treatments, suggesting a plastic response in both constitutive and induced resistance traits. Particular combinations of plastic responses defined three plant phenotypes: vigorous, elongated, and resistant plants. The ecological context in which plants grew modified the traits and the particular combinations of plastic responses that were favored by selection. Vigorous plants were favored by selection in all environments, except when they were damaged by herbivores in the absence of neighbors. The combination of responses defining an elongated plant phenotype was favored by selection in crowded conditions. Resistance was negatively selected in the absence of competition and herbivory but favored in the presence of both interactions. In addition, contextual analyses detected that population structure in heterogeneous environments can also influence the outcomes of selection. These findings suggest that natural selection can act on particular combinations of plastic responses, which may allow plants to adjust their phenotypes to those that promote greater fitness under particular ecological conditions.     

Responses of pond-breeding amphibians to wildfire: short-term patterns in occupancy and colonization.

Wildland fires are expected to become more frequent and severe in many ecosystems, potentially posing a threat to many sensitive species. We evaluated the effects of a large, stand-replacement wildfire on three species of pond-breeding amphibians by estimating changes in occupancy of breeding sites during the three years before and after the fire burned 42 of 83 previously surveyed wetlands. Annual occupancy and colonization for each species was estimated using recently developed models that incorporate detection probabilities to provide unbiased parameter estimates. We did not find negative effects of the fire on the occupancy or colonization rates of the long-toed salamander (Ambystoma macrodactylum). Instead, its occupancy was higher across the study area after the fire, possibly in response to a large snowpack that may have facilitated colonization of unoccupied wetlands. Naive data (uncorrected for detection probability) for the Columbia spotted frog (Rana luteiventris) initially led to the conclusion of increased occupancy and colonization in wetlands that burned. After accounting for temporal and spatial variation in detection probabilities, however, it was evident that these parameters were relatively stable in both areas before and after the fire. We found a similar discrepancy between naive and estimated occupancy of A. macrodactylum that resulted from different detection probabilities in burned and control wetlands. The boreal toad (Bufo boreas) was not found breeding in the area prior to the fire but colonized several wetlands the year after they burned. Occupancy by B. boreas then declined during years 2 and 3 following the fire. Our study suggests that the amphibian populations we studied are resistant to wildfire and that B. boreas may experience short-term benefits from wildfire. Our data also illustrate how naive presence-non-detection data can provide misleading results.     

How to toughen up your mussels: using mussel shell morphological plasticity to reduce predation losses

Improvements in stocking strategy and management could increase the yield of mussels that are on-grown from harvested wild seed mussel resources and thereby enhance the sustainability of this shellfishery. A field experiment was undertaken to ascertain shell characteristics (compression strength and thickness) of seed mussels grown at different shore heights, whether these characteristics changed after a period of growth under identical conditions, and if these characteristics reduced predation losses by crabs and birds. Results indicated that mussels grown at higher shore levels attained shell characteristics beneficial to predation resistance and that these were maintained after a period of growth at a lower shore level. A novel management plan for mussel cultivation was formulated from the results of this study by manipulating shore position according to the attainment of these predator resistant shell attributes and the spatial distribution of the main natural mussel predators (crabs and birds). This technique was expanded to address the mussel cultivation problem of low natural seed settlement.     

植物对干旱胁迫的响应研究进展

植物在经受干旱胁迫时,通过细胞对干旱信号的感知和传导,调节基因表达,产生新蛋白质,从而引起大量形态、生理和生化上的变化.干旱胁迫对植物在细胞、器官、个体、群体等水平上的形态指标有显著影响,也会影响其光合作用、渗透调节、抗氧化系统等生理生化指标.植物对干旱胁迫的分子响应较复杂,包括合成一些新的基因如NCED、dehydrin基因和CBF、DREB等转录因子.另外,干旱胁迫还能造成蛋白质组学的变化.参52     

Renal function in Antarctic teleost fishes: Serum and urine composition

The urine and serum compositions in 11 species of Antarctic teleost fishes were studied. The body fluids of these fishes were hypoosmotic to seawater. Both urine and serum osmoconcentrations were conspicuously greater than those of temperate species. The elevated serum osmolality was due to increased levels of NaCl, while elevated urine osmolality was due chiefly to increased concentrations of magnesium and chloride. The kidney was unable to form a urine hyperosmotic to serum under field conditions. The renal tubular epithelium secreted magnesium and calcium against a concentration gradient, while effectively preventing the renal excretion of significant amounts of sodium and potassium. Bladder urine of Antarctic teleosts may often be supercooled by as much as 0.8°C. Coelomic fluid and the integument probably act as a barrier in preventing ice propagation in the supercooled bladder urine, since they both contain glycoprotein “antifreeze” agents.