Trophic consequences of non‐native species for commercial fish in a backwater bay of the Three Gorges Reservoir,Southwest China

Trophic niche overlap in native and alien fish species can lead to competitive interactions whereby non‐native fishes outcompete indigenous individuals and eventually affect the viability of natural populations. The species Erythroculter mongolicus and Erythroculter ilishaeformis (belonging to the Culterinae), which are two commercially important fish species in the backwater bay of the Pengxi River in the Three Gorges Reservoir (TGR), were threatened by competition from the non‐native Coilia ectenes (lake anchovy). The latter is an alien species introduced into the lower reaches of the Yangtze River in China and now widespread in the TGR. The trophic consequences of non‐native lake anchovy invasion for E. mongolicus and E. ilishaeformis were assessed using stable isotope analysis (δ¹³C and δ¹⁵N) and associated metrics including the isotopic niche, measured as the standard ellipse area. The trophic niche of native E. mongolicus had little overlap (<15%) with the alien fish species and was significantly reduced in size after invasion by lake anchovy. This suggests that E. mongolicus shifted to a more specialized diet after invasion by lake anchovy. In contrast, the trophic niche overlap of native fish E. ilishaeformis with the alien fish species was larger (>50%) and the niche was obviously increased, implying that fish in this species exploited a wider dietary base to maintain their energetic requirements. Thus, marked changes for the native E. mongolicus and E. ilishaeformis were detected as the trophic consequences of invasion of non‐native lake anchovy.     

The role of alien fish (the centrarchid Micropterus salmoides) in lake food webs highlighted by stable isotope analysis

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Native richness and species level trophic traits predict establishment of alien freshwater fishes

Community level ecological traits are thought to affect invasibility as more diverse communities with complex trophic interactions may be associated with greater biotic resistance. Elucidation of the nature of this relationship is often hampered by difficulties in characterising food webs, particularly where field data are lacking. We attempted to overcome this by coupling food web modelling with information-theoretic analysis of the modelled webs. In addition, we also investigated the possibility that species level trends in trophic traits of established aliens might reflect exploitation of empty niches. We constructed hypothetical food webs of 26 natural and artificial lentic habitats from a data set consisting of 370 fish species representing 71 families. Using these food webs, we investigated associations at the community level between food web traits and network topology and number of alien fish species using an information-theoretic approach based on a set of competing a priori hypotheses. At the species level, we similarly tested for trends in trophic traits of established alien fishes using the information-theoretic approach in addition to nMDS of diet data. We found that native species richness in a community was the most important determinant of the number of alien fish taxa, displaying an inverse relationship. Our data also show that alien fish generally feed lower down the food web. Our findings suggest that the biotic resistance hypothesis, though scale dependent, can result in observable effects in animal communities. Moreover, we also found that the ability to exploit low energy yield food sources could favour the establishment of alien species via avoidance of resistive forces from native taxa.     

Introduction of Alien Host–parasite Complexes in a Natural Environment and the Symbiota Concept

When a species is introduced in a new territory its parasites may also follow, thus forming a sort of biotic unit. We propose the term symbiota to indicate this host–parasite complex. This concept is at the basis of the present research in which 11 species of alien and native freshwater fish were collected by electrofishing in the Po river area (Northern Italy). The alien host fish Ictalurus melas and Silurus glanis were parasitized by only one monogenean species each. Lepomis gibbosus, of North American origin, hosted four monogeneans and Carassius carassius three. All these parasites should be considered as alien species, except those collected from C. carassius that should be considered in part natives and in part aliens.     

Food niche variation of European and American mink during the American mink invasion in north-eastern Belarus

Understanding processes allowing the co-existence of ecologically similar species is important but difficult to study in community ecology. Introductions of alien species are unplanned experiments allowing investigation of co-adaptation of both native and invasive species over a short period. We analysed food niche differentiation between native European mink and alien American mink after invasion of the latter species in Belarus. European mink feed mainly on crayfish, frogs and fish whereas American mink prefer small mammals, fish and frogs. The diet of both species varied between seasons and during the period of alien mink invasion. Concurrent with the progress of American mink invasion, the European mink food niche has narrowed to feeding mainly on frogs, with the proportion of aquatic prey (fish and crayfish) in their diet drastically reduced. In contrast, the American mink food niche became wider during invasion. The breadth was stable but included a varied proportion of different prey categories: namely an increased proportion of aquatic prey and a decreased proportion of water vole and waterfowl. The increase in abundance of American mink saw a decrease in the proportion of larger prey in their diet. When American mink preyed more often on frogs, food niche overlap of both predators increased. This result suggests that arrival of an alien competitor reduced food abundance (exploitative competition) and caused a change in native mink diet.     

Dietary shifts in a stressed fish assemblage: Consequences of a bivalve invasion in the San Francisco Estuary

We compared dietary patterns within a temperate estuarine fish assemblage (Suisun Marsh, CA, U.S.A.) during a period of high mysid shrimp abundance and after a major decline in mysid abundance caused by the invasion of the overbite clam Potamocorbula amurensis. Prior to the invasion, high dietary overlap, high stomach fullness, and low niche breadth occurred among the fishes in spring when mysid populations were high. Dietary overlaps decreased and niche breadth increased for all species but the endemic splittail Pogonichthys macrolepidotus in fall when mysid populations were low. Eight native species exhibited lower overall collective overlaps and fuller stomachs than five alien species, suggesting more efficient resource partitioning. After mysid abundance declined, only alien striped bass Morone saxatilis preyed upon mysids in greater than trace amounts. An alien mysid became an important prey for small striped bass, but striped bass also switched to piscivory at a smaller size than when mysids were abundant. Eight of 13 species exhibited significant declines in abundance during the study period, which were concordant with the original importance of mysids in their diets. Our results suggest that altered lower food web dynamics in the San Francisco Estuary caused by the invasion of the overbite clam changed fish diets and have contributed to declines in fish abundance.     

Comparative nectar-foraging behaviors and efficiencies of an alien and a native bumble bee

Resource preemption by alien organisms can contribute to their invasion success and the demise of functionally equivalent native species, particularly when opportunistic foraging by aliens results in more efficient exploitation. In forests of NW Patagonia, the only native bumble bee and major pollinator, Bombus dahlbomii, declined almost to extinction as the alien B. ruderatus increased in abundance since its first appearance about 17 years ago. To explore whether resource competition might have driven this displacement we studied the behavior and foraging efficiency of both bumble bees while they harvested nectar from flowers of Alstroemeria aurea, the main summer food resource in the forests of NW Patagonia. We compared the nectar content of flowers that bees selected, recently visited, and rejected with that of randomly-chosen neighboring flowers and assessed differences in visitation rates. The native bumble bee selects flowers with abundant nectar and mostly exploits nectar-rich flower patches by rejecting a higher proportion of flowers with little or no nectar. On the other hand, the alien bumble bee discriminated less with respect to sugar content per visited flower, but visited more flowers per minute. Workers of the native bumble bee harvested ~70% more sugar per unit of time than those of the alien species in absolute terms, and a similar amount when sugar harvested was expressed as a percentage of body mass. In contrast to expectation, the opportunistic foraging of the alien bumblebee was not more efficient and therefore cannot explain the ecological extinction of the native species through exploitative competition. These findings suggest that the displacement of the native species by the alien may be driven by other factors, such as the associated introduction of novel diseases or parasites.     

Alien and native plant life‐forms respond differently to human and climate pressures

Aim To investigate whether differences in the elevational trend in native and alien species richness were dependent on climate or human pressures. Specifically we tested whether life‐form and/or alien/native status modifies the response of plant species richness to human population and temperature along: (1) a complete elevational gradient, and (2) within separate elevational bands that, by keeping temperature within a narrow range, elucidate the effects of human pressures more clearly. Location Two provinces (c. 7507 km²) on the southern border of the European Alps (Italy), subdivided into 240 contiguous sampling cells (c. 35.7 km²). Methods We used an extensive dataset on alien and native species richness across an elevation gradient (20–2900 m a.s.l.). Richness of natives and naturalized aliens were separately related to temperature, human population and Raunkiaer life‐form using general linear mixed models. Life‐form describes different plant strategies for survival during seasons with adverse cold/arid conditions. Results The relationship between species richness and temperature for natives was strongly dependent on life‐form, while aliens showed a consistent positive trend. Similar trends across alien and native life‐forms were found for the relationship between species richness and human population along the whole gradient and within separate elevational bands. Main conclusions The absence of life‐form‐dependent responses amongst aliens supports the hypothesis that the distribution of alien plant species richness was more related to propagule pressure and availability of novel niches created by human activities than to climatic filtering. While climate change will potentially contribute to relaxing species thermal constraints, the response of alien species to future warming will also be contingent on changes in anthropogenic pressures.     

Seed dispersal of alien and native plants by vertebrate herbivores

Seed dispersal is crucial for the success and spread of alien plants. Herbivores often establish a dual relationship with plants: antagonist, through herbivory, and mutualist, through seed dispersal. By consuming plants, herbivores may disperse large amounts of seeds, and can facilitate the spread of alien plants. However, seed dispersal of alien plants by herbivores has been largely uninvestigated. I studied factors associated with dispersal of alien and native seeds by the three most important vertebrate herbivores in SW Australia: emus (Dromaius novaehollandia), western grey kangaroos (Macropus fuliginosus) and European rabbits (Oryctolagus cuniculus). Overall frequencies of alien and native seeds dispersed by these herbivores were determined by differences among them in (1) the plant groups they predominantly disperse, that differed in frequencies of aliens versus natives, and (2) the predominant dispersal of aliens or natives within those plant groups. Emus and kangaroos (natives) tended to disperse predominantly alien seeds within plant groups (defined by life forms, dispersal syndromes, and diaspore size), whereas rabbits (alien) tended to disperse predominantly natives. This agrees with the hypothesis that herbivores will use predominantly plants that have evolved in different areas, because of less effective defences against new enemies. Overall frequencies were consistent with this pattern in kangaroos and rabbits, but not in emus. Kangaroos dispersed mostly plant groups that were mainly aliens (herbaceous species and small and medium sized dispersal units and seeds), which together with their predominant use of aliens over natives within groups resulted in the highest overall frequency of alien seeds (73%). Rabbits were similar to kangaroos in the type of plants dispersed, but their predominant use of natives over aliens within groups contributed to an overall predominance of native seeds in their pellets (88%). Emus dispersed mostly plant groups that were mainly natives (e.g. woody species with big diaspores), resulting in low overall frequency of alien seeds (11%), despite their predominant use of aliens over natives within plant groups. Thus, the within-groups trend pointed to a facilitative role of native herbivores of plant invasions through seed dispersal, but was obscured by the different use by herbivores of plant groups with different frequency of aliens.     

Contrasting response of native and alien plant species richness to environmental energy and human impact along alpine elevation gradients

Aim We tested whether the species–energy and species–human relationships vary between native and both naturalized and casual alien species richness when other environmental variables had been taken into account. Location Trento Province, a region (c. 6200 km²) on the southern border of the European Alps (Italy), subdivided into 156 contiguous (c. 37.5 km²) cells and ranging in elevation from 66 to 3769 m. Methods Data were separated into three subsets, representing richness of natives, naturalized aliens and casual aliens and separately related to temperature, human population and various environmental correlates of plant species diversity. We applied ordinary least squares and simultaneous autoregressive regressions to identify potential contrasting responses of the three plant status subsets and hierarchical partitioning to evaluate the relative importance of the predictor variables. Results Variation in alien plant species richness along the region was almost entirely explained by temperature and human population density. The relationships were positive but strongly curvilinear. Native species richness was less strongly related to either factor but was positively related to the presence of calcareous bedrock. Native species richness had a decelerating positive relationship with temperature (R²= 55%), whereas naturalized and casual aliens had a positive accelerating relationship explaining 86% and 62% of the variation in richness, respectively. Native species richness had a positive decelerating relationship with population density (R²= 42%), whilst both alien subsets had a positive accelerating relationship. Main conclusions Alien species richness was higher in areas with the most rich and diverse assemblages of native species. Areas at high altitudes are not especially prone to alien invasion due to energy constraints, low propagule pressure and disturbance, even considering a potential increased in temperature. Thus, if we consider future environmental change, we should expect a stronger response of aliens than natives in the currently warm, urbanized, low‐altitude areas than in cold, high‐altitude areas where human population density is low.     

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).     

Feeding ecology of the waterbirds in a tropical mangrove in the southeast Gulf of Mexico

ABSTRACT

Species that exploit the same type of environmental resources are defined as a guild, which have similar needs in the use of food or habitat. It was analyzed the diet of five waterbirds’ offsprings species (Neotropic Cormorant (Nannopterum brasilianus), Reddish Egret (Egretta rufescens), Boat-billed Heron (Cochlearius cochlearius), Snowy Egret (Egretta thula) and Great Egret (Ardea alba)), by prey identification and calculated the relative importance, overlap and breadth diet. The general diet of the piscivorous guild consisted of 17 fish species from 13 genera and eight families. The highest overlap was between the Reddish Egret and Boat-billed Heron. Fish species dominated the diets of all studied waterbirds, Poecilia velifera was the most abundant prey species in each of the birds, suggesting that they are abundant in the wetlands system of northern Yucatan. Diet overlap in waterbirds species depends on the use of resources and feeding habitat. Since reproductive success largely depends on the availability of food resources, mainly of resident or estuarine fishes. The information about diet is important for the conservation of waterbirds.     

Invasiveness of alien plants in Brussels is related to their phylogenetic similarity to native species

Aim Understanding the processes that drive invasion success of alien species has received considerable attention in current ecological research. From an evolutionary point of view, many studies have shown that the phylogenetic similarity between the invader species and the members of the native community may be an important aspect of invasiveness. In this study, using a coarse‐scale systematic sampling grid of 1 km², we explore whether the occupancy frequency of two groups of alien species, archaeophytes and neophytes, in the urban angiosperm flora of Brussels is influenced by their phylogenetic relatedness to native species. Location The city of Brussels (Belgium). Methods We used ordinary least‐squares regressions and quantile regressions for analysing the relationship between the occupancy frequency of alien species in the sampled grid and their phylogenetic distance to the native species pool. Results Alien species with high occupancy frequency in the sampled grid are, on average, more phylogenetically related to native species than are less frequent aliens, although this relationship is significant only for archaeophytes. In addition, as shown by the quantile regressions, the relationship between phylogenetic relatedness to the native flora and occupancy frequency is much stronger for the most frequent aliens than for rare aliens. Main conclusions Our data suggest that it is unlikely that species with very low phylogenetic relatedness to natives will become successful invaders with very high distribution in the area studied. To the contrary, under future climate warming scenarios, present‐day urban aliens of high occupancy frequency are likely to become successful invaders even outside urban areas.     

Comparisons of isotopic niche widths of some invasive and indigenous fauna in a South African river

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Relative abundance of an invasive alien plant affects native pollination processes

One major characteristic of invasive alien species is their occurrence at high abundances in their new habitat. Flowering invasive plant species that are visited by native insects and overlap with native plant species in their pollinators may facilitate or disrupt native flower visitation and fertilisation by forming large, dense populations with high numbers of flowers and copious rewards. We investigated the direction of such a proposed effect for the alien invasive Rhododendron ponticum in Irish habitats. Flower visitation, conspecific and alien pollen deposition, fruit and seed set were measured in a self-compatible native focal plant, Digitalis purpurea, and compared between field sites that contained different relative abundances of R. ponticum. Flower visitation was significantly lower at higher alien relative plant abundances than at lower abundances or in the absence of the alien. Native flowers experienced a significant decrease in conspecific pollen deposition with increasing alien abundance. Heterospecific pollen transfer was very low in all field sites but increased significantly with increasing relative R. ponticum abundance. However, lower flower visitation and lower conspecific pollen transfer did not alter reproductive success of D. purpurea. Our study shows that indirect interactions between alien and native plants for pollination can be modified by population characteristics (such as relative abundance) in a similar way as interactions among native plant species. In D. purpurea, only certain aspects of pollination and reproduction were affected by high alien abundances which is probably a result of high resilience due to a self-compatible breeding system. Native species that are more susceptible to pollen limitation are more likely to experience fitness disadvantages in habitats with high relative alien plant abundances.     

An invaded wet ecosystem in Central Italy: An arrangement and evidence for an alien food chain

The aim of this work is to provide an account of alien species richness and composition in a remnant Mediterranean coastal wetland of Central Italy. Data were obtained from different taxa-specific methods, in 2001–2005 (terrestrial vertebrates) and in 2005 (fish, arthropoda, mollusca). Among the 353 species sampled, 17 (4.8 %) are aliens (7.1% if we consider only vertebrates).Data on abundance, introduction type and status of alien species are reported. A crustacean, Procambarus clarkii, a fish, Gambusia sp. and three rodents, Myocastor coypus, Rattus norvegicus and Mus domesticus are the naturalized invasive species that locally could constitute a threat on economic and ecological levels. Data on Agapornis nigrigenis and Quelea cardinalis represent the first records for Italy. We also report evidence of a food chain on three levels, among the most invasive species (predation of Gambusia sp. by Procambarus clarkii and of Rattus norvegicus on Procambarus clarkii).      

Relationships among the seed rain,seed bank and vegetation of a Hawaiian forest

Abstract. Hawaiian ecosystems are prone to invasion by alien plant species. I compared the seed rain, seed bank, and vegetation of a native Hawaiian forest to examine the potential role that seed ecology plays in allowing alien species to invade native forest. Absolute cover of seed plants in the forest was 126 %, annual seed rain was 5 713 seeds m^-2 yr^-1, and the mean density of seedlings emerging from the seed bank averaged across four seasons was 1 020/m². The endemic tree Metrosideros polymorpha was the most abundant species in the vegetation, seed rain and winter seed bank. Overall, native seed plants comprised 95 % of the relative cover in the vegetation and 99 % of the seeds in the seed rain, but alien species comprised 67 % of the seeds in the seed bank. Alien species tended to form persistent seed banks while native species formed transient or pseudo-persistent seed banks. Dominance of the seed bank by alien species with persistent seed banks suggests that aliens are favorably placed to increase in abundance in the vegetation if the forest is disturbed.     

Comparisons of the food niches of three native and two introduced fish species in an Australian river

Synopsis The breadth, correspondence and overlap of the diets of the small and large size classes of the three native species (Galaxias occidentalis, Bostockia porosa and Edelia vittata) and two introduced species (Gambusia holbrooki and Perca fluviatilis) of fishes found in the shallows of the main channel and in the tributary creeks of a south-western Australian river have been compared in each season. Classification and ordination were used to examine the overall interrelationships of the diets across species, size groups, seasons and the location where the fish were caught (channel or creek). The smaller fish had a narrower dietary breadth than larger fish in the spring and summer, presumably reflecting the size limit imposed on prey size by their possession of a relatively small mouth in these seasons. Intraspecific dietary overlap between large and small size classes was usually high in G. occidentalis, but generally low in G. holbrooki and P. fluviatilis, and also in B. porosa and E. vittata when the difference between the lengths of the two size groups was greatest. Dietary overlap was least in autumn when the main prey taxa were most abundant. During winter, the diets of the three native species in the tributaries converged, probably reflecting a relatively low faunal diversity in these highly seasonal water bodies. The only relatively consistent interspecific overlap in diet was between B. porosa and E. vittata. Classification and ordination of the dietary samples separated G. occidentalis (which fed extensively on terrestrial organisms from the water surface) from the smaller P. fluviatilis (that concentrated on copepods in the plankton) and from B. porosa and E. vittata (which ingested primarily benthic organisms). Furthermore, B. porosa tended to ingest larger prey taxa than E. vittata. The diet of Gambusia holbrooki is sometimes dominated by terrestrial insects and at other times by benthic organisms, demonstrating that this species is an opportunistic carnivore. It is concluded that food partitioning by the three native and two introduced fish species in the Collie River is likely to be one of the principal factors facilitating the coexistence of substantial populations of these species in this system.     

Throwing the baby out with the bathwater: does laurel forest restoration remove a critical winter food supply for the critically endangered Azores bullfinch?

The invasive Clethra arborea has a dual-role in the diet of the Azores bullfinch, a critically endangered bird species endemic to the island of São Miguel (Azores, Portugal). This is a crucial winter food resource but it lowers the availability of native laurel forest species that compose most of the bird’s diet throughout the year. The removal of this and other invasive alien species is part of current laurel forest habitat restoration programmes, disregarding the impact on the Azores bullfinch population. In order to evaluate the first responses of the Azores bullfinch to habitat restoration, we studied bird diet, foraging behaviour, food availability and habitat occupancy in managed (without C. arborea) and control areas. Significant increases in the availability of native food resources in managed areas were noticeable in the diet, particularly the intake of Ilex perado ssp. azorica and Prunus lusitanica ssp. azorica flower buds. In most of the studied months birds heavily used and foraged in managed over control areas. The one exception was in December, when a resource-gap occurred in managed areas, which may be overcome in the short-term due to re-establishment of native plants following removal of invasive aliens.     

Are invaders disturbance‐limited? Conservation of mountain grasslands in Central Argentina

Abstract. Extensive areas in the mountain grasslands of central Argentina are heavily invaded by alien species from Europe. A decrease in biodiversity and a loss of palatable species is also observed. The invasibility of the tall‐grass mountain grassland community was investigated in an experiment of factorial design. Six alien species which are widely distributed in the region were sown in plots where soil disturbance, above‐ground biomass removal by cutting and burning were used as treatments. Alien species did not establish in undisturbed plots. All three types of disturbances increased the number and cover of alien species; the effects of soil disturbance and biomass removal was cumulative. Cirsium vulgare and Oenothera erythrosepala were the most efficient alien colonizers. In conditions where disturbances did not continue the cover of aliens started to decrease in the second year, by the end of the third season, only a few adults were established. Consequently, disturbances are needed to maintain alien populations in tall‐grass mountain grasslands. Burning also increased the species richness of native species. We conclude that an efficient way to control the distribution of alien species is to decrease grazing pressure while burning as a traditional management tool may be continued.