Density-dependent regulation of natural and laboratory rotifer populations

Density-dependent regulation of abundance is fundamentally important in the dynamics of most animal populations. Density effects, however, have rarely been quantified in natural populations, so population models typically have a large uncertainty in their predictions. We used models generated from time series analysis to explore the form and strength of density-dependence in several natural rotifer populations. Population growth rate (r) decreased linearly or non-linearly with increased population density, depending on the rotifer species. Density effects in natural populations reduced r to 0 at densities of 1–10 l^–1 for 8 of the 9 rotifer species investigated. The sensitivities of these species to density effects appeared normally distributed, with a mean r=0 density of 2.3 l^–1 and a standard deviation of 1.9. Brachionus rotundiformis was the outlier with 10–100× higher density tolerance. Density effects in laboratory rotifer populations reduced r to 0 at population densities of 10–100 ml^–1, which is 10⁴ higher than densities in natural populations. Density effects in laboratory populations are due to food limitation, autotoxicity or to their combined effects. Experiments with B. rotundiformis demonstrated the absence of autotoxicity at densities as high as 865 ml^–1, a much higher density than observed in natural populations. It is, therefore, likely that food limitation rather than autotoxicity plays a major role in regulating natural rotifer populations.     

Modeling the dynamics of natural rotifer populations: Phase-parametric analysis

A model of the dynamics of natural rotifer populations is described as a discrete non-linear map depending on three parameters, which reflect characteristics of the population and environment. Model dynamics and their change by variation of these parameters were investigated by methods of bifurcation theory. A phase-parametric portrait of the model was constructed and domains of population persistence (stable equilibrium, periodic and a-periodic oscillations of population size) as well as population extinction were identified and investigated. The criteria for population persistence and approaches to determining critical parameter values are described. The results identify parameter values that lead to population extinction under various environmental conditions. They further illustrate that the likelihood of extinction can be substantially increased by small changes in environmental quality, which shifts populations into new dynamical regimes.     

Density-dependent sexual reproduction in natural populations of the rotifer asplanchna girodi

The monogonont rotifer Asplanchna girodi was continuously present in daily and bi-daily plankton samples of Golf Course Pond during the spring of 1977. Two cycles of sexual reproduction occurred during this period. By isolation and culture of females it was possible to determine the reproductive type of collected individuals. Data thus obtained suggest that environmental cues associated with population density are responsible for the production of sexual females.     

Local adaptation in rotifer populations

The adaptation of organisms to their environment has been a subject of study for a long time. One method to study adaptations in populations involves comparing contemporary populations of the same species under different selective regimes, in what is known as a ??local adaptation?? study. A previous study of the cyclically parthenogenetic rotifer Brachionus plicatilis found high heritabilities for some life-history traits. Some of these life-history traits significantly differed among six populations from Eastern Spain and data suggested some traits to have higher evolutionary rates than neutral genetic markers. Here, by studying the same B. plicatilis populations, we examine the variation and possible local adaptation of their main life-history traits, closely related to fitness, in relation to habitat salinity and temperature. These environmental factors have been shown to play a key role in the ecological differentiation among co-generic species of B. plicatilis. The results obtained in this study show that: (1) the seasonality of rotifer populations from Eastern Spain has profoundly influenced sexual reproduction strategies; (2) salinity is probably a key factor in the ecological specialization of some populations; and (3) rotifer populations harbour high variability in their fitness components.     

Blue-green algae and selection in rotifer populations

Summary Observation in natural populations of the rotifer Asplanchna girodi suggested blue-green algae may have toxic effects on rotifers. The blue-greens Anabaena flos-aquae and Lyngbya sp. were isolated from a pond in Central Florida along with three electrophoretically identified genotypes of A. girodi. When tested Anabaena had marked effect on the reproductive rate of A. girodi and the effect was different among genotypes. Likewise, Lyngbya also depressed rotifer reproduction, but required an order of magnitude higher concentration to produce the same effects as Anabaena. The toxicity of these algae was corroborated by mouse bioassays. Results clearly show these blue-greens have the capacity to differentially reduce reproductive rates of rotifer genotypes. Because of this capability blue-greens could play an important role in the ecology of natural selection in rotifer populations.     

Dynamics of age- and size-structured populations in fluctuating environments: Applications of stochastic matrix models to natural populations

Recent developments of the theory of stochastic matrix modeling have made it possible to estimate general properties of age- and size-structured populations in fluctuating environments. However, applications of the theory to natural populations are still few. The empirical studies which have used stochastic matrix models are reviewed here to examine whether predictions made by the theory can be generally found in wild populations. The organisms studied include terrestrial grasses and herbs, a seaweed, a fish, a reptile, a deer and some marine invertebrates. In all the studies, the stochastic population growth rate (ln λ _s ) was no greater than the deterministic population growth rate determined using average vital rates, suggesting that the model based only on average vital rates may overestimate growth rates of populations in fluctuating environments. Factors affecting ln λ _s include the magnitude of variation in vital rates, probability distribution of random environments, fluctuation in different types of vital rates, covariances between vital rates, and autocorrelation between successive environments. However, comprehensive rules were hardly found through the comparisons of the empirical studies. Based on shortcomings of previous studies, I address some important subjects which should be examined in future studies.     

Effects of Daphnia longispina on rotifer populations in a natural environment: relative importance of food limitation and interference competition

To test the relative importance of exploitative and interferencecompetition from Daphnia limiting rotifer populations in Bermejalesreservoir, birth rates of rotifers, food abundance, zooplanktonbiomass and size distribution of Daphnia were analysed fromhigh frequency sampling at two depths. A depletion in food abundanceby Daphnia feeding should be reflected in the birth rates ofrotifers susceptible to exploitative competition, while interferenceshould produce direct negative relationships between the numberof individuals of the interacting species. Results show thatsignificant relationships between food (chlorophyll-a) and birthrates of rotifers are found when the ratio chlorophyll-a/herbivorouszooplankton biomass is used as a measure of food availability.In our study, Anuraeopsis fissa was only affected through exploitativecompetition by Daphnia, while only Synchaeta oblonga showedevidence of being suppressed through interference competitionby this cladoceran.     

Dynamics of finite populations

Summary A time-homogeneous stochastic process was used to derive exact expressions for the ultimate probability of fixation or loss and the expected time to fixation or loss of an allele in a haploid population whose size is a Poisson random variable. The treatment included selection and multiple alleles.Contribution no. 165 Department of Statistics, Kansas Agricultural Experiment Station, Manhattan, Kansas 66502.     

Dynamics of human populations

This study expands on earlier findings of racial/ethnic and education–allostatic load associations by assessing whether racial/ethnic differences in allostatic load persist across all levels of educational attainment. This study used data from four recent waves of the National Health and Nutrition Survey (NHANES). Results from this study suggest that allostatic load differs significantly by race/ethnicity and educational attainment overall, but that the race/ethnicity association is not consistent across education level. Analysis of interactions and education-stratified models suggest that allostatic load levels do not differ by race/ethnicity for individuals with low education; rather, the largest allostatic load differentials for Mexican Americans (p < .01) and non-Hispanic blacks (p < .001) are observed for individuals with a college degree or more. These findings add to the growing evidence that differences in socioeconomic opportunities by race/ethnicity are likely a consequence of differential returns to education, which contribute to higher stress burdens among minorities compared to non-Hispanic whites.     

Diel patterns of reproduction in rotifer populations from a tropical lake

SUMMARY. The diel patterns of reproduction were studied for three rotifer populations from Lake Valencia, Venezuela. Significant departures from uniformity were recorded for egg eclosion and egg deposition rates in Keratella americana and Brachianus calyciflorus , but only from egg deposition rates in B. havanaensis . The existence of non-random reproduction has practical ramifications for studies of population dynamics and secondary production.     

新建水库轮虫和甲壳类浮游动物动态特征

于2000~2003年调查了新建水库飞来峡水库(1999年蓄水)轮虫和甲壳类浮游动物的动态特征。共检到轮虫68种,枝角类18种和桡足类13种。轮虫、枝角类和桡足类的丰度均不高,分别为0.2~88.6,0.1~13和0.4~13.8 ind.L-1,最高丰度均出现在蓄水后的第4年(2002年3月)。在水库开始蓄水之后,轮虫先以螺形龟甲轮虫(Keratella cochlearis)为优势种;然后在蓄水后的第4年(2002年3月)由于长圆疣毛轮虫(Synchaeta pectinata)种群密度大幅度上涨,长圆疣毛轮虫取缔螺形龟甲轮虫成为优势种;此后,随着长圆疣毛轮虫种群回落,螺形龟甲轮虫又成为优势种。对于枝角类来说,水库蓄水初期,短尾秀体氵蚤(Diaphanosoma brachyurum)和长额象鼻氵蚤(Bosmina longirostris)交替成为枝角类的优势种;在蓄水后的第4年,长额象鼻氵蚤种群密度上涨,成为绝对优势种;此后,长额象鼻氵蚤种群回落,但仍然为绝对优势种。对于桡足类来说,在调查期间均以桡足幼体为主,成体丰度很低。飞来峡水库属河流型水库,在调查期间滞留时间为1.3~14.2 d,高的稀释和平流损失率造成浮游动物丰度很低。滞留时间的变化与轮虫、枝角类和桡足类种群波动密切相关;叶绿素a对它们的种群波动影响则不明显,但与长圆疣毛轮虫和长额象鼻氵蚤均显著正相关。在相对较长的滞留时间(14.2 d)和较丰富的食物的条件下,长圆疣毛轮虫和长额象鼻氵蚤种群密度大幅度上涨,并使总浮游动物丰度在蓄水后的第4年出现最高值。     

Analysing threshold effects in the sexual dynamics of cyclically parthenogenetic rotifer populations

Sex in some cyclically parthenogenetic rotifers is triggered when a threshold population density is achieved. In Brachionus, accumulation of a mixis-inducing protein (MIP) that the rotifers excrete into the medium is the proximal signal. Models explicitly accounting for MIP dynamics suggest that either (1) a positive feedback of MIP concentration on MIP production or (2) a switching relationship between the proportion of sexual females and MIP concentration is needed to produce a threshold response of mixis to population density. In order to examine both possibilities, experiments were conducted to estimate the sexual response to a dilution series of medium containing the MIP. The resulting sexual response patterns are consistent with the existence of a threshold response of mixis, but are insufficient to reject one hypothesized mechanism for the threshold response in favor of the other. However, the observed smooth increase of sexual offspring with MIP concentration is consistent with the hypothesis of positive feedback on MIP production, and deserves further investigation.     

Insight into incipient reproductive isolation in diverging populations of Brachionus plicatilis rotifer

Hydrobiologia - The emergence of reproductive isolation is key in maintaining within- and between-species diversity and one of the initial steps of speciation. In the Iberian Peninsula, the...     

The analysis of planktonic rotifer populations: A plea for long-term investigations

Short-term species succession, and long-term community development, of planktonic rotifer populations of the temperate zone and factors influencing species' abundance (ie., physical and chemical limitations, food and exploitative competition, mechanical interference competition, predation, parasitism) are described and discussed. The long-term development of plankton communities in three European lakes is described and the major events are discussed in relation to ecological interactions. Frequently, the shortcomings of our knowledge about population ecology and ecophysiology of rotifers prevent explanations of short-term and, especially, of long-term developments of these plankton organisms. The need for qualitative and quantitative observations in the field and in the laboratory over long periods becomes obvious.     

Seasonal dynamics of rotifer and crustacean zooplankton populations in a eutrophic,monomictic lake with a note on rotifer sampling techniques

The abundances, biomass, and seasonal succession of rotifer and crustacean zooplankton were examined in a man-made, eutrophic lake, Lake Oglethorpe, over a 13 month period. There was an inverse correlation between the abundance of rotifers and crustaceans. Rotifers were most abundant and dominated (>69%) the rotifer-crustacean biomass during summer months (June–September) while crustacean zooplankton dominated during the remainder of the year (>89%). Peak biomasses of crustaceans were observed in the fall (151 µg dry wt l^–1 in October) and spring (89.66 µg dry wt l^–1 in May). Mean annual biomass levels were 46.99 µg dry wt l^–1 for crustaceans and 19.26 µg dry wt l^–1 for rotifers. Trichocerca rousseleti, Polyarthra sp., Keratella cochlearis and Kellicottia bostoniensis were the most abundant rotifers in the lake. Diaptomus siciloides and Daphnia parvula were the most abundant crustaceans. Lake Oglethorpe is distinct in having an unusually high abundance of rotifers (range 217–7980 l^–1). These high densities can be attributed not only to the eutrophic conditions of the lake but also to the detailed sampling methods employed in this study.The research was supported by National Science Foundation grants DEB 7725354 and DEB 8005582 to Dr. K. G. Porter. It is lake Oglethorpe Limnological Association Contribution No. 25 and Contribution No. 371 of the Harbor Branch Foundation, Inc.     

Water quality and the rotifer populations in the Atchafalaya River Basin,Louisiana

We compiled distributional and ecological information on the class Rotifera from both flood controlled and uncontrolled reaches of the Atchafalaya River Basin, a large river-swamp in the south-central United States. In the minimally altered lower basin a variety of aquatic habitats within a small area resulted in a very diverse rotifer community consisting of an average of 46 taxa. In contrast, only an average of 28 different taxa were collected in leeved upper basin habitats. As a result of cluster analysis we were able to identify rotifer communities associated with areas of similar water quality. Variations in suspended solids, total dissolved solids, and organic carbon were most often significantly associated with variations in rotifer numbers from the lower basin. Seasonal flushing of backwater areas by mainstem waters is very important in maintaining the diversity of these lower basin rotifer communities.     

Evaluating decline parameters of rotifer Brachionus plicatilis populations as an interstitial water toxicity bioassay

Population decline of the rotifer Brachionus plicatilis has been evaluated as a sensitive and reliable bioassay for assessing toxicity of marine sediment interstitial water. Three environmental conditions were examined using both interstitial and seawater cultures: (a) adverse effects from dissolved contaminant concentrations; (b) effects from particulate organic matter; and (c) increase or decrease of bacterial biomass from addition of mixtures of antibiotics. Three different parameters were measured to characterize decline: (i) time required for 50% of inoculated population to die (TL₅₀); (ii) curve of the decline rate of Brachionus plicatilis (µ _BP and (iii) graphical area contained below plot of the egg:female ratio curve against time (A _BPM ).Results indicated that, for each of the different assays, the three parameters of the Brachionus plicatilis population decline test were sensitive to concentrations of contaminants dissolved in interstitial waters (principally: total ammonia, Cu, Cr and alkylbenzenesulphonates) but not to particulate organic matter. Nevertheless, the presence or absence of mixed antibiotics with the contaminants may influence the Brachionus plicatilis population decline test, principally by retarding the hatching of eggs. Based on these results, Brachionus plicatilis is confirmed as an appropriate organism for use as an indicator of interstitial water contamination, using either decline rate, TL₅₀ or both parameters. The presence of particulate matter has no effect on these parameters, but the bacterial population may be an influence, although to a lesser extent than the toxicants.     

Adult neurogenesis in natural populations

The dogma that the adult brain produces no new neurons has been overturned, but the critics are still asking, so what? Is adult neurogenesis a biologically relevant phenomenon, or is it perhaps harmful because it disrupts the existing neuronal circuitry? Considering that the phenomenon is evolutionarily conserved in all mammalian species examined to date and that its relevance has been well documented in non-mammalian species, it seems self-evident that neurogenesis in adult mammals must have a role. In birds, it has been established that neurogenesis varies dramatically with seasonal changes in song production. In chickadees, the learning behaviour related to finding stored food is also correlated with seasonal adult neurogenesis. Such studies are still nonexistent in mammals, but the related evidence suggests that neurogenesis does vary seasonally in hamsters and shows sexual differences in meadow voles. To promote studies on natural populations asking fundamental questions of the purpose and function of neurogenesis, we organized a Workshop on "Hippocampal Neurogenesis in Natural Populations" in Toronto in May 2000. The Workshop highlighted recent discoveries in neurogenesis from the lab, and focused on its functional consequences. The consensus at the Workshop was that demonstration of a role for neurogenesis in natural behaviours will ultimately be essential if we are to understand the purpose and function of neurogenesis in humans.