Cyanobacteria facilitate parasite epidemics in Daphnia.

The seasonal dominance of cyanobacteria in the phytoplankton community of lake ecosystems can have severe implications for higher trophic levels. For herbivorous zooplankton such as Daphnia, cyanobacteria have poor nutritional value and some species can produce toxins affecting zooplankton survival and reproduction. Here we present another, hitherto largely unexplored aspect of cyanobacteria, namely that they can increase Daphnia susceptibility to parasites. In a 12-yr monthly time-series analysis of the Daphnia community in Greifensee (Switzerland), we observed that cyanobacteria density correlated significantly with the epidemics of a common gut parasite of Daphnia, Caullerya mesnili, regardless of what cyanobacteria species was present or whether it was colonial or filamentous. The temperature from the previous month also affected the occurrence of Caullerya epidemics, either directly or indirectly by the promotion of cyanobacterial growth. A laboratory experiment confirmed that cyanobacteria increase the susceptibility of Daphnia to Caullerya, and suggested a possible involvement of cyanotoxins or other chemical traits of cyanobacteria in this process. These findings expand our understanding of the consequences of toxic cyanobacterial blooms for lake ecosystems and might be relevant for epidemics experienced by other aquatic species.

Natural selection for grazer resistance to toxic cyanobacteria: evolution of phenotypic plasticity?

We studied the selection response of the freshwater grazing zooplankter, Daphnia galeata, to increased abundance of cyanobacteria in its environment. Cyanobacteria are a poor-quality and often toxic food. Distinct genotypes of D. galeata were hatched from diapausing eggs extracted from three time horizons in the sediments of Lake Constance, Europe, covering the period 1962 to 1997, a time of change in both the prevalence of planktonic cyanobacteria and levels of phosphorus pollution. We assessed whether the grazers evolved to become more resistant to dietary cyanobacteria by exposing genetically distinct clones to two diets, one composed only of the nutritious green alga, Scenedesmus obliquus (good food), and the other a mixture of S. obliquus and the toxic cyanobacterium Microcvstis aeruginosa (poor food). Genotype performance was measured as the specific rate of weight gain from neonate to maturity (gj). We evaluated evolutionary change in the Daphnia population using an analysis of reaction norms based on relative (log-transformed) changes in gj. Log(gj) is a measure of the proportional effect of dietary cyanobacteria on other fitness components of the Daphnia phenotype. For comparison, we also analyze absolute (i.e., nontransformed) changes in gj and discuss the interpretations of the two approaches. Statistical results using a general linear model demonstrate a significant effect of genotype (showing differences in gj among genotypes), a significant genotype x food-type interaction (showing differences in phenotypic plasticity among genotypes), and, in the case of log-transformed data, a significant sediment-genotype-age x food-type interaction. The latter shows that phenotypic plasticity evolved over the period studied. Two constraints act on response to selection in the D. galeata-Lake Constance system. First, gj on a diet containing poor food is highly correlated with gj on a diet of good food, thus evolving resistance also meant evolving an increase in gj on both diets. Second, because genotypes with a high gj also grow to a large adult body size, which in turn increases Daphnia vulnerability to fish predation, we suggest that selection only acted to favor genotypes possessing a high potential gj after cyanobacteria became prevalent. The presence of cyanobacteria depressed realized gj and led to animals of small adult body size even if their genotypes had the potential for high gj and large size. With realized gj reduced, genotypes with an inherently high value could be selected even in the presence of predatory fish. The joint action of selection by dietary cyanobacteria and vulnerability to fish predation provides an explanation for the observed evolution of resistance to poor food through reduced phenotypic plasticity.

Crossing the hopf bifurcation in a live predator-prey system.

Population biologists have long been interested in the oscillations in population size displayed by many organisms in the field and laboratory. A wide range of deterministic mathematical models predict that these fluctuations can be generated internally by nonlinear interactions among species and, if correct, would provide important insights for understanding and predicting the dynamics of interacting populations. We studied the dynamical behavior of a two-species aquatic laboratory community encompassing the interactions between a demographically structured herbivore population, a primary producer, and a mineral resource, yet still amenable to description and parameterization using a mathematical model. The qualitative dynamical behavior of our experimental system, that is, cycles, equilibria, and extinction, is highly predictable by a simple nonlinear model.

Ecosystem size determines food-chain length in lakes.

Food-chain length is an important characteristic of ecological communities: it influences community structure, ecosystem functions and contaminant concentrations in top predators. Since Elton first noted that food-chain length was variable among natural systems, ecologists have considered many explanatory hypotheses, but few are supported by empirical evidence. Here we test three hypotheses that predict food-chain length to be determined by productivity alone (productivity hypothesis), ecosystem size alone (ecosystem-size hypothesis) or a combination of productivity and ecosystem size (productive-space hypothesis). The productivity and productive-space hypotheses propose that food-chain length should increase with increasing resource availability; however, the productivity hypothesis does not include ecosystem size as a determinant of resource availability. The ecosystem-size hypothesis is based on the relationship between ecosystem size and species diversity, habitat availability and habitat heterogeneity. We find that food-chain length increases with ecosystem size, but that the length of the food chain is not related to productivity. Our results support the hypothesis that ecosystem size, and not resource availability, determines food-chain length in these natural ecosystems.

Does food web complexity eliminate trophic-level dynamics?

Investigators are always divided into those who are looking for rules and those who are looking for exceptions. (Hackett 1937, p. 106).

The Interaction of Photoperiod and Temperature in Diapause Timing: A Copepod Example.

In many organisms, photoperiod and temperature are thought to be the most significant token cues for seasonally timed life history events, including diapause in arthropods. A common pattern in many species of terrestrial insects and several copepod species is the existence of a critical daylength on one side of which the animals do not enter diapause and on the other side of which they do. Temperature plays a secondary role as modifier of the critical daylength. In some species, however, including the freshwater copepod Diaptomus sanguineus, the fraction of females making subitaneous eggs (eggs that hatch immediately) undergoes a very gradual transition as daylength changes over the natural range of photoperiods experienced in nature. Here we show that temperature is as important as photoperiod in cuing diapause timing in a population of D. sanguineus living in Bullhead Pond, Rhode Island. When ecologically relevant photoperiod and temperature cues are provided in the laboratory, the copepods rapidly switch from producing subitaneous eggs to producing diapausing eggs in a way that is typical of the seasonal switch seen in the pond. We provide a graphical model that illustrates how copepod sensitivities to photoperiod and temperature interact to produce an abrupt transition, and we discuss how natural selection should act on D. sanguineus diapause response to produce the variation in diapause timing seen within and between natural populations.

Predator driven changes in community structure.

The zooplankton community of a small pond changed markedly with temporal variation in predation pressure. Long term changes in zooplankton community structure occurred following the replacement of planktivorous fish by phantom midge (Chaoborus americanus) larvae as the predominant predator of zooplankton. The interannual changes following the establishment of Chaoborus included the apparent or near extinction, of species ill adapted to the new predation pressure and the successful colonization of well adapted species. Seasonal changes in the species composition and size distribution of the zooplankton community correlate with temporal variation in predation intensity associated with temperature-activity patterns of the predator or changes in the stage structure of the predator population.

Population differences in the timing of diapause: a test of hypotheses.

The reproductive phenology of the freshwater copepod Diaptomus sanguineus differs markedly between populations residing in two Rhode Island ponds. In a permanent pond the population switches abruptly from making subitaneous (immediately hatching) eggs to diapausing eggs at the end of March each year. In contrast, a temporary pond population switches egg types in May, returns to production of subitaneous eggs in June, and concludes the reproductive season by making diapausing eggs in July. An ESS model suggests that the pattern of diapause expected of a copepod population is a function of annual variation in the onset of harsh conditions (catastrophe date). When variation is relatively low, the superior strategy is for diapause to begin a constant period before the mean catastrophe date. When variation is high, females should make first subitaneous eggs and then diapausing eggs irrespective of the expected catastrophe date. With discrete generations, such a population would alternate between egg types. In the permanent pond, variation of catastrophe date the spring onset of planktivory by sunfish is low, whereas in the temporary pond variation of the catastrophe (pond drying) is high. The model predicts well the phenology of the two copepod populations.In the research reported here, we tested the hypothesis that copepods from the permanent pond, which switch to diapause at the same time every year, are cued by the environment to begin diapause (i.e. by photoperiod, temperature, or both), whereas those from the temporary pond make both egg types regardless of environmental conditions. In opposition to our hypothesis, experimental results indicate that diapause in both populations is cued by the environment. The distinct reproductive phenologies documented in the two populations apparently result from the copepods responding to different environmental cues, rather than one being responsive to the environment while the other is not.

Rapid evolution of a life history trait.

The copepod Diaptomus sanguineus begins diapause in permanent ponds in late March as an adaptation to avoid summer fish predation. During a study of copepod populations in two Rhode Island ponds, a severe drought dried one pond killing all fish. The second (control) pond did not dry, and no fish were killed. Before the drought copepods in the two ponds entered diapause on nearly the same date. After the drought, the timing of diapause shifted to later in the year in the pond that had lost its fish, while no shift occurred in the control pond. The direction of this shift in the onset of diapause is that expected had the copepods been released from natural selection for early spring diapause imposed by summer fish predation.

Fish vision and the detection of planktonic prey.

Planktivorous sunfish of various sizes were studied to ascertain whether growth-related changes in the retina are related to the ability to capture small planktonic crustaceans. Behaviorally, the larger fish detected and captured crustaceans that subtended smaller visual angles. Histological examination of the retinas revealed that the distance between cones, measured in minutes of visual angle, decreased as the animals grew, suggesting that the larger retinas could resolve smaller objects. These correlated behavioral and anatomical results suggest that improved visual resolution contributes to improved predation. This finding provides a selective advantage for the continuous retinal growth noted in many fish.

On the inefficiency of transmission of Wuchereria bancrofti from mosquito to human host.

The high biting rate of Culex pipiens fatigans in Rangoon, combined with a low prevalence of microfilaraemia due to Wuchereria bancrofti, suggested a poor efficiency of transmission. Data obtained by the WHO Filariasis Research Unit in Rangoon were analysed, and the efficiency of the parasite from Stage III larva in the mosquito to the production of microfilariae was estimated as 6.04-6.71 x 10(-5), indicating that an average of around 15 500 bites by "infective" mosquitos is necessary to produce 1 case of microfilaraemia. This surprising result prompted a step-by-step analysis of the success of the parasite through departure from the mosquito, penetration of host tissues, survival to maturity, and encountering a mate in the human host. It was hoped that the second approach would identify some discrepancy in the original analysis, but when all sources of loss were combined, it was estimated that the expected efficiency was 4.78 x 10(-5). The two estimates are within observational error of each other. The degree to which the two approaches agree leads to the conclusion that survival of the parasite is reasonably well known at all stages, and the calculations indicate that a high proportion of the population of Rangoon must be carrying parasites that are either dead or immature. A complete quantitative statement of the epidemiology of W. bancrofti seems to be attainable.

The epidemiology of Schistosoma haematobium and S. mansoni infections in the Egypt-49 project area. 4. Measurement of the incidence of bilharziasis.

The measurement of incidence, or the rate at which people become positive, for Schistosoma haematobium and S. mansoni was carried out in four parts of the Egypt-49 project area near Alexandria. For S. haematobium, rates as high as 22.8% per year were found for children 0-6 years old in a rural area; in the same area, the incidence of S. mansoni was 8.5% per year. The true incidence is underestimated because many cases become negative spontaneously. This loss rate of S. haematobium cases is 0.476 per year for children 0-4 years old, and 0.049 per year for those aged 5 and 6 years; for S. mansoni, the rates are 0.580 and 0.327, respectively. Despite the error, incidence is the most accurate and sensitive method of assessing the success of control operations, and is an important measurable parameter in epidemiology.

The ecology of Bulinus truncatus and Biomphalaria alexandrina and its implications for the control of bilharziasis in the Egypt-49 project area.

The respective vectors of the two forms of bilharziasis in Egypt do not have the same ecological distribution. Bulinus truncatus is most abundant in large canals, and decreases in density as the water approaches and flows into drains. Biomphalaria alexandrina is most abundant in drains, and decreases in density upstream from these habitats. Both species are most abundant in the presence of aquatic vegetation, but they differ in their respective associations with the water hyacinth, Eichhornia crassipes. Biomph. alexandrina reaches maximum abundance in the presence of this plant, but Bul. truncatus is as uncommon in the absence of plants as in the presence of E. crassipes.Calculation of life-table parameters from field data shows that, under optimum field conditions, both species can double their populations in 14-16 days. The reproductive rates of both species are greatest in March and the death rates in midsummer. The observed peak densities in May and June give a false impression of optima because of undercollection of young snails, which are most abundant in March and April.Control operations should take advantage of the findings on population parameters. A single area-wide treatment with molluscicide in April is recommended. During the remainder of the year, search for isolated foci of snail breeding and individual treatment of these will effect large savings of chemical and will be effective in controlling the transmission of the parasites.

The effect of area-wide snail control on the endemicity of bilharziasis in Egypt.

Molluscicides applied to two areas near Alexandria had a significant effect in reducing both incidence and prevalence of Schistosoma haematobium and S. mansoni infections. No decrease in either measure of endemicity was found in an adjacent untreated area. Bayluscide and sodium pentachlorophenate were equally effective in interrupting the transmission of S. haematobium, but Bayluscide was more effective against S. mansoni, probably because of the difficulty of applying sodium pentachlorophenate to drains, which are the primary habitats for the intermediate hosts of S. mansoni. The authors believe that the present experiment provides strong evidence of the interruption of transmission of bilharziasis in the Nile Valley or Delta.