Population dynamic

Suppose that a large number of individuals are randomly matched into groups where each group plays a finite symmetric game. Individuals breed true according to their individual material payoffs, but the expected number of surviving offspring may depend on the material payoff vector to the whole group. We show that the mean-field equation for the induced population dynamic is equivalent to the replicator dynamic for a game with payoffs derived from those in the original game. We apply this selection dynamic to a number of examples, including prisoners' dilemma games, coordination games, hawk-dove games, a prisoners' dilemma with a punishment option, and common-pool games. For each of these, we provide conditions under which our selection dynamic leads to other outcomes than those obtained under the usual replicator dynamic. By way of a revealedpreference argument, we show how our selection dynamic can explain certain stable behaviors that are consistent with individuals having social preferences.

A cohort model for the European eel is presented, which enables the interpretation of observed catches of yellow eel and silver eel in the western Wadden Sea in terms of recruitment data of glasseel. The model builds on various assumptions on length-dependent mortality and silvering rates and on the standard Dynamic Energy Budget (DEB) model which predicts length growth. DEB parameter values are estimated on the basis of literature data. The model predictions are generally in good agreement with the data, though the final decline in numbers in the 1980s occurs earlier than predicted. This suggests that the decrease in eel stock is not just a consequence of lower glasseel immigration but that local conditions must have impoverished, a phenomenon earlier observed in fresh water.

The paper offers a policy-oriented discussion of the relationship between population, environment and economic development. Our particular focus is on the relationship between urbanization and environmental change. Their impact is often represented as strongly negative. We examine the relationship between nutrient levels in coastal lagoons, examining the degree to which these impacts are linked to population density in the watershed. We supplement this with information about environmental attitudes collected from a stratified, clustered random sample of households in six coastal districts of the Central Region. We conclude with some speculations about the implications for environmental change in the years after the Johannesburg Summit.

In an effort to improve knowledge of Cottonmouth (Agkistrodon piscivorus) life history and behavior, we conducted a capture-mark-recapture study in Tuskegee National Forest, Alabama over four years to examine the influence of body size on survival, recapture probabilities, and behavior; as well as monitor change in the population's body-size distribution over time. Our estimate of annual Cottonmouth survival (0.79) is among the highest reported for all snakes. Although we did not detect an effect of body size on survival, the probability of recapturing individuals increased with body size up to a snout-vent-length of 82 cm, after which it remained approximately constant. Relative to large snakes, small snakes were more likely to be found in a resting coil and less likely to move away from researchers, suggesting that our lower probability of recapturing small snakes was not due to behavioral avoidance of researchers but perhaps dispersal patterns or microhabitat use. Furthermore, the observed frequency of snakes ≤ 30 cm in length increased each year, indicative of an increase in recruitment over time. Our findings provide new information about Cottonmouth life history, and we suggest future paths of research that could further enhance knowledge of Cottonmouth demography and population dynamics.

We use six decades of catch-at-age data for perch (Perca fluviatilis) and pike (Esox lucius) in Windermere (UK) to estimate age-and sex-specific population sizes, natural mortalities and catchabilities in both species. Population sizes are estimated by fitting age structured population models to the catch-at-age data using standard maximum likelihood methods. We validate our methods using data simulations, and use our estimates of vital rates (natural mortality, recruitment and catahability) to address important aspects of fisheries biology. ...

In this paper we introduce stochasticity into a model of AIDS and condom use via the technique of parameter perturbation which is standard in stochastic population modelling. We show that the model established in this paper possesses non-negative solutions as desired in any population dynamics. We also carry out a detailed analysis on asymptotic stability both in probability one and in pth moment. Our results reveal that a certain type of stochastic perturbation may help to stabilise the underlying system.

PCR primers targeting loci in the current Burkholderia cepacia complex multilocus sequence typing scheme were redesigned to (i) more reliably amplify these loci from B. cepacia complex species, (ii) amplify these same loci from additional Burkholderia species, and (iii) enable the use of a single primer set per locus for both amplification and DNA sequencing.

Nitrogen fertilization and winter pruning are commonly used to control crop production in peach [Prunus persica (L.) Batsch] orchards. They are also known to affect the dynamics of Myzus persicae (Sulzer) (Homoptera: Aphididae) aphid populations via bottom-up regulation processes. Interactions between crops and pests can cause complex system behaviour in response to management practices. An integrated approach will therefore improve the understanding of the effects of these two cultural practices on aphid and peach performances.

The toxic haptophyte Prymnesium parvum is a harmful alga known to cause fish-killing blooms that occur worldwide. In Texas (United States), P. parvum blooms occur in inland brackish water bodies and have increased in frequency and magnitude in recent years. In this study we conducted three consecutive field experiments (Lake Whitney) to investigate the influence of zooplankton and viruses on P. parvum bloom dynamics during the time of year when blooms are still typically active in Texas (early spring). A localized P. parvum bloom developed during our study that involved increasing levels of toxicity (based on Pimephales promelas and Daphnia magna bioassays). Only in our last experiment, during later stages of bloom development and under highly toxic conditions, did the presence of grazers show a statistically significant, negative effect on P. parvum population dynamics. During this experiment, a rotifer-dominated zooplankton community emerged, composed mostly of Notholca laurentiae, suggesting that this species was less sensitive than other grazers to toxins produced by P. parvum. Microzooplankton may have also been important at this time. Similarly, only our final experiment demonstrated a statistically significant, negative effect of viruses on P. parvum. This exploratory study, resulting in observed impacts on P. parvum populations by both grazers and virus, enhances our understanding of P. parvum ecology and highlights direction for future studies on resistance of zooplankton to prymnesin toxins and algal-virus interactions.

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Although stony corals are most frequently studied on high-relief reef structures, many coral species occur in a wide variety of nearshore tropical habitats, including mangrove creeks, seagrass beds and rocky platforms. This study estimated overall population size, potential reproductive output, and population structure for several species across sandy-bottom, hard-bottom, and patch reef habitats. The density of coral colonies, area of available habitat, and size distribution of resident Siderastrea radians, Favia fragum, and Diploria clivosa colonies were recorded for several island bank systems across habitat types in the central Bahamas. Nearshore patch reefs generally exhibited larger population sizes and higher density of colonies, and contained more larger or older colonies. Nearshore hardbar communities had smaller population sizes and a lower density of colonies, and supported more small colonies, which may either be recruits or asexual clones. High numbers of larger colony s...

Population dynamics of Noctiluca scintillans (Macartney) Kof. et Swezy containing the photosynthetic endosymbiont Pedinomonas noctilucae (Subrahman.) Sweeney was investigated in relation to environmental conditions in the upper Gulf of Thailand. A clear association was observed between the abundance of N. scintillans and the monsoon cycle, with its blooms occurring during the southwest (SW) monsoon from May to September, and low abundance during the northeast (NE) monsoon from November to February. Nutrient concentrations were higher during the SW monsoon than during the NE monsoon due to the combined effect of increased river discharge into the northern upper gulf and the transport of the riverine inputs by the prevailing clockwise circulation of the water. These nutrient conditions favored the growth of both phytoplankton and the endosymbiont. Correlation analysis revealed that the higher abundance of N. scintillans in the SW monsoon was manifested primarily by higher growth throu...

The relationship between Fisher's fundamental theorem of natural selection and the ecological environment of density regulation is examined. Using a linear model, it is shown that the theorem holds when density regulation is caused by exploitative competition and that the theorem fails with interference competition. In the latter case the theorem holds only at the limit of zero population density and/or at the limit where the competitively superior individuals cannot monopolize the resource. The results are discussed in relation to population dynamics and life history evolution, where evidence suggests that the level of interference competition in natural populations is so high that the fundamental theorem does not apply.

Many of the simple mathematical models currently in use often fail to capture important biological factors. Here we extend current models of insect-pathogen interactions to include seasonality in the birth rate. In particular, we consider the SIR model with self-regulation when applied to specific cases-rabbit haemorrhagic disease and fox rabies. In this paper, we briefly summarize the results of the model with a constant time-independent birth rate, a, which we then replace with the time dependent birth rate a(t), to investigate how this effects the dynamics of the host population. We can split parameter space into an area in which the model without seasonality has no oscillations, in which case a simple averaging rule predicts the behaviour. Alternatively, in the area where oscillations to the equilibrium do occur in the non-seasonal model, disease persistence is more complicated and we get more complex dynamical behaviour in this case. We apply resonance techniques to discover the structure of the subharmonic modes of the SIR model with self-regulation. We then look at whether many biological systems are likely to display these ''resonant'' dynamics and find that we would expect them to be widespread.

We advance the hypothesis that cultural values such as high work ethic and thrift, "the Protestant ethic" according to Max Weber, may have been diffused long before the Reformation, thereby importantly affecting the pre-industrial growth record. The source of pre-Reformation Protestant ethic, according to the proposed theory, was the Catholic Order of Cistercians. Using county-level data for England we find empirically that the frequency of Cistercian monasteries influenced county-level comparative development until 1801; that is, long after the Dissolution of the Monasteries. The pre-industrial development of England may thus have been propelled by a process of growth through cultural change.

Arabinoxylan-oligosaccharides (AXOS) are a recently newly discovered class of candidate prebiotics as – depending on their structure – they are fermented in different regions of gastrointestinal tract. This can have an impact on the protein/carbohydrate fermentation balance in the large intestine and, thus, affect the generation of potentially toxic metabolites in the colon originating from proteolytic activity. In this study, we screened different AXOS preparations for their impact on the in vitro intestinal fermentation activity and microbial community structure. Short-term fermentation experiments with AXOS with an average degree of polymerization (avDP) of 29 allowed part of the oligosaccharides to reach the distal colon, and decreased the concentration of proteolytic markers, whereas AXOS with lower avDP were primarily fermented in the proximal colon. Additionally, prolonged supplementation of AXOS with avDP 29 to the Simulator of Human Intestinal Microbial Ecosystem (SHIME) reactor decreased levels of the toxic proteolytic markers phenol and p-cresol in the two distal colon compartments and increased concentrations of beneficial short-chain fatty acids (SCFA) in all colon vessels (25–48%). Denaturant gradient gel electrophoresis (DGGE) analysis indicated that AXOS supplementation only slightly modified the total microbial community, implying that the observed effects on fermentation markers are mainly caused by changes in fermentation activity. Finally, specific quantitative PCR (qPCR) analysis showed that AXOS supplementation significantly increased the amount of health-promoting lactobacilli as well as of Bacteroides–Prevotella and Clostridium coccoides–Eubacterium rectale groups. These data allow concluding that AXOS are promising candidates to modulate the microbial metabolism in the distal colon.

Pup mortality and the timing of birth of South American sea lions Otaria flavescens were investigated to determine the possible relationship between fluctuations in prey availability in the Peruvian upwelling ecosystem and current and future reproductive success of sea lions during six consecutive breeding seasons. Our study from 1997 to 2002 encompassed the strongest El Niño on record and one La Niña event. Pup mortality ranged from 13% before El Niño to 100% during El Niño, and was negatively correlated with prey availability. Abortions were also more frequent when prey availability was low. However, pup mortality remained high following El Niño due to the punctuated short-term effects it had on population dynamics and subsequent maternal behaviour. Births occurred later in the season after years of low food availability and earlier following years of high food availability. The peak of pupping occurred around the peak of mortality in all years, and may have been the product of intensive competition between bulls at the peak of the breeding season. The stronger and more frequent El Niños that appear to be occurring along the Peruvian coast may produce significant stochastic changes in future births and pup mortality, which may place the vulnerable South American sea lion population in Peru at greater risk.

We documented the loss of brown bear (Ursus arctos) cubs-of-the-year (cubs) in 2 Swedish populations for 11 years in the north and 12 years in the south, and made spatial and temporal comparisons to examine whether nutritional, social (sexually selected infanticide ...

Changing climate may impact wildlife populations in national parks and conservation areas. We used logistic and non-linear matrix population models and 35 years of historic weather and population data to investigate the effects of climate on the population dynamics of elk in Rocky Mountain National Park (RMNP), Colorado, U.S.A. We then used climate scenarios derived from Hadley and Canadian Climate Center (CCC) global climate models to project the potential impact of future climate on the elk population. All models revealed density-dependent effects of population size on growth rates. The best approximating logistic population model suggested that high levels of summer precipitation accelerated elk population growth, but higher summer minimum temperatures slowed growth. The best approximating non-linear matrix model indicated that high mean winter minimum temperatures enhanced recruitment of juveniles, while high summer precipitation enhanced the survival of calves. Warmer winters and wetter summers predicted by the Hadley Model could increase the equilibrium population size of elk by about 100%. Warmer winters and drier summers predicted by the CCC Model could raise the equilibrium population size of elk by about 50%. Managers of national parks have relied on effects of weather, particularly severe winters, to regulate populations of native ungulates and prevent harmful effects of overabundance. Our results suggest that these regulating effects of severe winter weather may weaken if climate changes occur as those that are widely predicted in most climate change scenarios.

This paper describes the linkage of a three-dimensional hydrodynamic circulation model with descriptive and experimental biological data concerning oyster (Crassostrea virginica) population dynamics in the Apalachicola Estuary (Florida, U.S.A.). Our intent was to determine the direct and indirect role of Apalachicola River flow in the maintenance of oyster production. Results of a monthly field sampling programme conducted on the oyster reefs in the Apalachicola system during 1985-1986 were used to develop statistical models relating several life-history characteristics of oysters to physical-chemical aspects of water quality. The same life-history characteristics were related statistically to output from a circulation model of Apalachicola Bay. Highest oyster densities and overall bar growth were found in the vicinity of the confluence of high salinity water moving westwards from St George Sound and river-dominated (low salinity) water moving south and eastwards from East Bay. With the exception of models for oyster mortality, the predictive capability of results from the parallel modelling efforts was low. A time-averaged model was developed for oyster mortality during the summer of 1985 by running a regression analysis with averaged predictors derived from the hydrodynamic model and observed (experimental) mortality rates throughout the estuary. A geographic information system was then used to depict the results spatially and to compare the extent of expected mortality in 1985 and 1986. High salinity, relatively low-velocity current patterns, and the proximity of a given oyster bar to entry points of saline Gulf water into the bay were important factors that contribute to increased oyster mortality. Mortality was a major determinant of oyster production in the Apalachicola Estuary with predation as a significant aspect of such mortality. By influencing salinity levels and current patterns throughout the bay, the Apalachicola River was important in controlling such mortality. Oyster production rates in the Apalachicola system depend on a combination of variables that are directly and indirectly associated with freshwater input as modified by wind, tidal factors, and the physiography of the bay. River flow reduction, whether through naturally occurring droughts, through increased upstream anthropogenous (consumptive) water use, or a combination of the two, could have serious adverse consequences for oyster populations. By coupling hydrodynamic modelling with descriptive and experimental biological data, we were able to determine the effects of potential freshwater diversions on oyster production in Apalachicola Bay.

Due to their boom and bust population dynamics and the enormous biomasses they can attain, jellyfish and ctenophores can have a large influence on the cycling of carbon (C), nitrogen (N) and phosphorus (P). This review initially summarises the biochemical composition of jellyfish, and compares and contrasts the mechanisms by which non-zooxanthellate and zooxanthellate jellyfish acquire and recycle C, N

Geonoma orbignyana is a common understory palm, whose leaves are used as foliage in Colombia. This palm has been used for more than 30 years; but there is no information about its life history or conservation status. Our objective was to characterize the life history of this palm and population dynamics of a natural population as a first step to assess the effect of leaf harvesting. More than 1600 palms were monitored from June 1999 to March 2000; we measured growth, mortality, and reproductive success. Population dynamics were evaluated using a Lefkovitch matrix model, and elasticity analysis. Results indicate that G. orbignyana grows at a rate of 1.98 ± 0.047 leaves/year, the palm has a long lifespan, and growth and reproduction are related to light conditions in the forest. The density of this palm is high (108–311 palms/100 m2), which is explained in part by its capacity to continue growing after damage, and to respond to the frequent gap formation in the forest. The population is growing (λ = 1.074, CI = [1.046–1.099]). Survival transitions account for 77% of population growth, a value that is higher compared to other understory plants. Because of the high density, the recovery capacity of these palms, and the correlations found between individual and population performances with forest dynamics, we hypothesize that G. orbignyana behaves as an opportunistic species taking advantage of forest gaps. This palm is a promising non-timber forest product (NTFP) with a high profile for further exploitation, although we recommend that harvesting regimens should consider time of recovery and forest dynamics. Without these considerations mortality could increase in all classes, which will compromise population persistence. Further studies should accurately estimate the recovery time after defoliation as well as to characterize forest dynamics identifying its most important features for population growth.

Marine reserves are promoted as an effective supplement to traditional fishery management techniques of harvest quotas and effort limitation. However, quantitative fishery models have ignored the impact of noncompliance (poaching). Here we link a model of a harvested fish population to a game-theoretic representation of fisherman behavior to quantify the effect of poaching on fishery yield and the enforcement effort required to maintain any desired level of reserve effectiveness. Although higher fish densities inside reserves will typically entice fishermen to poach, we show that the initial investment in enforcement efforts provides the greatest return on maintaining the benefits of the reserve to the fishery. Furthermore, we find that poaching eliminates the positive effect of fish dispersal on yield that is predicted by traditional models that ignore fisherman behavior. Our results broaden a fundamental insight from previous models of marine reserves, the effective equivalence of the harvest quota and reserve fraction, to the more realistic scenario in which fishermen attempt to maximize their economic payoffs.

All natural populations fluctuate in abundance and age structure through time; understanding why they do so is a critical step towards their effective management and conservation. However, the long-term data sets needed for such an understanding are rarely available, especially for tropical organisms. A 17-year capturemark-recapture study yielded detailed information on the demography of water pythons (Liasis fuscus) and their main prey, the dusky rat (Rattus colletti), on the Adelaide River flood plain in tropical Australia. The link between annual rainfall patterns and rat demography was highly non-linear. Rat numbers were low during years with low and high rainfall at the end of the wet season (April). Numbers of both predators and prey fluctuated considerably among years. Annual fluctuations in rat numbers generated a corresponding variation in rates of female python reproduction, python body condition and survival. Although variation in recruitment, survival and prey abundance all had a significant impact on annual fluctuations in python numbers, our analyses suggest that recruitment constituted the main determinant in driving the population dynamics of these large tropical predators. In combination with our other studies on this system, the data show that population dynamics of the water python population is ultimately driven by annual variation in rainfall, mediated via shifts in prey availability. The water pythons and the dusky rats of the Adelaide River flood plain thus demonstrate an unusually clear and direct link between an abiotic factor (rainfall) and predator-prey population dynamics.

In this paper, we study the population dynamics of the Mediterranean monk seal in the National Marine Park of Alonissos, Greece. As a first general approach, the well-known logistic model and a modified version are used. As a second approach, a θ-logistic type model with delay is adopted. Finally, a modified version of this delay model is proposed and compared with the others. The results are illustrated using computer simulations.

We studied the population dynamics of two species of tortoise beetles, Aspidomorplw miliaris (AM) and A. sanctaecnrcis (AS) (Coleoptera: Chrysomelidae: Cassidinae), feeding on a shrubby moming gluy, Ipomoea carnea (Convolvulaceae), in Purwodadi Bounic Garden, East Java, Indonesia. Rainfall in Purwodadi is distinctly seasonal with 5-7 months of harsh dry season, lasting from March or April to October or November. In March of 1993, we established three study sites by planting 10 cuttings of the host plants on each site. We carried out a weekly census including mark-recapture of adults with color paints and construction of life tables for immature stages, from June 1993 to March 1996, Total number of leaves on the study sites increased during ttre wet season and decreased during the dry season, although effects of dry seasons on the change in the total number of plant leaves depended on the site condition (sunshine and water). We marked all beetles found on the sites (136 and 436 beetles for AM and AS, respectively). Adult populations of the two species increased in the rainy season, decreased as the dry season started, and disappeued in the late dry season. Seasonal trends in the adult numbers mainly reflected the oviposition activity of adults, which was high in the rainy season and diminished or nil in the dry season. The sex ratio was l:l in AM and largely female-biased in AS. Average adult residence time (days) on each site was 6.6 (male) and 8.1 (female) in AM and 7.0 (male) and 9.8 (female) in AS. The current results ue discussed with our previous data of the two species in two localities with higher and less seasonal rainfall conditions, Padang (central Sumatra) and Bogor (West Java). Key Vlords: population dynamics / seasonality / Cassidinae / Asp ifumorplw / fustJava / Indqresia Climate in the tropics is characterized by small seasonal differences in temperature and by an altemation of rainy and dry seasons. Those regions of the tropics with a distinct altemation of rainy and dry season are called the "seasonal" tropics. Other regions within the tropics have little or no seasonal differences in either rainfall or temperature, although there is no completely "aseasonal" tropics on the eafth. Increasingly, studies of tropical insect populations indicate large population fluctuations ar€ not uncornmon and are often associated with the seasonal distribution of rainfall (for literature,

One of the most difficult problems in developing spatially explicit models of population dynamics is the validation and parameterization of the movement process. We show how movement models derived from capture-recapture analysis can be improved by incorporating them into a spatially explicit metapopulation model that is fitted to a time series of abundance data. 2. We applied multisite capture-recapture analysis techniques to photo-identification data collected from female grey seals at the four main breeding colonies in the North Sea between 1999 and 2001. The best-fitting movement models were then incorporated into state-space metapopulation models that explicitly accounted for demographic and observational stochasticity. 3. These metapopulation models were fitted to a 20-year time series of pup production data for each colony using a Bayesian approach. The best-fitting model, based on the Akaike Information Criterion (AIC), had only a single movement parameter, whose confidence interval was 82% less than that obtained from the capture-recapture study, but there was some support for a model that included an effect of distance between colonies. 4. The state-space modelling provided improved estimates of other demographic parameters. 5. The incorporation of movement, and the way in which it was modelled, affected both local and regional dynamics. These differences were most evident as colonies approached their carrying capacities, suggesting that our ability to discriminate between models should improve as the length of the grey seal time series increases. Fig. 2. Pup production data (circles) and smoothed estimates (solid line) for the best-fitting state-space model of the North Sea grey seal population. Dashed lines show the posterior 95% Bayesian credibility intervals.

This paper aims to establish a comprehensive Iist of exotic species introduced into Atlantic French waters over the last century as weil as their origin, and the vecklrs involved in the process. At least 104 nonindigenous species have been recorded in the Atlantic and Channel coastal wa· ters. Among those species, only a few (Bonamia ostreae, Crepidula fornicata) have resulted in highly significant negative impacts and side-effects on the coastal activity. In contrast, the intro duction of the Japanese oyster Crassostrea gigas can be considered to be successful, sustaining a large industry counting more than 4,000 companies and 150,000 tons of yearly production. Over the last decade, French and European laws have strenghtened the controls over exotic species introductions, limiting their impacts. These new regulations and increased awareness from the scientific community with regard to coastal industry and biodiversity have reduced significantly the cases of deliberate introduction, while the risk analysis requirement has improved the overall management. However, in spite of these improvements, cases remain significant, mostly by acci dentai and/or increased vectors of introduction such as the ballast waters and sediment. Moreover, global changes should now be considered as a major issue, Iikely inducing future geographic distribution changes over a large European coastal range. Understanding the invasion population dynamics for exotic species remains a challenging research issue currently addressed by the French INVABIO multidisciplinary program.

Hedgerows are an important element of the landscape both as a habitat and as a visual component contributing to the character of the countryside. The aim of this paper is to identify key areas of contemporary research into hedgerows in the UK, to assess progress being made in each area, and to highlight areas that need more attention. Key themes were extracted from Barr et al. (1995) and recent papers were assigned to eleven distinct groupings. Two areas were found to have had the most published papers: 'autecological studies' and 'landscape ecology'. Recommendations for future areas of research include the need for a national classification of hedgerow vegetation, long-term large-scale studies on the effects of management and different restoration techniques, and research into population dynamics at the landscape-scale including that into the role of hedgerows as supplementary woodland habitat and as movement corridors.

We study how the dynamics of a food chain depends on the biology of the top-predator. We consider two model food chains with specialist and generalist top-predators. Both types of food chains display same type of chaotic behavior, short-term recurrent chaos; but the generating mechanisms are drastically different. Food chains with specialist top-predators are dictated by exogenous stochastic factors. On the contrary, the dynamics of those with the generalist top-predator is governed by deterministic changes in system parameters. The study also suggests that robust chaos would be a rarity.

A Lagrangian model is used to simulate and quantify in the northern Humboldt upwelling ecosystem the processes of enrichment, concentration and retention, identified by Bakun [Bakun, A., 1996. Patterns in the ocean. Ocean processes and marine population dynamics. University of California Sea Grant, California, USA, in cooperation with Centro de Investigaciones Biologicas de Noroeste, La Paz, Baja California Sur, Mexico, 323 pp.] as being important for the survival and recruitment of early life stages of pelagic fish. The method relies on tracking the positions of particles within water velocity fields generated by a three-dimensional hydrodynamic model. Simple criteria for considering particles as participating to enrichment, concentration or retention are used to derive indices of the three processes. We analyse the spatial distribution of and seasonal variability in these indices. The results are discussed in relation to anchovy (Engraulis ringens) eggs and larvae distributions off Peru, and to a comparable study conducted in the southern Benguela upwelling ecosystem.

According to recent competition theory, the population dynamics of phytoplankton species in monoculture can be used to make a priori predictions of the dynamics and outcome of competition for light. The species with lowest ''critical light intensity'' should be the superior light competitor. To test this theory, we ran monoculture experiments and competition experiments with two green algae (Chlorella vulgaris and Scenedesmus protuberans) and two cyanobacteria (Aphanizomenon flos-aquae and a Microcystis strain) in light-limited continuous cultures. We used the monoculture experiments to estimate the critical light intensities of the species. Scenedesmus had by far the highest critical light intensity. The critical light intensities of Chlorella, Aphanizomenon, and Microcystis were rather similar. According to observation, Aphanizomenon had a slightly lower critical light intensity than Chlorella and Microcystis. However, according to a model fit to the monoculture experiments, Chlorella had a slightly lower critical light intensity than Microcystis, which in turn had a slightly lower critical light intensity than Aphanizomenon. These subtle differences between observed and fitted critical light intensities could be attributed to differences in the light absorption spectra of the species. The competition experiments were all consistent with the competitive ordering of the species according to the fitted critical light intensities: Chlorella displaced all three other species, Microcystis displaced both Aphanizomenon and Scenedesmus, and Aphanizomenon only displaced Scenedesmus. Not only the final outcomes, but also the time courses of competition predicted by the theory, were in excellent agreement with the experimental results for nearly all species combinations.

This report was prepared as an account of work sponsored by the United States Government. Neither the United States nor the United States Department of Energy (DOE), nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product or process disclosed, or represents that its use would not infringe privately owned rights.

The planthopper Hyalesthes obsoletus, is the vector of grapevine bois noir disease, caused by stolbur phytoplasma. This hopper is widespread in Europe and the Middle East but it is not a grapevine specialist, and its host range is still controversial. This paper reports on the population dynamics, host range and infectivity of H. obsoletus in northwestern Italy. Field sampling of adults was done with yellow sticky traps and sweep nets on grapevines and in nettle beds, whereas nymphs were sought on nettle and bindweed roots. PCR and dot-blot analyses were used to detect stolbur phytoplasma. The life span of the planthopper was tested in the laboratory on broadbean, periwinkle, nettle and grapevine. Nymphs were found only on roots of Urtica dioica L., and many more adults were captured in nettle beds than on grapevine. Flight activity started in the middle of June and ended middle of September, with a flight peak at the end of July. In the laboratory, the longest life span was observed on nettle whereas broadbean and grapevine were not suitable for survival. Natural infectivity was 6-8% for nymphs and 9-50% for adults. These results confirm the strong links between H. obsoletus, Bois Noir, and U. dioica in northwestern Italy.

Populations dominated by Cystoseira zosteroides, an endemic and threatened Mediterranean seaweed, colonize deep-water rocky habitats down to more than 50 m depth. Assemblages dominated by this species display high algal and invertebrate species richness. Algal biomass averages 1134 g dw m À2 . Erect and turf algae account for only 25% of total algal dry weight, while encrusting corallines are responsible for the remaining 75%. Sponges, bryozoans and ascidians constitute the dominant sessile macrofauna. Cystoseira zosteroides is the dominant erect algae, with a mean biomass of 60.6 g dw m À2 , and densities ranging from 4 to 7 plants m À2 . The alien turf alga Womersleyella setacea has a biomass of 104.2 g dw m À2 and covers most of the understory substrate. The size-frequency distribution of C. zosteroides populations shows differences over time. Mean annual growth of the main axis is around 0.5 cm and mean annual mortality rate is lower than 2%. Recruitment was almost nil during the studied period of time (10 years). Processes structuring these deep-water Cystoseira stands must be driven by episodic disturbances, afterdisturbance recruitment pulses, and long periods of steady growth that last at least 10 years. However, it is also possible that recruitment is irreversibly inhibited by the alien alga W. setacea in which case these old-growth stands are faced with extinction. The highly diversified assemblages and the low growth and low mortality rates of C. zosteroides indicate high vulnerability to natural and anthropogenic disturbances, and call for effective measures to ensure their conservation.

Variation of abundance, dominance and diversity of tardigrades were studied over a period of 54 months in a carpet of the moss Rhytidiadelphus squarrosus that covered a lawn in the Black Forest (Germany). Altogether 19,909 individuals belonging to 24 species were extracted from the moss. Macrobiotus hufelandi (56%), Macrobiotus richtersi (18%) and Diphascon pingue (12%) were the most abundant and dominant species. Dominances of the other species varied between 0.01 to 2.15%. Generally, species diversity (Shannon-Index and evenness) was highest during the winter. A temporal variation in numbers with a decline in winter and an increase in spring until fall was seen in M. hufelandi, D. pingue and less clear in M. richtersi. Three species (Diphascon rugosum, Hypsibius dujardini, Hypsibius cfr. convergens) showed a cyclic variation with clear peaks in wintertime. The moist season favoured species that were considered as hygrophilous in the literature, whereas the relatively dry sunny seasons promoted the relative increase of euryhygric species. The composition of the tardigrade community was strikingly robust over the years. Fourteen species were always present during the entire period of investigation. However, changes in the species composition over the years, and cyclical temporal presence of certain species stress the significance of long-term studies to estimate species richness in a given habitat. Application of the Spearman rank correlation test to the data set of the whole study period showed different correlations between the mean number of tardigrades as well as the mean number of individuals of selected species (dominance >1) and rainfall, humidity and temperature. Correlations confirmed some of the ecological dependences of tardigrade species known from literature. Conflicting classifications suggest dependence from other variables or perhaps presence of ecotypes.

The evolution of an infectious disease outbreak in an isolated population is split into two stages: a stochastic Markov process describing the initial contamination and a linked deterministic dynamical system with random initial conditions for the continued development of the outbreak. The initial contamination stage is well approximated by the randomized SI (susceptible/infected) model. We obtain the probability density function for the early behavior of the epidemic. This provides an appropriate distribution for the initial conditions with which to describe the subsequent deterministic evolution of the system. We apply the method of matching asymptotic expansions to link the two stages. This allows us to estimate the standard deviation of the number of infectives in the developed outbreak, and the statistical characteristics of the outbreak time. The potential trajectories caused by the stochastic nature of the contamination stage show greatest divergence at the initial and fade-out stages and coincide most tightly just after the peak of the epidemic. The time to the peak of the outbreak is not strongly dependent on the initial trajectory.