Influence of predation by fish and water turbidity on a Daphnia gessneri population in an Amazonian floodplain lake,Brazil

The population behavior of Daphnia gessneri Herbst, 1967 in a floodplain lake (Lago Grande) of the lower Rio Solimões was investigated between April 1979 and March 1980 with regard to 1) predation by the fish called tambaqui (Colossoma macropomum, Characidae), 2) water level fluctuation and 3) water transparency. Zooplankton density samples were collected at two sites near mid-lake, where water depth and Secchi disc transparency were measured. In addition, qualitative samples of zooplankton and fish collections were taken at several sites in the adjacent floodplain areas. The author concludes that fluctuations in Daphnia gessneri populations correlate most with intense predation by fish and water turbidity.     

Effects of hydroperiod and predation on a Mississippi River floodplain invertebrate community

 The objective of this study was to determine if pond permanence and vertebrate predation (by fish and waterfowl) affect invertebrate community structure in the mudflat habitat of floodplain ponds. Invertebrate communities were studied for 1 year in four Mississippi River floodplain ponds with different hydroperiods. Pond 1 experienced five dry periods, pond 2 experienced four, pond 3 dried once, and standing water remained in pond 4 for the entire year. Vertebrate predator exclusion treatments (all access, no access, small-fish access and cage controls) were placed in all ponds. As pond duration increased, predatory invertebrate richness and abundance increased while overall invertebrate richness and abundance decreased. With the exception of the cladoceran Diaphanosoma, all commonly encountered taxa were strongly affected by pond permanence in terms of abundance, biomass and, generally, individual biomass. Taxa were nearly early divided between those that were more abundant in less permanent ponds and those that were more abundant in longer-duration ponds. Invertebrate taxa richness, abundance, and total biomass were lower in the all-access treatment than in the treatments that restricted predator access, and these effects were stronger in the more permanent ponds. In general, there were no significant differences in responses to the treatments with small-fish access and no access. These results support models that predict relatively weak effects of predation in frequently disturbed habitats. Received: 30 May 1995 / Accepted: 21 June 1996     

The effect of fish predation on Cyclops life cycle

Two populations of Cyclops abyssorum tatricus studied in neighbouring alpine lakes in Tatra Mountains (southern Poland) differed in their life cycles. In the lake with planktivorous fishes, Cyclops was typically monocyclic, with highly synchronized reproduction during two winter months, while in the fishless lake its reproduction was asynchronous and continued for six months. Direct and indirect effects of fish predation on Cyclops life cycles are discussed.     

Dietary–morphological relationships in a fish assemblage of the Bolivian Amazonian floodplain

Morphological correlates of diet were examined in 48 species of freshwater fishes from floodplain lakes in the central part of the Mamoré River (Bolivian Amazon). The species were classified, according to the percentage occurrence of seven food items, into eight broad trophic categories: mud feeders, algivores, herbivores, terrestrial invertivores and omnivores, carnivores, zooplanktivores, aquatic invertivores and piscivores. There were significant relationships between the diet and morphology of the fishes even when the effect of taxonomical relatedness between species was eliminated. Relative gut length was the main morphological variable used to order species on a carnivore to mud feeder gradient. Standard length and head and mouth size were the morphological variables most closely associated with prey size. Mud feeder, algivore and piscivore species appeared as the most dietary and morphologically specialized. These results support both the hypotheses that species morphology influences the diet and that morphological similarity is conserved even in comparison with taxonomically unrelated species.     

Effects of an invasive anuran [the cane toad (Bufo marinus)] on the invertebrate fauna of a tropical Australian floodplain

The ways in which invasive organisms influence native ecosystems remain poorly understood. For example, feral cane toads Bufo marinus have spread extensively through tropical Australia over the last 70 years, but assessments of their ecological impact remain largely anecdotal. We conducted experimental trials to examine the effect of cane toad presence on invertebrate fauna in relatively small (2.4 × 1.2 m) outdoor enclosures on a floodplain near Darwin in the wet–dry tropics. Toads significantly reduced invertebrate abundance and species richness, but only to about the same degree as did an equivalent biomass of native anurans. Thus, if toads simply replaced native anurans, the offtake of invertebrates might not be substantially different from that due to native anurans before toad invasion. However, our field surveys suggest that toads cause a massive (fourfold) increase in total amphibian biomass. The end result is that cane toads act as a massive nutrient sink in the floodplain ecosystem because they consume vast numbers of invertebrates but (unlike native frogs) are largely invulnerable to predation by frog-eating predators.     

Adaptations of fish species to oxygen depletion in a central Amazonian floodplain lake

Pronounced seasonal and daily oxygen concentration changes are characteristic for Amazonian floodplain lakes. Studies on the fish fauna of the Lago Camaleão, Solimões River, Amazonas, Brazil, showed several fish species which are able to survive prolonged periods of heavy hypoxia. Twenty species belonging to eight families were observed in the laboratory in order to determine their respiratory adaptations to hypoxic conditions and oxygen concentrations at which the fish present respiratory adaptations. Finally, the fish species were distributed throughout the habitats of Lake Camaleão according to their adaptation responses. Ten fish species used the surface water for aquatic surface respiration, four species used atmospheric oxygen for aerial respiration, four species used oxygen supplied by the exudation of the roots of floating macrophytes and two exhibited a high tolerance to hypoxic conditions, and well-developed physiological biochemical mechanisms. The fish fauna is well adapted to low oxygen concentrations. The large variety of morpho-anatomical adaptations associated with biochemical and physiological mechanisms to tolerate hypoxic and anoxic conditions enable the 20 fish species to exploit several habitats of Lago Camaleão, such as floating aquatic macrophyte meadows, open water and near the shoreline.     

The influence of planktivorous fish on zooplankton communities in riverine slackwaters

1. Contrary to that for lakes and ponds, our knowledge of the influence of planktivorous fish on zooplankton communities in rivers is slight, largely because of the general assumption that such communities are overwhelmingly regulated by physical conditions. 2. In two separate but concurrent in situ enclosure experiments, we investigated the effects of carp gudgeon (Hypseleotris spp.) and Eastern Gambusia (Gambusia holbrooki) on zooplankton communities in slackwaters of a temperate Australian floodplain river. 3. A high biomass of Hypseleotris suppressed the density of daphniid microcrustaceans, but enhanced the total density of rotifers. A high biomass of Gambusia, on the other hand, suppressed the total density of both microcrustaceans and rotifers. 4. A high biomass of planktivorous fish also reduced the density of many of the ovigerous (egg‐carrying) zooplanktonic taxa. Indeed, ovigerous cyclopoid copepods were suppressed in the presence of a high biomass of Hypseleotris, even though there was no significant effect on overall (ovigerous plus non‐ovigerous) density. 5. Our results imply that a high biomass of planktivorous fish can potentially influence zooplankton communities in riverine slackwaters, as in many lakes and ponds.     

Benthivorous fish reduce stream invertebrate drift in a large-scale field experiment

Drift as a low-energy cost means of migration may enable stream invertebrates to leave risky habitats or to escape after encountering a predator. While the control of the diurnal patterns of invertebrate drift activity by fish predators has received considerable interest, it remains unclear whether benthivorous fish reduce or increase drift activity. We performed a large-scale field experiment in a second-order stream to test if invertebrate drift was controlled by two benthivorous fish species (gudgeon Gobio gobio and stone loach Barbatula barbatula). An almost fishless reference reach was compared with a reach stocked with gudgeon and loach, and density and structure of the invertebrate communities in the benthos and in the drift were quantified in both reaches. The presence of gudgeon and stone loach reduced the nocturnal drift of larvae of the mayfly Baetis rhodani significantly, in contrast to the findings of most previous studies that fish predators induced higher night-time drift. Both drift density and relative drift activity of B. rhodani were lower at the fish reach during the study period that spanned 3 years. Total invertebrate drift was not reduced, by contrast, possibly due to differences in vulnerability to predation or mobility between the common invertebrate taxa. For instance, Chironomidae only showed a slight reduction in drift activity at the fish reach, and Oligochaeta showed no reduction at all. Although benthic community composition was similar at both reaches, drift composition differed significantly between reaches, implying that these differences were caused by behavioural changes of the invertebrates rather than by preferential fish consumption. The direction and intensity of changes in the drift activity of stream invertebrates in response to the presence of benthivorous fish may depend on the extent to which invertebrate taxa can control their drifting behaviour (i.e. active versus passive drift). We conclude that invertebrate drift is not always a mechanism of active escape from fish predators in natural streams, especially when benthos-feeding fish are present.     

The effects of predation risk from crab spiders on bee foraging behavior

Recent studies have suggested that top–down effects ofpredation on plant–pollinator interactions may not be,as previously thought, rare and/or weak. In this paper, we explorethe effects of crab spiders (Araneae: Thomisidae) on the behaviorof 2 species of bee (Hymenoptera: Apidae) foraging for nectarand pollen on 3 different plant species in central Portugal.In 2 experiments, we found that the eusocial bee Apis melliferawas significantly less likely to inspect and accept a floweror inflorescence if it harbored a spider. In contrast, we foundno such effects of spiders on the behavior of the solitary beeEucera notata. Further experiments showed that the effects ofenvironmental cues associated with predators on flower visitationby A. mellifera were detectable even when no spider was presentat the moment a flower was encountered. Such indirect effectswere only identified, however, in bees foraging on 1 of 2 plantspecies studied. In a final experiment, A. mellifera was shownto respond negatively to the presence of the corpses of conspecificsglued to flowers. This suggests that prey corpses left exposedon petals or bracts by spiders provide an obvious cue that beescan use to avoid predators. These results add to a growing bodyof evidence that plant–pollinator interactions are notimmune to the effects of predation and suggest that the strengthof such effects vary both between and within species.     

Individual and combined effects of fish predation and bed disturbance on stream benthic communities: a streamside channel experiment

1. The composition and spatiotemporal dynamics of biological communities are influenced by biotic processes, such as predation and competition, but also by physical disturbances, such as floods in running waters. However, the interplay of disturbance with predation is still poorly understood, especially in frequently disturbed streams. Further, different predator species can affect prey communities in different ways depending on their feeding mode and efficiency. 2. We investigated the individual and combined effects of flood‐induced bed disturbance and fish predation on the benthos for 4 weeks in 18 streamside channels fed by a flood‐prone New Zealand river. Bed movements caused by floods were simulated by tumbling the substratum in half the channels. Six channels each were stocked with introduced brown trout (Salmo trutta) or native upland bully (Gobiomorphus breviceps) or had fish excluded. We studied algal biomass and both invertebrate density and daytime activity on surface stones on several dates after the disturbance, invertebrate community composition in the substrata of the entire channels on day 28 and leaf decomposition rates over the 28‐day period. 3. Disturbance affected algal biomass and density, richness and activity of surface stone invertebrates, and overall density and richness of channel invertebrates. Presence or absence of fish, by contrast, did not influence overall invertebrate standing stocks when subsurface substrata were included but did affect invertebrate densities on surface stones in 45% of all analysed cases and invertebrate activity on surface stones in all cases. Leaf decomposition rates were not influenced at all by the experimental manipulations. 4. Native upland bullies featured more often than exotic brown trout in causing invertebrate density changes and equally often in causing changes to grazer behaviour. Overall, our results imply that fish predation can have strong effects on the benthic invertebrate community in frequently disturbed streams, especially via behavioural changes.     

Habitat structure and fish predation: effects on invertebrate colonisation and breakdown of stream leaf packs

1. We investigated the effects of two features of leaf‐pack habitat structure (i.e. mass of a leaf pack and surface area of leaves comprising a leaf pack) and fish predation on colonisation of shredders and leaf breakdown rates in a coldwater stream. Packs were constructed of red maple (Acer rubrum) leaves. 2. A 2 × 3 × 3 factorial experiment was used to manipulate fish predation (exclusion and control cage), leaf‐pack mass (1, 3 and 5 g dry mass) and leaf surface area (small: approx. 17.9 cm², medium: approx. 34.6 cm², large: approx. 65.6 cm²). Exclusion cages had mesh on all sides, whereas control cages lacked mesh on two sides to provide access to fish. 3. Common shredders were Gammarus pseudolimnaeus, Pycnopsyche and Lepidostoma. Shredder biomass per leaf pack increased with the mass of a leaf pack (P < 0.001), but biomass per unit mass of leaf pack did not differ with leaf‐pack mass (P = 0.506). Shredder densities did not respond to the exclusion of fish (P > 0.7) or leaf surface area (P > 0.7), and interactions among treatment factors were not significant (P > 0.2). 4. Breakdown rates were lower for leaf packs comprised of small leaves (P < 0.001) and leaf packs with high mass (P = 0.001). Excluding fish did not significantly affect leaf breakdown rates (P = 0.293), and interactions among treatment factors were not significant (P > 0.3). Breakdown rates were highest when packs consisted of few leaves (i.e. leaf packs with large leaves and low mass) and were colonised by many shredders. 5. Fish predation was not an important factor controlling shredder densities in leaf packs over the spatiotemporal scale of our experiment. Nevertheless, we found shredder colonisation was proportional to leaf‐pack mass and breakdown rates were affected by leaf‐pack size (i.e. number of leaves in a pack). We suspect that fragmentation is the primary mechanism causing the breakdown rates to be dependent on leaf‐pack size.     

The influence of visibility and escape ability on sex-specific susceptibility to fish predation in Eudiaptomus gracilis (Copepoda,Crustacea)

The susceptibility to fish predation in males, ovigerous, and non-ovigerous females of the freshwater copepod Eudiaptomus gracilis is investigated in the laboratory by direct observation of the predation sequence of zebrafish, and by studying mortality when confronted with zebrafish or roach. Ovigerous females had the highest encounter rate indicating that the highly visible egg-clutch is a major determinant of their susceptibility. Males were least successful in escaping, the main reason being their inability or disinclination to react fast enough when attacked. This difference in escape reaction may have evolved because of sex-specific requirements in mate encounter and mate location. Ovigerous females and males had higher mortality than non-ovigerous females in all experiments while ovigerous mortality was higher than male mortality only in the zebrafish experiments. Neither experiment showed any difference in male and overall female mortality but as a consequence of the abrupt change in susceptibility between the ovigerous and non-ovigerous condition, it follows that sex-specific mortality rates may be dependent on the reproductive condition within diaptomid populations. A consequence of the sex-specific difference in escape ability is that the sex-specific mortality may be influenced by variation in the attack efficiency within and among predator populations.     

Responses to fish predation and nutrients by plankton at different levels of taxonomic resolution

1. To improve mechanistic understanding of plankton responses to eutrophication, a mesocosm experiment was performed in the shallow littoral zone of a south Swedish lake, in which nutrient and fish gradients were crossed in a fully factorial design. 2. Food chain theory accurately predicted total biomass development of both phyto‐ and zooplankton. However, separating zooplankton and algae into finer taxonomic groups revealed a variety of responses to both nutrient and fish gradients. 3. That both nutrients and fish are important for phytoplankton dynamics was seen more clearly when viewing each algal group separately, than drawing conclusions only from broad system variables such as chlorophyll a concentration or total phytoplankton biovolume. 4. In some taxa, physiological constraints (e.g. sensitivity to high pH and low concentrations of free CO₂) and differences in competitive ability may be more important for the biomass development than fish predation, grazing by herbivorous zooplankton, and nutrient availability. 5. We conclude that food chain theory accurately predicted responses in system variables, such as total zooplankton or algal biomass, which are shaped by the dynamics of certain strong interactors (‘keystone species’), such as large cladocerans, cyanobacteria and edible algae (<50 μm), whereas responses at finer taxonomic levels cannot be predicted from current theory.     

The influence of fish predation on the abundance cycles of an algal turf invertebrate fauna

Summary A fauna of small invertebrates in a perennial algal turf habitat showed marked seasonal fluctuations in abundance. These were characterised by a summer (December) peak in the abundance of gammarid amphipods and an autumn (April) peak in polychaete densities. Invertebrate abundance was lowest during the period February–March which coincided with an influx of very high densities of juveniles (0⁺ year class) of the sparid fish Chrysophrys auratus. Both juvenile C. auratus and also the mullid Upeneichthys porosus achieved their highest densities over coralline turf areas and fed on the associated invertebrates. Gammarid amphipods constituted the main food items of each species. All these observations suggested a key role of fish predators in determining the observed seasonal patterns of invertebrate abundance.This hypothesis was investigated by the use of replicated fish exclusion shields and cages which excluded both species from the algal turf and also permitted an assessment of Upeneichthys porosus feeding in the absence of Chrysophrys auratus. The experiment ran from November until June and covered the period of invertebrate and fish abundance changes. Little evidence of a fish predation effect on either the densities of or the timing of abundance peaks of the invertebrate fauna was detected. Sediment analysis of the cage and control experimental sites revealed no evidence of a cage effect on the microhabitat. It is suggested that seasonal changes in the abundance of invertebrates occurs independently of the high densities of predatory fish recorded in this habitat.     

Relative significance of direct and indirect effects of predation by planktivorous fish on zooplankton

One of the most obvious features of tropical lakes and reservoirs is the small body size of their zooplankton taxa. It is believed that this is the result of high and persistent predation by abundant planktivorous fish, which select large-bodied zooplankton prey thus making them more vulnerable to extinction in tropical as compared to temperate habitats. Do these extinctions result directly from fish predation? Could the high predation-induced mortality alone be responsible for an extermination of the population from a habitat? Or could indirect effects of predation be responsible? Some important indirect effects can be seen at the demographic level; these include reduced reproduction in the population resulting from higher vulnerability of ovigerous females to predation by visually oriented planktivores. Other important indirect effects can be observed at the individual level; these include shifts in behavior (from foraging to predator avoidance) and adjustments in physiology (from high to low feeding rate) in those planktonic animals which detect danger from their predators by sensing either the ‘predator odor’ or an ‘alarm substance’ originating from injured conspecific prey. Although a zooplankton species density may mostly result from the brutal force of direct predator impact on the population (mortality), it is more likely that its distribution in time and space could be attributed to a combination of indirect effects of predation on individual behavior and physiology. An example of periodicity in density and depth distribution patterns of Cahora Bassa zooplankton species and their periodic exterminations seems to confirm the role of indirect effects of predation by planktivorous fish.     

The influence of flood regime on fish catch and fish communities of the Cross River floodplain ecosystem,Nigeria

Synopsis The morphology, climate and flood regime of the Cross River floodplain are summarized and related to the catch, species distribution and abundance of the economically important fishes of the floodplain ecosystem. Two flood indices, FI-1 and FI-2, are used to assess, statistically. the relationship between fish catch and flood regime. There are good positive correlations between the catch (C_T) of year T and the flood index 1 of 1 year T-1 and between (C_T) and the sum of the flood indices 1 and 2 of year T-1 (r = 0.6516 and 0.6894, respectively, both at P = 0.05). Cross River is a relatively highly productive system (in terms of fish catch) and this productivity is attributable mainly to the yearly flooding of the plain when fish find allochthonous food materials in the expanded habitat. Changes in hydrometeorological conditions, such as the drought that affected the northern part of the Cross River basin in 1983–1984. if they persist can effectively reduce fish catch from the system.     

Voles on small islands: effects of food limitation and alien predation

Ecosystems of three trophic levels may be bottom-up (by food-plant availability) and/or top-down (by predators) limited. Top-down control might be of greater consequence when the predation impact comes from an alien predator. We conducted a replicated two-factor experiment with field voles (Microtus agrestis) during 2004-2005 on small islands of the outer archipelago of the Baltic Sea, south-west Finland, manipulating both predation impact by introduced American mink (Mustela vison) and winter food supply. In autumn 2004, we live-trapped voles on five islands from which mink had been consistently removed, and on four islands where mink were present, and provided half of these islands with 1.8 kg oats per vole. Body mass of female voles increased as a response to supplementary food, whereas both food supplementation and mink removal increased the body mass of male voles in subsequent spring. During winter, there was a positive effect of supplementary food, but in the subsequent summer, possible positive long-term impacts of food supplementation on field voles were not detected. Mink removal appeared not to affect density estimates of field voles during the winter and summer immediately after food addition. Trapping data from 2004 to 2005 and 2007 suggested, however, that in two out of three summers densities of voles were significantly higher in the absence than in the presence of mink. We conclude that vole populations on small islands in the archipelago of the Baltic Sea are mainly bottom-up limited during winter (outside the growing season of food plants), when food availability is low, and limited by mink predation during summer which slows population growth during the reproductive season of voles.     

The St. Lawrence River in winter: population structure,biomass, and pattern of its primary and secondary food web components

Plankton and benthic invertebrate populations were examined along a 160 km section of the St. Lawrence River between Lake Ontario and Waddington, New York during a period of ice cover. Algal biomass was low in winter with diatoms and cryptomonads the most prominent forms. Zooplankton biomass was about one-tenth the algal standing crop and cyclopoid copepods predominated. Phytoplankton and zooplankton biomass declined downriver providing evidence that most plankton was imported from Lake Ontario. Benthic invertebrate standing crop was greatest near Lake Ontario where molluscs were abundant. The downriver macroinvertebrate community shifted to coarse particle feeders, namely annelids and chironomids. Beds of accumulated detrital material derived primarily from dieback of macrophytes were particularly rich in benthic invertebrates.     

Responses of the fish community to the flood pulse and siltation in a floodplain lake of the Trombetas River,Brazil

The distributions of the fish species were examined in relation to environmental variables, to evaluate the effects of environmental degradation on the fish community of Batata Lake, a typical Amazonian clearwater lake. From 1979 to 1989, tailings composed of water and clay, extracted from bauxite by water jets, were discharged into Batata Lake. The tailings spread into about 30% of the lake’s area, where the level of the lake bottom rose and turbidity increased. In the present study, multivariate analyses were performed on data for environmental parameters and fish density and biomass. Fish were collected with gillnets during the annual hydrological cycle (filling, flood, drawdown and dry periods), in the silted area, the partly silted area (intermediate) and the natural area. Values of the Shannon index, density and biomass were compared among areas and periods to evaluate the effects of the tailings on community structure. Sediment resuspension, which reduces transparency, is accentuated in shallow water, and was the main factor regulating differences in the community structure between the natural and silted areas. The decrease in transparency occurs mainly during the filling period in the silted area and during the low-water period in the silted and intermediate areas, when sediment resuspension increases concentrations of nutrients and chlorophyll-a. The strong influence of migratory and piscivore species in low-transparency waters is likely associated with the greater bacterioplankton productivity and turnover rate observed by other authors in the silted area, increasing the importance of the heterotrophic food chain in Batata Lake. Reduction of transparency in the silted area was a selective factor for fish species. The death of part of the flooded forest vegetation was decisive in lowering densities of the igapó-associated species in the silted area. The unconsolidated substrate, the death of part of the igapó forest and the negative effects of low transparency – inhibiting resident visually oriented species in the affected areas – are the main factors causing the low diversity in the silted area as a whole. The correlations between CPUEs, conductivity and nutrients and chlorophyll-a concentrations do not appear to reflect cause–effect relationships, indicating that these environmental parameters are poor predictors of fish density in Batata Lake.