Non‐native species modify the isotopic structure of freshwater fish communities across the globe

Multiple anthropogenic pressures including the widespread introductions of non‐native species threaten biodiversity and ecosystem functioning notably by modifying the trophic structure of communities. Here, we provided a global evaluation of the impacts of non‐native species on the isotopic structure (δ¹³C and δ¹⁵N) of freshwater fish communities. We gathered the stable isotope values (n = 4030) of fish species in 496 fish communities in lentic (lakes, backwaters, reservoirs) and lotic (running waters such as streams, rivers) ecosystems throughout the world and quantified the isotopic structure of communities. Overall, we found that communities containing non‐native species had a different isotopic structure than communities without non‐native species. However, these differences varied between ecosystem types and the trophic positions of non‐native species. In lotic ecosystems, communities containing non‐native species had a larger total isotopic niche than communities without non‐native species. This was primarily driven by the addition of non‐native predators at the top of the food chain that increased δ¹⁵N range without modifying the isotopic niche size of native species. In lentic ecosystems, non‐native primary consumers increased δ¹⁵N range and this was likely driven by an increase of resource availability for species at higher trophic levels, increasing food chain length. The introduction of non‐native secondary consumers at the centre of the isotopic niche of recipient communities decreased the core isotopic niche size, the δ¹³C range of recipient communities and the total isotopic niche of coexisting native species. These results suggested a modified contribution of the basal resources consumed (e.g. multi‐chain omnivory) and an increase level of competition with native species. Our results notably imply that, by affecting the isotopic structure of freshwater fish communities at a global scale, non‐native species represent an important source of perturbations that should be accounted for when investigating macro‐ecological patterns of community structure and biotic interactions.     

Seabird species‐ and assemblage‐level isotopic niche shifts associated with changing prey availability during breeding in coastal Newfoundland

Shifting prey availability can lead to altered species interactions, indicated by variation in the dietary niche breadth and position of species within an assemblage. On the Newfoundland coast, annual inshore spawning migration of the dominant forage fish, Capelin Mallotus villosus, provides an excellent opportunity to investigate the influence of varying prey availability on dietary niche breadth and position among species. During June–August 2017, we investigated species‐ and assemblage‐level dietary responses to shifting Capelin availability of three Capelin‐eating, sympatrically breeding auk species, the Atlantic Puffin Fratercula arctica, Razorbill Alca torda and Common Murre Uria aalge. The diet of Leach's Storm Petrels Oceanodroma leucorhoa, which breed alongside the three auk species but are not known to rely on Capelin, was also examined to determine dietary shifts throughout breeding that were unrelated to Capelin availability. We quantified stable isotope ratios (δ¹⁵N, δ¹³C) in seabird blood components (plasma, cellular component) collected both before and after spawning Capelin arrived in the study area and compared isotopic niche breadth within a Bayesian framework. At the species level, auk trophic position increased and isotopic niche breadth narrowed after Capelin arrived, suggesting a more Capelin‐based diet. Simultaneously, trophic diversity of the auk assemblage, reflecting the extent of spacing among niches of species, decreased after spawning Capelin arrived inshore. Contrastingly, increased trophic position but broader isotopic niche breadth during higher relative to lower Capelin availability for Leach's Storm Petrel confirm that this species is probably not affected by the inshore arrival of Capelin, but instead that isotopic changes may be more related to a shift in breeding stage to chick‐rearing. Overall, our findings reiterate the importance of Capelin as a prey resource for breeding auks in coastal Newfoundland, but that the degree of reliance on Capelin varies among species, possibly allowing coexistence of these ecologically similar species. The findings highlight potential changing species interactions, such as increased competition, under declines in Capelin biomass.     

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.     

Trophic niche segregation allows range‐extending coral reef fishes to co‐exist with temperate species under climate change

Changing climate is forcing many terrestrial and marine species to extend their ranges poleward to stay within the bounds of their thermal tolerances. However, when such species enter higher latitude ecosystems, they engage in novel interactions with local species, such as altered predator–prey dynamics and competition for food. Here, we evaluate the trophic overlap between range‐extending and local fish species along the east coast of temperate Australia, a hotspot for ocean warming and species range extensions. Stable isotope ratios (δ¹⁵N and δ¹³C) of muscle tissue and stomach content analysis were used to quantify overlap of trophic niche space between vagrant tropical and local temperate fish communities along a 730 km (6°) latitudinal gradient. Our study shows that in recipient temperate ecosystems, sympatric tropical and temperate species do not overlap significantly in their diet—even though they forage on broadly similar prey groups—and are therefore unlikely to compete for trophic niche space. The tropical and temperate species we studied, which are commonly found in shallow‐water coastal environments, exhibited moderately broad niche breadths and local‐scale dietary plasticity, indicating trophic generalism. We posit that because these species are generalists, they can co‐exist under current climate change, facilitating the existence of novel community structures.     

Comparative trophic impacts of two globally invasive cyprinid fishes reveal species‐specific invasion consequences for a threatened native fish

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Stable isotope analysis reveals variation in trophic niche depending on altitude in an endemic alpine gecko

Interspecific competition is considered a major determinant of ecological niche. It is hypothesized that increased competition should reduce niche breadth. However, there are scarce field tests on this hypothesis. Here, we test this central hypothesis in ecology by using the Atlas day gecko Quedenfeldtia trachyblepharus. This alpine gecko faces fewer competitors as altitude increases, and thereby, we predict that this species should increase niche breadth and relevant fitness parameters with altitude. We tested this prediction by analysing the isotopic signature of carbon (δ¹³C) and nitrogen (δ¹⁵N). Our results reveal that specimens from higher altitudes showed higher values for both carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopes, had better body condition and a greater isotopic breadth when compared to specimens from lower altitudes. Altitudinal variation in carbon values was not explained by variation in isotopic concentration in the baseline of the trophic chain. Therefore, our findings support the prediction that relaxed interspecific competition favours increased trophic niche breadth. These results also suggest that global warming may represent an important threat for this species, as it may provoke the ascent in altitude of competitors, with negative consequences for the conservation of this endemism.     

Temporal variation in trophic relationships among three congeneric penguin species breeding in sympatry

Penguins are a monophyletic group in which many species are found breeding sympatrically, raising questions regarding how these species coexist successfully. Here, the isotopic niche of three sympatric pygoscelid penguin species was investigated at Powell Island, South Orkney Islands, during two breeding seasons (austral summers 2013–2014 and 2015–2016). Measurements of carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope ratios were obtained from blood (adults) or feather (chicks) samples collected from Adélie Pygoscelis adeliae, chinstrap P. antarctica, and gentoo P. papua penguins. Isotopic niche regions (a proxy for the realized trophic niches) were computed to provide estimates of the trophic niche width of the studied species during the breeding season. The isotopic niche regions of adults of all three species were similar, but gentoo chicks had noticeably wider isotopic niches than the chicks of the other two species. Moderate to strong overlap in isotopic niche among species was found during each breeding season and for both age groups, suggesting that the potential for competition for shared food sources was similar during the two study years, although the actual level of competition could not be determined owing to the lack of data on resource abundance. Clear interannual shifts in isotopic niche were seen in all three species, though of lower amplitude for adult chinstrap penguins. These shifts were due to variation in carbon, but not nitrogen, isotopic ratios, which could indicate either a change in isotopic signature of their prey or a switch to an alternative food web. The main conclusions of this study are that (1) there is a partial overlap in the isotopic niches of these three congeneric species and that (2) they responded similarly to changes that likely occurred at the base of their food chain between the 2 years of the study.     

Fifty years later: trophic ecology and niche overlap of a native and non-indigenous fish species in the western basin of Lake Erie

Since the introduction of white perch (Morone americana) into Lake Erie over 50 years ago, the population size of native yellow perch (Perca flavescens) has decreased up to 79 % and significant changes to the ecosystem have occurred. We examined long-term population estimates and used stable isotopes of carbon (δ ¹³C) and nitrogen (δ ¹⁵N) paired with stomach content analysis to quantify the trophic ecology and niche overlap of adult yellow perch and white perch in the western basin of Lake Erie. We found that changes in yellow perch abundance since 1979 appeared to be better correlated with changes in fishery exploitation rates than with food competition effects from white perch. At the time of this study, yellow perch were found to have higher δ ¹³C values, indicating greater utilization of benthic food resources than white perch, and white perch occupied higher trophic positions based on δ ¹⁵N. The diets of both species varied spatially and seasonally based on stable isotopes and stomach contents, likely driven by changes in prey abundance. Comparison of niche widths using stable isotope population metrics and Schoener diet similarity index suggested a low to moderate degree of niche overlap between species. Isotopic niches of white perch were generally larger than those of yellow perch demonstrating broader resource utilization by this non-indigenous species. We submit that isotopic niche overlap comparisons are more appropriate for studies seeking to understand interactions among populations over course temporal scales, while diet overlap indices, such as the Schoener index provide a means to study fine-scale interactions such as ontogenetic and seasonal diet shifts.     

Questioning assumptions of trophic behavior in a broadly ranging marine predator guild

We evaluated whether existing assumptions regarding the trophic ecology of a poorly‐studied predator guild, northwest (NW) Atlantic skates (family: Rajidae), were supported across broad geographic scales. Four hypotheses were tested using carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotope values as a proxy for foraging behavior: 1) species exhibit ontogenetic shifts in habitat and thus display a shift in ¹³C with differential use of the continental shelf; 2) species exhibit ontogenetic prey shifts (i.e. from smaller to larger prey items) and become enriched in ¹⁵N; 3) individuals acquire energy from spatially confined local resource pools and exhibit limited displacement; and 4) species exhibit similarly sized and highly overlapping trophic niches. We found some evidence for ontogenetic shifts in habitat‐use (δ¹³C) for thorny and little skate and diet (δ¹⁵N) of thorny and winter skate and hypothesize that individuals exhibit gradual trophic niche transition, especially in δ¹⁵N space, rather than a clear and distinct shift in diet throughout ontogeny. Spatial isoscapes generated for little, thorny, and winter skate highlighted distinct spatial patterns in isotopic composition across the coastal shelf. For little and thorny skate, patterns mimicked expected spatial variability in the isotopic composition of phytoplankton/POM, suggesting limited displacement and utilization of spatially confined resource pools. Winter skate, however, exhibited a much narrower range of δ¹³C and δ¹⁵N values, suggesting individuals may use resources from a more confined latitudinal range. Although high total trophic niche overlap was observed between some species (e.g. little and thorny skate), sympatric species (e.g. little and winter skate) exhibited a degree of trophic niche separation. These findings offer new insight into the trophic dynamics of a poorly‐studied, vulnerable group of predators, and highlight a need to re‐examine assumptions pertaining to aspects of their ecology.     

Identifying climatic niche shifts using coarse‐grained occurrence data: a test with non‐native freshwater fish

Aim We tested whether coarse‐grained occurrence data can be used to detect climatic niche shifts between native and non‐native ranges for a set of widely introduced freshwater fishes. Location World‐wide. Methods We used a global database of freshwater fish occurrences at the river basin scale to identify native and non‐native ranges for 18 of the most widely introduced fish species. We also examined climatic conditions within each river basin using fine‐grained climate data. We combined this information to test whether climatic niche shifts have occurred between native and non‐native ranges. We defined climatic niche shifts as instances where the ranges of a climatic variable within native and non‐native basins exhibit zero overlap. Results We detected at least one climatic niche shift for each of the 18 studied species. However, we did not detect common patterns in the thermal preference or biogeographic origin of the non‐native fish, hence suggesting a species‐specific response. Main conclusions Coarse‐grained occurrence data can be used to detect climatic niche shifts. They also enable the identification of the species experiencing niche shifts, although the mechanisms responsible for these shifts (e.g. local adaptation, dispersal limitation or physiological constraints) have yet to be determined. Furthermore, the coarse‐grained approach, which highlights regions where climatic niche shifts have occurred, can be used to select specific river basins for more detailed, fine‐grained studies.     

Let’s eat out,there’s crayfish for dinner: American bullfrog niche shifts inside and outside native ranges and the effect of introduced crayfish

In invasion ecology, niche width has been recognized as a crucial factor for the outcome of an invasion. A common characteristic of successful invaders seems to be a broad niche width, and their impact on native communities may increase with increasing niche size. Overall, successful invader predators are predicted to shift their niche width by broadening it from native to invaded conditions. The scarcity of ecological studies examining invasive species in their native ranges prevents researchers from knowing if the prevalence of generalist invaders represents conservatism of broad native-range niches or instead niche shifts as a result of different processes acting in the invaded areas. Here we reviewed literature on trophic niche of the predatory invader American bullfrog (Lithobates catesbeianus) in both native and invaded ranges. We used statistical and graphic tools to analyse possible shifts in dietary niche width and the effect of introduced crayfish on the feeding strategy of L. catesbeianus. Globally, our results indicate that food sources used by the species differed in native and invaded sites, with a narrower trophic niche width in invaded areas. However, this pattern was disrupted by the occurrence of introduced crayfish that represents the major driver of the observed niche-width variation. Our data shed light on possible complications in interpreting and predicting patterns of biological invasions due to the interaction among species from different trophic levels that apparently disrupt general patterns that are likely bound to be idiosyncratic and complex.     

Trophic redundancy among fishes in an East African nearshore seagrass community inferred from stable‐isotope analysis

Stable‐isotope analysis supplemented with stomach contents data from published sources was used to quantify the trophic niches, trophic niche overlaps and potential trophic redundancy for the most commonly caught fish species from an East African nearshore seagrass community. This assessment is an important first step in quantifying food‐web structure in a region subject to intense fishing activities. Nearshore food webs were driven by at least two isotopically distinct trophic pathways, algal and seagrass, with a greater proportion of the sampled species feeding within the seagrass food web (57%) compared with the algal food web (33%). There was considerable isotopic niche overlap among species (92% of species overlapped with at least one other species). Narrow isotopic niche widths of most (83%) species sampled, low isotopic similarity (only 23% of species exhibited no differences in δ¹³C and δ¹⁵N) and low predicted trophic redundancy among fishes most commonly caught by fishermen (15%), however, suggest that adjustments to resource management concerning harvesting and gear selectivity may be needed for the persistence of artisanal fishing in northern Tanzania. More detailed trophic studies paired with information on spatio‐temporal variation in fish abundance, especially for heavily targeted species, will assist in the development and implementation of management strategies to maintain coastal food‐web integrity.     

Impacts of zebra mussels (Dreissena polymorpha) on isotopic niche size and niche overlap among fish species in a mesotrophic lake

Zebra mussels (Dreissena polymorpha) filter feed phytoplankton and reduce available pelagic energy, potentially driving fish to use littoral energy sources in lakes. However, changes in food webs and energy flow in complex fish communities after zebra mussel establishment are poorly known. We assessed impacts of zebra mussels on fish littoral carbon use, trophic position, isotopic niche size, and isotopic niche overlap among individual fish species using δ¹³C and δ¹⁵N data collected before (2014) and after (2019) zebra mussel establishment in Lake Ida, MN. Isotope data were collected from 11 fish species, and from zooplankton and littoral invertebrates to estimate baseline isotope values. Mixing models were used to convert fish δ¹³C and δ¹⁵N into estimates of littoral carbon and trophic position, respectively. We tested whether trophic position, littoral carbon use, isotopic niche size, and isotopic niche overlap changed from 2014 to 2019 for each fish species. We found few effects on fish trophic position, but 10 out of 11 fish species increased littoral carbon use after zebra mussel establishment, with mean littoral carbon increasing from 43% before to 67% after establishment. Average isotopic niche size of individual species increased significantly (2.1-fold) post zebra mussels, and pairwise-niche overlap between species increased significantly (1.2-fold). These results indicate zebra mussels increase littoral energy dependence in the fish community, resulting in larger individual isotopic niches and increased isotopic niche overlap. These effects may increase interspecific competition among fish species and could ultimately result in reduced abundance of species less able to utilize littoral energy sources.

     

Effects of invasive macrophyte on trophic diversity and position of secondary consumers

Invasive species are one of the widespread stressors of aquatic ecosystems. Several studies document food web effects of invasive fish, but little information is available on the effects of invasive macrophytes. We studied differences in food chain length as well as trophic position and trophic diversity of fish and odonates in lakes dominated by native plants or invasive Eurasian watermilfoil. Trophic position and food chain length were determined using baseline-adjusted δ¹⁵N isotope signatures. Trophic diversity, or isotope niche width, was estimated from convex hull area analysis. Results show that trophic position of secondary consumers was not affected by the invasive macrophyte, whereas trophic diversity was greater in watermilfoil-dominated lakes. The direction of isotopic niche expansion was different in fish and odonates, suggesting potential decoupling in predator–prey interactions. This study shows that dominant non-native macrophytes may cause significant changes in food web structure of invaded ecosystems. Trophic diversity may be a more sensitive indicator of environmental stress than trophic position and has the potential to be used for assessment of invasive species impacts and restoration success.     

Comparative feeding ecology of invasive Ponto-Caspian gobies

Invasions of Ponto-Caspian gobiid fishes are suspected to cause regime shifts in freshwater ecosystems. This study compared the trophic niche differentiations of Neogobius melanostomus and Ponticola kessleri in the upper Danube River using stable isotope analyses (δ¹³C and δ¹⁵N), gut content analyses and morphometric analyses of the digestive tract. Both species were identified as predacious omnivores with high dietary overlap and a generalistic feeding strategy. Amphipods (especially invasive Dikerogammarus spp.) contributed 2/3 to the index of food importance. δ¹⁵N-signatures of N. melanostomus revealed an ontogenetic diet shift and significantly exceeded those in P. kessleri by ~1.5‰, indicating a niche separation of half a trophic level. P. kessleri had shorter uncoiled intestinal tracts than N. melanostomus, indicating a narrower niche and adaptation to animal food. Trophic niches in both species expanded during the growth period with increasing intraguild predation and cannibalism in P. kessleri and increasing molluscivory in N. melanostomus. P. kessleri showed a higher degree of specialization and more stable feeding patterns across seasons, whereas N. melanostomus adapted its diet according to the natural prey availability. The feeding patterns of both species observed in the upper Danube River strongly differ from those in their native ranges, underlining their great plasticity. Both goby species consumed mainly other non-native species (~92% of gut contents) and seemed to benefit from previous invasions of prey species like Dikerogammarus villosus. The invasive success of gobies and their prey mirror fundamental ecological changes in large European freshwater ecosystems.     

Trophic ecology of the Atlantic blue crab Callinectes sapidus as an invasive non-native species in the Aegean Sea

Invasive non-native crustaceans are a biodiversity and management concern in the Mediterranean Sea. The Atlantic blue crab (Callinectes sapidus) was first recorded in the Mediterranean Sea in 1949, but may have arrived as early as in the 1930’s. Blue crabs in the Mediterranean Sea are of concern due to their presumed potential for negative consumptive and competitive interactions with native fauna. The aim of this study was to provide a first assessment of the trophic ecology of non-native blue crab in the Northern Aegean Sea using stable carbon (δ¹³C) and nitrogen isotope (δ¹⁵N) analysis. We found limited isotopic niche overlap between blue crabs and seven native species examined at Gökçeada Island in April, June, and August of 2017. In addition, the range of calculated trophic positions of blue crabs at Gökçeada Island (2.0 to 4.4), while broad, is in general agreement with prior studies in both native and non-native ranges. We also observe that trophic position declined and the relative importance of pelagic carbon sources to blue crabs increased from April to August. However, we also found that differing assumptions as to the number and type of food web baselines and trophic discrimination factors led to differing estimates of trophic position in blue crabs at Gökçeada Island by as much as one to two trophic levels. These methodical differences make it challenging to directly compare results within and between studies, and thus limit our ability to assess negative consumptive and competitive interactions of invasive blue crab with native coastal species in the Mediterranean Sea.

     

Niche dynamics of sympatric non-breeding shearwaters under varying prey availability

Variation in prey availability can cause changes in species interactions among marine predators. Foraging theory predicts that niche breadth will expand when resources become limited, possibly leading to higher niche overlap among sympatric species; however, a species’ niche can become constrained by interactions with other similar species, resulting in an inability to shift niche breadth or position. In coastal Newfoundland, Capelin Mallotus villosus is the main forage fish species and its availability (i.e. biomass) during the summer has varied considerably following a population collapse in the 1990s. During the summer, non-breeding Great and Sooty Shearwaters Ardenna gravis and A. grisea migrate and aggregate at annually persistent Capelin spawning sites. We used stable isotope ratios (δ¹³C, δ¹⁵N) of blood components (plasma, red blood cells) to investigate variation in isotopic niche breadth (95% prediction ellipse areas) and overlap of the two shearwater species during 2014, 2015 and 2016. Capelin availability varied among years, illustrated by lower peak biomass in 2015 (0.126 g/m²) and 2016 (0.027 g/m²) relative to 2014 (0.254 g/m²). The isotopic niche breadth (plasma) of both shearwater species expanded similarly from 2014 (0.65–0.66‰²) to 2015 (2.22–2.57‰²) and 2016 (1.15–1.42‰²), suggesting the incorporation of alternative prey types into their diet during years of lower prey availability. Isotopic niche overlap between Great and Sooty Shearwaters remained high across years (44–63%), however, providing little evidence for dietary niche partitioning during years of lower prey availability. Findings suggest that both shearwater species are flexible foragers and can modify their diet during the non-breeding season to accommodate fluctuations in prey availability.     

Gut content and stable isotope analyses provide complementary understanding of ontogenetic dietary shifts and trophic relationships among fishes in a tropical river

1. Despite widespread recognition of the role of body size in fish trophic ecology, little attention has been focused on this issue in isotopic studies, particularly in tropical systems. 2. We used analyses of stomach contents and stable isotopes to examine size‐related shifts in diet in a terapontid fish assemblage in the Australian wet–dry tropics. Stomach content analysis identified substantial ontogenetic dietary shifts in all species, corresponding to changes in body size–isotope trajectories for two species. Shifts away from relatively specialised diets of heavily ¹³C‐depleted insect larvae to consumption of a range of items across multiple basal carbon sources appeared to be the proximate cause of observed isotopic changes. 3. Allochthonous organic matter in the form of C3 riparian vegetation was particularly important to smaller terapontids before larger fish shifted to a broad range of dietary items and similarly broad range of basal carbon sources. 4. While there was general agreement between δ¹³C and stomach content analysis, there was minimal concurrence between the latter and δ¹⁵N isotopic derivation of estimates of trophic position. Due to factors such as omnivory, isotopically overlapping basal sources and uncertainties about rates of isotopic fractionation in both predator and prey species, stomach content analysis provides an essential complement to isotopic methodologies in tropical systems. 5. Given that basal sources supporting any individual species can change markedly with ontogeny, consideration of intraspecific, size‐related variation is necessary in isotopic studies of food web structure.