A species invasion mediated through habitat structure, intraguild predation and parasitism.

With field, laboratory, and modeling approaches, we examined the interplay among habitat structure, intraguild predation (IGP), and parasitism in an ongoing species invasion. Native Gammarus duebeni celticus (Crustacea: Amphipoda) are often, but not always, replaced by the invader Gammarus pulex through differential IGP. The muscle-wasting microsporidian parasite Pleistophora mulleri infects the native but not the invader. We found a highly variable prevalence of P. mulleri in uninvaded rivers, with 0–91% of hosts parasitized per sample. In addition, unparasitized natives dominated fast-flowing riffle patches of river, whereas parasitized individuals dominated slower- flowing, pooled patches. We examined the survivorship of invader and native in single and mixed-species microcosms with high, intermediate, and zero parasite prevalence. G. pulex survivorship was high in all treatments, whereas G. duebeni subsp. celticus survivorship was significantly lower in the presence of the invader. Further, parasitized G. duebeni subsp. celticus experienced near-total elimination. Models of the species replacement process implied that parasite-enhanced IGP would make invasion by G. pulex more likely, regardless of habitat and parasite spatial structure. However, where heterogeneity in parasite prevalence creates a landscape of patches with different susceptibilities to invasion, G. pulex may succeed in cases where invasion would not be possible if patches were equivalent. The different responses of parasitized and unparasitized G. duebeni subsp. celticus to environmental heterogeneity potentially link landscape patterns to the success or failure of the invasion process.

Predation on mayfly nymph, Baetis rhodani, by native and introduced Gammarus: direct effects and the facilitation of predation by salmonids.

1. In a series of laboratory experiments, we assessed the predatory nature of the native Irish amphipod, Gammarus duebeni celticus, and the introduced G. pulex, towards the mayfly nymph Baetis rhodani. We also investigated alterations in microhabitat use and drift behaviour of B. rhodani in the presence of Gammarus, and indirect predatory interactions with juvenile Atlantic salmon, Salmo salar. 2. In trials with single predators and prey, B. rhodani survival was significantly lower when Gammarus were free to interact with nymphs as than when Gammarus were isolated from them. The invader G. pulex reduced the survival of B. rhodani more rapidly than did the native G. d. celticus. Both Gammarus spp. were active predators. 3. In `patch' experiments, B. rhodani survival was significantly lower both when G. pulex and G. d. celticus were present, although the effect of the two Gammarus species did not differ. Again, active predation of nymphs by Gammarus was observed. Significantly more nymphs occurred on the top and sides of a tile, and per capita drifts were significantly higher, when Gammarus were present. Baetis rhodani per capita drift was also significantly higher in the presence of the introduced G. pulex than with the native G. d. celticus. 4. Gammarus facilitated predation by salmon parr of B. rhodani by significantly increasing fish–nymph encounters on exposed gravel and in the drift. There were no differential effects of the two Gammarus spp. on fish –B. rhodani encounters or consumption. 5. We conclude that Gammarus as a predator can have lethal, nonlethal, direct and indirect effects in freshwaters. We stress the need for recognition of this predatory role when assigning Gammarus spp. to a `Functional Feeding Group'.

Invasion by the amphipod Gammarus pulex alters community composition of native freshwater macroinvertebrates

Assessing the effects of invading species on native community structure is often confounded by environmental factors and weakened by lack of replicated, long-term pre- and post-invasion monitoring. Here, we uncouple the community effects of a freshwater amphipod invader from environmental differences. In Irish rivers, the introduced Gammarus pulex replaces the native Gammarus duebeni celticus. However, the River Lissan in Northern Ireland is dissected by a weir that has slowed the upstream invasion by G. pulex. This allowed us in 2000 to sample three contiguous 150-m reaches that were (1) G. pulex dominated; (2) mixed Gammarus spp.; and (3) G. duebeni celticus only. In 2003, we resampled these reaches and one additional of mixed Gammarus species and one with only G. duebeni celticus further upstream. In temperature, conductivity, and pH, there were statistically significant but no biologically relevant differences among the five reaches of 2003, and between the three reaches surveyed in both years. Although there was evidence of recovery in macroinvertebrate diversity and richness in invaded reaches between years, continued upstream invasion was associated with sustained reductions in these community metrics as compared to un-invaded sites. Community ordination indicated (1) different associations of community composition attributed to the distribution, abundance, and biomass of the invader; and (2) increasing similarity of invaded communities over time. The impact mechanisms of G. pulex on macroinvertebrate community composition may include predation and competition. The consequences of the observed community changes for ecosystem functioning require further investigation.

Effects of environment and an introduced invertebrate species on the structure of benthic macroinvertebrate species at the catchment level

Biotic interactions such as predation and competition can influence aquatic communities at small spatial scales, but they are expected to be overridden by environmental factors at large scales. The continuing threat to freshwater biodiversity of biological invasions indicates that biotic factors do, however, have important structuring roles. In Irish rivers, the native amphipod Gammarus duebeni celticus has become locally extinct, ostensibly through differential predation by the more aggressive and introduced G. pulex. This mechanism explains impacts of G. pulex at within-river spatial scales on native macroinvertebrate community diversity, including declines in ephemeropterans, plecopterans, dipterans and oligochaetes. To determine if these patterns are predictable at larger spatial scales, we assessed patterns in native macroinvertebrate communities across river sites of the Erne catchment in 1998 and 1999, in conjunction with the distribution of G. pulex and G. d. celticus. In both years, G. pulex dominated invaded sites, whereas G. d. celticus occurred at low abundance in uninvaded sites. In both years, invaded sites had lower diversity and fewer pollution sensitive invertebrate species than un-invaded sites. Community ordination in 1998 showed that invaded sites had higher conductivity, smaller substrate particle size and comprised a lower proportion of pollution sensitive taxa including Ephemeroptera and Plecoptera. In contrast, in 1999, conductivity was the only variable explaining site ordination along axis 1, but was unable to separate sites with respect to invasion status. A second explanatory axis separated sites with respect to invasion status, with invaded sites having fewer taxa, including lower abundance of ephemeropterans, dipterans and plecopterans. Laboratory experiments examined the potential role of differential predation between the two Gammarus species in explaining these taxon specific patterns in the field. Survival of the ephemeropterans, Ephemerella ignita and Ecdyonurus venosus and the isopod, Asellus aquaticus, was lower when interacting with G. pulex than with G. d. celticus. This study indicates that G. putex may alter invertebrate community structure at scales beyond those detected within individual rivers. However, effects may be influenced by gradients in physico-chemistry, which may be temporal or depend on catchment characteristics. Invasions by amphipods have increased globally, thus comprehensive assessments of their impacts and of other aquatic invaders, may only be apparent when studies are conducted at a range of spatio-temporal scales.

Factors influencing the distribution of native and introduced Gammarus spp. in Irish river systems

Populations of Gammarus duebeni celticus, previously the only amphipod species resident in the rivers of the Lough Neagh catchment, N. Ireland, have been subjected to invasion by G. pulex from the British mainland. Numerous previous studies have investigated the potential behavioural mechanisms, principally differential mutual predation, underlying the replacement of G. d. celticus by G. pulex in Irish waters, and the mutually exclusive distributions of these species in Britain and mainland Europe. However, the relative degree of influence of abiotic versus biotic factors in structuring these amphipod communities remains unresolved. This study used principal component analysis (PCA) to distinguish physico-chemical parameters that have significant roles in determining the current distribution of G. pulex relative to G. d. celticus in L. Neagh rivers. We show that the original domination of rivers by the native G. d, celticus has changed radically, with many sites in several rivers containing either both species or only G. pulex. G. pulex was more abundant than the G. d. celticus in sites with low dissolved oxygen levels. This was reflected in the macroinvertebrate assemblages associated with G. pulex in these sites, which tended to be those tolerant of low biological water quality. The present study thus emphasizes the importance of the habitat template, particularly water quality, for Gammarus spp. interactions. If rivers become increasingly stressed by organic pollution, it is probable the range expansion of G. pulex will continue. Because these two species are not ecological equivalents, the outcomes of G. pulex incursions into G. d. celticus sites may ultimately depend on the prevailing physico-chemical regimes in each site.

Comparison of the functional responses of invasive and native amphipods

While we can usually understand the impacts of invasive species on recipient communities, invasion biology lacks methodologies that are potentially more predictive. Such tools should ideally be straightforward and widely applicable. Here, we explore an approach that compares the functional responses (FRs) of invader and native amphipod crustaceans. Dikerogammarus villosus is a Ponto-Caspian amphipod currently invading Europe and poised to invade North America. Compared with other amphipods that it actively replaces in fresh-waters, D. villosus exhibited significantly greater predation, consuming significantly more prey with a higher type II FR. This corroborates the known dramatic field impacts of D. villosus on invaded communities. In another species, FRs were nearly identical in invasive and native ranges. We thus propose that if FRs of other taxa and trophic groups follow such general patterns, this methodology has potential in predicting future invasive species impacts.

Cryptic species diversity in a widespread bumble bee complex revealed using mitochondrial DNA RFLPs

Cryptic species diversity is thought to be common within the class Insecta, posing problems for basic ecological and population genetic studies and conservation management. Within the temperate bumble bee (Bombus spp.) fauna, members of the subgenus Bombus sensu stricto are amongst the most abundant and widespread. However, their species diversity is controversial due to the extreme difficulty or inability morphologically to identify the majority of individuals to species. Our character-based phylogenetic analyses of partial CO1 (700 bp) from 39 individuals spread across their sympatric European ranges provided unequivocal support for five taxa (3-22 diagnostic DNA base pair sites per species). Inclusion of 20 Irish specimens to the dataset revealed >= 2.3% sequence divergence between taxa and 200 m) whilst B. cryptarum was relatively more abundant at higher altitudes. Bombus magnus was rarely encountered at urban sites. Both B. lucorum and B. terrestris are nowadays reared commercially for pollination and transported globally. Our RFLP approach to identify native fauna can underpin ecological studies of these important cryptic species as well as the impact of commercial bumble bees on them.

The replacement of a native freshwater amphipod by an invader: roles for environmental degradation and intraguild predation

We assessed the extent to which an invader, Gammarus pulex (Crustacea: Amphipoda), has replaced a native, Gammarus duebeni celticus, over a 13-year period in a European river system and some of the abiotic and biotic factors that could account for this. Between 1988 and 2001, 56% of mixed-species sites had become invader-only sites, whereas no mixed sites had become native only again. The native dominated areas of higher dissolved oxygen and water quality, with the reciprocal true for the invader. Field transplant experiments revealed that native survivorship was lower in areas where it had been replaced than in areas where the invader does not yet occur. In invader-only areas, native survivorship was lower than that of the invader when kept separately and lowest when both species were kept together. We also observed predation of the native by the invader. Laboratory oxygen manipulation experiments revealed that at 30% saturation, the native's survivorship was two thirds that of the invader. We conclude that decreasing water quality favours replacement of the native by the invader.

Lethal and sublethal toxicity of ammonia to native, invasive, and parasitised freshwater amphipods

In lethal and sublethal ammonia toxicity tests, we examined differences in tolerance of three species of freshwater amphipods, one native and two invasive in Ireland. The native Gammarus duebeni celticus was slightly less tolerant to ammonia than the invasive G. pulex (96h LC50 = 1.155 and 1.544 mg l(-1), respectively), while another invader, Crangonyx pseudograeilis, had the lowest tolerance (LC50 = 0.36 mg l(-1)). Parasitism of G. pulex by the acanthocephalan Echinorhynchus truttae greatly reduced the tolerance of the invader to ammonia (LC50 = 0.381 mg l(-1)). Further, precopula pair disruption tests indicated that G. d. celticus was more sensitive to ammonia than G. pulex at sublethal levels. We discuss these results in the context of the ecological replacements of native by invader amphipods. (C) 2004 Elsevier Ltd. All rights reserved.

Differential physico-chemical tolerances and intraguild predation among native and invasive amphipods (Crustacea); a field study

We used field surveys and transplant experiments to elucidate the relative roles of physico-chemical regime and intraguild predation in determining the generally mutually exclusive distributions of native and invader freshwater amphipod species. Field surveys showed that the native Gammarus duebeni celticus dominates the shoreline of Lough Neagh, N. Ireland, with some co-occurrence with the N. American invader G. tigrinus. However, the latter species dominates the deeper areas of the mid-Lough. Transplant experiments showed no difference in survival of the native and invader in single species 'bioassay tubes' placed along the shoreline. However, there was significantly higher survival of the invader compared with the native in single species tubes placed in the mid-Lough. In mixed species tubes on the shoreline, the native killed and ate the invader, with no reciprocal interaction, leading to significant reductions of the invader. However, the invader had significantly higher survival than the native in mixed species tubes in the mid-Lough, with no evidence. of predation between the two species. These results indicate that, whereas differential intraguild predation may determine domination of the shoreline by the native, differential physico-chemical tolerances may be major determinants of the domination of the mid-Lough by the invader. This study emphasises the need to consider the habitat template in conjunction with biotic interactions before attempting to draw conclusions about mechanisms determining relative distribution patterns of native and invasive species.

Predator-prey interactions between brown trout Salmo trutta and native and introduced amphipods; their implications for fish diets

An attempt to improve the food base for brown trout Salmo trutta in Northern Ireland was made in 1958.59 by deliberately introducing English Gammarus pulex into several Irish rivers. In addition. another amphipod Crangonyx pseudogracilis, was later accidently introduced into II ish waters. Our study represents the first attempt to examine the trophic interactions between a native fish predator (S. trutta) and an array of these native (Gammarus duebeni celticus) and introduced (G. pulex and C. pseudogracilis) amphipods. Feeding experiments, involving young brown trout predators and ampiphod prey, revealed that the fish actively selected C. pseudogracilis relative to two alternative Gammarus prey species. Although the trout encountered the Gammarus species more than C. pseudogracilis, they were eaten less than Crangonyx. Difficulties in handling and ingestion of Gammarus by trout may be a. key component of the preference fbr the smaller, more easily handled Crangonyx. The microdistribution of the species was altered by the fish, due to predation being greater in particular microhabitats, Our study showed that the introduction of the herbivorous C. pseudogracilis into Irish freshwaters may represent a useful addition to fish diets. particularly for small and/or juvenile fish. The reprecussions of the deliberate introduction of G. pulex are less clear. It may improve feeding for fish. but only if it can coexist with indigenous macroinvertebrates and thus ultimately improve the range and quantity of possible food items in predator diets. Alternatively, being highly predatory towards other macroinvertebrates including G. d. celticus and C. pseudogracilis. G. pulex may be deleterious to the diversity of the resident benthic community and hence reduce the diversity of prey available to fish predators.

Distribution of the common lizard (Zootoca vivipara) and landscape favourability for the species in Northern Ireland

The common lizard (Zootoca vivipara) is Ireland’s only native reptile, forming a key part of the island’s biodiversity. However, there is a general paucity of distributional and abundance data for the species. In this study, we collated incidental records for common lizard sightings to define the distribution of the species in Northern Ireland. Maximum entropy modelling was employed to describe species-habitat associations. The resulting predicted landscape favourability was used to evaluate the current status of the species based on the distribution of its maximum potential range in relation to the degree of fragmentation of remaining suitable habitat. In common with previous studies in the Republic of Ireland, sightings were highly clustered indicating under-recording, observer bias, and fragmentation of suitable habitat. A total of 98 records were collated from 1905 to 2009. The species was recorded in 63 (ca. 34%) of 186 × 10 km Northern Irish grid squares. Lizard occurrence was strongly and positively associated with landscapes dominated by heathland, bog and coastal habitats. The single best approximating model correctly classified the presence of lizards in 84.2% of cases. Upland heath, lowland raised bog and sand dune systems are all subject to Habitat Action Plans in Northern Ireland and are threatened by conversion to agriculture, afforestation, invasive species encroachment and infrastructural development. Consequently, remaining common lizard populations are likely to be small, isolated and highly fragmented. Establishment of an ecological network to preserve connectivity of remaining heath and bog will not only benefit remaining common lizard populations but biodiversity in general.

Direct and indirect effects of species displacements; an invading freshwater amphipod can disrupt leaf litter processing and shredder efficiency

Invasive species may threaten the fundamental role played by native macroinvertebrate shredders in determining energy flow and the trophic dynamics of freshwater ecosystems. Functionally, amphipods have long been regarded as mainly shredders, but they are increasingly recognized as major predators of other macroinvertebrate taxa. Furthermore, intraguild predation (IGP) between native and invasive amphipods underlies many species displacements. We used laboratory mesocosms to investigate what might happen to shredders and leaf-litter processing in water bodies invaded by the highly predatory Ponto-Caspian amphipod Dikerogammarus villosus, which is spreading rapidly throughout Europe and may soon invade the North American Great Lakes. The leaf-shredding efficiency of D. villosus was significantly lower than that of 3 Gammarus species (2 native and 1 invasive) that D. villosus has either already displaced or may be currently displacing in The Netherlands. In addition, D. villosus was a major predator of all of these native and invasive amphipod shredders and of a common isopod shredder Asellus aquaticus. Leaf processing in Gammarus and Asellus mesocosms declined rapidly in the presence of D. villosus and ceased altogether within 4 d because by then, all potential shredders had been killed and consumed. Furthermore, the shredding efficiency of surviving amphipods and isopods declined significantly within 2 d of the release of D. villosus, a result indicating that predator-avoidance behavior may override leaf processing. We discuss the implications of these direct and indirect effects of D. villosus invasions and species displacements on community structure and litter processing in aquatic ecosystems. © 2011 The North American Benthological Society.


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Reaxys Database Information|

Fronts, jumps and secondary introductions suggested as different invasion patterns in marine species, with an increase in spread rates over time

Not all introduced (invasive) species in a region will spread from a single point of introduction. Long-distance dispersal or further introductions can obscure the pattern of spread, but the regional importance of such processes is difficult to gauge. These difficulties are further compounded when information on the multiple scale process of invasive species range expansion is reduced to one-dimensional estimates of spread (e. g. km yr(-1)). We therefore compared the results of two different metrics of range expansion: maximum linear rate of spread and accumulation of occupied grid squares (50 x 50 km) over time. An analysis of records for 54 species of introduced marine macrophytes in the Mediterranean and northeast Atlantic revealed cases where the invasion process was probably missed (e. g. Atlantic Bonnemaisonia hamifera) and suggested cases of secondary introductions or erratic jump dispersal (Dasysiphonia sp. and Womersleyella setacea). A majority of species analysed showed evidence for an accumulation of invaded sites without a clear invasion front. Estimates of spread rate are increasing for more recent introductions. The increase is greater than can be accounted for by temporally varying search effort and implies a historical increase in vector efficiency and/or a decreased resistance of native communities to invasion.

Impacts of unregulated harvesting on a recovering stock of native oysters (Ostrea edulis)

In Strangford Lough, Northern Ireland stocks of Ostrea edulis collapsed in the 1890s and the species was rarely recorded again until 1998 when the wild stock was estimated to be 100,000. The stock increased to 1.2 million in 2003 but declined to 650,000 by 2005. In 2007 the stock exceeded 1 million. The initial recovery of wild stocks is attributed to the combined effects of spawning commercial O. edulis stocks of and larval retention due to local hydrography. The stock decline between 2003 and 2005 is attributed to unregulated harvesting. Significant differences in abundances between sites over this period may be explained by the exploitation of more-readily accessible sites initially and of less accessible sites later. Oysters at sites where there was minimal exploitation probably contributed to widespread recruitment in 2007. Sustainable management of recovering native oyster stocks in Strangford Lough and elsewhere and will be impossible without appropriate legislation and enforcement.