A model of the seasonal pattern of carbon acquisition in two woodland herbs,Mercurialis perennis L. and Geum urbanum L.

Summary Seasonal changes in the light and temperature dependence of photosynthesis were investigated in field grown plants of Mercurialis perennis and Geum urbanum. In both species changes in photosynthetic capacity were closely related to the development of the overstorey canopy. In G. urbanum there was a marked shift in the temperature dependence of photosynthesis through the season whereas no such pattern was found in M. perennis. Model predictions of field rates of photosynthesis were made using the measurements of light and temperature dependence in the laboratory and validated against field observations. Long term continuous records of light and temperature in the field were used in conjunction with the model to make predictions of carbon acquisition in shoots of the two species through the season. These calculations indicated that G. urbanum was able to take advantage of high light levels just prior to canopy closure through a combination of high photosynthetic capacity, the ability to maintain photosynthesis at relatively low temperatures and the presence of overwintering leaves. In M. perennis leaf development was early enough to utilise the high spring light period. After canopy closure M. perennis maintained a higher average rate of CO₂ flux due to a combination of high apparent quantum efficiency and low rates of respiration.     

植物群落物种共存机制:负密度制约假说

负密度制约假说主要描述由于资源竞争、有害生物侵害(比如病原微生物、食草动物捕食)等, 同种个体之间发生的相互损害行为; 它主要强调同种个体之间的相互作用, 解释自然群落物种共存的机理; 负密度制约机制主要在小尺度上降低群落内同种个体生长率, 同时提高个体死亡率, 从而为其他物种的生存提供空间和资源, 促进物种共存。目前负密度制约假说的检验研究主要侧重密度制约、距离制约、群落补偿效应等三个方面。最近, 研究者又探讨了近缘物种之间由于对相似资源的竞争所产生的负效应, 扩展了负密度制约假说, 进而提出异群保护假说和谱系多样性制约假说。负密度制约假说引起生态学家长久的探讨和关注, 世界范围内大尺度森林动态样地的建立, 又为探索不同尺度上密度制约效应的研究提供了条件。然而目前的研究仍然存在不足, 比如负密度制约假说的检验受到其他因素的干扰、区域研究不平衡等。因此, 生态学家们仍然怀疑负密度制约效应调节群落物种共存的重要性, 但是目前的研究还没有发现否定负密度制约假说的充分证据。     

Negative frequency-dependent selection is intensified at higher population densities in protist populations

Natural populations of free-living protists often exhibit high-levels of intraspecific diversity, yet this is puzzling as classic evolutionary theory predicts dominance by genotypes with high fitness, particularly in large populations where selection is efficient. Here, we test whether negative frequency-dependent selection (NFDS) plays a role in the maintenance of diversity in the marine flagellate Oxyrrhis marina using competition experiments between multiple pairs of strains. We observed strain-specific responses to frequency and density, but an overall signature of NFDS that was intensified at higher population densities. Because our strains were not selected a priori on the basis of particular traits expected to exhibit NFDS, these data represent a relatively unbiased estimate of the role for NFDS in maintaining diversity in protist populations. These findings could help to explain how bloom-forming plankton, which periodically achieve exceptionally high population densities, maintain substantial intraspecific diversity.     

Active sulfate absorption in rabbit ileum: Dependence on sodium and chloride and effects of agents that alter chloride transport

Summary Unidirectional fluxes of³⁵SO₄ across and into rabbit ileal epithelium were measured under short-circuit conditions, mostly at a medium SO₄ concentration of 2.4mm. Unidirectional mucosa (m)-to-serosa (s) ands-to-m fluxes (J _ms,J _sm) were 0.456 and 0.067 moles hr^–1 cm^–2, respectively.J _ms was 2.7 times higher in distal ileum than in mid-jejunum. Ouabain abolished net SO₄ transport (J _net) by reducingJ _ms. Epinephrine, a stimulus of Cl absorption, had no effect on SO₄ fluxes. Theophylline, a stimulus of Cl secretion, reducedJ _ms without affectingJ _sm, causing a 33% reduction inJ _net. Other secretory stimuli (8-Br-cAMP, heat-stable enterotoxin, Ca-ionophore A23187) had similar effects. Replacement of all Cl with gluconate markedly reducedJ _net through both a decrease inJ _ms and an increase inJ _sm. The anion-exchange inhibitor, 4-acetoamido-4-isothiocyano-2,2-sulfonic acid stilbene (SITS), when added to the serosal side, reducedJ _ms by 94%, nearly abolishingJ _net. SITS also decreasedJ _sm by 75%. Mucosal SITS (50 m) was ineffective. 4,4-diisothiocyano-2,2-sulfonic acid stilbene (DIDS) had effects similar to SITS but was less potent. Measurements of initial rates of epithelial uptake from the luminal side (J _me) revealed the following: (1)J _me is a saturable function of medium concentration with aV _max of 0.94 moles hr^–1 cm^–2 and aK _1/2 of 1.3mm; (2) replacing all Na with choline abolishedJ _me; (3) replacing all Cl with gluconate increasedJ _me by 40%; (4) serosal SITS had no effect onJ _me; and (5) stimuli of Cl secretion had no effect onJ _me or increased it slightly. Determination of cell SO₄ with³⁵SO₄ indicated that, at steady-state, the average mucosal concentration is 1.1 mmoles per liter cell water, less than half the medium concentration. Cell SO₄ was increased to 3.0mm by adding SITS to the serosal side. Despite net transport rates greater than 1.4 Eq hr^–1 cm^–2, neither addition of SO₄ to the SO₄-free medium nor addition of SITS to SO₄-containing medium altered short-circuit current. The results suggest that (1) ileal SO₄ absorption consists of Na-coupled influx (symport) across the brush border and Cl-coupled efflux (antiport) across the basolateral membrane; (2) the overall process is electrically neutral; (3) the medium-to-cell Cl concentration difference may provide part of the driving force for net SO₄ absorption; and (4) since agents affecting Cl fluxes (both absorptive and secretory) have little effect on SO₄ fluxes, the mechanisms for their transcellular transports are under separate regulation.     

Woodland caribou calf mortality in Newfoundland: insights into the role of climate,predation and population density over three decades of study

The rates and causes of juvenile mortality are central features of the dynamics and conservation of large mammals, like woodland caribou (Rangifer tarandus caribou (Gmelin, 1788)), but intrinsic and extrinsic factors may be modified by variations in animal abundance. We tested the influences of population size, climate, calf weight and sex on survival to 6 months of age of 1241 radio-collared caribou calves over three decades, spanning periods of population growth (1979–1997) and decline (2003–2012) in Newfoundland, Canada. Daily survival rates were higher and rose more quickly with calf age during the population growth period compared to the decline. Population size (negatively) and calf weight (positively) affected survival during the decline but neither had a detectable influence during the growth phase. Sex, climate and plant productivity (the latter two derived from the North Atlantic Oscillation and Normalized Difference Vegetation Index, respectively) exerted minimal influence during either phase. Predation was the dominant source of mortality. The mean percentage of calves killed by predators was 30 % higher during the decline compared to the growth phase. Black bears (Ursus americanus) and lynx (Lynx canadensis) were the major predators during the population increase but this changed during the decrease to black bears and coyotes (Canis latrans). Our findings are consistent with the hypothesis that Newfoundland caribou experienced phase-dependent survival mediated proximally by predation and competition for food.     

Analysis of species coexistence co-mediated by resource competition and reproductive interference

Reproductive interference is any interspecific sexual interaction that reduces the reproductive success of females through promiscuous reproductive activities of heterospecific individuals. This phenomenon is ubiquitous in nature in both plants and animals, and is frequently observed in biological invasions. However, its effects on interspecific competition remain incompletely understood despite growing concern. To study the interactive effects of resource competition and reproductive interference on species coexistence and exclusion, we analyzed a unified competition model including both processes in symmetric and asymmetric scenarios. The results of our model showed that resource competition and reproductive interference act synergistically to promote competitive exclusion. We also found that when the two processes are asymmetric, the species that is superior in reproductive interference can coexist with or even exclude the species that is superior in resource competition. Therefore, coexistence is possible via an unbalanced trade-off between resource use and reproduction. Our results suggest that integration of reproductive interference and resource competition will contribute to a better understanding of interspecific competition and to more effective biodiversity conservation against management of biological invasions.     

Temperature dependence of two parameters in a photosynthesis model

The temperature dependence of the photosynthetic parameters V_cmax, the maximum catalytic rate of the enzyme Rubisco, and J_max, the maximum electron transport rate, were examined using published datasets. An Arrehenius equation, modified to account for decreases in each parameter at high temperatures, satisfactorily described the temperature response for both parameters. There was remarkable conformity in V_cmax and J_max between all plants at T_leaf < 25 °C, when each parameter was normalized by their respective values at 25 °C (V_cmax0 and J_max0), but showed a high degree of variability between and within species at T_leaf > 30 °C. For both normalized V_cmax and J_max, the maximum fractional error introduced by assuming a common temperature response function is < ± 0·1 for most plants and < ± 0·22 for all plants when T_leaf < 25 °C. Fractional errors are typically < ± 0·45 in the temperature range 25–30 °C, but very large errors occur when a common function is used to estimate the photosynthetic parameters at temperatures > 30 °C. The ratio J_max/V_cmax varies with temperature, but analysis of the ratio at T_leaf = 25 °C using the fitted mean temperature response functions results in J_max0/V_cmax0 = 2·00 ± 0·60 (SD, n = 43).     

The Janzen-Connell effect on the population dynamics of a Fagus engleriana- Cyclobalanopsis oxyodon community in a subtropical zone of China

The Janzen-Connell (J-C) hypothesis provides a mechanism explaining the high species diversity in tropical rainforests. It postulates that predation could cause greater mortality on seeds and seedlings near their parental trees. In this study, we tested the hypothesis in a subtropical zone, a mixed evergreen-deciduous broad-leaved forest dominated by the Fagus engleriana and Cyclobalanopsis oxyodon. The study area was in the Shennongjia region, a key area of biodiversity conserva-tion in both China and the world. The recruitment probability index was used to detect the J-C effect on nine species of the community, which were more than 50 individuals. Six large adults of each species were selected, and the numbers of saplings and adults were counted at the distance intervals of 0-5, 5-10, 10-15, 15-20, and 20-25 m from each focal tree. Two species in saplings stage and six in adult stage supported the J-C hypothesis, but their χ2 was not significant. Three species, the F. engleri-ana, Rhododendron hypoglaucum, and Toona sinensis, showed a strong Hubbell pattern in the adult stage. Because of these results, we reject the J-C hypothesis and conclude that species could recruit near the conspecific trees in subtropical forest. The reasons why the J-C hypothesis fails to explain the species diversity in this community are the shortage of seed-consuming agents of subtropical forest and the influence of microsite topo-graphic variation.     

A third DNA-dependent ATPase from Bacillus cereus free of ATP-dependent DNase activity

The purification of ATP-dependent DNase from Bacillus cereus led to the isolation and characterization of a third DNA-dependent ATPase. The enzyme called ATPase III has been purified free of nuclease activity. None of the expected ATPases proved to be identical with ATP-dependent DNase-DNA-dependent ATPase. Separation of ATPase I, II and III and a DNase specific for single-stranded DNA from the same source excludes the possibility of ATP-dependent DNase being the action of a single enzyme molecule.     

Long-term population dynamics of two Carex curvula species in the Central Alps on native and alien soils

The demography of two closely related alpine sedges, Carex curvula subsp. curvula and Carex curvula subsp. rosae (=C. curvula and C. rosae) has been investigated on their typical sites in the Central Alps. Both species proliferate vegetatively and develop dense tussocks but they show different dominance behaviours in their respective grasslands. It was hypothesized that this may be caused by different growth abilities. The main aim of the study was to compare the vegetative growth of the species under field conditions, under competition-free conditions and under changed soil conditions. An attempt was also made to clarify whether vegetative growth is density dependent. Permanent plots were established in the respective grasslands of the two species and the ramet density was counted over 3 years. Groups of 10 and of 30 ramets of each species were grown in pots with typical and with alien substrate and their growth was observed for 5 years at the field site. The grassland populations of both species were very stable and the overall ramet growth rate (λ) was close to 1.0. Within the pots, both species reached a high ramet number. Only the group of 30 ramets of C. curvula on alien soil could not recover from the transplantation shock. Within the pots, C. rosae showed a greater ramet turnover and a higher increase in ramets than C. curvula. On their native substrate, both species had a significantly higher ramet increase than on the alien substrate. Ramet growth was found to be density dependent for both species, the increases recorded for the groups of 10 being significantly greater than for the groups of 30. Although C. curvula produced fewer ramets than C. rosae, the aboveground dry weight of the former was significantly higher. This may be decisive for its greater competitive success in closed grasslands. Received: 12 April 1997 / Accepted: 12 December 1997     

On the use of EBT3 film for relative dosimetry of kilovoltage X ray beams

EBT3 films were evaluated for relative dosimetry in water, in the energy range of therapeutic kV X ray beams. A film batch was calibrated in air for all nine beam qualities of a clinical unit (XStrahl 200). Monte Carlo (MC) simulations using MCNP v.6 facilitated the calculation of the film absorbed dose (f), and beam quality (k_bq) energy dependences in air. Results were found in agreement with corresponding data in the literature. Film samples from the same batch were irradiated in water along the central beam axis for each beam quality. Experimental percentage depth dose (PDD) results obtained using calibration data in air showed quality and depth dependent differences from corresponding MC simulations. These differences increased beyond film dosimetry uncertainty (<3.3%), reaching up to 8% at increased depth. The observed differences reduced only slightly when spectral variation as a function of measurement point was accounted for, using photon effective energy. PDD measurements and corresponding MC results facilitated the determination of f and k_bq in water. Results showed that the origin of the observed differences between experimental and MC PDD results is the difference between film response in air and water, as a result of radiation field perturbation from the film oriented along the central beam axis. This implies a directional dependence of film response which necessitates that the angular distribution of photons impinging on the film is the same in the calibration and measurement geometries.     

Difference in frequency spectrum of extremely-low-frequency effects on the genome conformational state of AB1157 and EMG2 E. coli cells

The effect of weak extremely-low-frequency (ELF) magnetic fields (sinusoidal, 30 μT amplitude) on the genome conformational state (GCS) of E. coli mutant and wild type cells was studied by using the method of anomalous viscosity time dependency (AVTD) in the 6–37 Hz frequency range. We confirmed the existence of three resonance frequencies of 8.9, 15.5, and 29.4 Hz when mutant cells of K12 AB1157 strain were exposed. In the same frequency range, the wild type K12 EMG2 cells displayed only two effective windows, with resonance frequencies of 8.3 and 27 Hz. The resonance frequencies differed significantly (P < .001–.000001) in the strains studied, whereas other resonance parameters did not. It was concluded that mutations in the AB1157 strain resulted in a significant rearrangement in the ELF action spectrum, including the appearance of a new resonance. © 1996 Wiley-Liss, Inc.     

Detection of density dependence from annual censuses of bracken-feeding insects

Summary A variety of techniques were used to test for density dependence in 32 time series from bracken-feeding insects. Seventeen taxa (primarily species, but including some pooled data from two or more closely related species whose larvae could not be distinguished in frond surveys) occurred on an open site; a woodland site held 15 taxa. For series of 12 years, collected on the open habitat, direct density dependence was detected by one or more of the techniques in 10 (58.8%) of 17 taxa, compared to only 5 (33.3%) of 15 taxa with time series of 8 years in length from the woodland habitat. Delayed density dependence was detected in 6 cases for the open site and in no cases at the woodland site. Either direct or delayed density dependence was found in 13 (76.5%) of 17 taxa for the open site and 13 (86.7%) of the 15 taxa which occurred on both sites. Although these results suggest a high frequency of density dependence in the species making up the bracken insect community, results from individual tests were extremely variable. Density dependence was detected least often by Vickery and Nudds' (1984) test, and most frequently by Varley and Gradwell's (1960) test, although the latter is prone to high rates of detecting spurious density dependence. Direct density dependence was detected most frequently in taxa that were univoltine and did not have delayed diapause, i.e. in those taxa whose life-histories conform most closely to the assumptions of the models underlying the analyses. Delayed density dependence occurred more frequently in species with more complex life-histories at the open site (taxa that were either bivoltine or multivoltine, or had delayed diapause). The results are consistent with the view that that the bracken herbivore assemblage consists of populations which are independently regulated by density dependent processes, although the present analyses suggest that we cannot rely on these tests to firmly show whether density dependence is present or not in an individual time series of the lengths considered here.     

Plant species richness and species-area relations in a shortgrass steppe in Colorado

Abstract. Plant species richness and species-area relations were examined for three landscapes (toposequences), each with a summit or upland, a midslope and a toeslope or lowland, in a shortgrass steppe in Colorado. The number of plant species in the largest plot size (0.16 ha) varied from 38 to 53. Neither the exponential relationship: s = a + b log A, nor the power function: S = cAz fit the data equally well in all situations. The processes acting upon species diversity here seem to operate at two spatial scales. The number of species in plots smaller than 3 m² was independent of the total number of species in the 0.16-ha plots and was constrained by the presence of the dominant bunchgrasses. Beyond 3 m², species number in each plot size was a function of the total number of species in the 0.16-ha plot.     

Density-dependent effects on allelopathic interactions in yeast

The ability of rare types to invade populations is important for the maintenance of diversity and spread of beneficial variants. Spatial structure promotes strategies of interference competition by limiting diffusion of interference toxins and resources, often allowing interference competitors to invade when rare. Consistent with previous results in other microbial systems, toxin production by Saccharomyces cerevisiae is advantageous in spatially structured, high-density environments, but not in unstructured environments. However, at low density and at low frequency, rare toxin producers cannot invade populations of common, sensitive, toxin nonproducers. This is because the likelihood of interaction between toxin producers and sensitives depends upon the density and frequency of both competitors.     

Status‐dependence and morphological trade‐offs in the expression of a sexually selected character in the mite,Sancassania berlesei

Abstract In the male dimorphic mite Sancassania berlesei, fighter males kill rivals with a pair of armoured legs whereas scrambler males are benign with unmodified legs. In an adaptive response mediated by colony pheromones, fighter expression increases at low colony density. Under the status‐dependent evolutionarily stable strategy (ESS) model we expected heavier final instar nymphs to become fighters. This was supported in group reared nymphs. In individually reared nymphs fighter expression was experimentally suppressed using two concentrations of colony pheromone. Here, male morph expression again depended on tritonymphal body mass and contact is therefore unnecessary for individuals to judge their status. Fighter suppression was greater in the higher pheromone treatment, but morph determination remained status dependent. The weight and length of fighters was lower than scramblers of same‐weight final instar nymphs, indicating a developmental trade‐off, and a cost not recouped at the adult stage.     

Empirical validation of a two-state kinetic model for the B-Z transition of double-stranded poly[d(G-m5C)]

5 C)] in terms of the salt concentration, the absolute temperature, and the cooperativity index. The validity of the formula was tested using circular dichroism spectroscopy after variation of the type of salt (NaCl, MgCl₂), the salt concentration, and the temperature of the polynucleotide solution. A consequence of the proposed function is that in conditions of high salt there is a predictable salt threshold which determines the particular molecular mechanism of the B-Z transition. The paper also describes the way in which this threshold level is temperature dependent. A detailed comparison of our data with the experimental data found by other authors is given. The function agrees quantitatively with the experiments and explains the contrasting results found in the literature about the influence in the B-Z transition of both the temperature and the polymer size. Received: 21 April 1999 / Accepted: 9 September 1999     

The demise of the population regulation controversy?

For many decades a controversy in population dynamics theory has been raging concerning whether or not populations are regulated by density dependent factors, without reaching a satisfactory conclusion. It is suggested that this failure to solve the problem is due to asking the wrong question and it is suggested that this controversy should be buried and forgotten.     

Computer Dependence of An Improvement of The Cell Neighbour-Table Method For Short-Range Potentials

Abstract

A comparative test is presented for molecular dynamics, MD, computer simulation between the original Cell Neighbour-Table method, CNT, and a later development, the Link-Cell Neighbour-Table, LCNT. The test was simultaneously carried out on a vector Convex 210 computer and on a scalar Pentium 120. The comparison of the two methods for very large systems (up to 100000 particles) and for two short-range pair potentials (Weeks-Chandler-Andersen, WCA, and Lennard-Jones, LJ) showed that on the Convex, in vectorial mode, the LCNT method is about 25% more efficient in time than the CNT method for the shorter-range WCA potential, the difference being unnoticeable (less than 5%) for the LJ potential. However, when the scalar mode is on in the Convex, the difference disappears. In the Pentium, which was found to be systematically faster than the Convex even in vectorial mode, no significant difference of any kind between the both methods was found, with the either of the two potentials.     

Sexual selection is influenced by both developmental and adult environments

Sexual selection is often assumed to be strong and consistent, yet increasing research shows it can fluctuate over space and time. Few experimental studies have examined changes in sexual selection in response to natural environmental variation. Here, we use a difference in resource quality to test for the influence of past environmental conditions and current environmental conditions on male and female mate choice and resulting selection gradients for leaf‐footed cactus bugs, Narnia femorata. We raised juveniles on natural high‐ and low‐quality diets, cactus pads with and without ripe cactus fruits. New adults were again assigned a cactus pad with or without fruit, paired with a potential mate, and observed for mating behaviors. We found developmental and adult encounter environments affected mating decisions and the resulting patterns of sexual selection for both males and females. Males were not choosy in the low‐quality encounter environment, cactus without fruit, but they avoided mating with small females in the high‐quality encounter environment. Females were choosy in both encounter environments, avoiding mating with small males. However, they were the choosiest when they were in the low‐quality encounter environment. Female mate choice was also context dependent by male developmental environment. Females were more likely to mate with males that had developed on cactus with fruit when they were currently in the cactus with fruit environment. This pattern disappeared when females were in the cactus without fruit environment. Altogether, these results experimentally demonstrate context‐dependent mate choice by both males and females. Furthermore, we demonstrate that simple, seasonal changes in resources can lead to fluctuations in sexual selection.