A system-dynamic model on the competitive growth between Potamogeton malaianus Miq. and Spirogyra sp.

A system-dynamic model has been built to evaluate the competition between submerged macrophytes Potamogeton malaianus Miq. (PM) and filamentous green algae Spirogyra sp. (SP). The data background is based on a spring–summer and an autumn–winter experiment carried out in artificial field ponds. The experiments had the aim to acquire a knowledge base necessary to a successful restoration of submerged macrophyte vegetation in Lake Taihu, China by use of P. malaianus Miq. The model mainly focuses on variations in water volume; biomass dynamics of P. malaianus Miq., Spirogyra sp. and zoobenthos; nutrients cycling between water column, P. malaianus Miq., Spirogyra sp., zoobenthos, detritus and sediment. Sixteen state variables are included in the model: biomass of P. malaianus Miq., Spirogyra sp. and zoobenthos; nitrogen in sediments, detritus, in P. malaianus Miq., in Spirogyra sp. and zoobenthos; total dissolved nitrogen; phosphorus in sediments, detritus, in P. malaianus Miq., in Spirogyra sp. and in zoobenthos; total dissolved phosphorus, and water volume of the experiment pond. The calibration and validation of the model show a good accordance with the results of the spring–summer experiment and the autumn–winter experiment.     

Are wolves saving Yellowstone's aspen? A landscape-level test of a behaviorally mediated trophic cascade

Behaviorally mediated trophic cascades (BMTCs) occur when the fear of predation among herbivores enhances plant productivity. Based primarily on systems involving small-bodied predators, BMTCs have been proposed as both strong and ubiquitous in natural ecosystems. Recently, however, synthetic work has suggested that the existence of BMTCs may be mediated by predator hunting mode, whereby passive (sit-and-wait) predators have much stronger effects than active (coursing) predators. One BMTC that has been proposed for a wide-ranging active predator system involves the reintroduction of wolves (Canis lupus) to Yellowstone National Park, USA, which is thought to be leading to a recovery of trembling aspen (Populus tremuloides) by causing elk (Cervus elaphus) to avoid foraging in risky areas. Although this BMTC has been generally accepted and highly popularized, it has never been adequately tested. We assessed whether wolves influence aspen by obtaining detailed demographic data on aspen Stands using tree rings and by monitoring browsing levels in experimental elk exclosures arrayed across a gradient of predation risk for three years. Our study demonstrates that the historical failure of aspen to regenerate varied widely among stands (last recruitment year ranged from 1892 to 1956), and our data do not indicate an abrupt cessation of recruitment. This pattern of recruitment failure appears more consistent with a gradual increase in elk numbers rather than a rapid behavioral shift in elk foraging following wolf extirpation. In addition, our estimates of relative survivorship of young browsable aspen indicate that aspen are not currently recovering in Yellowstone, even in the presence of a large wolf population. Finally, in an experimental test of the BMTC hypothesis we found that the impacts of elk browsing on aspen demography are not diminished in sites where elk are at higher risk of predation by wolves. These findings suggest the need to further evaluate how trophic cascades are mediated by predator-prey life history and ecological context.     

Reproductive strategy and singing activity: blue tit and great tit compared

The costs and benefits of bird song are likely to vary among species, and different singing patterns may reflect differences in reproductive strategies. We compared temporal patterns of singing activity in two songbird species, the blue tit (Cyanistes caeruleus) and the great tit (Parus major). The two species live side by side year round, and they have similar breeding ecology and similar rates of extra-pair paternity. However, they differ in two aspects of reproductive strategy that may have an influence on song output: blue tits are facultatively polygynous and have a fairly short breeding season with almost no second broods, whereas great tits are socially monogamous but more commonly raise second broods. We found that great tit males continued singing at high levels during the egg-laying and incubation periods, while monogamously paired blue tit males strongly reduced singing activity after the first days of egg-laying by their female. Since males of both species sang much more intensely shortly before sunrise than after sunrise, at midday or in the evening, this difference was most conspicuous at dawn. No differences in singing activity were found within species when testing for male age. We suggest that in contrast to blue tits, great tit males continued singing after egg-laying to defend the territory and to encourage the female for a possible second brood.     

Dispersal, density dependence, and population dynamics of a fungal microbe on leaf surfaces

Despite the ubiquity and importance of microbes in nature, little is known about their natural population dynamics, especially for those that occupy terrestrial habitats. Here we investigate the dynamics of the yeast-like fungus Aureobasidium pullulans (Ap) on apple leaves in an orchard. We asked three questions. (1) Is variation in fungal population density among leaves caused by variation in leaf carrying capacities and strong density-dependent population growth that maintains densities near carrying capacity? (2) Do resident populations have competitive advantages over immigrant cells? (3) Do Ap dynamics differ at different times during the growing season? To address these questions, we performed two experiments at different times in the growing season. Both experiments used a 2 x 2 factorial design: treatment 1 removed fungal cells from leaves to reveal density-dependent population growth, and treatment 2 inoculated leaves with an Ap strain engineered to express green fluorescent protein (GFP), which made it possible to track the fate of immigrant cells. The experiments showed that natural populations of Ap vary greatly in density due to sustained differences in carrying capacities among leaves. The maintenance of populations close to carrying capacities indicates strong density-dependent processes. Furthermore, resident populations are strongly competitive against immigrants, while immigrants have little impact on residents. Finally, statistical models showed high population growth rates of resident cells in one experiment but not in the other, suggesting that Ap experiences relatively "good" and "bad" periods for population growth. This picture of Ap dynamics conforms to commonly held, but rarely demonstrated, expectations of microbe dynamics in nature. It also highlights the importance of local processes, as opposed to immigration, in determining the abundance and dynamics of microbes on surfaces in terrestrial systems.     

Characterization of model error in a simulation of fine particulate matter exposure distributions of the working age population in Helsinki, Finland

Exposure models are needed for comparison of scenarios resulting from alternative policy options. The reliability of models used for such purposes should be quantified by comparing model outputs in a real situation with the corresponding observed exposures. Measurement errors affect the observations, but if the distribution of these errors for single observations is known, the bias caused for the population statistics can be corrected. The current paper does this and calculates model errors for a probabilistic simulation of 48-hr fine particulate matter (PM2.5) exposures. Direct and nested microenvironment-based models are compared. The direct model requires knowledge on the distribution of the indoor concentrations, whereas the nested model calculates indoor concentrations from ambient levels, using infiltration factors and indoor sources. The model error in the mean exposure level was <0.5 microg m(-3) for both models. Relative errors in the estimated population mean were +1% and -5% for the direct and nested models, respectively. Relative errors in the estimated SD were -9% and -23%, respectively. The magnitude of these errors and the errors calculated for population percentiles indicate that the model errors would not drive general conclusions derived from these models, supporting the use of the models as a tool for evaluation of potential exposure reductions in alternative policy scenarios.     

Modelling 137Cs uptake in plants from undisturbed soil monoliths

A model predicting 137Cs uptake in plants was applied on data from artificially contaminated lysimeters. The lysimeter data involve three different crops (beans, ryegrass and lettuce) grown on five different soils between 3 and 5 years after contamination and where soil solution composition was monitored. The mechanistic model predicts plant uptake of 137Cs from soil solution composition. Predicted K concentrations in the rhizosphere were up to 50-fold below that in the bulk soil solution whereas corresponding 137Cs concentration gradients were always less pronounced. Predictions of crop 137Cs content based on rhizosphere soil solution compositions were generally closer to observations than those based on bulk soil solution composition. The model explained 17% (beans) to 91% (lettuce) of the variation in 137Cs activity concentrations in the plants. The model failed to predict the 137Cs activity concentration in ryegrass where uptake of the 5-year-old 137Cs from 3 soils was about 40-fold larger than predicted. The model generally underpredicted crop 137Cs concentrations at soil solution K concentration below about 1.0 mM. It is concluded that 137Cs uptake can be predicted from the soil solution composition at adequate K nutrition but that significant uncertainties remain when soil solution K is below 1 mM.     

Recent and future signatures of climate change in Europe

A set of six regional climate model experiments is investigated for future changes in daily temperature and precipitation in Europe. Changes in the probability distributions for these variables are studied. It is found that the asymmetry of these distributions change differently depending on location and season. Large summertime changes in extremely high temperatures in central, eastern and southern Europe are followed by higher than average temperature increases on warm days in general. Likewise, temperatures on cold days increase much more than the average temperature increase during winter in eastern and northern Europe. A comparison with historical data on wintertime temperature shows that the model simulated and observed daily variability are similar. In particular, the much stronger increase in temperatures on cold days, compared to the average temperature increase as observed in warm compared to cold historical periods, is simulated also by the model. The contribution from heavy precipitation events is simulated to increase over most parts of Europe in all seasons.     

Formation of Reactive Gaseous Mercury in the Arctic: Evidence of Oxidation of Hg° to Gas-Phase Hg-II Compounds after Arctic Sunrise

We have measured total gaseous mercury concentrations(Hg°) at Point Barrow, Alaska since September 1998 in aneffort to determine the geographic extent and reaction mechanismof the so-called mercury depletion events (MDE) previouslyreported in the high Arctic at Alert, Canada. Hg° has beensampled now for nearly 2 years at Barrow. In September, 1999, webegan making the first automated measurements of reactive gaseousmercury (RGM) attempted in the Arctic, along with measurements ofHg accumulation in snowpack to determine the fate of the depleted Hg°. During the fall and early winter, Hg°and RGM exhibit only minor variation, Hg° remaining within10% of global background, near 1.6–1.8 ng m^-3. The MDEperiods are quite different, however; within days of Arcticsunrise in January, Hg° exhibits major variations from themean, rapidly dropping as low as 0.05 ng m^-3 and then cyclingback to typical levels, sometimes exceeding global background. These events continue throughout Arctic spring, then end abruptlyfollowing snowmelt, in early June. Prior to Arctic sunrise, RGMremains near detection (<2 pg m^-3), but after sunriseincreases dramatically (to levels as high as 900 pg/m³) insynchrony with the depletion of Hg°. Both phenomenaexhibit a strong diel cycle, in parallel with UV-B. We concludethat MDE's involve rapid in-air oxidation of Hg° to aspecies of RGM by photochemically-driven reactions, probablyinvolving the same reactive bromine and chlorine compoundsinvolved in ozone destruction. Sharp increases in Hg in thesurface snowpack after sunrise coincident with periods of peakRGM suggest surface accumulation of the RGM by dry deposition.