Zooplankton of Fishless Ponds of Northern Patagonia: Insights into Predation Effects of Mesostoma ehrenbergii

The turbellarian predator Mesostoma ehrenbergii, a common inhabitant of fishless ponds of northern Patagonia, can consume prey larger than 1 mm. Because the feeding strategy of M. ehrenbergii includes mucus trapping and external digestion, this predator may exploit a broad range of prey sizes. We hypothesize that M. ehrenbergii could exert a strong effect on zooplankton composition and body size spectra in Patagonian fishless ponds. We investigated this hypothesis by analyzing the crustacean zooplankton composition and size spectra in five fishless ponds of northern Patagonia, and we carried out experiments to assess predation rates of M. ehrenbergii on potential prey species from 0.8 mm to 6 mm. These ponds were colonized by macrophytes, which favored habitat heterogeneity, especially in the smaller ponds that had higher species richness. The surveyed ponds showed distinctive crustacean zooplankton assemblages and sizes, but all were dominated by calanoid copepods of the genus Boeckella. Our results indicated that M. ehrenbergii consumed the whole size range of offered prey, from ∼0.8 mm (Ceriodaphnia dubia) to ∼6 mm (Parabroteas sarsi). Predation rates were higher for intermediate‐bodied copepods (∼1.5 mm) and C. dubia (0.8 mm), but we did not find conclusive evidence that variations in size spectra of crustacean zooplankton are a result of M. ehrenbergii predation. We suggest that an interaction between prey body size and its evasion tactic might be important to determine the true effect Mesostoma on zooplankters. The structural complexity created by macrophytes in Patagonian fishless ponds may also help account for the lack of a strong predation effect of Mesostoma in the field survey. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Chemical signals and habitat selection by three zooplankters in Andean Patagonian ponds

1. Zooplankton may react differently to chemical signals produced by macrophytes in shallow systems. They may be attracted by macrophytes, as the plants may be used as a refuge against predators, or the plants may have a repellent effect (e.g. when the plants are a habitat for numerous invertebrate predators or fish). In fishless Patagonian ponds, the structural complexity provided by macrophytes modulates the rate of predation on zooplankton by the invertebrate predator Mesostoma ehrenbergii (Turbellaria). 2. We performed a field study to analyse the coexistence of M. ehrenbergii and three of its prey (two copepods, the calanoid Boeckella gracilis and the cyclopoid Acanthocyclops robustus, and the cladoceran Ceriodaphnia dubia) in four ponds. In two of the ponds, we carried out day and night sampling to evaluate the influence of macrophytes on the distribution of these zooplankters. 3. In laboratory experiments, we analysed the response of the zooplankters to the chemical signals produced by macrophytes (the emergent Juncus pallescens and the submerged Myriophyllum quitense), the predator M. ehrenbergii and the ‘alarm signal’ provided by a homogenate of conspecifics. 4. Our field studies demonstrated the coexistence of M. ehrenbergii and the selected prey in different seasons and that A. robustus and C. dubia choose the vegetated area (a mixed bed of J. pallescens and M. quitense) over the non‐vegetated area. The habitat choice experiments indicated that the presence of M. ehrenbergii may directly affect the habitat selection of B. gracilis, because this zooplankter swam away from the predator. In addition, Mesostoma may indirectly affect the habitat selection of the cyclopoid copepod A. robustus and the cladoceran C. dubia as both zooplankters exhibited a negative response to the alarm signal produced by crushed conspecifics. 5. The presence of the submerged M. quitense did not affect the horizontal movements of any of the zooplankters studied. In contrast, the emergent macrophyte J. pallescens elicited a positive response of B. gracilis, suggesting that this aquatic plant may act as a predation refuge. 6. Our results suggest that predator avoidance behaviour can occur in fishless environments in response to a tactile invertebrate predator like Mesostoma. In addition, the refuge effect of emergent macrophytes, enhancing the survival of pelagic zooplankters, may act as a key factor in stabilizing predator–prey interactions in fishless Patagonian ponds, as has been widely recorded in northern temperate lakes with fish.     

The influence of flatworm predation on zooplankton inhabiting small ponds

Experiments were performed in 1977 to determine which large zooplankton in a series of high altitude ponds can be consumed by the predatory flatworm Mesostoma ehrenbergii. This predator consumes Daphnia at a high rate and the fairy shrimp Branchinecta at a low rate, but does not consume Diaptomus. Experiments were performed in 1978 and 1979 to determined the rate of predation on Daphnia in 30 liter tubs and to determine if predation rate is correlated with surface to volume ratio of experimental containers. There is a clear correlation between surface to volume ratio and predation rate. Determinations of Mesostoma and Daphnia densities were made in a series of eight high altitude ponds, and pond surface to volume ratios were determined. Examination of these parameters lends credence to the argument that Mesostoma predation affects Daphnia dynamics in some circumstances. The results suggest that benthic invertebrate predators may affect zooplankton dynamics, especially in shallow ponds.     

The effects of group size on per capita ingestion in flatworms

1. We evaluated the effect of group size on the per capita ingestion rates of three species of flatworm, two of which actively group with conspecifics (Dugesia tigrina, D, dorotocephala) and employ mucus to capture prey, and a third species (Mesostoma ehrenbergii) that does not actively group but does use mucus to capture prey. 2. As flatworm group size increased, daily per capita ingestion first increased and then decreased for D. tigrina and D. dorotocephala. In the case of D. tigrina this pattern was observed even at low predator densities. Ingestion rates of M. ehrenbergii were largely unaffected by group size. 3. Results suggest that the observed changes in per capita ingestion rates with changes in group size previously reported for D. tigrina are related to their tendency for active grouping and are not directly a consequence of prey capture technique or experimental design. 4. We argue that freshwater triclads in general, and D. tigrina in particular, represent an ideal model system for the development and testing of group foraging theory.     

The colonial cyanobacterium Trichodesmium as a physical and nutritional substrate for the harpacticoid copepod Macrosetella gracilis

The pelagic harpacticoid copepod Macrosetella gracilis usesthe colonial cyanobacterium Trichodesmium not only as a physicalsubstrate for juvenile development, but also as a food source.By associating itself with a buoyant colonial cyanobacterium,M.gracilis has developed a successful mode of life for existencein the plankton. Further evidence of M.gracills' dependenceon Trichodesmium as a physical substrate is demonstrated bypreviously undescribed microscopic observations of a gravidM.gracilis female attaching eggs to a Trichodesmium colony.Shipboard experiments investigating the ingestion and assimilationof Trichodesmium carbon (C) were conducted in September 1991and January/February 1992 in waters of the Bahamas and the Caribbean,respectively. Macrosetella gracilis not only ingested, but rapidlyincorporated, cyanobacterial organic matter into its own cellularmaterial. Utilization of ingested Trichodesmium by M.graciliswas investigated by assessing the metabolic partitioning andincorporation of ¹⁴C-labelled Trichodesmium into copepod lipids,proteins, polysaccharides and low-molecular-weight (LMW) compoundsusing sequential biochemical fractionation techniques. Despitevariations in grazing rates between the two sites and times(September 1991,0.017 µg C^* µg^–1 C h^–1;January 1992, 0.134 µg C ^* µg^–1 C h^–1,the partitioning of incorporated C into the different biochemicalfractions was relatively consistent. There was rapid assimilationof ingested C into the LMW ({small tilde}60%) and polysaccharidefractions ({small tilde}30%) in the first few hours, with asubsequent increase in the percent C incorporated into protein.On average, {small tilde}21% of the Trichodesmium C ingestedby M.gracilis was assimilated. Therefore, M.gracilis is an importantsecondary link in the food web of oligotrophic waters whereTrichodesmium is abundant.     

Predation potential of three flatworm species (Platyhelminthes: Turbellaria) on mosquitoes (Diptera: Culicidae)

We conducted a field survey for flatworms to select species as potential biological control agents against Aedes aegypti and Culex pipiens (Diptera, Culicidae) breeding in artificial containers. Laboratory experiments were performed to determine the daily predation rate, differential predation on each mosquito larval instar, selective predation on either A. aegypti or C. pipiens, and predator tolerance to water from artificial containers. Girardia anceps (Tricladida, Paludicola, Dugesiidae), Mesostoma ehrenbergii and Bothromesostoma cf. evelinae (Rhabdocoela, Typhloplanoida, Typhloplanidae) were found in temporary puddles and permanent pools. In the laboratory, they killed between 52% and 100% of immature mosquitoes coexisting in the same habitat. No preference of flatworms for mosquito preys was detected. Predation rate was related to predator size and instar of preys. Girardia anceps and B. evelinae survived after a dry period and when re-flooding occurred, they laid eggs. Tolerance to water from artificial containers was highest in G. anceps and this species could be a suitable predator to reduce mosquito populations from artificial containers using an inoculative approach.     

六斑月瓢虫对菊小长管蚜的捕食作用

六斑月瓢虫对菊小长管蚜的捕食功能反应符合HollingⅡ型方程。功能反应受到温度、容器大小和捕食者密度的影响。在同一温度下,六斑月瓢虫的捕食量随着猎物密度的增加而增大,寻找效应随着猎物密度的增加而降低。在15℃～25℃范围内,随着温度的升高,捕食的菊小长管蚜高龄若蚜头数增多,而在25℃～35℃有相反的趋势,以25℃下的捕食数量最大,平均达95头/天,捕食上限达392.1头。相同猎物密度条件下,温度与六斑月瓢虫捕食作用的关系可用二次曲线拟合,捕食的最适温度（25℃左右）与菊小长管蚜发生高峰季节的温度相吻合。六斑月瓢虫的捕食作用有较强的种内干扰反应,随着捕食者密度的增大,平均捕食量逐渐减少,捕食作用率也相应地降低,搜索常数Q为0.9003,干扰系数m为0.9816,E=0.9003P^-0.9816。实际应用时,要充分考虑气象因子、瓢蚜密度比等对防效的影响,以期获得最佳防治效果。     

Laboratory studies of predation by the ctenophore Mnemiopsis leidyi on the early stages in the life history of the bay anchovy, Anchoa mitchilli

The lobate ctenophore, Mnemiopsis leidyi, consumed eggs andlarvae of the bay anchovy, Anchoa mitchilli, in laboratory experiments.This ctenophore exhibited a type I functional response to increasesin egg densities without reaching saturation at high prey densities.Clearance rate increased with increasing experimental containervolume. There was a 3-fold increase in the volume of water clearedby 2.0–2.5-cm ctenophores and a >5-fold increase for4.5–5.0-cm ctenophores in 15–1 versus 100–200–1containers. Clearance rate was dependent on tenophore lengthbut was probably underestimated for the larger animals due tocontainer effects. The presence of various densities of alternateprey, Acartia hudsonica or Anemia sp. nauplii, in addition toAnchoa mitchilli eggs did not affect the clearance rates onthe eggs alone. Comparison of clearance rates of 2.0–2.5-cmctenophores on various ages of starved and fed bay anchovy larvaeindicated that predation may be higher on yolk-sac larvae thanon eggs but decreases as the larvae grow. After 3 days posthatch starved anchovy larvae become more vulnerable to predationthan fed larvae. The ctenophore, M.leidyi has the potentialto inflict substantial predation pressure on early stages inthe life history of bay anchovy. ¹ Present address: University of Maryland, Center for Environmentaland Estuarine Studies, Chesapeake Biological Laboratory, POBox 38, Solomons, MD 20688-0038, USA     

An example of niche partitioning in three coexisting freshwater cyclopoid copepods

Vertical distributions at noon and midnight of copepodites,males and females of three coexisting species of freshwatercyclopoid copepods, Cyclops vicinus (a cold-water form), Thermocyclopscrassus and Mesocyclops leuckarti (warm-water forms), were studiedin Dreiangel Lake, a small eutrophic gravel pit. The largestspecies, C. vicinus, inhabits deep water layers, both at noonand midnight, while the smaller species, T.crassus and M.leuckarti,inhabit upper water layers. In all three species, copepoditesand males live closer to the water surface than females. Insummer, abundance of the smallest species (T.crassus) is highest,although finite birth rates are lowest. The distribution patternsand population characteristics suggest that the vertical distributionof cyclopoid copepods in Oreiangel Lake is mainly governed byfish predation; different temperature tolerances and preferencesof the three species may also be of significance.     

Effects of structural heterogeneity provided by the floating macrophyte <Emphasis Type="Italic">Eichhornia azurea</Emphasis> on the predation efficiency and habitat use of the small Neotropical fish <Emphasis Type="Italic">Moenkhausia sanctaefilomenae</Emphasis>

Macrophytes have a fundamental structuring role in aquatic environments. Several authors have suggested that trophic interactions are particularly mediated by aquatic plants. In the current article, we evaluated the effects of the structural heterogeneity provided by Eichhornia azurea (Kunth) roots on predation and habitat use by the small fish Moenkhausia sanctaefilomenae (Steindachner). We tested the hypotheses that (i) high structural heterogeneity protects macroinvertebrates against predation by M. sanctaefilomenae; (ii) distinct prey types are differently protected by the refuge provided by roots; and (iii) the behavior of M. sanctaefilomenae is affected by the structural heterogeneity provided by macrophyte roots. To test these hypotheses, we performed an experiment in 20 l aquaria in which macroinvertebrates (Cypricercus sp. and Chironomus sp.) were exposed to M. sanctaefilomenae predation for 4 h under three structural heterogeneities, represented by different root densities. High structural heterogeneity protected macroinvertebrates against predation. Additionally, E. azurea roots similarly protected different prey species. The macrophyte spatial structure substantially changed the habitat use of M. sanctaefilomenae. In general, our results corroborated the hypothesis that the structural heterogeneity provided by E. azurea roots significantly affects predation and habitat use by M. sanctaefilomenae. Handling editor: S. Declerck     

Livestock exclusion increases the spatial heterogeneity of vegetation in Colorado shortgrass steppe

Abstract. Spatial heterogeneity, an important characteristic in semi‐arid grassland vegetation, may be altered through grazing by large herbivores. We used Moran's I, a measure of autocorrelation, to test the effect of livestock grazing on the fine scale spatial heterogeneity of dominant plant species in the shortgrass steppe of northeastern Colorado. Autocorrelation in ungrazed plots was significantly higher than in grazed plots for the cover of the dominant species Bouteloua gracilis, litter cover and density of other bunchgrasses. No species had higher autocorrelation in grazed compared to ungrazed sites. B. gracilis cover was significantly auto‐correlated in seven of eight 60‐yr ungrazed exclosures, four of six 8‐yr exclosures, and only three of eight grazed sites. Autocorrelograms showed that B. gracilis cover in ungrazed sites was frequently and positively spatially correlated at lag distances less than 5 m. B. gracilis cover was rarely autocorrelated at any sampled lag distance in grazed sites. The greater spatial heterogeneity in ungrazed sites appeared linked to patches characterized by uniformly low cover of B. gracilis and high cover of C₃ grasses. This interpretation was supported by simple simulations that modified data from grazed sites by reducing the cover of B. gracilis in patches of ca. 8 m diameter and produced patterns quite similar to those observed in ungrazed sites. In the one exclosure where we intensively sampled soil texture, autocorrelation coefficients for sand content and B. gracilis cover were similar at lag distances up to 12 m. We suggest that the negative effect of sand content on B. gracilis generates spatial heterogeneity, but only in the absence of grazing. An additional source of heterogeneity in ungrazed sites may be the negative interaction between livestock exclusion and B. gracilis recovery following patchy disturbance.     

Predation by a carnivorous marine copepod, Euchaeta elongata Esterly, on eggs and larvae of the Pacific hake, Merluccius productus

The predatory behavior of the carnivorous marine copepod, adultfemale Euchaeta elongata Esterly, feeding on eggs and larvaeof the Pacific hake, Merluccius productus, was examined in thelaboratory and in a natural setting. E. elongata did not feedon eggs. Predation on larvae is believed to depend on larvalswimming behavior: (1) predation rates were low on early stageyolk-sac larvae which are inactive swimmers and are relativelyundetected by the predator; (2) rates were high on middle stageyolk-sac larvae which are more active swimmers yet have a poorlydeveloped escape response; and (3) rates were low on largerlarvae which are able to escape the predator effectively. Starvedhake larvae were more vulnerable to predation due to a poorescape ability although they were less active and not easilydetected. The presence of naturally occurring alternative prey,Pseudocalanus sp., depressed the rate of E. elongata predationon hake larvae. In an analysis of field data, hake larvae andE. elongata were found to occupy the same depths in Dabob Bay.A high percentage of E. elongata collected had apparently beenfeeding on hake larvae, as indicated by the presence of pigmentsin their guts. Survival of hake larvae in late spring appearsto be relatively poor compared with early spring; poorer survivalin late spring may be due partly to an increase in the abundanceof invertebrate predators, such as E. elongata. ^*Current address: Northwest and Alaska Fisheries Center, 2725Montlake Blvd, Seattle, WA 98112, USA.     

Zooplankton community structure and copepod egg production in coastal waters of the central Great Barrier Reef lagoon

We describe zooplankton community structure and copepod eggproduction in the vicinity of the coastal boundary zone of theGreat Barrier Reef lagoon, Australia. The abundance and eggproduction rate of constituents of the zooplankton assemblagecharacteristic of the coastal zone rapidly increase subsequentto events such as flooding and upwelling. Our sampling spannedtwo summer monsoonal seasons, the first of which (1990-91) wasvery wet. The second monsoonal season (1991-92) was very dryand was characterized by intrusive upwelling events from theCoral Sea. Chlorophyll a concentrations did not rise in thewet year, probably because of light limitation, but did riseas a result of upwelling. Terrestrial run-off in the wet yearhad a greater apparent effect on zooplankton abundance patternsthan did upwelling in the dry year, except where coastal trappingallowed sufficient time for increases in zooplankton abundanceto occur. Egg production rates by the copepods Acrocalanus gibberand Acrocalanus gracilis showed haphazard spatial differences.Nitrogen-specific egg production ranged between 0.03 and 0.21day^–1 for A.gibber, and between 0.13 and 0.41 day^–1for A.gracilis. The egg production rate by A.gibber was foodlimited for most of the year and showed a poor correlation withtemperature. ³Present address: Department of Biological Sciences, FloridaTech, 150 W University Boulevard, Melbourne, FL 32901, USA     

Effects of large Saduria entomon (Isopoda) on spatial distribution of their small S. entomon and Monoporeia affinis (Amphipoda) prey

We performed laboratory experiments to investigate the effects of predator avoidance and numerical effects of predation on spatial distribution of small Saduria entomon (Isopoda) and Monoporeia affinis (Amphipoda), with large S. entomon as predators. The horizontal distribution and mortality of the prey species, separately and together, were studied in aquaria with a spatial horizontal refuge. We also estimated effects of refuge on mortality of small S. entomon and M. affinis by experiments without the refuge net. In addition, we investigated whether predation risk from large S. entomon influenced the swimming activity of M. affinis, to clarify the mechanisms behind the spatial distribution. Both small S. entomon and M. affinis avoided large S. entomon. The avoidance behaviour of M. fffinis contributed about 10 times more to the high proportion in the refuge than numerical effects of predation. Due to the low mortality of small S. entomon the avoidance behaviour of this species was even more important for the spatial distribution. The combined effect of avoidance behaviour and predation in both species was aggregation, producting a positive correlation between the species in density. M. affinis showed two types of avoidance behaviour. In the activity experiments they reduced activity by 36% and buried themselves in the sediment. In the refuge experiments we also observed avoidance behaviour with the emigration rate from the predator compartment being twice the immigration rate. The refuge did not lower predation mortality in M. affinis, probably due to the small scale of the experimental units in relation to the mobility of the species. Predation mortality in small S. entomon was higher in absence of a refuge and especially high in absence of M. affinis.     

Clutch detachment in a copepod after capture by a predator

When the egg-carrying copepod, Eudiaptomus gracilis, is capturedby larvae of predatory Chaoborus species (C.obscuripes or C.flavicans)in laboratory experiments, the external egg clutch detachesin 16-29% of caw, thereby allowing the eggs to escape from ingestion.Clutch detachment may be due to active removal by the femalebefore she is eaten or the result of accidental loss by thepredator during handling. Detached clutches were significantlylarger than thase that were ingested along with the female.Clutch detachment is advantageous to the female since ingestedeggs are lost, whereas detached eggs hatch normally and maypotentially propagate the genotype. Chaoborus predation maytherefore influence the evolution of clutch size and the degreeof iteroparity in Eudiaptomus.     

Habitat heterogeneity and mammalian predator–prey interactions

• 1 In predator–prey theory, habitat heterogeneity can affect the relationship between kill rates and prey or predator density through its effect on the predator's ability to search for, encounter, kill and consume its prey. Many studies of predator–prey interactions include the effect of spatial heterogeneity, but these are mostly based on species with restricted mobility or conducted in experimental settings.
• 2 Here, we aim to identify the patterns through which spatial heterogeneity affects predator–prey dynamics and to review the literature on the effect of spatial heterogeneity on predator–prey interactions in terrestrial mammalian systems, i.e. in freely moving species with high mobility, in non‐experimental settings. We also review current methodologies that allow the study of the predation process within a spatial context.
• 3 When the functional response includes the effect of spatial heterogeneity, it usually takes the form of predator‐dependent or ratio‐dependent models and has wide applicability.
• 4 The analysis of the predation process through its different stages may further contribute towards identifying the spatial scale of interest and the specific spatial mechanism affecting predator–prey interactions.
• 5 Analyzing the predation process based on the functional response theory, but separating the stages of predation and applying a multiscale approach, is likely to increase our insight into how spatial heterogeneity affects predator–prey dynamics. This may increase our ability to forecast the consequences of landscape transformations on predator–prey dynamics.

     

Copepod and cladoceran success in an oligotrophic lake

Cladocerans have proved to be much less successful members ofthe Lake Tahoe limnetic community than the calanoid copepods.An analysis of temporal and spatial heterogeneity of the cladoceranBosmina longirostris and the herbivorous calanoid copepod Diaptomustyrrelli revealed important differences between the two typesof zooplankton. Although the productivity of both species islimited to varying degree by the abundance of certain algalspecies and the availability of particulate nitrogen, they differstrongly in the mechanisms responsible for mortality. Correlationanalysis suggests that B. longirosris is severely limited temporallyand spatially by predation from the omnivorous calanoid copepodEpischura nevadensis. In contrast, D. tyrrelli is not noticeablylimited by predators. Instead, abiotic seasonal events and starvationcontrol its population dynamics. Mortality and natality differencesbetween Bosmina and Diaptomus in Lake Tahoe help to explainthe general lack of success of cladocerans in oligotrophic lakes.     

Clutch size and body size at first reproduction in Daphnia pulicaria at different levels of food and predation

Field data from seven alpine lakes in Serra da Estrela. Portugal.show that reproduction in Daphnia may be as efficiently controlledby fish predation and copepod predation on eggs in brood cavitiesas it is by food limitation. Body length and clutch size estimatesin Daphnia pulicaria revealed high inter- and intra-populationvariability in maturation size (body size at first reproduction).and in number of eggs per clutch. Daphnia at first maturationin lakes stocked with rainbow trout were half the size of thosefound in fishless lakes (body length of 0.86–0.95 and1.55–1.81 mm. respectively). The mean number of eggs perclutch was reduced to a similar degree by food limitation, predationby fish and copepod predation on eggs in brood cavities, butthe underlying mechanisms of this reduction were different.Food limitation caused smaller clutch sizes in all individuals,so variation remained the same. Fish predation caused the selectiveremoval of individuals with maximum clutches, so variation decreased.Copepod predation caused removal of eggs from brood cavitiesof randomly infested females, so that variation increased, particularlyat a high food level when non-infested females carried largeclutches of eggs.     

The effect of UV-B radiation and humic substances on growth and motility of the flagellate, Euglena gracilis

The effects of ultraviolet-B radiation (UV-B, 250–315nm) were determined on Euglena gracilis with respect to speed,phototactic orientation, specific growth rate, and in the presenceof humic substances. Humic substances had a protective effectwhen studying the speed and specific growth rate. However, thedegree of phototactic orientation decreased in UV-B radiationboth with and without humic substances. The inhibition of O₂evolution and speed was most pronounced when using cutoff filtersWG280 and WG295. The photosynthetic inhibitor DCMU (10^–6M) did not have any effect on the speed, but the 0₂ evolutiondecreased to zero. The effect of different wavelengths in theUV-B region on the speed of E.gracilis showed the maximum sensitivityat 280 and 290 nm.     

Competition, predation and the relative abundances of two species of Daphnia

We investigated the factors controlling the relative abundancesof two Daphnia species, D.pulex and D.laevis, in a small Wisconsinpond. D.pulex was the dominant Daphnia species in fall 1977and summer-fall 1978; D.laevis was the only Daphnia speciespresent in summer 1979. The abundance of D.laevis was positivelycorrelated with the abundance of the notonectid, Buenoa confusa.In predation trials, notonectides exhibited a distinct preferencefor D.pulex over similarly-sized D.laevis, but Chaoborus larvaefed at similar rates on both Daphnia species. Behavioral observationsrevealed that Buenoa adults were much less efficient at capturingD.laevis than D.pulex. Quantitative results of these predationtrials were combined with estimates of predator and prey densityand distribution to evaluate the effect of predation on thedaphnid populations. The effect of predation varied throughtime and microhabitat, and only infrequently could predationaccount for total prey mortality. D.laevis was most abundantat times and in places where Buenoa predation was most intense.Competition experiments illustrated the competitive superiorityof D.pulex over D.laevis. D.pulex was able to competitivelyexclude D.laevis in long term experiments, and D.pulex's fecunditywas higher than that of D.laevis in shorter experiments. Inlong-term experiments, Chaoborus larvae at natural densitieswere able to keep both Daphnia species at low, constant levelsand neither species clearly dominated when Chaoborus was present.The relative abundances of D.pulex and D.laevis were controlledby a complex of biotic and abiotic factors. Pond depth and predatordensity determined the intensity of predation on daphnid populations.When notonectid predation was intense, D.laevis dominated; whenthe intensity of predation by notonectids was low, D.pulex dominateddue to its superior competitive abilities. At different timesselective predation or high resource levels promoted the co-existenceof these two species. ¹Current address of both authors: Department of Biological Sciences,University of California, Santa Barbara, CA 93106, USA