Water temperature and mixing depth affect timing and magnitude of events during spring succession of the plankton

Individual-level diet variation can be easily quantified by gut-content analysis. However, because gut contents are a ‘snapshot’ of individuals’ feeding habits, such cross-sectional data can be subject to sampling error and lead one to overestimate levels of diet variation. In contrast, stable isotopes reflect an individual’s long-term diet, so isotope variation among individuals can be interpreted as diet variation. Nevertheless, population isotope variances alone cannot be directly compared among populations, because they depend on both the level of diet variation and the variance of prey isotope ratios. We developed a method to convert population isotope variances into a standardized index of individual specialization (WIC/TNW) that can be compared among populations, or to gut-content variation. We applied this method to diet and carbon isotope data of four species of frogs of the Brazilian savannah. Isotopes showed that gut contents provided a reliable measure of diet variation in three populations, but greatly overestimated diet variation in another population. Our method is sensitive to incomplete sampling of the prey and to among-individual variance in fractionation. Therefore, thorough sampling of prey and estimates of fractionation variance are desirable. Otherwise, the method is straightforward and provides a new tool for quantifying individual-level diet variation in natural populations that combines both gut-content and isotope data. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The complex hatching response of Aedes eggs to larval density

Abstract. 1. We investigated the effects of hatching medium, larval density and larval instar on egg hatching in Aedes triseriutus (Say) (Diptera: Cuficidae).
2. In a nutrient broth medium, hatching rates, responded positively to low densities of large larvae, but were suppressed by high densities.
3. The magnitude of suppression was moderated through the use of a yeast hatching medium, which provides a food source for the large larvae. Small instars generally exerted positive effects on hatching, with the exception of first in star larvae in the yeast medium, which inhibited hatching at low and moderate densities.
4. The results demonstrate an ability of eggs to assess indirectly the potential risks of hatching through their responses to changes in dissolved oxygen concentration.     

Asymmetric competition in larval amphibian communities: conservation implications for the northern crawfish frog, Rana areolata circulosa

Asymmetric competition in larval amphibians can influence population dynamics and community structure. This density-dependent regulatory mechanism may be of particular importance for rare or endangered species such as the northern crawfish frog, Rana areolata circulosa. Interspecific competition of R. areolata with two congenerics, R. blairi and R. sphenocephala, was examined in artificial ponds. Analysis of covariance (differential mortality covariate) indicated that interspecific competition increased larval period length and decreased metamorphic body mass of R. areolata. The number of metamorphs produced was lower for R. blairi ponds when reared with R. areolata at high density. Body mass at metamorphosis was larger for R. sphenocephala when reared with R. areolata, suggesting that R. areolata facilitates larval growth in R. sphenocephala. These results indicate that the larval performance of R. areolata was reduced in the presence of interspecific competitors. Although many conservation efforts emphasize the preservation of critical habitat or particular rare species, interactive effects of biotic components in the focal community may also be important demographic regulators. Received: 11 December 1997 / Accepted: 15 April 1998     

Sexual differences in response to larval food stress in two nuptial feeding orthopterans – implications for sexual selection

The extent to which a trait deteriorates in response to stress can indicate its fitness importance. Food limitation is a naturally-occurring stress in the katydid Kawanaphila nartee (Orthoptera: Tettigoniidae) and should cue larvae as to the level of sexual competition expected; hunger drives adult females to compete for copulations and thus acquire spermatophore meals. This increases sexual selection on females relative to males for increased body mass. As predicted, under experimental food-stress larval female K. nartee showed little loss of mass whereas male mass decreased. In contrast, the sizes of body parts less critical to mating success showed similar decreases in males and females. For katydids such as Conocephalus nigropleurum , where reversals in mating roles do not occur, larval food stress should result in a greater preservation of male body mass, an important trait in male mating success. This prediction was supported; male mass decreased less than that of females.     

The measurement of subjective response to stressful life events

A scale was developed to assess subjective responses to stressful life events. Over 400 subjects (divided between psychiatric patients and nonpatients) completed the 14-item response-to-stress scale for events perceived as personally important. Optimal scale item weights were assigned on the basis of Nishisato's dual scaling procedure. Four items, typically associated with responses to stress, did not contribute to the item-weight solution and were dropped from the scale. Factor analyses showed that responses to stress loaded on one bipolar factor composed of four emotional distress items and six control-management items. In contrast to previous methods developed to assess response to stress, there was less than 4% overlap between the subjective response to stress mean scores and the total number of events endorsed by each subject. Also, the correlations between simple event counts and symptom index scores were smaller than those between the subjective stress-response scores and the symptom variable. The influence of demographic factors on both the patients' and nonpatients' responses to stress were analyzed.     

Differential response to soil salinity in endangered key tree cactus: implications for survival in a changing climate

Understanding reasons for biodiversity loss is essential for developing conservation and management strategies and is becoming increasingly urgent with climate change. Growing at elevations <1.4 m in the Florida Keys, USA, the endangered Key tree cactus (Pilosocereus robinii) experienced 84 percent loss of total stems from 1994 to 2007. The most severe losses of 99 and 88 percent stems occurred in the largest populations in the Lower Keys, where nine storms with high wind velocities and storm surges, occurred during this period. In contrast, three populations had substantial stem proliferation. To evaluate possible mortality factors related to changes in climate or forest structure, we examined habitat variables: soil salinity, elevation, canopy cover, and habitat structure near 16 dying or dead and 18 living plants growing in the Lower Keys. Soil salinity and elevation were the preliminary factors that discriminated live and dead plants. Soil salinity was 1.5 times greater, but elevation was 12 cm higher near dead plants than near live plants. However, distribution-wide stem loss was not significantly related to salinity or elevation. Controlled salinity trials indicated that salt tolerance to levels above 40 mM NaCl was related to maternal origin. Salt sensitive plants from the Lower Keys had less stem growth, lower root:shoot ratios, lower potassium: sodium ratios and lower recovery rate, but higher δ (13)C than a salt tolerant lineage of unknown origin. Unraveling the genetic structure of salt tolerant and salt sensitive lineages in the Florida Keys will require further genetic tests. Worldwide rare species restricted to fragmented, low-elevation island habitats, with little or no connection to higher ground will face challenges from climate change-related factors. These great conservation challenges will require traditional conservation actions and possibly managed relocation that must be informed by studies such as these.     

Stomatal response to humidity: implications for transpiration

Abstract. Transpiration rates from apple leaves are analysed in terms of the ratio of latent heat flux (λ E ) to leaf net radiation ( Q ₁) and the climatological resistance ( r_i ). Increases in stomatal resistance with increasing leaf to air vapour pressure gradient ( D ), described by an empirical model, are incorporated in the analysis. This humidity effect causes the proportion of energy dissipated as latent heat to fall as Q ₁ increases, so that leaf transpiration rates in high energy environments are likely to be similar to those in lower energy environments. Boundary layer resistance ( r _a) exerts an increasingly important effect on transpiration rates as Q ₁ increases. At constant Q ₁ stomatal closure in response to increasing D results in very small changes in leaf temperature ( T ₁) across a wide range of ambient vapour pressure deficits (δ e ); r _a is then the major factor determining T ₁. The implications of these results are discussed.     

An ecosystem model for the assessment of plankton production in relation to the survival of young fish

A mixed upper layer ecosystem simulation model is used to assessthe effect of changes in the zooplankton stock on young salmonidgrowth and survival. The effect of physical factors such aslight extinction and mixed layer depth cause salmonid productionto optimize at conditions which favour maximum zooplankton standingstock. However, the overwintering stock of ctenophores may decimatethe zooplankton and cause a reduction in young salmonid growthand survival, particularly in unseasonally warm years. The standingstock of salmonids and the time of release have less effecton growth than either physical factors or ctenophores.     

Predictability and possible mechanisms of plankton response to reduction of planktivorous fish

The predictability of plankton response to reductions of planktivorous fish was investigated by comparing the plankton community in three biomanipulated lakes and ten unmanipulated lakes differing in intensity of fish predation. Data collected on total phosphorus, phytoplankton and zooplankton biomass and share of cyanobacteria and large grazers, as well as specific growth rate of phytoplankton, were further used to test some of the proposed underlying response-mechanisms. In the biomanipulated lakes the algal biomass and share of cyanobacteria decreased, specific growth rate of phytoplankton increased, and zooplankton biomass and share of large grazers increased or remained unchanged. This pattern was largely reflected in the differences in food-chain structure between the unmanipulated lakes with highversus those with low fish predation. The qualitative response to planktivorous fish reduction thus seems largely predictable. The biomanipulated lakes differed, however, in magnitude of response: the smallest hypertrophic, rotenone-treated lake (Helgetjern) showed the most dramatic response, whereas the large, deep mesotrophic lake (Gjersjøen), which was stocked with piscivorous fish, showed more moderate response, probably approaching a new steady state. These differences in response magnitude may be related to different perturbation intensity (rotenone-treatmentversus stocking with piscivores), food-chain complexity and trophic state. Both decreased phosphorus concentration and increased zooplankton grazing are probably important mechanisms underlying plankton response to biomanipulation in many lakes. The results provide tentative support to the hypothesis that under conditions of phosphorus limitation, increased zooplankton grazing can decrease algal biomassvia two separate mechanisms: reduction of the phosphorus pool in the phytoplankton, and reduction of the internal C:P-ratio in the phytoplankton cells.

     

Transient response of sap flow to wind speed

Transient responses of sap flow to step changes in wind speedwere experimentally investigated in a wind tunnel. A Granier-typesap flow sensor was calibrated and tested in a cylindrical tubefor analysis of its transient time response. Then the sensorwas used to measure the transient response of a well-wateredPachira macrocarpa plant to wind speed variations. The transientresponse of sap flow was described using the resistance–capacitancemodel. The steady sap flow rate increased as the wind speedincreased at low wind speeds. Once the wind speed exceeded 8.0m s^–1, the steady sap flow rate did not increase further.The transpiration rate, measured gravimetrically, showed a similartrend. The response of nocturnal sap flow to wind speed variationwas also measured and compared with the results in the daytime.Under the same wind speed, the steady sap flow rate was smallerthan that in the daytime, indicating differences between diurnaland nocturnal hydraulic function, and incomplete stomatal closureat night. In addition, it was found that the temporal responseof the Granier sensor is fast enough to resolve the transientbehaviour of water flux in plant tissue. Key words: Nocturnal transpiration, sap flow, transient response, wind speed Received 31 July 2008; Revised 6 October 2008 Accepted 20 October 2008     

The determination of nucleic acids in freshwater plankton and its ecological implications *

Nucleic acid concentrations show large variations between different planktonic species. RNA concentration is much higher in phytoplankton than in zooplankton. DNA varies to a considerable extent, being five to six times higher in copepods than in cladocerans. In Daphnia hyalina, nucleic acid contents are proportional to dry weight during the whole life cycle except in newborn Daphnia where DNA concentration is abnormally high. Seasonal variations affect, to a large extent, nucleic acid concentrations. These results rule out the possibility of using nucleic acids as indicators of biomass in mixed planktonic populations.     

The survival response of a biological system to mixed radiations

The concept of additive radiation action is applied to the process of merging of the intermediate lesions at a common stage in the radiation inactivation pathways for lesions produced by different radiations. This gives rise to a natural nonindependent effect for combined irradiation. Even though the exact nature of this common intermediate lesion is unknown, the effect of this lesion additivity can still be formulated into a mathematical model using the assumptions: (1) there exists a stage in the chain of radiation inactivation events where different types of lesion precursors, produced by different types of radiations in a mixture, inflict lesions which become functionally indistinguishable and hence additive thereafter, to produce the same end point observed; (2) all precursors of all types are simultaneously competing for the opportunity to inflict lesions at the stage indicated in assumption 1, and each precursor has equal opportunity regardless of its origin; (3) if the radiations are delivered sequentially within a sufficiently short time, the lesion precursors of both radiations arrive at the above stage at about the same time and hence inflict lesions which are additive as described in assumptions 1 and 2. The model is quantitative but contains no free-fitting parameters. It is shown to be capable of explaining a large variety of apparently unrelated published experimental results observed for mixtures of high- and low-LET radiations.     

Effects of founder events on the genetic variation of translocated island populations: implications for conservation management of the northern quoll

Translocation is a strategy commonly used to maximize the persistence of threatened species, but it may sometimes lead to undesirable genetic consequences. The northern quoll (Dasyurus hallucatus) is a carnivorous marsupial that is critically endangered in Australia’s Northern Territory due to rapid population declines in areas recently colonized by the exotic cane toad Chaunus [Bufo] marinus. In 2003, 64 quolls were translocated to two offshore islands to establish insurance populations and reduce the species’ risk of extinction. In this study, we assessed genetic diversity at five microsatellite loci in the translocated populations, two endemic islands and three mainland populations. In the short-term (three generations), the translocated populations showed a slight but non-significant reduction in genetic diversity (A = 4.1–4.2; H _e = 0.56–0.59) compared to the mainland source populations (A = 5.0–8.4; H _e = 0.56–0.71). In comparison, high genetic erosion was observed in the endemic island populations (A = 1.5–2.9; H _e = 0.11–0.34). Genetic bottlenecks were detected on both endemic islands and in one mainland population, indicating recent reductions in population size. Our results are consistent with previous studies describing greater losses of genetic diversity on islands compared to mainland populations. Divergence from ancestral allele frequencies in the translocated populations also suggests effects due to founder events. This study, although short-term, highlights the importance of continued monitoring for detecting changes in genetic diversity over time and makes a significant contribution to our understanding of the effects of founder events on island populations.     

Developmental inflexibility of larval tapeworms in response to resource variation

The timing of habitat switching in organisms with complex life cycles is an important life history characteristic that is often influenced by the larval growth environment. Under starvation, longer developmental times are frequently observed, probably as a consequence of developmental thresholds, but prolonged ontogeny sometimes also occurs under good conditions, as organisms may take advantage of the large potential gains in body size. I investigated whether variation in growth conditions affects the larval development time of a complex life cycle tapeworm (Schistocephalus solidus) in its copepod first host. Moreover, I reviewed patterns of developmental plasticity in larval tapeworms to assess the generality of my findings. Copepod starvation weakly retarded parasite growth but did not affect development. Worms grew bigger in larger copepods, but they developed at a similar rate in large and small hosts. Thus, S. solidus does not delay ontogeny under good conditions nor does it fail to reach a developmental threshold under poor conditions. Although unusual in comparison to free-living organisms, such inflexibility is common in tapeworms. Plasticity, namely prolonged ontogeny, has been mainly observed at high infection intensities. For S. solidus, there were large cross-environment genetic correlations for development, suggesting there may be genetic constraints on the evolution of developmental plasticity.     

Aerodynamics of saccate pollen and its implications for wind pollination

Pollen grains of many wind-pollinated plants contain 1-3 air-filled bladders, or sacci. Sacci are thought to help orient the pollen grain in the pollination droplet. Sacci also increase surface area of the pollen grain, yet add minimal mass, thereby increasing dispersal distance; however, this aerodynamic hypothesis has not been tested in a published study. Using scanning electron and transmission electron microscopy, mathematical modeling, and the saccate pollen of three extant conifers with structurally different pollen grains (Pinus, Falcatifolium, Dacrydium), we developed a computational model to investigate pollen flight. The model calculates terminal settling velocity based on structural characters of the pollen grain, including lengths, widths, and depths of the main body and sacci; angle of saccus rotation; and thicknesses of the saccus wall, endoreticulations, intine, and exine. The settling speeds predicted by the model were empirically validated by stroboscopic photography. This study is the first to quantitatively demonstrate the adaptive significance of sacci for the aerodynamics of wind pollination. Modeling pollen both with and without sacci indicated that sacci can reduce pollen settling speeds, thereby increasing dispersal distance, with the exception of pollen grains having robust endoreticulations and those with thick saccus walls. Furthermore, because the mathematical model is based on structural characters and error propagation methods show that the model yields valid results when sample sizes are small, the flight dynamics of fossil pollen can be investigated. Several fossils were studied, including bisaccate (Pinus, Pteruchus, Caytonanthus), monosaccate (Gothania), and nonsaccate (Monoletes) pollen types.