Differential predatory and interference interactions between native and invasive freshwater amphipods and a co-occurring mysid (Crustacea)

Animal introductions can often have unexpected and complex consequences for both native and invader species. Freshwater crustaceans such as Gammarus spp. (amphipods) and Mysis relicta (an ‘opossum shrimp’) have frequently come into contact because of deliberate and accidental introductions. However, there remains poor understanding of mechanisms leading to the co-existence and/or exclusion among amphipods and mysids. Here, we examined predatory and interference interactions between native (Gammarus duebeni celticus) and invasive (Gammarus tigrinus and Crangonyx pseudogracilis) amphipods and the native M. relicta in Britain’s largest expanse of freshwater, Lough Neagh. Laboratory mesocosm experiments simulating near-shore/mid-lough benthic habitats showed that both Gammarus species, but not C. pseudogracilis, preyed on M. relicta, which itself preyed on C. pseudogracilis. Further, M. relicta micro-distribution and habitat use changed because of interference from G. d. celticus and to a lesser extent G. tigrinus, with C. pseudogracilis having no such impact. In smaller microcosms, predation of M. relicta adults and juveniles by Gammarus spp. was significant. Although predation of Gammarus spp. by M. relicta was low, adult C. pseudogracilis were killed by M. relicta and its predation of juvenile C. pseudogracilis was high. We discuss the concurrence of these laboratory interactions with known field patterns of co-existence amongst these species.     

The enemy of my enemy is my friend: intraguild predation between invaders and natives facilitates coexistence with shared invasive prey

Understanding and predicting the outcomes of biological invasions is challenging where multiple invader and native species interact. We hypothesize that antagonistic interactions between invaders and natives could divert their impact on subsequent invasive species, thus facilitating coexistence. From field data, we found that, when existing together in freshwater sites, the native amphipod Gammarus duebeni celticus and a previous invader G. pulex appear to facilitate the establishment of a second invader, their shared prey Crangonyx pseudogracilis. Indeed, the latter species was rarely found at sites where each Gammarus species was present on its own. Experiments indicated that this may be the result of G. d. celticus and G. pulex engaging in more intraguild predation (IGP) than cannibalism; when the ‘enemy’ of either Gammarus species was present, that is, the other Gammarus species, C. pseudogracilis significantly more often escaped predation. Thus, the presence of mutual enemies and the stronger inter- than intraspecific interactions they engage in can facilitate other invaders. With some invasive species such as C. pseudogracilis having no known detrimental effects on native species, and indeed having some positive ecological effects, we also conclude that some invasions could promote biodiversity and ecosystem functioning.     

Differential physico-chemical tolerances of amphipod species revealed by field transplantations

Physico-chemical regimes of river systems are major determinants of the distributions and relative abundances of macroinvertebrate taxa. Other factors, however, such as biotic interactions, may co-vary with changes in physico-chemistry and concomitant changes in community composition. Thus, direct cause and effect relationships may not always be established from field surveys. Equally, however, laboratory studies may suffer from lack of realism in extrapolation to the field. Here, we use balanced field transplantation experiments to elucidate the role of physico-chemical regime in determining the generally mutually exclusive distributions of two amphipod taxa, Gammarus (two species) and Crangonyx pseudogracilis. Within two river systems in Ireland, the former species dominate stretches of well oxygenated, high-quality water, whereas the latter dominates stretches of poorly oxygenated, low-quality water. G. pulex and G. duebeni celticus did not survive in bioassay tubes in areas dominated by C. pseudogracilis, which itself survived in tubes in such areas. However, both C. pseudogracilis and Gammarus spp. survived equally well in tubes in areas dominated by Gammarus spp. Physico-chemical regime thus limits the movement of Gammarus spp. into C. pseudogracilis areas, but some other factor excludes C. pseudogracilis from Gammarus spp. areas. Since previous laboratory experiments showed high predation rates of Gammarus spp. on C. pseudogracilis, we propose that predation by the former causes exclusion of the latter. Hence, presumed effects of physico-chemical regime on macroinvertebrate presence/abundance may often require experimental field testing and appreciation of alternative explanations. Received: 14 June 1999 / Accepted: 20 January 2000     

An acanthocephalan parasite mediates intraguild predation between invasive and native freshwater amphipods (Crustacea)

1. The balance of predation between closely related invasive and native species can be an important determinant of the success or failure of biological invasions. In Irish freshwaters, the introduced amphipod Gammarus pulex has replaced the native G. duebeni celticus, possibly through differential mutual intraguild predation (IGP). Theoretically, parasitism could mediate such predation and hence the invasion outcome. However, this idea remains poorly studied. 2. In a field survey, we show that the acanthocephalan parasite Echinorynchus truttae is present in more G. pulex populations than G. d. celticus populations. In addition, within parasitised populations, E. truttae is more prevalent in the invader than in the native. 3. We show for the first time that an acanthocephalan parasite mediates predation between its intermediate macroinvertebrate hosts. In a field experiment, E. truttae parasitism of the invader lowered IGP upon the unparasitised native. In laboratory experiments, parasitism of G. pulex significantly reduced their predatory impact on recently moulted female G. d. celticus. Parasitism also appeared to cause reduction in predatory behaviour, such as attacks per contact on precopula guarded female natives. 4. We conclude that higher parasite prevalence in invaders as compared with natives, by mediation of interspecific interactions, could promote species coexistence, or at least slow species replacements, in this particular biological invasion.     

A long-term study (1949–2005) of experimental introductions to an island; freshwater amphipods (Crustacea) in the Isle of Man (British Isles)

Aim Using data from field introduction experiments with Gammarus spp. conducted in the rivers of a small island, commencing in 1949, with resampling in the 60s, 70s, 80s, 90s and finally in 2005, we aimed to examine the long‐term interaction of the native freshwater amphipod Gammarus duebeni celticus with the introduced G. pulex. Using physico‐chemical data from a 2005 island‐wide survey, we also aimed to find what environmental factors could influence the distribution of the two species. Location The Isle of Man, British Isles. Methods We used species distribution data from 10 river sites from a field introduction study in 1949–69, resampling of these sites in 1974/5, 1988, 1995 and 2005. We sampled a further 82 sites in 2005, where physico‐chemical characteristics of each site were obtained. ANOVA was used to ascertain significant differences in environmental variables between native and introduced amphipod assemblages. Multiple discriminant analysis was used to relate Gammarus distribution to environmental gradients in rivers. Results Field introductions of G. pulex into previously G. d. celticus dominated sites failed, despite the large propagule pressure in terms of numbers of individuals released and repeated introductions. The island‐wide survey revealed that G. d. celticus was found in areas of higher water quality than G. pulex. Sites where the two species co‐occurred also tended to be of lower water quality than native only sites. Continuing metal pollution from past mining activity may be a major factor excluding all amphipods from many rivers. The North American amphipod Crangonyx pseudogracilis, which was first discovered on the Isle of Man in one river site in 1995, was found in a further five sites. Main conclusions This ecological experiment, through long‐term monitoring, thus continues to elucidate factors influencing the distributions of introduced and native species. Under some physicochemical regimes, G. d. celticus is able to withstand its usual displacement by G. pulex, whereas increased river pollution could promote replacement of the native by the introduced species.     

Physicochemical tolerance,habitat use and predation are drivers of patterns of coexistence and exclusion among invasive and resident amphipods

相似文献   

The functional role of Gammarus(Crustacea, Amphipoda): shredders, predators, or both?

Gammarus spp. are traditionally viewed under the functional feeding group (FFG) concept as herbivorous `shredders'. Although recent studies suggest that Gammarus should also be viewed as predators, this latter role remains contentious. Here, in a laboratory experiment, we objectively examine the balance between shredder and predator roles in a common freshwater species. Gammarus pulex preyed significantly on mayfly nymph, Baetis rhodani, in both the presence and absence of excess leaf material. There was no significant difference in predation where the alternative food, that is, leaf material, was present as compared to absent. Also, G. pulex shredded leaf material in the presence and absence of B. rhodani. However, shredding was significantly reduced where alternative food, that is, B. rhodani prey, was present as compared to absent. Further, G. pulex had a clear leaf species preference. Our results suggest that Gammarus function as both predators and shredders, with the balance of the two roles perhaps depending on food availability and quality. We discuss implications for the use of the FFG concept in assessing freshwater processes, and the role that Gammarus predation may play in structuring macroinvertebrate communities.     

Effects of coexistence on habitat use and trophic ecology of interacting native and invasive amphipods

1. Invasive species in aquatic systems are major drivers of changes in biodiversity. Amphipods are key species in freshwaters, with invasive amphipods either replacing or coexisting with native species and often damaging local biodiversity. However, the consequences of interactions among native and invasive amphipods for their habitat use and feeding ecology and ecosystem function are not yet well understood. 2. We examined a number of streams in Brittany and Northern Ireland, with native and invasive amphipods, to evaluate the consequences of species interactions for both habitat use and diet. Our field studies centred on testing two proposed models: a cohabitation model without competition between two native species (Gammarus pulex vs Echinogammarus berilloni), and a competition model between an invasive and a native species (Gammarus pulex vs Gammarus duebeni celticus). For these three species, alone and in combination, we assessed their habitat use and feeding patterns, the latter through gut contents and stable C and N isotope analyses of their tissues. 3. When existing as single‐species populations, all three species used stream habitats broadly similarly, although G. pulex was more strongly associated with leaf litter and vegetation compared to pebble substrata than the other species. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two changed to using all habitats equally, whereas the former retained its habitat preferences. 4. Similarly, all three species when alone had similar gut contents, with inorganic material predominating, followed by leaf and woody material and more rarely algae and invertebrates. When G. pulex coexisted with E. berilloni, the diet of the latter did not change; however, the frequency of inorganic matter, leaves and wood declined in the gut contents of G. pulex. When G. pulex coexisted with G. d. celticus, the pattern of gut contents did not change in either species. 5. When existing as single‐species populations, G. pulex had a broader range of isotopic signatures, both for δ¹³C and for δ¹⁵N, than the two other species, indicating a more variable diet among individuals. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two had similar ranges of δ¹³C and δ¹⁵N, whereas for G. pulex the range was much less for δ¹³C and δ¹⁵N, suggesting a less diverse diet. 6. Our results infer two different modes of coexistence between native and non‐native amphipods. We have shown that the native species, which coexist stably, appear to show interference competition, leading to spatial habitat segregation, whereas competition for food and possible intraguild predation by G. pulex on G. d. celticus would explain why the distribution and density of the latter is affected by G. pulex. However, since all the species have a similar diet and feeding habit, we expect no great overall effect on ecosystem processes as a consequence of species interactions and displacements.     

The dynamics of predation on Gammarus spp. (Crustacea: Amphipoda)

Gammarus spp. (Crustacea: Amphipoda) are widespread throughout a diverse range of marine, freshwater and estuarine/brackish habitats, often dominating benthic macroinvertebrate communities in terms of both numbers and/or biomass. Gammarus spp. are the dominant macroinvertebrate prey items of many fish, whether as a seasonal food source or a year-round staple. Selective predation by fish on Gammarus spp. is often linked to parasitism and the body size of the prey. Gammarus spp. populations are under increasing threat from both pollution and replacement/displacement by introduced species. Loss of populations and species invasions/replacements could have significant impacts on native predator species if the predator(s) cannot successfully adapt their feeding patterns to cope with non-indigenous Gammarus prey species. Despite this, many fish predation studies do not identify Gammarus prey to species level. This lack of precision could be important, as Gammarus spp. exhibit wide variations in physiochemical tolerances, habitat requirements, abilities to invade and susceptibility to replacement. Although rarely acknowledged, the impacts of nonpiscean predators (particularly macroinvertebrates) on Gammarus prey species may frequently be stronger than those exerted by fish. A major aim of this review is to ascertain the current importance of Gammarus as a prey species, such that the implications of changes in Gammarus spp. populations can be more accurately assessed by interested groups such as ecologists and fisheries managers. We also review the dynamics of Gammarus spp. as prey to a diverse array of mammals, birds, amphibians, insects, flatworms, other crustaceans such as crabs and crayfish and, perhaps most importantly, other Gammarus spp.     

Invader abundance and contraction of niche breadth during replacement of a native gammarid amphipod

The introduction of non‐native species to new locations is a growing global phenomenon with major negative effects on native species and biodiversity. Such introductions potentially bring competitors into contact leading to partial or total species replacements. This creates an opportunity to study novel species interactions as they occur, with the potential to address the strength of inter‐ and intraspecific interactions, most notably competition. Such potential has often not been realized, however, due to the difficulties inherent in detecting rapid and spatially expansive species interactions under natural field conditions. The invasive amphipod crustacean Gammarus pulex has replaced a native species, Gammarus duebeni celticus, in river and lake systems across Europe. This replacement process is at least partially driven by differential parasitism, cannibalism, and intraguild predation, but the role of interspecific competition has yet to be resolved. Here, we examine how abundance of an invasive species may affect spatial niche breadth of a native congeneric species. We base our analyses of niche breadth on ordination and factor analysis of biological community and physical parameters, respectively, constituting a summative, multidimensional approach to niche breadth along environmental gradients. Results derived from biological and environmental niche criteria were consistent, although interspecific effects were stronger using the biological niche approach. We show that the niche breadth of the native species is constrained as abundance of the invader increases, but the converse effect does not occur. We conclude that the interaction between invasive G. pulex and native G. d. celticus under natural conditions is consistent with strong interspecific competition whereby a native, weaker competitor is replaced by a superior invasive competitor. This study indicates a strong role of interspecific competition, alongside other known interactions such as differential intraguild predation, in rapid and expansive species replacements following biological invasions.     

Habitat-mediated cannibalism and microhabitat restriction in the stream invertebrate <Emphasis Type="Italic">Gammarus pulex</Emphasis>

In cannibalistic species, small individuals often shift habitats to minimize risk of predation by larger conspecifics. The availability of diverse size-structured habitats may mediate the incidence of cannibalism by larger individuals on smaller individuals and increase fitness of smaller individuals. We tested these hypotheses in a series of laboratory studies with Gammarus pulex, a freshwater amphipod inhabiting substrates with varying interstitial pore space sizes. In the absence of larger, potentially cannibalistic individuals, small Gammarus actively used all pore space sizes offered. They used only substrates containing food and preferred food items that provided cover to food items that did not. In the presence of larger G. pulex, small individuals almost exclusively used smaller pore spaces from which larger individuals were excluded. Small individual survival was significantly lower in the presence of larger Gammarus than in controls without larger individuals regardless of substrate size, but availability of mixed pore sizes significantly increased survival. Food consumption and growth per individual were not affected by the presence of larger individuals or substrate composition. Our results suggest that the distribution and availability of complex and high-quality habitats may affect the occurrence and significance of cannibalism in size-structured populations. Handling editor: K. Martens     

Sex ratio of Nysius huttoni White (Hemiptera: Lygaeidae) in field and laboratory populations

Abstract

We studied the parasitoids of three insect seed predators of the mast-seeding genus Chionochloa spp. (snow tussocks) at Mt Hutt, New Zealand. Megacraspedus calamogonus (Lepidoptera: Gelechiidae) is parasitised by four species, three Hymenoptera (Zealachertus tortriciphaga [Eulophidae], Diadegma sp. [Ichneumonidae] and Dolichogenidea sp. [Braconidae]) and one Diptera (Uclesiella sp. [Tachinidae]). Diplotoxa similis (Diptera: Chloropidae) is parasitised by Callitula sp. (Hymenoptera: Pteromalidae). Eucalyptodiplosis chionochloae (Diptera: Cecidomyiidae) is parasitised by two Hymenoptera, Gastrancistrus sp. (Pteromalidae) and Zelostemma chionochloae (Platygastridae); all three species have extended diapause. Overall parasitism was 68.5% in M. calamogonus, 1% in D. similis, and 41% in E. chionochloae. Such parasitism in M. calamogonus and E. chionochloae may reduce seed predation in Chionochloa and alter the selective benefit of mast seeding (predator satiation) to the plant. However, seed predation is still high at many sites, so some of the seed predators and parasitoids may be food-limited (bottom-up regulation).     

Uncoupling strong predator–prey interactions in streams: the role of marginal macrophytes

Predation refugia can facilitate the coexistence of predators and prey within an ecosystem by weakening trophic links between the two. The marginal macrophytes of shallow lakes are used facultatively by zooplankton to escape fish predation, leading to the stabilisation of lentic food webs. Little is known about such a role for the marginal macrophytes of lotic systems. In this paper, we examine whether the marginal macrophytes of chalk streams help buffer the interaction between the freshwater shrimp, Gammarus pulex and a benthic fish predator, Cottus gobio, both characteristically abundant in such systems. Quantitative surveys were taken of Gammarus and bullheads in winter and summer in a southern English chalk stream. These indicated that Gammarus seasonally switched their habitat preference, from marginal macrophytes in summer, to midchannel habitats in winter. Bullheads exhibited an opposite trend, preferring midchannel habitats in summer and all habitats, particularly marginal macrophytes, in winter. Large Gammarus and precopula pairs were found almost exclusively in the margins in summer. This spatial separation between Gammarus and bullheads in summer, though not in winter, was reflected in bullhead diets, as determined by gut analysis. In field manipulation experiments, bullhead presence was the strongest factor explaining the between‐habitat distribution of Gammarus in both summer and winter, indicating that the habitat shift of Gammarus was driven by the distribution of fish. Other abundant invertebrate taxa, including Asellus, mayflies and chironomids, exhibited little avoidance of habitat patches of high bullhead density. We conclude that marginal macrophytes in chalk streams can potentially facilitate the coexistence of high densities of both Gammarus and bullheads by spatially separating predator and prey in summer. They may further allow large populations of Gammarus to persist in the presence of high bullhead density at stretch‐wide spatial scales, by reducing the predation by bullheads of large breeding and newly born individuals of Gammarus in summer.     

Impact of olfactory non-host predator cues on aggregation behaviour and activity in <Emphasis Type="Italic">Polymorphus minutus</Emphasis> infected <Emphasis Type="Italic">Gammarus pulex</Emphasis>

Parasites with a complex life cycle are supposed to influence the behaviour of their intermediate host in such a way that the transmission to the final host is enhanced, but reduced to non-hosts. Here, we examined whether the trophically transmitted bird parasite Polymorphus minutus increases the antipredator response of its intermediate host, the freshwater amphipod Gammarus pulex to fish cues, i.e. non-host cues (‘increased host abilities hypothesis’). Aggregation behaviour and reduced activity are assumed to decrease the predation risk of gammarids by fishes. Uninfected G. pulex are known to aggregate in the presence of a fish predator. In the present study, gammarids were allowed to choose either to join a group of conspecifics or to stay solitary (experiment 1) or between two groups differing in infection status (experiment 2), both in the presence or absence of fish odour. The perception of the groups was limited to mainly olfactory cues. Contrary to the ‘increased host abilities hypothesis’, in infected gammarids of experiment 1, fish cues induced similar aggregation behaviour as in their uninfected conspecifics. In experiment 2, uninfected as well as infected gammarids did not significantly discriminate between infected and uninfected groups. Although only uninfected gammarids reduced their activity in the presence of predator cues, infected G. pulex were generally less active than uninfected conspecifics. This might suggest that P. minutus manipulates rather the general anti-predator behaviour than the plastic response to predation risk.     

Nomenclature,redescription, and new record from Okinawa of Cymbasoma moriiSekiguchi,1982 (Monstrilloida)

In 1990 and 1991 the distribution of the genus Gammarus was examined in the River Hunte and its tributaries (Lower Saxony, northern Germany). In contrast to data from the literature, Gammarus fossarum and G. roeseli were just as abundant as G. pulex, which had been known to inhabit rivers and brooks in northern Germany. G. pulex was most frequent (1900 at 57 out of 70 and 1991 at 37 out of 39 sampling sites) followed by G. fossarum (at 52 and 32 sampling sites, respectively), whereas G. roeseli was locally restricted (at 7 and 6 sampling sites, respectively). G. fossarum and G. pulex were distributed up to the level of the city of Oldenburg; further north their distribution is limited by tidal influences and they are replaced by G. zaddachi. The possibility of using gammarids as indicators of water quality, as it has been done in low mountain ranges, is discussed.     

Presence of sculpins (Cottus gobio) reduces drift and activity of Gammarus pulex (Amphipoda)

Introduction of sculpins into a stream previously devoid of these predators significantly reduced drift rate of Gammarus pulex. The drift of insect larvae was not affected. High amounts of exudates after implantation of sculpins were probably responsible for the low number of drifting G. pulex specimens. Laboratory experiments confirmed reduced locomotory activity of G. pulex when exposed to caged sculpins, an observation that excludes reduced drift activity as a result only of predation.The average size of drifting G. pulex specimens was larger during the night than during the day. This result is in accordance with the hypothesis that large individuals should, in relation to small ones, turn nocturnal because of greater predation risk during daytime. Presence of sculpins did not alter the size composition of drifting G. pulex.     

THE TROPHIC ECOLOGY OF FRESHWATER GAMMARUS SPP. (CRUSTACEA:AMPHIPODA): PROBLEMS AND PERSPECTIVES CONCERNING THE FUNCTIONAL FEEDING GROUP CONCEPT

Gammarus spp. are widespread throughout a diverse range of freshwater habitats and can be the dominant part of many benthic macroinvertebrate assemblages, in terms of both numbers and/or biomass. Although the vast majority of studies have emphasized the herbivorous nature of Gammarus spp. and their ‘shredder’ functional feeding group (FFG) classification, we show that a far wider food base is exploited than has been previously acknowledged. This ‘plasticity’ as herbivore/predator is linked to the success of Gammarus spp. in persisting in and colonizing/invading disturbance-prone ecosystems. Intraguild predation and cannibalism are more common than previously realized. This behaviour appears to be a causal mechanism in many amphipod species replacements. Additionally, Gammarus spp. are major predators of other members of the macroinvertebrate community. Furthermore, while many studies have emphasized fish predation on Gammarus spp., we illustrate how this fish: amphipod, predator: prey interaction may be a two-way process, with Gammarus spp. themselves preying upon juvenile and wounded/trapped fish. We urge that a new realism be adopted towards the trophic ecology of Gammarus spp. and their role as predators and prey and that previously established FFG assumptions of both the food and the feeder be questioned critically.     

Interference competition among native and invader amphipods

Aquarium experiments were used to study indications of interference competition, such as substratum choice shifts, swimming activities and mortality of invasive and indigenous gammarids in each other's presence. The more recent invaders Gammarus tigrinus and Dikerogammarus villosus were more likely to prefer stone substratum, whereas the native Gammarus pulex and an earlier invader Gammarus roeseli were found more frequently in the water layer. Sand was the least likely substratum to be chosen by any of the species. G. pulex and G. roeseli did not alter their substratum preference in each other's presence. In the presence of D. villosus, G. pulex shifted towards smaller stones and increased its swimming activities, whereas D. villosus did not change its behaviour in the presence of G. pulex. These shifts may indicate interference competition, with D. villosus being the stronger competitor. The greatest shifts in substratum preference arose when one species had occupied a substratum before the other one was introduced, especially when D. villosus was already present before G. pulex was introduced, possibly indicating pre-emptive competition. Swimming activities of G. pulex increased in the presence of D. villosus, whereas D. villosus spent little time swimming. Mortality was comparable between the different experiments without any indication of predation. The effect of Intra Guild Predation (IGP) may not be reflected adequately by short-time experiments as moults occurred seldom during the experiments. Although no IGP was observed during our experiments, habitat shifts occurred, which may indicate that competitive interactions are apparent before IGP starts. Such shifts may serve to avoid intraguild competition.     

Interspecific differences in drift behaviour between the native <Emphasis Type="Italic">Gammarus pulex</Emphasis> and the exotic <Emphasis Type="Italic">Gammarus roeseli</Emphasis> and possible implications for the invader’s success

“Drifting” is known to subject aquatic invertebrates to intense predation by drift feeding fish. Consequently, interspecific variations in drifting behaviour could lead to differences in predation pressure between coexisting prey species. Predation being an important factor determining the success of invaders, differences in drift patterns could advantage either native or exotic invertebrates through differential predation by native fish predators. The exotic freshwater amphipod (Gammarus roeseli) has now largely colonized Western Europe where it is often found in sympatry with a native species (Gammarus pulex). Here we documented interspecific differences in drifting behaviour that might have favored the invader’s success through differential predation. Benthic and drifting amphipods were sampled three times at the same site to compare the proportion of each species within and between sample types (benthos or drift) across time. Compared with the benthos, where the invader (G. roeseli) was significantly less abundant than the native (G. pulex), G. roeseli was proportionally overrepresented in the drift but displayed a very different drifting pattern. While G. pulex drift rates remained roughly constant over a 24 h period, G. roeseli showed a marked diel periodicity with low diurnal and high nocturnal drift rates. Such drifting behaviour could procure this species with a competitive advantage regarding predation as most drift feeding fish are diurnal. As a result, the native appears more disadvantaged with respect to drift. This may partly explain the ability of G. roeseli to coexist with G. pulex in a habitat more suitable to the native.     

Lack of oxygen and low pH as limiting factors for Gammarus in Hessian brooks and rivers

During two decades (1968–1988) the occurrence of Gammarus was studied in Hessian running waters draining some 7700 km² of the Fulda-Eder-basin and two adjacent areas. About 1530 sites were sampled, most of them in 1968, 1978, 1981/82 and 1985, which made comparisons possible, primarily with respect to space, but partly also to time. After consideration of the relief of the landscape, geological conditions, and water-quality in the neighbourhood of human settlements, lack of oxygen (due to organic pollution) and low pH (souring) can be recognized as the most important factors which influence and alter natural distribution patterns of Gammarus fossarum, G. pulex and G. roeseli. Both vanishing and recovering populations were observed in the course of twenty years.