Selective predation on Cladocera by brown trout Salmo trutta L.

The size and species composition of zooplankton ingested by brown trout in Lough Sheelin was studied. Zooplankton and fish samples were taken simultaneously. The zooplankton samples consisted of Daphnia hyalina var. galeata , Copepoda and Bythotrephes longimanus in the proportion of 246: 10: 1. The entire stomach contents of the fish were examined with particular attention paid to the zooplankton component. The results indicated a high degree of selection for B. longimanus .     

Diel and seasonal patterns of food intake and prey selection by Coregonus sp. in re-oligotrophicated Lake Lucerne, Switzerland

Feeding intensity by whitefish Coregonus sp., in oligotrophic Lake Lucerne in Switzerland was high during dusk when the bulk of potential prey items were in the depth zone occupied by the fish. Diet composition was fairly uniform throughout the day but changed substantially over the seasons. The fish fed opportunistically; differences between seasons reflected changes in prey availability. During the intensive feeding and growing period (May-September), fish were found in the upper 20 m of the lake feeding primarily on cladocerans. Large and non-evasive species, Daphnia spp. and Bythotrephes longimanus , were the most numerous and frequent organisms in the diet during the major part of the growing season. Smaller ( Bosmina spp.) as well as evasive species (cyclopoid copepods) were consumed in large numbers when larger, non-evasive species were rare in the lake. The fish showed strong preference for the least abundant crustacean, B. longimanus , while the most abundant crustaceans, calanoid copepods, were rare in the diet. The fish not only selected particular species but, within each species, selected the larger individuals. Diel vertical migration of the prey items in this lake could be, at least in part, attributed to fish predation pressure. The observed selectivity patterns shown by the fish are explained in terms of prey visibility, escape ability, the overlap in distribution of predators and prey in time and space, the profitability of the prey and the present trophic state of the lake.     

Shifts in phenology of Bythotrephes longimanus and its modern success in Lake Maggiore as a result of changes in climate and trophy

In Lake Maggiore, the density of the invertebrate predator Bythotrepheslongimanus increased following lake re-oligotrophication inthe late 1980s. This "invasion" was followed by dramatic changesin the pelagic food web, consistent with those that followedthe establishment of B. longimanus in North American lakes whereit is not native. In this contribution, we explore the modernsuccess of B. longimanus in Lake Maggiore by investigating itsphenology and population density, and their correlations withabiotic and biotic factors during the period from 1981 to 2003.A 10-fold increase in the abundance of B. longimanus followedan earlier start and longer duration of annual population growth.Increased prey resources and decreased predation pressure werenot observed during the B. longimanus density increase. Instead,a rise in lake temperature may have altered the reproductivecycle of this species. Furthermore, the depth and duration ofa refuge from visually orienting fish predators increased duringthese 20 years, as a result of changes in the thermal stratificationregime of the lake. This case study provides a timely exampleof how climatic changes may interact with biotic drivers (e.g.fish predation) to influence the density and phenology of aninvertebrate predator.     

An introduced invertebrate predator (Bythotrephes) reduces zooplankton species richness

Rarely do ecologists have the data needed to assess the impacts of invading species on biodiversity, i.e. pre- and post-invasion census information from both invaded and control sites. Using a 21-year time series, we demonstrate that the invasion of Harp Lake, Ontario, Canada, by the Eurasian spiny water flea, Bythotrephes longimanus , a zooplanktivore, was accompanied by a rapid and long-lasting reduction in the average species richness of crustacean zooplankton, particularly of cladoceran taxa. No such reduction was observed in seven nearby un-invaded lakes over the same two decades. If the Harp Lake results are typical, we predict a widespread reduction in crustacean zooplankton richness on the Canadian Shield for three reasons. Shield lakes provide the invader with good habitat. Its dispersal rates and colonization success are high. Zooplankton richness in Harp Lake is now unusually low for a Shield Lake of its size and acidity.     

Experimental evidence for emergent facilitation: promoting the existence of an invertebrate predator by killing its prey

1. Recent theoretical insights have shown that predator species may help each other to persist by size-selective foraging on a shared prey. By feeding on a certain prey stage, a predator may induce a compensatory response in another stage of the same prey species, thereby favouring other predators; a phenomenon referred to as emergent facilitation. 2. To test whether emergent facilitation may occur in a natural system, we performed an enclosure experiment where we mimicked fish predation by selectively removing large zooplankton and subsequently following the response of the invertebrate predator Bythotrephes longimanus. 3. Positive responses to harvest were observed in the biomass of juvenile individuals of the dominant zooplankton Holopedium gibberum and in Bythotrephes densities. Hence, by removing large prey, we increased the biomass of small prey, i.e. stage-specific biomass overcompensation was present in the juvenile stage of Holopedium. This favoured Bythotrephes, which preferentially feed on small Holopedium. 4. We argue that the stage-specific overcompensation occurred as a result of increased per capita fecundity of adult Holopedium and as a result of competitive release following harvest. If shown to be common, emergent facilitation may be a major mechanism behind observed predator extinctions and patterns of predator invasions.     

Endozoochory by ducks: influence of developmental stage of Bythotrephes diapause eggs on dispersal probability

Abstract. This paper evaluates whether the recent expansion of Bythotrephes longimanus Leydig (Crustacea: Onychopoda) may be a result of endozoochorous avian dispersal. In three experiments, we investigated the survival of diapause eggs in newly produced and end-of-winter developmental stages after their passage through the digestive tract of four species of ducks. Resistance of eggs to digestion was low for both developmental stages (< 12%). Most eggs (67–95%) were recovered by four hours after ingestion, with a longer maximum retention time for newly produced (22 h) than end-of-winter eggs (4–6 h). Ingestion had a detrimental effect on viability: only eggs retrieved one hour after ingestion hatched and the proportion of those ingested that produced viable neonates was low (0.5% for newly produced and 0.3–3% for end-of-winter eggs). We conclude that the probability of endozoochorous dispersal by individual ducks is low but becomes significant when considering the thousands of ducks moving among wetlands. Endozoochorous dispersal of B. longimanus would most likely involve the transport of newly produced diapause eggs from north to south, during the autumn migration of ducks. Based on flight speed estimates of ducks we estimate dispersal probability to drop sharply at distances over 60–80 km.     

Contrasting patterns and mechanisms of spatial turnover for native and exotic freshwater fish in Europe

Aim We compare the distribution patterns of native and exotic freshwater fish in Europe, and test whether the same mechanisms (environmental filtering and/or dispersal limitation) govern patterns of decrease in similarity of native and exotic species composition over geographical distance (spatial species turnover). Locations Major river basins of Europe. Methods Data related to geography, habitat diversity, regional climate and species composition of native and exotic freshwater fish were collated for 26 major European river basins. We explored the degree of nestedness in native and exotic species composition, and quantified compositional similarity between river basins according to the beta‐sim (independent of richness gradient) and Jaccard (dependent of richness gradient) indices of similarity. Multiple regression on distance matrices and variation‐partitioning approaches were used to quantify the relative roles of environmental filtering and dispersal limitation in shaping patterns of decreasing compositional similarity over geographical distance. Results Native and exotic species exhibited significant nested patterns of species composition, indicating that differences in fish species composition between river basins are primarily the result of species loss, rather than species replacement. Both native and exotic compositional similarity decreased significantly with increasing geographical distance between river basins. However, gradual changes in species composition with geographical distance were found only for exotic species. In addition, exotic species displayed a higher rate of similarity decay (higher species turnover rate) with geographical distance, compared with native species. Lastly, the majority of explained variation in exotic compositional similarity was uniquely related to geography, whereas native compositional similarity was either uniquely explained by geography or jointly explained by environment and geography. Main conclusions Our study suggests that large‐scale patterns of spatial turnover for exotic freshwater fish in Europe are generated by human‐mediated dispersal limitation, whereas patterns of spatial turnover for native fish result from both dispersal limitation relative to historical events (isolation by mountain ranges, glacial history) and environmental filtering.     

Alewife planktivory controls the abundance of two invasive predatory cladocerans in Lake Michigan

1. We sampled along a nearshore transect (10-m bathymetric contour) in Lake Michigan to determine diet, 24-h feeding periodicity, daily ration and food requirements of an invasive fish, the alewife, Alosa pseudoharengus , relative to zooplankton abundance and production. Our objective was to determine whether the alewife controls the abundance of two invasive, predatory cladocerans, Bythotrephes longimanus and Cercopagis pengoi .
2. Bosminidae was the most abundant prey taxon and Chydoridae, Leptodora , Chironomidae and Bythotrephes were the least abundant. Neither Bythotrephes nor Cercopagis were important prey for small alewives (≤100 mm). Bythotrephes was eaten by over 50% of large alewives (>100 mm) and accounted for 10–27% of the diet weight. Cercopagis was eaten by about 30% of the large alewives but only accounted 1% of the diet weight.
3. Food weight in stomachs was highest early in the night for small alewives and lowest at night for large alewives. Chironomidae and large Chydoridae were the preferred prey of small alewives. Bythotrephes and large Chydoridae were the preferred prey for large alewives.
4. Food requirements of alewife were much less than production for most prey taxa, although the consumption of Bythotrephes greatly exceeded production on both dates. Alewives consumed only 3% of Cercopagis production. High selectivity and food requirements of alewife for Bythotrephes , and low selectivity and food requirements for Cercopagis , probably explain the difference in abundance between these two invasive cladocerans at our nearshore site in Lake Michigan.     

Invasion genetics of the Eurasian spiny waterflea: evidence for bottlenecks and gene flow using microsatellites

The Eurasian spiny waterflea (Bythotrephes longimanus) is a predacious zooplankter that has increased its range in Europe and is rapidly invading inland water-bodies throughout North America's Great Lakes region. To examine the genetics of these invasions, we isolated five microsatellite DNA loci with between 5 and 19 alleles per locus. We sampled three populations where B. longimanus has been historically present (Switzerland, Italy, and Finland) as well as an introduced European population (the Netherlands) and three North American populations (Lakes Erie, Superior, Shebandowan). Consistent with a bottleneck during colonization (i.e. founder effect), average heterozygosities of the four European populations ranged from 0.310 to 0.599, and were higher than that of three North American populations (0.151-0.220). Pairwise F(ST) estimates among North American populations (0.002-0.063) were not significantly different from zero and were much lower than among European populations (0.208-0.474). This is consistent with a scenario of high gene flow among North American populations relative to that of European ones. Contrary to an invasion bottleneck, however, Erie and Superior populations contained similar numbers of rare alleles as European populations. Assignment tests identified several migrant genotypes in all introduced populations (the Netherlands, Erie, Superior, Shebandowan), but rarely in native ones (Switzerland, Italy and Finland). A large number of genotypes from North America were assigned to our Italian population suggesting a second, previously unidentified, invasion source somewhere in the region of northern Italy. Together, our results support an invasion bottleneck for North American populations that has been largely offset by gene flow from multiple native sources, as well as gene flow among introduced populations.     

Influence of Chironomus plumosus larvae on ammonium flux and denitrification (measured by the acetylene blockage- and the isotope pairing-technique) in eutrophic lake sediment

Bythotrephes invaded Harp Lake, Ontario, Canada, in the early 1990s. Here we describe seasonal changes in the size, abundance and life history of the invader. The weight of Bythotrephes could be accurately estimated (r² =0.90) from the length of its body and the state of development of its brood, but substantial (22%) corrections for shrinkage in sugarformalin were required. The seasonality of the size and abundance of Bythotrephes was apparently regulated by the interaction of fish predation, temperature-mediated growth, the availability of the invader's prey, and switches in its reproductive biology. The vernal increase in the growth of the Bythotrephes population lagged behind increases in water temperature and abundances of preferred prey, most probably because of predation by lake herring (Coregonus artedi). Bythotrephes abundance increased rapidly in early July; however, the population maximum was brief. After the abundance of their preferred prey crashed in mid-July, Bythotrephes body size declined, and adult females switched from parthenogenic to resting egg production. Population size declined in consequence. In the summer, female Bythotrephes committed most of their energy to growth between instar I and II, but to egg production for instars II and III. The biology of the Harp Lake population differed from that observed in other recently invaded lakes in several ways – the lag in population growth in the spring, the small change in body size between instars II and III, the decline in body size in August, and the co-incident switch by mature females to resting egg production. These inter-lake differences in biology indicate we have more to learn about Bythotrephes before we can predict its influence on the inland lakes of North America.     

Selective predation on zooplankton by pelagic Arctic charr, Salvelinus alpinus, in six subarctic lakes

Selective predation by planktivore fish appears to be an important regulatory factor of zooplankton communities, potentially causing large changes in species composition and size distributions within populations. In this study, prey preferences and size-selective predation on zooplankton by Arctic charr were examined in six subarctic lakes with Arctic charr as the dominant pelagic fish species. Most of the lakes had a zooplankton community dominated by copepods (Cyclops scutifer and Eudiaptomus graciloides), but the pelagic charr evidently selected cladoceran species (Bythotrephes longimanus, Daphnia sp. and Bosmina sp.), likely because the copepods have a higher mobility and evasiveness than the cladocerans. Furthermore, a strong size selection was also revealed for both Bosmina sp. and Daphnia sp., as individual prey from Arctic charr stomachs were exclusively larger than individuals sampled in the environment. Additionally, visibility due to size, morphology and pigmentation (egg-carrying females) was also a major factor for the selection of zooplankton prey. In conclusion, Arctic charr was found to be highly selective on zooplankton both in respect to species composition and individual size of Bosmina sp. and Daphnia sp.     

Distribution and community structure of Ostracoda (Crustacea) in shallow waterbodies of southern Kenya

The current study presents the ostracod communities recovered from 26 shallow waterbodies in southern Kenya, combined with an ecological assessment of habitat characteristics. A total of 37 waterbodies were sampled in 2001 and 2003, ranging from small ephemeral pools to large permanent lakes along broad gradients in altitude (700–2 800 m) and salinity (37–67 200 µS cm^?1). Between 0 and 12 species were recorded per site. Lack of ostracods was associated with either hypersaline waters, or the presence of fish in fresh waters. Three of the 32 recovered ostracod taxa, Physocypria sp., Sarscypridopsis cf. elizabethae and Oncocypris mulleri, combined a wide distribution with frequent local dominance. Canonical correspondence analysis on species–environment relationships indicated that littoral vegetation, altitude, surface water temperature and pH best explain the variation in ostracod communities. Presence of fish and water depth also influence species occurrence, with the larger species being more common in shallow waterbodies lacking fish. Based on Chao’s estimator of total regional species richness, this survey recovered about two-thirds (60–68%) of the regional ostracod species pool. Scanning electron micrographs (SEM) of the valve morphology of 14 ostracod taxa are provided, in order to facilitate their application in biodiversity and water-quality assessments and in palaeoenvironmental reconstruction.     

The food and feeding habits of young perch, Perca fluviatilis, in Windermere

SUMMARY. The food of 0+ perch, Perca fluviatilis L. has been investigated in Windermere, the largest lake in the English Lake District. Whole guts and stomachs from 800 fish were investigated during four 24-h periods between June and September in 1975 and in 1976. Ciliates, algae, rotifers and cyclopoid nauplii constituted the first food of larval perch in Windermere. Vertical samples were taken from the zooplankton, at the same time and in the same location where the fish were sampled, and the concentration of the different food organisms studied. Electivity of the different food organisms was studied from lvlev's index, which showed positive electivity, or selection for some organisms, e.g. Bosmina obtusirostris (Sars.) and copepods (both cyclopoid and calanoid), and negative for others such as Daphnia hyalina var. galeata (Sars.), Leptodora kindti (Focke) and Bythotrephes longimanus (Leydig). In studying predatorprey relationships, Daphnia hyalina was chosen as the 'standard prey', being the most abundant of all food organisms in the diet of young perch, also because its maximum body-depth (carapace width (mm)) was among the largest of the cladocera and copepods in Windermere. Comparison of the maximum carapace width of Daphnia eaten with that of the plankton revealed that smaller specimens were taken by juvenile perch in July, but they tended to select larger organisms during August and September.     

Agonistic behavior in a schooling fish: form,function and ontogeny

Synopsis Although juvenile chum salmon,Oncorhynchus keta, are generally regarded as a schooling fish, when presented with a defensible point-source of food, some individuals abandon schooling, aggressively subordinate competitors, and monopolize food. When food is removed, fish gradually abandon solitary agonistic behaviors and return to schooling behavior. Agonism increases in frequency and intensity as juveniles age. The ability to alternate facultatively between schooling and solitary agonistic behavior may enable juvenile chum to respond to local patterns of food distribution and predation risk. The ontogenetic increase in agonism may result in school dispersal as fish move from the estuary into coastal waters, and may well reflect a shift in the costs versus the benefits of schooling as fish mature and become less vulnerable to predation.     

Evidence for size and sex-specific dispersal in a cooperatively breeding cichlid fish

African Great Lake cichlid populations are divided into thousands of genetic subpopulations. The low gene flow between these subpopulations is thought to result from high degrees of natal philopatry, heavy predation pressure, and a patchy distribution of preferred habitats. While predation pressure and habitat distribution are fairly straightforward to assess, data on dispersal distances and rates are scarce. In fishes, direct observations of dispersal events are unlikely, but dispersal can be studied using molecular markers. Using seven microsatellite loci, we examined dispersal in the cooperatively breeding cichlid fish, Neolamprologus pulcher. As this species is found in well-defined groups clustered into subpopulations, we could assess dispersal on a narrow (within subpopulation) and broad (between subpopulation) scale. While fish were generally more related to others in their own subpopulation than they were to fish from other subpopulations, large males diverged from this pattern. Large males were more related to other large males from different subpopulations than they were to large males from their own subpopulation, suggesting more frequent dispersal by large males. Across subpopulations, relatedness between large males was higher than the relatedness among large females; this pattern was not detected in small males and small females. Within a subpopulation, individuals appeared to be preferentially moving away from relatives, and movement was unrestricted by the physical distance between groups. Our results highlight the importance of examining multiple spatial scales when studying individual dispersal biases.     

Predation and the evolution of complex oviposition behaviour in Amazon rainforest frogs

Summary Terrestrial oviposition with free-living aquatic larvae is a common reproductive mode used by amphibians within the central Amazonian rainforest. We investigated the factors presently associated with diversity of microhabitats (waterbodies) that may be maintaining the diversity of reproductive modes. In particular, desiccation, predation by fish, competition with other anurans and water quality were examined in 11 waterbodies as possible forces leading to the evolution of terrestrial oviposition. Predation experiments demonstrated that fish generally do not eat anuran eggs, and that predacious tadpoles and dytiscid beetle larvae are voracious predators of anuran eggs. The percentage of species with terrestrial oviposition was only weakly correlated with the occurrence of pond drying, pH and oxygen concentration, suggesting that anurans in this tropical community are able to use the range of water quality available for egg development. There was a tendency for terrestrial oviposition to be associated with the number of species of tadpoles using the waterbody, but we consider this to be spurious as there was no obvious competitive mechanism that could result in this relationship. The percentage of species with terrestrial oviposition was significantly positively related to our index of egg predation pressure, and negatively related to our index of fish biomass. Egg predation pressure was also negatively related to the index of fish biomass. These results allow us to discount as improbable the hypothesis that predation by fish on anuran eggs was an important selective pressure leading to terrestrial oviposition in this community. The strong positive relationship between terrestrial oviposition and our index of egg predation pressure indicates that these predators have exerted, and are exerting, a significant selective pressure for terrestrial oviposition. The strong negative relationship between the occurrence of fish and the egg predators suggests the surprising conclusion that the presence of fish actually protects aquatic anuran eggs from predation in this tropical system, and allows aquatic oviposition to dominate only in those waterbodies with moderate to high densities of fish. Our results suggest that terrestrial oviposition is a fixed predator avoidance trait.     

Higher zooplankton species richness associated with an invertebrate top predator

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Taxonomic resolution of the genus Bythotrephes Leydig using molecular markers and re-evaluation of its global distribution

Bythotrephes Leydig is a predatory, onychopod cladoceran native to Eurasia that typically inhabits oligo‐ and mesotrophic water bodies of the Palaearctic region. It recently invaded 70 North American lakes, prompting a re‐evaluation of the taxonomic status, global distribution, and determinants of local occurrence and abundance. European studies have reported two distinct species, B. longimanus, which lacks a kink on the caudal process, and B. cederstroemi, which possesses one. We employed sequencing of the mitochondrial gene cytochrome c oxidase subunit I (COI) in addition to Directed Amplification of Minisatellite‐region DNA (DAMD) using polymerase chain reactions (PCRs) to assay the genetic nature of these ‘species’. These analyses revealed that the two taxa are consistent with a single, common species, Bythotrephes longimanus Leydig 1860, thus resolving the nomenclatural issue. Furthermore, a common haplotype was identified between Lake Ontario and Lake Puruvesi, Finland, suggesting a potential invasion corridor via the nearby Baltic Sea. Statistical analysis revealed that the two forms also occur in similar habitats in Eurasia and North America. Habitat characteristics of water bodies in Northern Europe, where both forms are found and occasionally co‐occur, do not differ statistically. Similarly, no significant differences were detected between characteristics of Eurasian habitats that support longimanus‘forms’ and those of inland lakes in North America that support cederstroemi‘forms’. Human activities have had a strong effect on the distribution of Bythotrephes in both Europe and North America. Global and local distributions have been affected by ballast water transfer and by boating and fishing activities, respectively. Cultural eutrophication, oligotrophication and acidification alter the suitability of habitats for fish predators, and may indirectly influence Bythotrephes’ local occurrence and abundance.     

Investigations on fecundity of Bythotrephes longimanus in Lake Lucerne (Switzerland) and on Niche Segregation of Leptodora kindti and Bythotrephes longimanus in Swiss lakes

In Lake Lucerne, Switzerland, the predaceous cladocerans Leptodora kindti and Bythotrephes longimanus segregate along spatial and temporal dimensions. In spring (April–May/June), Bythotrephes longimanus occurs below 0–20 m, while Leptodora is absent. In summer and early autumn (July–September/October), when Leptodora dominates during daytime in the 0–20 m depth, Bythotrephes longimanus also lives in deeper zones. Food competition and fish predation pressure may be the cause of differences in ecology of Leptodora and Bythotrephes acquired during evolution. Due to its transparency and tolerance of higher temperature, Leptodora could avoid fish predation and, therefore, competes with Bythotrephes longimanus successfully. In addition, the differences between the two species may account for the spatial and temporal niche segregation in oligotrophic Swiss Lakes. But spatial niche segregation is less important in mesotrophic lakes with high prey density than in oligotrophic lakes with low prey density. In small, eutrophic lakes importance of temporal niche segregation also decreases, and Bythotrephes is seldom or not present. The preference of Bythotrephes to live in deeper water to avoid fish predation during summer may be the cause of its difficulties to establish itself in small and eutrophic lakes with high prey densities, where the hypolimnion is missing or anoxic.In the spring, Bythotrephes exhibits r-strategy (smaller body size and a higher fecundity), the female is already fertile after the first molt. In the summer, a K-strategy prevails (larger body length and lower fecundity than in the spring), and female Bythotrephes are fertile only after the second molt. Shortage of prey (biomass of Bosmina and Daphniadecreased after June especially in the surface layers) and the maximum fish predation pressure in summer may change the life strategy of Bythotrephes: while fecundity decreases from generation to generation, body length increases. Enhanced prey densities (e.g. during mesotrophic conditions in L. Lucerne) lead to larger individuals in summer and autumn.     

Reaction distance of Bythotrephes longimanus, encounter rate and index of prey risk for Harp Lake, Ontario

SUMMARY 1. Some predatory zooplankton, such as the large cladoceran Bythotrephes longimanus (Leydig), have a large compound eye, hence vision may play an important role in prey detection and encounter. Our objectives were to quantify, under different light regimes, the reaction distance of Bythotrephes to an assortment of zooplankton prey and to model encounter rate with prey from Harp Lake, Ontario. Reaction distance to prey increased at higher light intensity.
2. Results from the encounter model show that small, slow-moving prey faced the greatest risk from Bythotrephes and most encounters occurred in the upper 10 m of the water column throughout the 24-h period. The model was highly sensitive to ambient light. Encounter rate and prey risk were two to three orders of magnitude more sensitive to swimming velocity of the predator than to that of the prey.