The Effects of Zebra Mussels (Dreissena polymorpha) on the Foraging Success of Eurasian Perch (Perca fluviatilis) and Ruffe (Gymnocephalus cernuus)

Complex habitat structures can influence the foraging success of fish. Competition for food between fish species can therefore depend on the competitors' abilities to cope with structural complexity. In laboratory experiments, we comparatively assessed effects of zebra mussels (Dreissena polymorpha Pall .) on the foraging success of Eurasian perch (Perca fluviatilis L.) and ruffe (Gymnocephalus cernuus (L.)). In single‐species and mixed‐species experiments, the fish were fed caddisfly larvae (Tinodes waeneri (L.)) over complex (mussel‐covered stones) and less‐complex (bare stones) substrates. With intraspecific competition, food consumption by perch and ruffe decreased significantly when the complex substrate was used. With interspecific competition, food consumption by perch and ruffe did not change with substrate complexity, but perch clearly out‐competed ruffe on both substrates. Zebra mussel beds provide a refuge for macrozoobenthos against predation by ruffe and probably also by perch. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

The foraging behavior of the central mudminnow and yellow perch: the influence of foraging site,intraspecific and interspecific competition

Summary The central mudminnow (Umbra limi) and yellow perch (Perca flavescens) are two species of fishes that commonly co-occur in small bog lakes in the Great Lakes region of North America. Both species are dietary generalists with a high degree of dietary overlap, and perch populations have been shown to have a strong negative effect on mudminnow populations. I compared the foraging rates of the two species in a series of laboratory experiments in which four foraging sites were simulated (open bottom substrate, water column, submerged aquatic macrophytes, and water surface). Fish were observed as they foraged singly, in monospecific pairs, and in mixed species pairs to evaluate the effects of intra- and interspecific competition on foraging success and the potential for resource partitioning based on foraging site. Single species trials showed that each species had similar foraging rates at each of the four sites. Across all sites combined, interspecific competition had a greater negative effect on the foraging success of mudminnows than did intraspecific competition. Conversely, intraspecific competition had a greater negative effect on the foraging success of perch than did interspecific competition. In mixed species trials perch took more food items from each of the four foraging sites than did mudminnows and caused shifts in site-use patterns of mudminnows. Differences in the foraging success of the two species reflected the superior ability of perch to discover the presence of food and to search rapidly for additional items, but were unrelated to handling times. These differences were linked to the species' foraging modes. The competitive superiority of perch, demonstrated in these experiments, could contribute to the negative impact of perch on mudminnow populations under natural conditions.     

Some like it deep: Intraspecific niche segregation in ruffe (Gymnocephalus cernua)

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Coexistence of Fish Species in a Large Lowland River: Food Niche Partitioning between Small-Sized Percids,Cyprinids and Sticklebacks in Submersed Macrophytes

In the spring and summer of each year, large patches of submersed aquatic macrophytes overgrow the bottom of the alluvial Warta River downstream of a large dam reservoir owing to water management practices. Environmental variables, macroinvertebrates (zoobenthos and epiphytic fauna, zooplankton) and fish abundance and biomass were assessed at this biologically productive habitat to learn intraseasonal dynamics of food types, and their occurrence in the gut contents of small-sized roach, dace, perch, ruffe and three-spined stickleback. Gut fullness coefficient, niche breadth and niche overlap indicated how the fishes coexist in the macrophytes. Chironomidae dominated in the diet of the percids. However, ruffe consumed mostly benthic chironomids, while perch epiphytic chironomids and zooplankton. The diet of dace resembled that in fast flowing water although this rheophilic species occurred at unusual density there. The generalist roach displayed the lowest gut fullness coefficient values and widest niche breadth; consequently, intraspecific rather than interspecific competition decided the fate of roach. Three-spined stickleback differed from the other fishes by consuming epiphytic simuliids and fish eggs. The diet overlap between fishes reaching higher gut fullness coefficient values was rather low when the food associated with the submersed aquatic macrophytes was most abundant; this is congruent with the niche overlap hypothesis that maximal tolerable niche overlap can be higher in less intensely competitive conditions.     

Diet niche relationships among predator and prey fish species in their early life stages in Lake Võrtsjärv (Estonia)

In Lake Võrtsjärv pikeperch was observed not to shift to piscivory in their first autumn of life, although juvenile stages of a variety of fish species were abundant in the lake. It was hypothesized that the diets of predator and prey fish fry overlap and that coarse fish species are important food competitors for juvenile piscivores and thus, pikeperch and perch fry do not shift to piscivory during their first growing season. To discover the possible linkages in this pattern, in 2009 the feeding relationships of pikeperch, perch, ruffe and roach fry were analysed. The stomach content analyses showed that in the summer period, Mesocyclops leuckarti was the most frequent prey for perch and ruffe, pikeperch consumed Leptodora kindti in large quantities, and roach ate mostly plant material. Towards autumn, M. leuckarti was the most abundant prey for all percids. However, average stomach content weight and the number of prey items eaten by ruffe were considerably higher than for other fish fry. Since the feeding opportunities of fish fry are considered poor in the examined lake, the prey has the potential to restrict the recruitment to piscivory of their predators, as prey fish seem to have better abilities to persist in this ecosystem. Furthermore, supposed competition in the juvenile stage may result in a reduced top‐down effect on coarse fish.     

Diel variations in the assemblage structure and foraging ecology of larval and 0+ year juvenile fishes in a man‐made floodplain waterbody

This study investigated diel variations in zooplankton composition and abundance, and the species composition, density, size structure, feeding activity, diet composition and prey selection of larval and 0+ year juvenile fishes in the littoral of a man‐made floodplain waterbody over five 24 h periods within a 57 day period. There was a significant difference in the species composition of diurnal and nocturnal catches, with most species consistently peaking in abundance either during daylight or at night, reflecting their main activity period. There were no consistent diel patterns in assemblage structure or the abundance of some species, however, most likely, respectively, due to the phenology of fish hatching and ontogenetic shifts in diel behaviour or habitat use. There were few clear diel patterns in the diet composition or prey selection of larval and 0+ year juvenile roach Rutilus rutilus and perch Perca fluviatilis, with most taxa consistently selected or avoided irrespective of the time of day or night, and no obvious shift between planktonic and benthic food sources, but dietary overlap suggested that interspecific interactions were probably strongest at night. It is essential that sampling programmes account for the diel ecology of the target species, as diurnal surveys alone could produce inaccurate assessments of resource use. The relative lack of consistent diel patterns in this study suggests that multiple 24 h surveys are required in late spring and early summer to provide accurate assessments of 0+ year fish assemblage structure and foraging ecology.     

Competitive Interactions between Hydrilla (Hydrilla verticillata) and Vallisneria (Vallisneria americana) as Influenced by Insect Herbivory

Two experiments (winter and summer) were conducted in outdoor tanks using addition-series methods to evaluate the impact of specialized feeding by two biological control agents,Hydrellia pakistanaeDeonier andBagous hydrillaeO'Brien, on competitive interactions between hydrilla [Hydrilla verticillata(L.f.) Royle] and vallisneria (Vallisneria americanaMichx). Competitive abilities of each plant species were determined using the reciprocal-yield model of mean plant weight. In the absence of the biocontrol agents, intraspecific competition from hydrilla on itself was 8.3 times stronger than interspecific competition from vallisneria.Hydrellia pakistanaeinterfered with hydrilla canopy formation by removing as much as 80% of the plant biomass in the top 30 cm of the water column. Damage byH. pakistanaealso caused a 43% reduction in hydrilla tuber production during the winter experiment. Similarly,B. hydrillaecaused up to a 48% reduction in hydrilla plant weight in the summer experiment. Neither insect species damaged vallisneria. As a result, there were significant shifts in the competitive balance between hydrilla and vallisneria due to selective insect feedings. In the presence ofH. pakistanae, hydrilla intraspecific competition was nearly equal to interspecific competition from vallisneria, indicating that hydrilla had lost its competitive edge over vallisneria.Bagous hydrillaealso produced similar, but smaller, shifts in the relative competitive abilities of hydrilla and vallisneria. These results indicate that biological control agents can disrupt the competitive balance between plant species in favor of native species, thus adding another element to the weed biological control strategies.     

Do migrants influence the foraging behaviour of the insectivorous Cyprus Wheatear,Oenanthe cypriaca,at a stopover site? (Aves: Passeriformes)

During their staging at stopover sites, migrants may compete with resident species over food resources (“resource competition hypothesis”), or migrants feed on surplus that is not utilised by the residents. Here, I studied foraging behaviour of the Cyprus Wheatear, Oenanthe cypriaca, a ubiquitous and common species with a broad niche range, on Cyprus, which is an important stopover site during intercontinental spring migration. During adverse weather conditions, perch height and aerial sallying were lower and perch/pounce activity was higher. Average perch height and sallying increased over the season, while perch-pounce foraging decreased. There was no influence of clock time. Bivariate correlations suggested an influence of migrants on perch height but not on other variables. Using a series of regression analyses with sex, time of day, date, weather conditions and number of migrants as independent variables revealed no influence of the number of migrants on the foraging behaviour of the Cyprus Wheatear. This suggests that the abundance of migrants does not lead to a change in the behaviour of the Cyprus Wheatear.     

Overwinter changes in mass and lipid content of Perca fluviatilis and Gymnocephalus cernuus

The body condition, lipid reserves and mortality of 0 and 1 year‐old perch Perca fluviatilis and ruffe Gymnocephalus cernuus , sampled during the winter in Lake Constance, Germany, were compared. Length‐frequency analyses did not provide evidence for overwinter mortality in either species. The fresh and dry mass of perch as well as their lipid contents decreased during winter, while ruffe were heavier and contained more lipid at the end of the winter. The superior performance of ruffe was mainly attributed to its sensory capabilities, which allowed it to ingest zoobenthos throughout the winter, while the zooplankton feeding of perch was constrained by low light levels. In lakes that undergo a process of re‐oligotrophication, this advantage of ruffe over perch may be even more pronounced, since lower food supply during the growth season and thus lower fish lipid content at the start of winter is probably better tolerated by ruffe than by perch.     

Variation in the relative magnitude of intraspecific and interspecific competitive effects in novel versus familiar environments in two<Emphasis Type="Italic">Drosophila</Emphasis> species

Models of competitor coevolution, especially the genetic feedback hypothesis, suggest that a negative correlation between intraspecific and interspecific competitive effects may be important in sustaining competitor coexistence, and can give rise to oscillatory dynamics with repeated reversals of competitive superiority. I reanalyzed previously published census data from an experiment in which populationsof Drosophila melanogaster andD. simulans underwent competitive coevolution in one familiar and two novel environments, to specifically look for any evidence of a negative relationship between intraspecific and interspecific competitive effects on population growth rates, and for any indication of short period cycling in the relative magnitude of intraspecific and interspecific competitive effects. While there was considerable variation in the relative magnitude of intraspecific and interspecific competitive effects over generations, among both populations and environments, there was no clear evidence supporting the genetic feedback hypothesis. Intraspecific and interspecific competitive effects on population growth rates were strongly positively correlated in novel environments, and uncorrelated in the familiar environment. Data from the familiar environment indicated that indices of competition of populations of the initially superior competitor,D. melanogaster, might be showing some cyclic behaviour, but I argue that this is likely to be transient, and not suggestive of sustained oscillatory dynamics predicted by the genetic feedback model. I discuss the results in the context of the importance of the genetic architecture of intraspecific and interspecific competitive abilities in determining the coevolutionary trajectory of competitive interactions.     

Influence of predation risk on individual competitive ability and growth in Eurasian perch, Perca fluviatilis

We studied the potential influence of predation risk on the competitive ability and habitat use of foraging perch and the effect of these interactions on growth. Groups of four similar-sized young-of-the-year perch were in visual contact with a piscivorous perch during feeding. The fry had the choice of vegetation and open habitat, with food presented in the open habitat. Competitive ability, defined as proportion of prey attacks, varied between perch individuals and was unaffected by predation risk. The variation in proportion of prey attacks was affected by relative size within each replicate group, despite small size differences (±1 mm), with the largest individual being a better competitor than the smallest ones. The degree of boldness, measured as the proportion of time spent in the open habitat, was significantly related to both competitive ability and prey attack order. Observations of aggressive behaviour indicated a possible occurrence of interference competition, which may contribute to the appearance of different competitive abilities between individuals within a group of perch. A significant correlation was found between competitive ability and growth. Growth variation within groups was not affected by predation risk.     

Temperature-dependent differences in foraging ability of two percids,Perca fluviatilis and Gymnocephalus cernuus

Synopsis The effects of temperature on capture rate, handling time, capture probability and routine swimming performance were studied in two fish species, perch Perca fluviatilis and ruffe Gymnocephalus cernuus. In addition, the reaction distance (RD) of both species was measured. Handling time decreased and capture rate increased with increasing temperature for both perch and ruffe. The response of handling time and capture rate to temperature was more pronounced for perch than for ruffe. Routine swimming performance increased with increasing temperature for perch but not significantly so for ruffe. The laboratory study showed that the foraging ability of ruffe was less sensitive to temperature than it was for perch. These results were consistent with the vertical distributions of both species in thermally stratified lakes; ruffe, which occurred at all depths, utilized a wider temperature range than perch.     

The ecology of ruffe,Gymnocephalus cernuus (Pisces: Percidae) introduced to Mildevatn,western Norway

Synopsis Diet, habitat use, diel and seasonal activity and a number of population parameters were studied on ruffe,Gymnocephalus cernuus, introduced to Mildevatn, western Norway. This lake is sited outside the natural range of the ruffe and has a lower fish diversity and a different fish species composition than within its native range. From June through September the ruffe was planktivorous and mainly caught at 4 to 6 m depth in the benthic zone. At other times of year ruffe was feeding on zoobenthos and caught deeper in the benthic zone. Ruffe was mainly day active. Zooplankton feeding during summer is the clearest difference compared to ruffe populations living within its natural range. Presence of large zooplankton organisms available for ruffe is suggested as the main reason for the difference found in food choice. The availability of large zooplankton is probably due to community structure caused by a predator and lack of interspecific competition for zooplankton in the deeper parts of the lake. Piscivorous brown trout.Salmo trutta, restrict the habitat of threespined stickleback,Gasterosteus aculeatus, to the zone of littoral vegetation, allowing high densities of larger zooplankton species likeBythotrephes longimanus to be present in the lake. Brown trout is present only in the upper light and well oxygenated parts of the lake, leaving a refuge for the ruffe, where they can feed on the rich zooplankton community.     

Intraspecific interference influences the use of prey hotspots

Resource clumping (prey hotspots) and intraspecific competition may interact to influence predator foraging behaviour. Optimal foraging theory suggests that predators should concentrate most of their foraging activity on prey hotspots, but this prediction has received limited empirical support. On the other hand, if prey concentration at hotspots is high enough to allow its use by several individuals, increased competition may impose constraints on foraging decisions of conspecifics, resulting in a temporal segregation in the use of shared resources. We investigated how artificial prey hotspots influence foraging behaviour in the Iberian lynx Lynx pardinus, and examined variations in the use of prey hotspots by individuals with different competitive abilities. All lynx, irrespective of their position in the competitive hierarchy, focused their activity on prey hotspots. Supplemented lynx concentrated higher proportions of active time on 100 m circular areas around hotspots than non‐supplemented lynx did on any area of the same size. However, we found evidence for a dynamic temporal segregation, where inferior competitors may actively avoid the use of prey hotspots when dominants were present. Dynamic temporal segregation may operate in solitary and territorial predators as a mechanism to facilitate coexistence of conspecifics in the absence of opportunities for spatial segregation when they use extremely clumped resources.     

Temperature modulates the effects of predation and competition on mosquito larvae

1. Predation risk affects interspecific competition by decreasing foraging activity and relative competitive ability. Predation risk is determined by predators' prey choice and prey responses, both of which can be influenced by temperature. Temperature is especially important for larval prey and can result in a trade‐off between predator‐induced decreases in foraging activity and growth. Interspecific competition must also be examined in relation to intraspecific density‐dependent competition; weaker interspecific competition leads to coexistence of competitors. 2. This study explored how temperature (15 and 25 °C) could affect a focal species, larvae of the mosquito Culex quinquefasciatus, by examining prey choice in a shared predator (mosquitofish; Gambusia holbrooki) and the effects of predation risk on interspecific competition with Limnodynastes peronii tadpoles. Intraspecific density‐dependent competition in C. quinquefasciatus at these temperatures was also examined. 3. At 25 °C, G. holbrooki consumption of both C. quinquefasciatus and L. peronii increased; however, the effects of interspecific competition on mosquito survival did not decrease with L. peronii exposure to predation risk. The relationship between intraspecific density‐dependent competition and interspecific competition was temperature‐dependent, with competitive dominance of L. peronii at 25 °C. Male and female mosquitoes had different temperature‐dependent responses, indicating sex‐specific intrinsic responses to starvation and differential selection pressures. At 25 °C, females were susceptible to interspecific competition by L. peronii, while males were susceptible to intraspecific competition. 4. The use of competitors as biological controls has implications for mosquito disease transmission, and these results suggest that control effectiveness may be modified by climate change.     

Feeding of fish in Lake Glubokoe

Lake Glubokoe is inhabited by 13 species of fish. The commonest are roach, bream, perch and ruff. Comparison of seine catches for 1950 and 1984 revealed a considerable decrease in the share of perch (from 31% to 2%) and ruff (from 18.5 to less than 0.1%) in 1984. The richest food resource in the lake is crustacean plankton. In summer, it is mostly consumed by roach. Estimation of the electivity shows that, unlike perch, roach selectively consumes small crustaceans (Bosmina and Polyphemus). Poor growth rate of all common species of fish points to their food limitation. In summer the diets of common species of fish are differentiated to a considerable extent. Of 36 pairs of combinations a high food overlap was found only in 2 cases: 1) O + perch and Leucaspius delineatus, 2) yearlings and adult roach. Low diet overlap in fish during intensive feeding in summer probably reflects a feedback between the volume and intensity of food competition. The populations of bream and ruff probably compete for chironomid larvae which inhabit the sublittoral, as perch, roach and Leucaspius delineatus probably do for large littoral insects. However, intraspecific competition appears to be more important than interspecific for the roach and bream populations (the two most numerous species in the lake).     

Competition in Umbra-Perca fish assemblages: experimental and field evidence

Summary One potentially important effect of interspecific competition in freshwater fish communities is to increase predation intensity from gape-limited piscivores by lowering growth rates of prey species. We investigated the operation and consequences of competition between central mudminnows (Umbra limi) and yearling yellow perch (Perca flavescens) in a system where size-limited predation on mudminnows by larger perch is a principle structuring mechanism. During laboratory experiments in which mudminnows foraged for patchily-presented food in the presence and absence of yearling perch, the food intake of mudminnows decreased at both the population and individual-fish levels when perch were present. Mudminnows were neither less active nor did they occupy lower-quality food patches in the presence of perch; exploitation competition, rather than interference or predator avoidance, appeared responsible for reduced feeding success. To assess effects of competition in the field, we examined size distributions and condition factors of yearling mudminnows and perch in small Wisconsin lakes having mudminnow-only and mudminnow-perch assemblages. Yearling mudminnows were smaller and in poorer condition in an assemblage composed (by mass) of 45% yearling perch than in two mudminnow-only assemblages. Conversely, yearling perch were larger and in better condition when the mudminnow-perch assemblage contained only 45% perch then when it contained 92% perch. Our experimental and field results indicated that an asymmetrical competitive relationship could contribute directly to the interspecific population dominance of perch over mudminnows in Umbra-Perca assemblages through reduced food intake, growth, and condition of mudminnows, and indirectly through increased vulnerability of mudminnows to size-limited predation.     

Intraspecific heritable variation in life-history traits can alter the outcome of interspecific competition among insect herbivores

Competition is one of the most important biotic factors determining the structure of ecological communities. In this study, we show that there is variation in competitive ability between two clones of the pea aphid, Acyrthosiphon pisum, both of which out-compete a clone of the vetch aphid, Megoura viciae, in the laboratory. We tested whether this variation in competitive ability would alter the outcome of interspecific competition in the field. While one pea aphid clone followed the pattern set in the laboratory, out-competing the Megoura viciae clone, another showed the reverse effect with Megoura viciae dominating. These differences appear to be the result of variation in early population growth rate between the pea aphid clones, rather than predation, although predation did lead to the eventual extinction of colonies. We also questioned whether intra- and interspecific differences in predator escape behaviour could affect the outcome of competition in the field. All three clones responded similarly to the presence of foraging hoverfly larvae (Episyrphus balteatus), but the Megoura viciae clone dropped from the plant significantly less often in response to the presence of a foraging two-spot ladybird (Adalia bipunctata). This work provides evidence that intraspecific variation in competitive ability can alter the outcome of interspecific competitive interactions in nature and suggests that species–specific behavioural traits may have the potential to modify the outcome of these interactions.     

AN EXPERIMENTAL STUDY OF COMPETITIVE PERFORMANCE IN BRASSICA RAPA (CRUCIFERAE)

Growth chamber experiments with rapid-cycling Brassica rapa were designed to estimate the signs and magnitudes of the genetic correlations for plant performance in each of three conditions: no-competition (isolated plants), intraspecific competition, and interspecific competition with Raphanus sativa. Biomass and flower number were highest in the no-competition treatment, intermediate under intraspecific competition, and lowest under interspecific competition. Significant among-family variation in biomass and flower number was found under each regime. The mean family performance (biomass or flower number) in the no-competition treatment was significantly positively correlated with the performance in only one of the competitive treatments (for biomass in the intraspecific treatment). For both biomass and flower number there was a significant positive correlation between family means in the intra- and interspecific regimes. These correlations were greater in magnitude than those for the comparison between no-competition and competition (intra- or interspecific) treatments. Our results suggest that the importance of traits affecting plant performance is environment-dependent; the performance of a family grown without competition was a poor predictor of performance with competition, while the performance of families grown under intra- and interspecific competition was positively correlated.     

Segregation in space and time explains the coexistence of two sympatric sub‐Antarctic petrels

Biological communities are shaped by competition between and within species. Competition is often reduced by inter‐ and intraspecific specialization on resources, such as differencet foraging areas or time, allowing similar species to coexist and potentially contributing to reproductive isolation. Here, we examine the simultaneous role of temporal and spatial foraging segregation within and between two sympatric sister species of seabirds, Northern Macronectes halli and Southern Macronectes giganteus Giant Petrels. These species show marked sexual size dimorphism and allochrony (with earlier breeding by Northern Giant Petrels) but this is the first study to test for differences in foraging behaviours and areas across the entire breeding season both between the two species and between the sexes. We tracked males and females of both species in all breeding stages at Bird Island, South Georgia, to test how foraging distribution, behaviour and habitat use vary between and within species in biological time (incubation, brood‐guard or post‐brood stages) and in absolute time (calendar date). Within each breeding stage, both species took trips of comparable duration to similar areas, but due to breeding allochrony they segregated temporally. Northern Giant Petrels had a somewhat smaller foraging range than Southern Giant Petrels, reflecting their greater exploitation of local carrion and probably contributing to their recent higher population growth. Within species, segregation was spatial, with females generally taking longer, more pelagic trips than males, although both sexes of both species showed unexpectedly plastic foraging behaviour. There was little evidence of interspecific differences in habitat use. Thus, in giant petrels, temporal segregation reduces interspecific competition and sexual segregation reduces intraspecific competition. These results demonstrate how both specialization and dynamic changes in foraging strategies at different scales underpin resource division within a community.