Submerged vegetation development in two shallow, eutrophic lakes

Submerged macrophyte vegetation in two shallow lakes in the Netherlands, Lake Veluwemeer and Lake Wolderwijd, has been affected by eutrophication in the late 1960's and 1970's. Recent changes in the vegetation occurred in the period following lake restoration measures. Between 1987 and 1993, the dominance of Potamogeton pectinatus decreased, while Charophyte meadows expanded over the same time interval. The pattern of change of the dominant macrophyte species might result from changes in the underwater light climate. Seasonally persistent clear water patches associated with the Chara meadows have been observed in the last few years. The interaction between submerged macrophyte vegetation succession and water transparency in the lakes is discussed.     

Nutrient-induced competition between two species of marine diatoms

Competition experiments betweenPhaeodactylum tricornutum andSkeletonema costatum showed that even at temperatures higher than 10°C (i.c. 14°C), the development ofSkeletonema can be favoured by adjusting nutrient levels and nutrient ratios. Low NSi ratios were found to favourSkeletonema. Additionally, high NP ratios further enhanced the ability ofSkeletonema to dominate the cuftures. Contrary to some statements in literature, it seems that high concentrations of silicates are more important for the dominance ofSkeletonema costatum in large-scale cultures than just low temperatures. This finding is important with regard to stimulating the blooming ofSkeletonema costatum in natural phytoplankton populations as food for bivalve molluscs.     

Competitive relationships between two water plant species: Najas marina L. and Myriophyllum spicatum L.

The interrelationships between two submerged macrophytes, Najas marina and Myriophyllum spicatum, were investigated. Double-reciprocal analysis was used to distinguish between the possible types of negative relationships. Results have demonstrated that interspecific inhibition was more severe than the intraspecific one. The intensity of inhibition was negatively correlated with the density of the examined species. The mutual inhibitory effect and the pattern of the obtained results suggest that competitive relationships exist between Najas marina and Myriophyllum spicatum.     

Experimental tests of nest site competition in two Peromyscus species

The importance of interspecific competition for nest sites between the white-footed mouse (Peromyscus leucopus noveboracensis) and the cloudland deermouse (P. maniculatus nubiterrae) were investigated in the montane forests of southwestern Virginia over 3 years. Trials were conducted for both species using large, outdoor enclosures in order to examine: (i) nest site preference in isolation and (ii) nest site selection made in the presence of potential competitors. Both species demonstrated a strong preference for arboreal nest sites when tested without competitors present. After the introduction of a heterospecific intruder, P. leucopus often shifted to a non-arboreal nest while P. maniculatus continued to use arboreal nests. Intruding P. maniculatus displaced resident P. leucopus from P. leucopus' preferred nest sites in all 3 years while intruding P. leucopus never displaced P. maniculatus. Neither species was routinely displaced in conspecific trials. Resident P. maniculatus also excluded P. leucopus from access to preferred nesting sites in all three years while P. leucopus were only able to exclude P. maniculatus in the 3rd year. Both species exhibite relatively low frequencies of exclusion in conspecific trials with the exception of P. maniculatus which excluded high proportions of conspecific intruders in the second year. We suggest that the asymmetrical advantage enjoyed by P. maniculatus in nest site selection may result from both site-specific effects and a species-specific influence on P. leucopus. Nonetheless, the intensity of such influence varied between years, perhaps as a function of population density, and did not appear to drastically influence continued coexistence of these congeners.     

Effect of competition on stable carbon isotope ratios of two tussock grass species

Summary Previous studies have shown that plant carbon isotope composition varies when plants experience differences in water and nutrient availability. However, none have addressed the effect of root interactions, including competition for these soil resources, on carbon isotope ratios. We studied the effect of interspecific root interactions on the productivity and carbon isotope ratios of two Great Basin tussock grass species (Agropyron desertorum and Pseudoroegneria spicata). We compared grasses grown in mixture with sagebrush (Artemisia tridentara) to grasses in similar mixtures but where root interactions with sagebrush were limited by fiberglass partitions. During both years of the study, tussocks growing in competition with sagebrush produced tissue with more negative ¹³C values than grasses experiencing limited root interaction with sagebrush. The magnitude of this difference (0.5 to 0.9%) is similar to that found in other studies when soil fertility and moisture availability were altered.     

两种本地植物种子萌发对飞机草的化感耐受性及其相互竞争

为探讨本地物种假地豆和白饭树对入侵植物飞机草的替代控制潜力,利用培养皿法和同质园种植实验分别研究了两个本地物种种子萌发对飞机草的化感耐受性及其与飞机草的竞争关系。结果显示:除了假地豆的萌发率在高浓度(2.5%)的飞机草叶提取液下受到显著抑制外,两个本地种的萌发在不同浓度飞机草根、茎、叶提取液下均不受抑制。飞机草与假地豆混种时,飞机草的株高、地下生物量比及根冠比显著降低,假地豆的株高无显著变化,但生物量显著增加;飞机草的竞争参数相对产量(RY)显著小于1,竞争攻击力系数显著小于零,表明其竞争力弱于假地豆。飞机草与白饭树混种时,飞机草的根冠比也显著降低,但株高和生物量均显著增加,而白饭树的株高和生物量却显著降低;飞机草的相对产量(RY)显著大于1,竞争攻击力系数显著大于零,表明其竞争力强于白饭树。结合以上结果,本地植物假地豆可以一定程度上竞争抑制飞机草的生长,具有替代控制飞机草的潜力,而白饭树可以在清除飞机草后的入侵地辅助植被修复。实验结果为飞机草替代控制的目标物种的筛选提供了科学依据,对被飞机草入侵生境生物多样性的恢复和重建具有一定指导意义。     

Predator-mediated apparent competition between two herbivores that feed on grapevines

We have been releasing economically unimportant herbivorous mites of one species early in the season and protecting grapevines against another, more damaging herbivorous mite throughout the growing season. In this experiment, releases of economically unimportant Willamette mites alone, or of predatory mites alone, failed to reduce populations of the damaging Pacific spider mite. However, where both herbivorous Willamette mites and predatory mites were released together populations of Pacific mites were reduced. This interaction between effects of Willamette mites and predatory mites suggests that predation against Pacific mites was more effective where alternate prey (Willamette mites) were available for the predators. The apparent competition between Willamette mites and Pacific mites, mediated through their shared predator, can be an important force in the agroecosystem although its importance varies from year to year and vineyard to vineyard.     

A comparative study on planktonic ciliates in two shallow mesotrophic lakes (China): species composition, distribution and quantitative importance

The species richness and seasonal development of planktonic ciliates were studied and compared in two shallow mesotrophic lakes, one covered with dense submerged macrophytes, the other macrophyte poor. Considerable differences in ciliate species composition, dominant taxa, abundance and biomass were observed. Ciliates were much more species rich in the macrophyte-rich lake, while they were more abundant numerically in the macrophyte-poor lake. Altogether, 96 species, included in 53 genera, 14 orders were identified. Among them, 80 species (included in 45 genera, 14 orders) observed from the macrophyte-rich lake, against 49 species (36 genera, 12 orders) were from the macrophyte-poor lake. In the macrophyte-rich lake, the mean abundance and biomass were 13.5 cells ml^-1 and 547.10 g l^-1 f.w.; abundance and biomass were higher in spring and winter; naked oligotrichs dominated total ciliate abundance and Peritrichida dominated the biomass. In the macrophyte-poor lake, ciliate mean abundance and biomass were 35.5 cells ml^-1 and 953.39 g l^-1 f.w.; abundances peaked in autumn; Scuticociliates dominated the abundance and Tintinnids dominated the biomass. Possible causes for the observed differences are discussed.     

Variation in the carbon content of two Asplanchna species

The rotifers of the genus Asplanchna were sampled four times during the summer from eight lakes of different types. The mean individual carbon content in the population varied between 0.15–0.66 µg C ind.^-1 (n = 21) for A. priodonta and 1.0–1.6 µg C ind.^-1 (n = 3) for A. herricki. The carbon content and the size of A. priodonta varied considerably between the populations of both different lakes and dates.The carbon level of both Asplanchna species (sample mean 0.2–1% of wet weight) was considerably lower than is generally found for rotifers. Much of the variation of carbon level could be explained by an inverse relationship with wet weight. The high variation in the carbon content of individuals suggests that Asplanch population may adapt their mean body size to fit prevailing environmental conditions.     

Polarity and competition between megaspores in the ovule ofOenothera hybrids

New data on the development of polarity in the ovules during megasporogenesis and early stages of embryo sac development inOenothera-hybrids are presented. It is confirmed that allOe. hookeri-hybrids show a strong tendency to form heteropolar tetrads, with the micropylar megaspore developing into an embryo sac. This preference is seen in the delay of the second meiotic division on the chalazal side, the absence of callose in the lateral wall of the micropylar megaspore, and the accumulation of starch in this megaspore. However, homopolar tetrads, chalazal preference, and ovules with two developing embryo sacs are also observed with considerable frequency. Quantitative data on the frequency of the different developmental types are compared with earlier genetic results about competition in the haplophase. There is sufficiently good agreement to support the hypothesis ofRenner that there is a correlation between the developmental processes in the megaspore tetrad and the genetic phenomena of competition in the haplophase.     

The distribution and prevalence of helminths,coccidia and blood parasites in two competing species of gecko: implications for apparent competition

Across the Pacific the invading gecko species Hemidactylus frenatus has competitively displaced the resident gecko species Lepidodactylus lugubris in urban/surburban habitats. Do parasites enhance, inhibit, orhave no effect on this invasion? Parasites can confer an advantage to an invading species when the invader (1) introduces a new parasite to a resident species that has a greater detrimental effect on the resident than the invader, (2) is less susceptible to endemic parasites than the resident, and/or (3) increases the susceptibility of the resident to parasites. Conversely, parasites may protect a resident against invasion when endemic parasites have a greater impact on the invader than the resident. We screened more than one thousand H. frenatus and L. lugubris in areas of sympatry and allopatry from 28 islands and 5 sites on mainland Asia for a broad array of blood parasites, coccidia and helminths in order to evaluate the potential for parasites to affect their interaction. We found that 1) There were no parasites which appear to protect L. lugubris against invasion by H. frenatus. 2) H. frenatus does not introduce the same parasite to L. lugubris in every location where the two come in to contact, but probably has introduced different parasites in different locations. L. lugubris also seems to have introduced at least one parasite to H. frenatus. 3) The prevalence of parasite species shared by the two hosts is generally higher in H. frenatus; however, prevalence is determined by many factors and cannot be directly translated as susceptibility. We discuss the implications of this difference in prevalence for the Red Queen hypothesis. 4) The prevalence of the cestode Cylindrotaenia sp. is significantly higher in L. lugubris that are sympatric with H. frenatus than those which are allopatric.     

Heterospecific larval competition and host discrimination in two species of aphid parasitoids: Aphidius ervi and Aphidius smithi

Females of the solitary aphid parasitoids Aphidius ervi Haliday and A. smithi Sharma & Subba Rao (Hymenoptera: Aphidiidae) discriminated between unparasitized pea aphids and those parasitized by the other species. Oviposition restraint varied with the attack sequence and the length of the interval between successive attacks. The tendency to reject a previously parasitized host increased with interval length; A. smithi females rarely oviposited in aphids that had been parasitized 30 h earlier by A. ervi. Early first-instar larvae of A. ervi physically attacked and killed older A. smithi larvae, and older A. ervi larvae killed younger A. smithi, possibly by physiological suppression. Neither species appeared to have a competitive advantage when their eggs hatched at the same time. The evolution of heterospecific host discrimination in A. ervi and A. smithi is discussed. It is suggested that avoidance of multiparasitism is adaptive for both parasitoid species: for A. smithi because it is the inferior larval competitor, and for A. ervi because immatures develop more slowly in multiparasitized than in initially unparasitized hosts.

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Compétition interspécifique et discrimination des hôtes chez deux parasitoïdes de pucerons: Aphidius ervi et A. smithi

Résumé Les femelles des parasitoïdes de pucerons: Aphidius ervi Haliday et A. smithi Sharma & Subba Rao (Hyméno. Aphidiidae) distinguent les pucerons du pois sains des parasités par d'autres espèces. La rétention de la ponte dépend de la séquence de l'attaque et du temps écoulé entre des attaques successives. La tendance au rejet d'un puceron précédemment parasité augmente avec l'importance du délai; A. smithi a rarement pondu dans des pucerons qui avaient été parasités 30 h avant par A. ervi. Les jeunes larves de premier stade de A. ervi ont attaqué physiquement et tué les larves plus âgées de A. smithi, et les larves plus âgées de A. ervi ont tué des larves plus jeunes de A. smithi par élimination physiologique. Aucune espèce ne semble avoir un avantage quand les oeufs ont éclos en même temps. L'évolution de la discrimination interspécifique de l'hôte chez A. ervi et A. smithi est discutée. On estime que la tendance à éviter le multiparasitisme est adaptative chez les 2 espèces: pour A. smithi parce qu'il est dominé dans la compétition larvaire et pour A. ervi parce que les larves se développent plus lentement dans un hôte multiparasité que dans un hôte initialement sain.

     

Interannual and between-site variability in the occurrence of clear water phases in two shallow Mediterranean lakes

It is widely accepted that clear water phases constitute a regular stage in the seasonal succession of plankton in dimictic lakes and reservoirs (i.e. PEG Model). The occurrence of such a phenomenon in Mediterranean shallow lakes is characterised by a marked interannual variability, which makes it difficult to establish reliable predictions on the dynamics and functioning of plankton in these ecosystems. In the present paper we analyse the factors influencing the occurrence of the clear water phases in the two shallow lakes of the Albufera of Adra, a coastal wetland region of south-eastern Spain: Lake Honda and Lake Nueva. Despite their geographical proximity, both lakes depicted large hydrological and limnological differences. Lake Honda is an epigenic and recharge lake that is strongly influenced by the hydrological conditions in its watershed, while Lake Nueva can be classified as a hypogenic and discharge lake and, as such, is less affected by the hydrological regime. In contrast, the morphometry, exposure and fetch of Lake Nueva make this ecosystem especially sensitive to wind forcing. Clear water phases in these shallow lakes were linked with periods of low thermal stability and the dominance of small-edible algae in the phytoplankton community, both of which allowed a Daphnia magna population to grow up and induce the algae collapse by grazing. In Lake Honda, those conditions were met during the spring of 2002 under the influence of intense rainfall-events, while in Lake Nueva the clear water phase was induced in the spring of 2003 by the occurrence of strong and frequent wind events. In both lakes, a relatively high water column thermal stability and the abundance of cyanobacteria early in the spring prevented the development of the Daphnia magna population and the occurrence of the clear water phase.     

Habitat-specific estimates of competition in stream salmonids: A field test of the isodar model of habitat selection

Summary The population densities of sympatric Atlantic salmon,Salmo salar and brook charr,Salvelinus fontinalis, were measured in riffle and pool stream habitats to test whether non-linear isodars, a multispecific model of habitat selection based on ideal distribution assumptions, could (1) predict the distribution of densities between habitats and (2) reproduce the processes postulated to underlie spatial segregation and species interactions in previous laboratory and field studies. The model provided a good fit to observed density patterns and indicated that habitat suitability declined non-linearly with increased heterospecific competitor densities. Competitive effects in riffles appeared to be due to exploitative resource use, with salmon always emerging as the superior competitor. No evidence was found for interference competition in riffles. In contrast, interspecific competition in pools seemed to occur through exploitation and interference. The specific identity of the superior competitor in pools depended on the density of both species; pools provided the charr with refuge from competition with the salmon, presumably through the adoption by the charr of density-dependent behaviours, such as schooling and group foraging, that mitigated the negative impact of the salmon. Charr were displaced from the riffles toward the pools as the total salmon density increased. The isodar analysis, based on limited density data, successfully reproduced the processes suggested to underlie spatial segregation in previous field and laboratory studies and provided new insights into how changes in competitor densities modify habitat suitability in this system.     

Life history comparison between two competitive Aleochara species in the cabbage root fly, Delia radicum: implications for their use in biological control

This study evaluates the efficacy of Aleochara bilineata Gyll and Aleochara bipustulata L. (Coleoptera: Staphylinidae) as biological control agents against the cabbage root fly, Delia radicum L. (Diptera: Anthomyiidae). Biological and demographic characters are documented and comparisons made between the two species. The following results were obtained:(1) The reproductive potential and longevity of A. bipustulata females are significantly higher than those of A. bilineata. These differences are correlated with the biology and ecology of the hosts of both species.(2) Under laboratory conditions the rate of increase (r_m) of both staphylinids species is significantly higher than that of the pest and their major competitor Trybliographa rapae West. (Hymenoptera:Figitidae). This may indicate that A. bilineata and A. bipustulata can be effective biological control agents.(3) Aleochara bilineata appears to be the most interesting biological control agent against D. radicum, because of its higher rate of increase, higher host specificity and host acceptance, and a development time which is well synchronised with that of its host. With its higher competitive ability, a population of A. bilineata may increase even if competition occurs. However, A. bipustulata presents interesting biocontrol characteristics, but the best strategy may be only to use the potential of its natural populations.     

土壤水分及其垂直分布对内蒙古草原冰草和冷蒿竞争的影响

内蒙古退化冷蒿草原围封恢复演替3-4年,植物群落更替出现半灌木冷蒿（Artemisia frigida）向禾草冰草（Agropyron cristatum）群落突然转变的现象,这对内蒙古草原植被恢复具有积极的作用,但目前关于冰草-冷蒿演替和竞争机制的研究鲜见报道。水分作为草原植被生长的主要限制因子,是影响群落演替的重要因素。气候干旱和过度放牧导致草原土壤水分垂直分布发生明显变化。由于冰草和冷蒿的根系分布深浅不同,土壤水分垂直变化可能会影响冰草-冷蒿的竞争。为探讨土壤水分及其垂直分布对冰草和冷蒿竞争的影响,本盆栽试验设置冰草、冷蒿单种以及混种处理,并进行不同土壤深度（0-30cm,30-60cm）的水分处理（上干下湿、上湿下干、上干下干）,结果表明：（1）与冷蒿相比,冰草受土壤水分变化影响显著,相对于上干下干处理,冰草在湿润（上干下湿或者上湿下干）处理的地上、地下生物量均显著增加,而冷蒿没有显著变化;（2）上干下湿和上湿下干处理间,冰草、冷蒿的地上、地下总生物量和根长差异均不显著;（3）不同水分处理,冰草和冷蒿的根系均可分布在30-60cm土壤中,且土壤30-60cm层冰草单种的根生物量和根长显著高于冷蒿单种;（4）相对竞争强度和竞争攻击力系数表明：湿润处理冰草的竞争力大于冷蒿;上干下干处理冷蒿的竞争力大于冰草。土壤水分变化引起冰草、冷蒿的明显竞争,上湿下干与上干下湿处理间冰草与冷蒿竞争力差异不显著。由于冰草和冷蒿均为多年生物种,冰草-冷蒿的竞争实验仍需在今后的研究中反复地验证。     

A model for wave control on coral breakage and species distribution in the Hawaiian Islands

The fringing reef off southern Molokai, Hawaii, is currently being studied as part of a multi-disciplinary project led by the US Geological Survey. As part of this study, modeling and field observations were utilized to help understand the physical controls on reef morphology and the distribution of different coral species. A model was developed that calculates wave-induced hydrodynamic forces on corals of a specific form and mechanical strength. From these calculations, the wave conditions under which specific species of corals would either be stable or would break due to the imposed wave-induced forces were determined. By combining this hydrodynamic force-balance model with various wave model output for different oceanographic conditions experienced in the study area, we were able to map the locations where specific coral species should be stable (not subject to frequent breakage) in the study area. The combined model output was then compared with data on coral species distribution and coral cover at 12 sites along Molokais south shore. Observations and modeling suggest that the transition from one coral species to another may occur when the ratio of the coral colonys mechanical strengths to the applied (wave-induced) forces may be as great as 5:1, and not less than 1:1 when corals would break. This implies that coral colonys mechanical strength and wave-induced forces may be important in defining gross coral community structure over large (orders of 10s of meters) spatial scales.     

起始生物量比对3种海洋微藻种间竞争的影响

为深入了解饵料微藻与赤潮微藻间的种间竞争关系,通过微藻共培养的方法,研究了起始生物量比(1:4、1:1和4:1)对3种海洋微藻(塔玛亚历山大藻、蛋白核小球藻和湛江等鞭金藻)两两之间种间竞争的影响,并对其作用机制进行了探讨。结果表明:①3种海洋微藻表现出种间竞争的相互抑制效应;②在与塔玛亚历山大藻(简称A)的种间竞争中,蛋白核小球藻(简称C)和湛江等鞭金藻(简称I)均在竞争中占优势,蛋白核小球藻随自身起始生物量比的提高,其竞争优势越加明显,湛江等鞭金藻在A:I=1:1时竞争优势最为明显;在蛋白核小球藻和湛江等鞭金藻的种间竞争中,当C:I=1:4时,湛江等鞭金藻在竞争中占优势,C:I=1:1时,初期湛江等鞭金藻占竞争优势,随蛋白核小球藻的迅速生长,后期蛋白核小球藻占竞争优势,C:I=4:1时,蛋白核小球藻占绝对竞争优势;③由种间竞争抑制参数比较得出:3种微藻的种间竞争强弱依次为蛋白核小球藻>湛江等鞭金藻>塔玛亚历山大藻。蛋白核小球藻和湛江等鞭金藻在起始比例C:I=1:1时,可共培养利用,在海产经济动物育苗中可对其进行适时采收投喂;两种饵料藻对塔玛亚历山大藻具有明显的抑制作用,可为开发利用饵料藻进行赤潮生物防控提供一定的科学依据。     

The relationship between cladoceran body size and the growth of underyearling roach (Rutilus rutilus) (L.) in two shallow lowland lakes: a mechanism for density-dependent reductions in growth

Large planktonic Cladocera are typically the most important components of the diet of underyearling roach. Selection for large species and individuals by fish can result in a shift in the species composition of the cladoceran community as well as a reduction in the mean size of the individuals of large species and in the assemblage as a whole. Laboratory experiments demonstrated that underyearlings feeding on Daphnia hyalina smaller than 1 mm in length has a significantly lower intake of prey volume per unit time than when feeding on prey greater than 1.5 mm. A decrease in the nutritional quality of zooplankton prey, brought about by increasing predation pressure is suggested as the mechanism for density-dependent reductions in the growth of underyearling roach in eutrophic water bodies.     

Influence of drought on competition between selected Rhizobium meliloti strains and naturalized soil rhizobia in alfalfa

Drought is an important environmental factor that can affect rhizobial competition and N₂ fixation. Three alfalfa (Medicago sativa L. and M. falcata L.) accessions were grown in pots containing soil from an irrigated (Soil 1) and a dryland (Soil 2) alfalfa field in northern Utah, USA. Mutants of three strains of Rhizobium meliloti Dang. from Pakistan (UL 136, UL 210, and UL 222) and a commercial rhizobial strain 102F51a were developed with various levels of resistance to streptomycin. Seeds inoculated with these individual streptomycin-resistant mutants were sown in the two soils containing naturalized rhizobial populations. Soils in the pots were maintained at −0.03, −0.5, and −1.0 MPa. After 10 weeks, plants were harvested and nodule isolates were cultured on agar medium with and without streptomycin to determine nodule occupancy (proportion of the nodules occupied by introduced rhizobial strains). Number of nodules, nodule occupancy, total plant dry weight, and shoot N were higher for Soil 1 than Soil 2. Number of nodules, plant dry weight, and shoot N decreased as drought increased from −0.03 to −1.0 MPa in the three alfalfa accessions. Rhizobial strains UL 136 and UL 222 were competitive with naturalized alfalfa rhizobia and were effective at symbiotic N₂ fixation under drought. These results suggest that nodulation, growth, and N₂ fixation in alfalfa can be improved by inoculation with competitive and drought-tolerant rhizobia and may be one economically feasible way to increase alfalfa production in water-limited environments. Joint contribution from USDA-ARS and the Utah Agric. Exp. Sta., Utah State Univ., Logan, UT 84322-4810, USA. Journal Paper No. 4931. Joint contribution from USDA-ARS and the Utah Agric. Exp. Sta., Utah State Univ., Logan, UT 84322-4810, USA. Journal Paper No. 4931.