The effect of fish on planktonic rotifers

The dynamics, community structure, and productivity of planktonic rotifers were studied during 3 years in two lakes near St. Petersburg (Russia). One lake was repeatedly stocked with larvae of the fish Coregonus peled; the other contained no fish. Fish addition led to a shift in plankton community structure. Population densities of some rotifer species (Keratella cochlearis, K. irregularis, Asplanchna priodonta) increased as a result of the elimination of large crustaceans by fishes during summer and autumn. An inverse relationship was found between the biomass of rotifers and Daphnia.     

Food limitation and body size in the life cycles of planktonic rotifers and cladocerans

This review considers what is known about the effects of food limitation upon the life cycle characteristics of rotifers and planktonic cladocerans. The characteristics considered in rotifers are the size of eggs, juveniles and adults and the durations of the juvenile phase and period of egg production. In cladocerans, the life history features dealt with are their length-weight relationships, the body size, instar stage, age and fecundity of the primiparous female and their fecundity-adult size relationship. The influence of limiting food conditions is demonstrated for these characteristics by comparison with the situation in non-limiting circumstances; the comparison is confined to experiments where food concentrations are quantified. A direct comparison is made between rotifers and cladocerans in conditions of defined food resource availability in terms of their length-weight relationships, the daily allocation of adults or near-adults to growth and reproduction and their threshold food concentrations. These comparisons are discussed in relation to the following topics: the high cost of cumulated respiration resulting from prolongation of the juvenile phase of body growth; the fundamentally different nature of growth in the two taxonomic groups; the body size of species and the size that must be attained for reproduction; the ecological implications of the very different threshold food concentrations.     

Distribution and abundance of zooplankton in the Waikato River,New Zealand

Potamoplankton is often a well developed component in large lowland rivers, yet little is known about its structure in New Zealand's longest river, the Waikato River. To redress this gap we sampled bimonthly at seven sites along the length of the river over 12 months. Rotifers were the dominant zooplankton in the Waikato River making up 85% of the total densities. Cladocerans represented 9% and copepods only 6%. Rotifers were also the most taxonomically rich group with 41 species in 20 genera identified throughout the study. Thirty rotifer species and nine genera represent new records for the river – two cladoceran species were also recorded for the first time. The highest densities of crustaceans and rotifers were found in the hydro lakes. Densities of crustaceans decreased with increasing distance downstream and densities of rotifers were on average 15 times greater than crustaceans in the lower river. The seasonality of Crustacea was similar to that in New Zealand lakes and rivers with high densities in summer and minimum densities over the winter period. Total rotifer densities showed a similar trend although there were marked seasonal differences between individual species.     

Complementary impact of copepods and cladocerans on phytoplankton

The differences in the impact of two major groups of herbivorous zooplankton (Cladocera and Copepoda) on summer phytoplankton in a mesotrophic lake were studied. Field experiments were performed in which phytoplankton were exposed to different densities of two major types of herbivorous zooplankton, cladocerans and copepods. Contrary to expectation, neither of the two zooplankton groups significantly reduced phytoplankton biomass. However, there were strong and contrasting impacts on phytoplankton size structure and on individual taxa. Cladocerans suppressed small phytoplankton, while copepods suppressed large phytoplankton. The unaffected size classes compensated for the loss of those affected by enhanced growth. After contamination of the copepod mesocosms with the cladoceran Daphnia , the combined impact of both zooplankton groups caused a decline in total phytoplankton biomass.     

Competition between grassland plants of different initial sizes

Summary Four species of grassland plant, Plantago lanceolata, Holcus lanatus, Lolium perenne and Rumex acetosa, were grown as monocultures and mixtures in pots of nutrient poor soil in a glasshouse for 8 months. There were four plants per pot and these were arranged in two competition modes: either root and shoot interactions were permitted, or only roots allowed to interact by using above-ground partitions. Time of introduction of seedlings was varied to give a range of plant size ratios at the start of the experiment. The factorial design catered for all combinations of species, competition modes and planting times, replicated in four blocks. The shoots were clipped at a fixed height at each of five harvests. Rumex grew badly and was mostly omitted from analysis of the data.By (i) following the change in the relationship of clip dry weights against planting time with successive harvests, (ii) plotting the change in the logarithm of the ratio of cumulative clip dry weights with time and (iii) the use of de Wit logarithmic ratio plots it was demonstrated that each monoculture and mixture combination's ratios of plant weights converged towards stable equilibrium values. Three hypotheses are put forward to explain why in monocultures a smaller plant was at a competitive advantage relative to a larger neighbour and was not suppressed in its growth by the latter. In mixtures this plant size effect was superimposed to different extents on the relative aggressiveness of the species considered. It was concluded that in a nutrient poor soil, when competition for light was low, root interactions can promote the co-existence of neighbouring plant species.     

Diversity and dominance in planktonic rotifers

The diversity of planktonic species in a body of water can be estimated in two ways: momentary diversity and long term diversity. The former represents the number capable of coexisting at any one time, and the latter includes a seasonal or other temporal component. The relationship between the two varies mainly with salinity. Dominance is generally inversely related to diversity. In considering the total possible range of dominance for a given number of species the mean values are surprisingly restricted. An abnormally high dominance for a number of species can be an indication of pollution or some other form of environmental stress.     

轮虫和枝角类的种间竞争及其影响因子研究进展

轮虫和枝角类是浮游动物群落的重要组成部分和优势类群,它们之间的竞争互作是调节水生态系统结构和功能的主要动力之一。普遍的观点认为,轮虫和大型枝角类难以共存,往往被竞争排斥,而和小型枝角类可以共存;实际上,轮虫和枝角类的种间竞争结局存在一定的不确定性。介绍了轮虫和枝角类的种间竞争方式及其相对重要性,对影响轮虫和枝角类种间竞争结局的因素,包括温度、食物、相对起始密度、个体大小、食物临点、耐饥饿能力、捕食及竞争者和捕食者释放的化感物质等,进行了系统的梳理和分析,并提出了今后亟待解决的科学问题和研究切入点。     

Biotic relations affecting species structure in zooplankton communities

Mathematical modelling and laboratory experiments were used to study how exploitative competition and predation influence the species structure in cladoceran community. For five species of Cladocera (Sida crystallina, Daphnia magna, Simocephalus vetulus, Daphnia longispina, and Diaphanosoma brachyurum), representing a gradient of body size, population characteristics were described as functions of food concentration. Abundance dynamics were simulated in mixed species cultures and invasion experiments under different levels of food supply corresponding to oligo-, meso-, and eutrophic conditions. Separate simulations were also run including and excluding (fish) predation. The competitive ability of each species was estimated as the values of the population equilibrium food concentration. Simulation results showed that for the no-predator scenario, increases in the level of food supply promoted species coexistence while under lower food concentrations only one species remained at the end of the simulation runs. When predation was allowed, the number of species that coexisted at the end of the simulations increased up to four species, indicating that predator pressure facilitated species coexistence because it shortened periods of food depletion. Simulation results were verified in laboratory experiments which suggested that population equilibrium food concentration can be used as an estimate of competitive ability. Finally, species structure and relative abundance in Lake Naroch (Belarus) during the summer of 2004 was found to be consistent with our results from computer simulation and laboratory experiments with regard to competition and predation impacts on zooplankton community. Handling editor: S. I. Dodson     

不同温度条件下几种枝角类浮游动物的抑藻净水效应研究

研究了22℃、16℃两个温境、实验室系统中长刺潘、短腹平直潘、多刺裸腹潘、透明潘4种枝角类对于小球藻及铜绿微囊藻混合藻液的牧食能力及其在恒定光照条件下的净水能力。结果显示：4种枝角类在藻液初始吸光度大于0．160的情况下均可使藻液浓度下降,其摄食速度随藻类生物量增加而增加,当藻类生物量超过饱和值时,摄食速度随之下降。徐州长刺潘牧食及净水能力最强,在底泥背景下滇池短腹平直澄摄食及净水能力有所增强,透明潘摄食净水能力差,多刺裸腹潘在处理后期自身的排泄物会污染水体。     

The effect of body size on the thermoregulation of lizards on hot,dry Mediterranean islands

Body size shapes the overall biology of organisms. We assessed the impact of size on temperature regulation in populations of normal-sized and large-bodied insular Mediterranean lizards (Podarcis gaigeae, Lacertidae). We hypothesized that large lizards would achieve higher body temperatures and thermoregulate more effectively than their smaller kin. Large- and small-bodied lizards share the same thermoregulation pattern, achieving similar body temperatures in the field. Large lizards, however, prefer higher set-point temperatures. Lizards in both populations thermoregulate effectively, but large lizards thermoregulated less effectively than normal-sized lizards. The particular conditions at the islet that harbors the large-bodied population (harsh intraspecific competition) seem to account for this pattern.     

Contribution analysis of body mass dynamics in <Emphasis Type="Italic">Daphnia</Emphasis>

The concept of body mass dynamics can be viewed as part of life history theory, but its potential has remained largely untapped due to a lack of analytical methodology. We therefore propose a method, called contribution analysis, which enables us to decompose a change in body mass into contributions associated with variations in individual egg mass, clutch size, and standard somatic mass (somatic mass adjusted to body length). The advantage of contribution analysis is that various contributions are expressed in the same units (units of mass) and show the amount of resources committed to changes in the individual traits, while the traits themselves are measured in different units and thus incomparable on their own. The method is tuned to study zooplankton, and is applied to examine body mass dynamics in Daphnia galeata. We found that when recovering from a poor-resource environment just above the threshold food concentration, Daphnia primarily increase their standard somatic mass, that is, restore body condition. When the trophic environment improves further but remains below the incipient limiting level, resources are invested equally to enhance body condition and reproduction in terms of clutch size. Finally, when food is no longer a limiting factor, almost all resources are committed to increase clutch size. While individual egg mass also varies, it never attracts more resources than the shift in the most prioritized trait. We suggest that the significance of this shift in resource allocation priorities is to keep an adult female alive in a poor environment and thus to allow her to retain her reproductive potential for better conditions in the future. Contribution analysis of body mass dynamics may allow us to detect flexible allocation strategies in a changing natural environment.     

Competition between two planktonic rotifer species at different temperatures: an experimental test

1. The effect of temperature on the outcome of resource competition between two planktonic rotifers (Synchaeta pectinata and Brachionus calyciflorus) was investigated in laboratory experiments. In addition to the competition experiments, several physiological variables and their temperature‐dependence were characterised, including ingestion rate and starvation tolerance. 2. Because of a lower threshold food level (TFL) for population growth for the food algae Cryptomonas erosa, Synchaeta was predicted to be the superior competitor at low temperatures (12 °C). In contrast, Brachionus had a lower TFL at 20 °C and was predicted to be competitively superior at this temperature. 3. In both rotifer species, ingestion rates increased with temperature, but the increase was much more pronounced in Brachionus. Ingestion rates of Brachionus at temperatures from 8 to 24 °C were always higher than in Synchaeta (up to 4.6‐fold). 4. Starvation resistance reduced with temperature in both rotifer species. At all temperatures investigated (12, 16 and 20 °C) Brachionus could survive starvation for longer than Synchaeta. This difference was strongest at 12 °C (5.8 days versus 2.5 days). 5. In the first competition experiment, food was supplied at 48 h‐intervals. Brachionus displaced Synchaeta at both experimental temperatures (12 and 20 °C). Competitive exclusion of Synchaeta at the lower temperature was probably because of large fluctuations in algal densities that resulted from the long intervals between feeding, a condition that favoured Brachionus because of its higher starvation resistance. 6. In the second competition experiment, one third of the food suspension was renewed every 8 h, resulting in a much better approximation to a continuous resource supply. At 12 °C Synchaeta and Brachionus coexisted for more than 1 month and the densities of both rotifer species were significantly lower in the presence of their competitor. In contrast to expectations, Brachionus was able to persist even when Cryptomonas concentrations fell below its TFL. This was probably because Brachionus was using detritus and associated bacteria as additional food sources, which were present in the cultures during the later phase of the experiment. 7. Autocorrelation analysis of the temporal changes in egg ratios revealed significant periodic cycles in Synchaeta during the second competition experiment. A possible explanation for this is the fecundity schedule of Synchaeta, in which reproduction is highly concentrated in a few age classes. According to demographic theory, such a life cycle feature can cause slower convergence to a stable age distribution.     

Community structure of rotifers in two environments of the upper River Paraná floodplain (MS) - Brazil

We evaluate the influence of abiotic and biotic factors on the community structure of rotifers across a regional hydrological cycle in lotic and lentic environments of the upper River Paraná. Depth, transparency, temperature, pH, electrical conductivity, dissolved oxygen, chlorophyll a and densities of rotifers were measured at two stations in Lake Guaraná (littoral and open water regions) and at one station in the River Baía (open water region). Highest densities of rotifers were found at the lake littoral. Canonical correlation analysis related environmental variables with the densities of the most abundant rotifers. The strongest relationship was with chlorophyll a, dissolved oxygen, hydrological level and water temperature. Diversity of rotifers at each station was mainly explained by fluctuations in hydrological level. Results of grouping analysis suggested the formation of groups according to phases of the hydrological cycle.     

Evolution and ecology of lizard body sizes

Aim Body size is instrumental in influencing animal physiology, morphology, ecology and evolution, as well as extinction risk. I examine several hypotheses regarding the influence of body size on lizard evolution and extinction risk, assessing whether body size influences, or is influenced by, species richness, herbivory, island dwelling and extinction risk. Location World‐wide. Methods I used literature data and measurements of museum and live specimens to estimate lizard body size distributions. Results I obtained body size data for 99% of the world's lizard species. The body size–frequency distribution is highly modal and right skewed and similar distributions characterize most lizard families and lizard assemblages across biogeographical realms. There is a strong negative correlation between mean body size within families and species richness. Herbivorous lizards are larger than omnivorous and carnivorous ones, and aquatic lizards are larger than non‐aquatic species. Diurnal activity is associated with small body size. Insular lizards tend towards both extremes of the size spectrum. Extinction risk increases with body size of species for which risk has been assessed. Main conclusions Small size seems to promote fast diversification of disparate body plans. The absence of mammalian predators allows insular lizards to attain larger body sizes by means of release from predation and allows them to evolve into the top predator niche. Island living also promotes a high frequency of herbivory, which is also associated with large size. Aquatic and nocturnal lizards probably evolve large size because of thermal constraints. The association between large size and high extinction risk, however, probably reflects a bias in the species in which risk has been studied.     

Culturing of some bdelloid rotifers

A method for culturing 19 species of Bdelloid Rotifers is described. The most suitable food for bdelloids are bacteria and yeast. Algae do not seem to be an adequate food. Some specificity, in terms of different growths on different diets, can be observed. Food concentration is very important when determining growth capacity of a species.     

Convergent succession of phytoplankton in microcosms with different inoculum species composition

Summary Different initial mixtures of phyto-and zooplankton from different lakes were grown under identical chemical and physical conditions in medium size (8-and 12–1) laboratory microcosm cultures until convergence of phytoplankton species composition was attained. Five such experiments with four (four experiments) or three (one experiment) microcosm cultures were run. Three experiments were performed with weak stirring which permitted sedimentary elimination of the diatoms. Two experiments were conducted with stronger stirring to prevent sedimentation. In the three sedimentation intensive experiments, the final phytoplankton community was composed of the filamentous chlorophyte Mougeotia thylespora together with a smaller biomass of nanoplanktic algae. In the two sedimentation free experiments the final phytoplankton community consisted of pennate diatoms. Both dissolved nutrient concentrations and the chemical composition of biomass suggested strong nutrient limitation of algal growth rates in the final phase of the experiments. The zooplankton communities at the end of the experiments were composed of species that were apparently unable to ingest the large, dominant algae and that presumably fed on the nanoplanktic undergrowth and the bacteria. There was a distinct sequence of events in all experiments: first, the large zooplankton species (Daphnia and Copepoda) were replaced by smaller ones (Chydorus, Bosmina, rotifers); second, all cultures within one experiment developed the same nutritional status (limitation by the same nutrient); and third, the taxonomic composition of phytoplankton of the different cultures within one experiment converged. The last took 7–9 weeks, with is about 2–3 times as long as the time needed in a phytoplankton competition experiment to reach the final outcome.     

Inconsistent estimates of diversity between traditional and DNA taxonomy in bdelloid rotifers

Microscopic animals offer great potential in the analysis of spatial patterns of diversity, as they may provide different scenarios for biogeography and macroecology, but understanding diversity of microscopic animals is hampered by lack of comprehensive data on species distribution and by unreliable taxonomy. DNA taxonomy may prove useful in obtaining reliable data in the future, but we still do not know to what extent traditional and DNA taxonomy can be comparable for microscopic organisms. In this paper, we compare analyses and estimates of diversity at the level of species assemblage between traditional and DNA taxonomy for a group of moss-dwelling microscopic animals, bdelloid rotifers. The results are straightforward: Traditional species identification underestimates diversity by factors of 2 at the local and 2.5 at the regional scale. We discuss the results in the framework of current hypotheses on the distribution of microscopic animals.     

Competition between Dunaliella species at high salinity

Information on the distribution of more than 225 species of planktonic, periphytic, and benthic rotifers from diverse waters in south and central Sweden was analyzed for pH preference. No particularly strong correlation was found between pH and any other environmental factor, with regards to rotifer distribution. However, species indicating oligotrophy generally have their pH optima at or below 7.0, while those indicating eutrophy occur at or above this level. Rotifers found in acidic waters are often non-planktonic or semiplanktonic.     

Distribution of rotifers in African fresh and inland saline waters

The distribution of rotifers in African fresh and inland saline waters is discussed in the light of historical expeditions and modern limnological research. Number of papers published and number of rotifer species described for each country are given. Some peculiarities of African rotifer fauna are described.