Low Food Levels Research Articles

Tissue expression of two α-amylase genes in the Pacific oyster Crassostrea gigas. Effects of two different food rations

A semi-quantitative reverse transcription-polymerase chain reaction (RT-PCR) assay was developed to estimate tissue-specific expression of two amylase genes in five different tissues of the Pacific oyster Crassostrea gigas and the experimental effects of food level on the cellular, enzymatic and gene expression changes of the digestive gland. Results indicated that both genes were transcribed at different levels depending on tissue type. A strong preferential expression of both amylase genes was observed in the digestive gland leading to high amylase activities, in accordance with the digestive function of this organ. Traces of gene expression and activity were observed in other analysed tissues. These appeared higher in labial palps than in other non-digestive tissues. After 4 weeks of feeding two groups of oysters on either a high or low food level, cellular growth of the digestive gland (+56%) and increase in the total activity of amylase enzymes and in specific amylase activity (+50%) were observed under high food conditions compared to low food conditions. A significant increase (+18%) in the expression of gene A was observed in the digestive gland of oysters fed the high ration compared to that of oysters fed on the low food level whereas no changes in the expression of gene B were observed. Gene A was expressed at a higher level than gene B in all tissues and at all conditions. A role of the amylase gene A in digestive processes and changes in its expression according to food availability is demonstrated in this study. The functional role of the α-amylase B gene remains to be elucidated.

Influence of food abundance on individual behaviour strategy and growth rate in juvenile brown trout (Salmo trutta)

We studied the benefit of being territorial as an effect of food abundance by measuring the proportions of individuals that displayed a territorial, floating (individuals occasionally displayed territorial behaviour), or nonterritorial (shoaling) behaviour strategy, and individual growth rates. Also, swimming activity was monitored as an indicator of emigration. Replicate groups of 12 juvenile brown trout (Salmo trutta) were released into an artificial stream channel, fed according to one of four food regimes for 10 days, and allocated to one of three behaviour categories. There was no significant relationship between food abundance and the number of territorial individuals. Instead, the proportions of individuals that displayed the alternative behaviour strategies, i.e., floating and nonterritorial, changed with food abundance. At the lowest food level, more individuals displayed nonterritorial than floating behaviour, but these two strategies were equally represented at the highest food level. The difference in growth rates with respect to behaviour category was highest at intermediate food levels. At the highest and lowest food levels, nonterritorial fish and floaters grew nearly as fast or as poorly, respectively, as the territorial individuals. Swimming activity between the two sections of the stream tank was greatest at the lowest food level, and there was a general tendency (not significant) for the floaters to exhibit the greatest activity.

Nutrient limitation on a planktonic rotifer: Life history consequences and starvation resistance

Nutrient content of plants is low relative to that of herbivores. Dietary nutrients can limit the growth and reproduction of herbivores. We studied life history consequences and starvation resistance for a common planktonic rotifer, Keratella cochlearis, feeding on algae grown on nutrient‐limited media. A strain of Cryptomonas was grown on three types of growth medium with nitrogen (N) and phosphorus (P) adjusted to produce N‐limiting, P‐limiting, and nutrient‐sufficient conditions. The analysis of nutrient content of Cryptomonas cells grown on nutrient‐limited conditions showed that those algae exhibited higher P content than algae growing on a nutrient‐sufficient medium. However, Cryptomonas reached lower densities in nutrient‐limited media. The life history responses of Keratella to food quality were examined with low and high food availability. Our study showed that the interaction of food quantity and quality had a significant effect on rotifer growth rates. We observed, at low food levels, that Keratella growth rates were highest when the algae were grown under P limitation. The lowest rotifer growth rates were obtained when the rotifer fed on Cryptomonas grown in nutrient‐sufficient conditions. Our study also showed that starvation resistance was higher for those rotifers fed on N‐limited Cryptomonas. In natural situations, rotifers could even grow better under nutrient‐limited conditions if this group of algae is dominant in the phytoplankton community. Moreover, differences in starvation resistance could be critical in determining competitive outcome and community structure in nutrient‐variable environments.

Longevity of the freshwater anostracan Streptocephalus mackini (Crustacean: Anostraca) in relation to food (Chlorella vulgaris) concentration

SUMMARY 1. Temporary ponds are inhabited by a variety of invertebrates, of which anostracans are an important group. We studied the lifetables of male and female anostracan Streptocephalus mackini at 3 algal concentrations (0.5 × 106, 1.0 × 106 and 1.5 × 106 cells mL−1).2. Regardless of sex, S. mackini showed better survivorship at lower food levels. The longest average lifespan observed was 85 ± 2 days for males fed Chlorella at 0.5 × 106 cells mL−1.3. Both net reproductive rate and generation time decreased with increasing food level. The highest net reproductive rate was about 120 cysts per female. The longest generation time of about 40 days, observed at 0.5 × 106 cells mL−1, was more than three times that at 1.5 × 106 cells mL−1.4. The rate of population increase (r) was nearly the same (0.31 ± 0.06) at high (1.5 × 106 cells mL−1) and intermediate (1.0 × 106 cells mL−1) food levels. The r‐value at low food level (0.5 × 106 cells mL−1 of Chlorella) was 0.20 ± 0.01 per day.

Hungry predators render predator‐avoidance behavior in tadpoles ineffective

Behavioral responses of prey to their predators can critically alter community dynamics. Whether or not a prey responds, clearly depends on the effectiveness of that response. The effectiveness on the other hand is predicted to depend on predator behavior. Actively searching predators can render the behavioral responses in their prey ineffective. Nevertheless, most studies investigating the optimal reaction of prey treated predators as immobile elements of the environment. I experimentally manipulated activity of poolfrog (Rana lessonae) tadpoles by keeping them at low and high food levels, and exposed them to three species of invertebrate predators (Aeshna cyanea, Anax imperator, and Dytiscus marginalis), whose activity also was manipulated through different food levels. Satiated, less active predators were more likely to kill hungry, more active tadpoles, but hungry predators killed hungry and satiated tadpoles about equally often. This result suggests that reducing their activity is a more effective strategy for tadpoles if the predators themselves are less active. On the other hand, against hungry, highly motivated predators, the behavioral avoidance strategies were essentially ineffective. Antipredator behavior is generally thought to stabilize the dynamics of predator–prey systems. The results presented here, however, suggest that the community dynamical consequences of antipredator behavior also critically depend on decisions made by predators.

Seasonal differences in citrate synthase and digestive enzyme activity in larval and postlarval antarctic krill, Euphausia superba

Antarctic krill maintain large population sizes despite dramatic seasonal fluctuations in food availability, but the mechanisms for this are still debated. The aim of this study was to compare seasonal differences in enzyme activity and respiration rates of larval and postlarval krill to provide insights into their overwintering strategies. Respiration rates, activity of the metabolic enzyme citrate synthase (CS), and those of the digestive enzymes laminarinase and total proteinase were measured in austral summer west of the Antarctic Peninsula, and in autumn in the southwestern Lazarev Sea. The 100-fold difference in chlorophyll a concentrations between the two studies is representative of the classic transition from a summer bloom to sparse winter conditions. Correspondingly, adult krill showed reduced respiration rates and CS activity in autumn. However, their digestive enzyme activity was significantly higher, suggesting more efficient assimilation of food at low food levels. Similar-sized larvae showed no summer-autumn differences in respiration rates and enzyme activity, supporting suggestions that they need to feed and grow year-round. However, trends in enzymatic activity varied between the larval stages measured, implying ontogenetic changes in body structure and function.

The effect of dispersal on permanence in a predator-prey population growth model

In this paper, we consider a periodic predator-prey system where the prey can disperse between one patch with a low level of food and without predation and one patch with a higher level of food but with predation. We assume a Volterra within-patch dynamic. Under the assumption that the average of dispersal rate from Patch 1 to Patch 2 is less than that of the intrinsic growth rate of prey in Patch 1, we provide a sufficient and necessary condition to guarantee the prey and predator species to be permanent by using the main techniques in [1].

Effects of food abundance and predictability on body condition and health parameters: experimental tests with the Hooded Crow

It has been shown that small passerines can counteract variability of food resources by actively regulating their body reserves through an increase of mass. However, the effects of food predictability on body mass regulation and other body parameters, such as immune functions, in larger species have been little studied. To analyse the response of the Hooded Crow Corvus corone to food abundance and predictability, we performed three experiments with controlled food provisioning under laboratory conditions. Body mass, TOBEC (total body electrical conductivity) lean mass index, blood parameters and immune organ masses were measured at the beginning and end of a 15‐day period. In the first experiment, the food release was predictable (same amount each day) but the quantity of food delivered to five groups of birds varied (37, 75, 100, 150 or 300 g/day). Low food levels induced a greater decrease in mass accompanied by an increase in erythrocyte sedimentation rate. In the second experiment, the same average quantity of food (100 g) was supplied according to either predictable or unpredictable (random) schedules. In this case, the crows lost more mass, and their erythrocyte sedimentation rate increased when food was unpredictable. In the third experiment, the same average quantity of food (150 g) was supplied according to either a predictable schedule or two schedules with different levels of variability. The group with a low level of variability did not differ from the control, while the group with a highly variable feeding schedule lost more mass. In this group, the higher mass loss was associated with greater variation of the erythrocyte sedimentation rate and a reduced Phytohaemagglutinin index of immunocompetence and haematocrit value. The results of experiments 2 and 3 contrast with findings in other passerines, which increase their mass when food availability is unpredictable. It appears that a body mass decrease in the Hooded Crow can be induced by a reduction of either food abundance (exp. 1) or food predictability (exp. 2, 3), and it is accompanied by a worsening of health state.

LIFE AT THE EDGE: IS FOOD QUALITY REALLY OF MINOR IMPORTANCE AT LOW QUANTITIES?

There is increasing evidence that the quality of nutrient-limited algae is suboptimal for zooplankton production. These effects of nutrient limitation are supposed to be important mainly at higher concentrations of food because at lower quantities the overall energetic limitation of body growth should hide the effect of mineral limitations. This has been hypothesized in a variety of papers, but experimental evidence is still weak. In the present study we carried out a set of growth experiments investigating the effect of food quality at very low food levels ranging from 30 μg C/L up to 150 μg C/L. In all of the experiments, the growth rates of Daphnia magna neonates were lower when grown on P-limited Scenedesmus obliquus. This effect disappeared when phosphorus was added to the P-limited algae prior to feeding, indicating that mineral limitation can occur even at very low levels of food. Neonates born to mothers raised on either high- or low-P Scenedesmus were analyzed for body mass and lipid content as well as mass-specific phosphorus content to check for possible differential investment into neonates in different environments. Although mass-specific phosphorus content was higher in animals born from mothers grown on high-P algae, when fed low quantities of P-limited algae, growth rates of neonates born under low-P conditions were higher than those of animals born under high-P conditions. This can be explained by an increased body lipid content of low-P neonates, even though there were no differences in neonate body mass between treatments. These results illustrate the importance of the incorporation of low food concentrations in ecological stoichiometry models.

Evaluation of biochemical characters of broiler chickens during dietary aflatoxin (50 and 100 ppb) and clinoptilolite exposure

Aflatoxin ( af) and clinoptilolite ( cli, a natural zeolite) were added to broiler food and some biochemical values and enzyme activities were evaluated. The experimental design consisted of six dietary treatments. (1) Control: basal diet; (2) cli: basal diet plus 15 g clinoptilolite kg −1 diet; (3) 50 ppb af: basal diet plus 50 μg total aflatoxin ( af; the composition given below) kg −1 diet; (4) 50 ppb af + cli: basal diet plus 50 μg af plus 15 g cli kg −1 diet; (5) 100 ppb af: basal diet plus 100 μg af kg −1 diet; (6) 100 ppb af + cli: basal diet plus 100 μg af plus 15 g cli kg −1 diet. A commercially available cli was provided from the west region of Turkey and its chemical formula is “KNa 2Ca 2(Si 29AL 7)O·7 2.32H 2O”. For this a total of 576 1-day-old Ross broiler chicks were housed in six treatment groups from days 1 to 42. af treatment significantly increased the serum Na levels and the aspartate-amino-transferase ( asat) and alanine-amino-trasferase ( alat) enzyme activities, while total protein, albumin, total cholesterol uric acid, and K levels were not significantly different between groups. These results suggest that these low af levels in food did not change the serum biochemistry but significantly affected the enzyme activities in broilers.

Variations of body mass and immune function in response to food unpredictability in magpies

The effects of food unpredictability on body mass regulation have been studied in a few species of birds. It has been shown that small passerines can counteract variability of food resources by actively regulating their body reserves through an increase of mass. However, it was recently found that, with unpredictable food availability, quails and hooded crows do not adopt an adaptive fattening strategy but lose mass. At present, little is known about the effects of food unpredictability on other body parameters, such as immune functions and blood variables. In this study, we analysed the responses to food abundance and unpredictability by performing two experiments of controlled food administration in the magpie, a passerine intermediate in size between small passerines and crows. Body mass, leukocyte abundance, haematocrit, spleen mass and immune reaction to phytohaemagglutinin (PHA) were assessed during a 15-d period. In the first experiment, the food release was predictable (same amount each day), but the quantity of food varied in four groups of birds. Low food levels induced a greater decrease in mass. In the second experiment, the same average quantity of food was supplied, but with predictable or unpredictable (random) schedules. As previously reported for crows and quails, magpies did not adaptively increase their mass in response to food unpredictability. Furthermore, there was a decrease of haematocrit and immune response to PHA. It appears that a body mass decrease in the magpie can be induced by a reduction of either food abundance or food predictability, and it is accompanied by a worsening of health state.

Demographic Responses of a Tropical Cladoceran to Cadmium: Effects of Food Supply and Density

Life table response experiments (LTRE) were performed to investigate effects of food supply on density-dependent effects on life history and population responses of the tropical cladoceran Moinodaphnia macleayi to cadmium. Its short life cycle and relatively constant age-specific birth and mortality rates allowed population growth rates (λ) to be accurately estimated from short life table experiments and by using a simplified two-stage demographic model. Decomposition and regression analyses of λ showed that density-dependent effects on population responses to cadmium were modified by food availability through density-dependent effects on mortality and reproductive rate. At moderate food levels (1.8 μg C/mL of Chorella vulgaris) and low densities (<125 animals/L), the effects of cadmium on λ decreased with increasing population density, due to antagonistic effects between the contributions of age at first reproduction and daily reproduction on λ. At high densities (>125 animals/L), the negative effects of cadmium on daily reproduction rates increased with density; hence density promoted the negative effects of cadmium on λ. At low food levels (0.4 μg C/mL of Chorella vulgaris), increasing population density reduced juvenile survival, increasing food per head. Thus adverse effects of cadmium on reproduction, which had the greatest contribution to λ, were buffered by increasing population density. Regression analysis performed on population responses across increasing population density levels and cadmium concentrations showed that at high densities and low food levels ecological compensation will prevent populations at the steady-state equilibrium size from being driven to extinction by toxicity effects at the individual level. Alternatively, at low densities, when food availability is not limiting, exposure to toxic substances can increase extinction probability. These results indicate that risk assessment procedures based on demographic analysis performed at low densities and high food levels may overestimate the ecological risk posed by toxic substances.

DEMOGRAPHIC RESPONSES OF A TROPICAL CLADOCERAN TO CADMIUM: EFFECTS OF FOOD SUPPLY AND DENSITY

Life table response experiments (LTRE) were performed to investigate effects of food supply on density-dependent effects on life history and population responses of the tropical cladoceran Moinodaphnia macleayi to cadmium. Its short life cycle and relatively constant age-specific birth and mortality rates allowed population growth rates (λ) to be accurately estimated from short life table experiments and by using a simplified two-stage demographic model. Decomposition and regression analyses of λ showed that density-dependent effects on population responses to cadmium were modified by food availability through density-dependent effects on mortality and reproductive rate. At moderate food levels (1.8 μg C/mL of Chorella vulgaris) and low densities (<125 animals/L), the effects of cadmium on λ decreased with increasing population density, due to antagonistic effects between the contributions of age at first reproduction and daily reproduction on λ. At high densities (>125 animals/L), the negative effects of cadmium on daily reproduction rates increased with density; hence density promoted the negative effects of cadmium on λ. At low food levels (0.4 μg C/mL of Chorella vulgaris), increasing population density reduced juvenile survival, increasing food per head. Thus adverse effects of cadmium on reproduction, which had the greatest contribution to λ, were buffered by increasing population density. Regression analysis performed on population responses across increasing population density levels and cadmium concentrations showed that at high densities and low food levels ecological compensation will prevent populations at the steady-state equilibrium size from being driven to extinction by toxicity effects at the individual level. Alternatively, at low densities, when food availability is not limiting, exposure to toxic substances can increase extinction probability. These results indicate that risk assessment procedures based on demographic analysis performed at low densities and high food levels may overestimate the ecological risk posed by toxic substances.

Comparative growth and feeding in zebra and quagga mussels (Dreissena polymorphaandDreissena bugensis): implications for North American lakes

In laboratory experiments, quagga mussels (Dreissena bugensis) survived as well as zebra mussels (Dreissena polymorpha) and equaled or exceeded their growth rate (–3 to 242% change in wet mass) when reared at 6 or 23°C and fed natural seston or Chlamydomonas at food levels ranging from 0.05–7.4 µg·L–1chlorophyll a (chl a). Superior growth of quagga mussels was most pronounced at low food levels. We found no significant differences in per capita clearance rates (CR), functional responses, or feeding behavior between zebra and quagga mussels fed Chlamydomonas, Nannochloris, or mixed suspensions of Nannochloris and clay. Per capita CR ranged from 0.018 to 0.402 L·mussel–1·h–1for zebra mussels and from 0.010 to 0.407 L·mussel–1·h–1for quagga mussels. Because quagga mussels had more biomass per unit shell length, we found lower biomass-specific CR for quagga mussels. When fed natural seston, zebra and quagga mussels could selectively reject inorganic material and at the lowest seston level the assimilation efficiency of quagga mussels (81%) was significantly higher than that of zebra mussels (63%). Our experiments suggest that quagga mussels can survive, grow, and feed as well or better than zebra mussels in epilimnetic waters with either low or high productivity.

Detection and Isolation of Listeria monocytogenes from Food Samples: Implications of Sublethal Injury

Detection of L. monocytogenes is often limited by the performance of the enrichment media used to support bacterial growth to detectable levels. Because Listeria may exist at extremely low levels in foods, sample enrichment protocols must amplify these low initial populations to detectable limits. Listeria may also exist in an injured state in food products as a result of processing treatments such as heating, freezing, exposure to acids, or exposure to sanitizing compounds. Selective agents in enrichment media normally used for recovery of Listeria may inhibit repair and detection of sublethally injured Listeria, which may go on to repair, grow, and regain pathogenicity. Simple modifications to existing regulatory protocols, such as those that use more than one enrichment broth, raise sensitivity of detection to 90%. This review shows the efficacy of repair/enrichment strategies, which increase sensitivity of detection to 97.5-98.8% compared with 65-70% by standard regulatory protocols. Ribotype analysis of isolates obtained from meat samples reveals a complex microbial ecology, with striking differences in both number and distribution of distinct genetic types of Listeria, depending upon whether samples are enriched in selective or repair/enrichment media. In studies on enrichment of dairy environmental samples in University of Vermont medium and Listeria repair broth (UVM and LRB), combining these 2 primary enrichment media into a single tube of Fraser broth for dual secondary enrichment yielded a significantly higher percentage (p < 0.05) of Listeria-positive samples than did use of either LRB or UVM alone. Refinement of conventional Listeria recovery methods should consider the importance of the enrichment step, the nutritional needs of specific genetic types, and the physiological condition of Listeria isolates in foods.

Comparative Life Table Demography and Population Growth of Alona rectangula and Macrothrix triserialis (Cladocera: Crustacea) in Relation to Algal (Chlorella vulgaris) Food Density

ABSTRACT We conducted survivorship and population growth studies with two littoral-benthic cladocerans, Alona rectangula and Macrothrix triserialis, littoral-benthic cladocerans, under low and high food levels (0.5×106 and 2.0×106 Chlorella vulgaris cells ml−1). The net reproductive rate ranged from 13 to 15 offspring/female in the case of A. rectangula and between 19 and 27 for M. triserialis. The generation time was also lower (10 d) for A. rectangula but higher (16–18 d) for M. triserialis. Thus, the growth rates of M. triserialis were around 0.25 d−1 while those of A. rectangula around 0.32. The rates of population increase from the population growth experiments were lower than those from the life table demography studies.

Size and age at first copulation and spawning of the apple snail, Pomacea canaliculata(Gastropoda: Ampullariidae).

The size and age at first copulation and spawning of the apple snail, Pomacea canaliculata, were determined at three food levels under laboratory conditions (14L:10D; 25°C). Males fed at a low food level started copulation at smaller sizes than males at higher food levels. However, age at first copulation was similar among snails at all food levels. The food level appeared to affect the penis sheath length at maturity, although the difference did not reach a statistically significant level. Females fed at the low food level delayed both first copulation and spawning longer than those at higher food levels. Consequently, their sizes at first copulation and spawning were smaller. The albumen gland of poorly fed snails was smaller at first copulation but comparable at maturity. The adaptive significance of these patterns in sexual maturity and their possible implications for the integrated management of this snail are discussed.

Ecological interactions of the microparasite Caullerya mesnili and its host Daphnia galeata

It has been repeatedly suggested that parasites play an important role in the ecology and evolution of Daphnia populations; however, little is known about Daphnia—parasite interactions in lakes with vertebrate predation on Daphnia. Between September 1997 and April 1998, an epidemic of the protist parasite Caullerya mesnili in Daphnia galeata and Daphnia hyalina occurred in Lake Constance (Germany), infecting up to 50% of all individuals. Using laboratory experiments, we investigated the epidemiological interactions between this parasite and its host D. galeata at the individual and the population level. C. mesnili was found to be transmitted directly and horizontally through waterborne infection stages. Transmission of the parasite was dependent on the host density, and all life stages of female D. galeata were susceptible to infection. In a life table experiment at low and high food levels, the life expectancy and fecundity of infected D. galeata were dramatically reduced at both food levels as compared to the uninfected controls. Additionally, we found a significant interaction between infection and food level, indicating a stronger parasitic effect in well‐fed hosts. To test the effects of the parasite at the population level, we compared the size of D. galeata populations infected with C. mesnili with the size of parasite‐free microcosm populations. The population size of infected D. galeata was significantly lower than that of uninfected populations after 4 weeks. In all four infected replicate populations, the parasite drove the host population to extinction.

Combined effects of copper and microalgal (Tetraselmis suecica) concentrations on the population growth of Brachionus plicatilis müller (Rotifera)

Copper compounds are commonly used to kill algal blooms in fish and shrimp ponds. However, indiscriminate use of copper in the aquaculture ponds may have a negative impact on the production of rotifers, which, in turn, can influence the final yield. In this work we studied the combined effects of chronic toxicity of copper (0, 0.125, 0.25, 0.5 and 1.0 mg L-1 of copper sulphate) and algal (Tetraselmis suecica) levels (0.05 × 106 and 0.10 × 106 cells ml-1) on the population growth of the saline water rotifer B. plicatilis at 25 °C, pH 8.5 and salinity 36‰. The population of B. plicatilis increased with increasing levels of T. suecica. Regardless of the food level, rotifers exposed to 1 mg L-1 of copper did not survive beyond 3 days. Copper of 0.125 mg L-1 caused a reduction in the population of B. plicatilis but only at low algal food level; at the high (0.2 × 106) food concentration, the growth was comparable to that observed in the control suggesting that higher algal levels had an ameliorating effect on the toxicity of copper to B. plicatilis. The relation between the rotifer density and the egg-ratio was inverse and statistically significant in controls. However, this relation was non-significant in rotifers subjected to the copper concentration. The peak population density of B. plicatilis in controls varied from 36 to 251 ind. ml-1 depending on the food level. The rate of population increase per day (r) in the controls varied from 0.24–0.63 depending on the food level. In general, an increase in food level resulted in higher peak population abundance and growth rates. The presence of copper resulted in the reduction of both r values and peak population densities of B. plicatilis. The results have been discussed in light of protective role of algae in reducing the toxicity of heavy metals to rotifers.

Combined effects of food level and inoculation density on competition between Brachionus patulus (Rotifera) and the cladocerans Ceriodaphnia dubia and Moina macrocopa

Competition among cladocerans and rotifers is of considerable interest not only due to their close similarity in life history strategies, but also due to the considerable overlap they exhibit in their feeding habits. In tropical waterbodies, several genera of cladocerans, including Ceriodaphnia and Moina occur, simultaneously with rotifers. We tested over a period of 3 weeks the combined effects of food (0.5×106 and 1.5×106 cells ml−1 of Chlorella) level and rotifer density on the competition between B. patulus and C. dubia and M. macrocopa using population growth experiments. For each cladoceran species we used 30 test jars of 50 ml capacity. The initial density of cladocerans was 0.2 ind ml−1, while for B. patulus it was either 1 ind ml−1 or 5 ind ml−1. Neither the maximal population density nor the rate of population increase (r) of C. dubia was significantly affected by B. patulus. However, for M. macrocopa, both these variables were negatively affected by the rotifers. The combined effects of low food level and high initial density of B. patulus resulted in a 50% reduction in the peak population density of M. macrocopa. The population growth of B. patulus was negatively influenced by the presence of C. dubia and M. macrocopa. The results of the competition experiments conducted in the present study between cladocerans and rotifers suggest the existence of a more complex and delicate interaction than is generally thought.