Predicting the competitive interactions and trophic niche consequences of a globally invasive fish with threatened native species

Trophic niche partitioning and intraspecific variation in food resource use in the genus Pangasianodon in a reservoir revealed by stable isotope analysis of multiple tissues

Understanding the mechanism by which non-native fish species integrate into native communities is crucial for evaluating the possibility of their establishment success. The genus Pangasianodon, comprising Pangasianodon gigas and Pangasianodon hypophthalmus, has been introduced into reservoirs, which are non-native habitats, for fishery stock enhancement. P. gigas and P. hypophthalmus often successfully establish and co-occur in several Thai reservoirs, but there is little information on differences in food resource use between the two species. To investigate the trophic niche width of P. gigas and P. hypophthalmus in a Thai reservoir, we conducted stable carbon and nitrogen ratio (δ13C and δ15N) analyses. We examined the degree of individual specialization in both species using the δ13C and δ15N values of muscle and liver tissues, which provides long- and short-term diet information. The isotopic niches did not overlap between P. gigas and P. hypophthalmus. The δ15N value of P. gigas was significantly higher than that of P. hypophthalmus, whereas the δ13C value did not significantly differ between the two species. The isotopic niche sizes were larger in P. hypophthalmus than in P. gigas. Individual specialization was observed in P. hypophthalmus but not in P. gigas, indicating that intraspecific variation in food resource use was larger in P. hypophthalmus compared to P. gigas. These findings suggest that trophic niche partitioning was one of the factors facilitating the establishment success of P. gigas and P. hypophthalmus in a reservoir, but the establishment process may differ between the two species.

Assessing the role of non-native species and artificial water bodies on the trophic and functional niche of Mediterranean freshwater fish communities

Habitat alterations and the introduction of non-native species have many ecological impacts, including the loss of biodiversity and a deterioration of ecosystem functioning. The effects of these combined stressors on the community trophic web and functional niche are, however, not completely clear. Here, we investigated how artificial ecosystems (i.e. reservoirs) and non-native species may influence the trophic and functional niche space of freshwater fish communities. To do so, we used carbon and nitrogen stable isotope and abundance data to compute a set of isotopic, trait, and functional metrics for 13 fish communities sampled from 12 distinct ecosystems in Türkiye. We show that in reservoirs, fish were more similar in their trophic niche compared to lakes, where the trophic niche was more variable, due to higher habitat complexity. However, there were no differences in the trait and functional metrics between the two ecosystem types, suggesting a higher prey diversity than assumed in reservoirs. We also found that the number of non-native species did not affect the trophic niche space, nor the trait or functional space occupied by the fish community. This indicates that non-native species tended to overlap their trophic niche with native species, while occupying empty functional niches in the recipient community functional space. Similarly, the proportion of non-native species did not affect any trophic, trait, or functional metric, suggesting that changes in community composition were not reflected in changes in the community niche space. Moreover, we found that trait richness, but not functional richness, was positively related to the isotopic niche width and diversity, indicating that a wider occupied trait niche space corresponded with a wider occupied trophic niche and lesser interspecific similarity. Our findings underscore the complexity of ecological relationships within freshwater ecosystems and highlight the need for comprehensive management strategies to mitigate the impacts of human activities and biological invasions.

Use of trophic ecology of omnivorous fish and abiotic factors as supporting tools for assessing environmental impacts in a neotropical river

The study of diet is one of the mechanisms by which competition for resources between species that cohabit in the same ecosystem can be inferred. Therefore, the relationships of the indices that measure specialization in the diet of fish species are necessary to characterize the nutritional quality of these populations and the ecosystem's environmental health. Three species of catfish were selected: one invasive (Clarias gariepinus) and two natives (Trachelyopterus striatulus and Rhamdia quelen), with similar distribution along the Guapi-Macacu River, in the Guapimirim Protection Area (Rio de Janeiro). Fifty-nine catfish of the three species were collected in total, along 32 collection points in the Guapi-Macacu River in two periods (dry and rainy) in 2018. Non-parametric statistics showed the partition of resources between species and the influence of abiotic factors (temperature, pH, transparency, and dissolved oxygen) contributing to the selection of available resources in the environment. Diet-related indices-repletion index (RI), condition factor (K), niche width, and trophic position (TP) of the specimens collected-contributed to measuring the nutritional status of each of these catfish species, showing that R. quelen has a relationship between RI and K, tending to absorb and metabolize nutrients faster than other species. In addition, the invasive species occupies a wide range of TPs compared to native species, confirming its feeding plasticity. On the contrary, T. striatulus needs large amounts of terrestrial insects to maintain its poor condition factor. Also, the RI showed direct influences of abiotic variables, with the temperature being the most prominent. Our results suggest that the invasive species can benefit from this environment that shows signs of environmental degradation.

Inter - and intraspecific phenotypic variation in co-occurring invasive and introduced cyprinid fish species

A species introduced outside of its native range will likely encounter unusual abiotic and biotic conditions,and may exhibit phenotypic traits that may facilitate survival and persistance. Phenotypic plasticity drives non-native species' development of adaptive traits in the new environment, increases their fitness, and as a result, contributes to invasion success. In this study, we examined inter and intraspecific phenotypic variation (body size and shape) for an invasive (Carassius gibelio) and introduced (Cyprinus carpio) cyprinid fish species (Teleostei: Cyprinidae) in the Düden Stream, Turkey, which is a small-scale river system. We hypothesized that interspecific phenotypic variation correlates with fish-specific variables and river site. We further hypothesized that these two species may exhibit similar phenotypic variation patterns between populations. The MANCOVA revealed that species-specific traits, river site, had significant effects on body shape variation and size along the stream. The differences in the shape of the head, the central portion of the body, and fins in both species most probably reflected differences in the swimming and feeding of the fish, possibly to avoid interspecies competition. The intraspecific phenotypic variation observed in both species may indicate rapid local adaptation, triggered by multiple founding event, or/and phenotypic plasticity.

Characterizing functional morphology and trophic niches in a neotropical Characiforms (Actinopterygii: Teleostei) assemblage in middle Munim River basin, Maranhão, Brazil

Understanding how functionally similar species segregate resources to minimize competition is vital for predicting evolutionary factors and patterns of coexistence. We conducted a study in Mata de Itamacaoca, in the middle Munim River basin, Maranhão, northeastern Brazil, to characterize the functional morphology and trophic niches of five coexisting Characiform species in this area - including a recently described species, and to investigate whether their functional morphology is a key determinant of their trophic niches. Our analysis of functional morphology and diet, employing linear measurements to predict dietary specializations, showed that these species are predominantly generalist insectivores with a significant morphological overlap. This study underscores the influence of species' natural history on their ecological characteristics, contributing to more effective conservation strategies.

Contemporary perspectives on the ecological impacts of invasive freshwater fishes

Introductions of non-native freshwater fish continue to increase globally, although only a small proportion of these introductions will result in an invasion. These invasive populations can cause ecological impacts in the receiving ecosystem through processes including increased competition and predation pressure, genetic introgression and the transmission of non-native pathogens. Definitions of ecological impact emphasize that shifts in the strength of these processes are insufficient for characterizing impact alone and, instead, must be associated with a quantifiable decline of biological and/or genetic diversity and lead to a measurable loss of diversity or change in ecosystem functioning. Assessments of ecological impact should thus consider the multiple processes and effects that potentially occur from invasive fish populations where, for example, impacts of invasive common carp Cyprinus carpio populations are through a combination of bottom-up and top-down processes that, in entirety, cause shifts in lake stable states and decreased species richness and/or abundances in the biotic communities. Such far-reaching ecological impacts also align to contemporary definitions of ecosystem collapse, given they involve substantial and persistent declines in biodiversity and ecosystem functions that cannot be recovered unaided. Thus, while not all introduced freshwater fishes will become invasive, those species that do develop invasive populations can cause substantial ecological impacts, where some of the impacts on biodiversity and ecosystem functioning might be sufficiently harmful to be considered as contributing to ecosystem collapse.

Isotopic Overlap of Invasive and Native Consumers in the Food Web of Lake Trasimeno (Central Italy)

An advanced characterization of the trophic niche of non-indigenous species (NIS) may provide useful information on their ecological impact on invaded communities. Here, we used carbon and nitrogen stable isotopes to estimate pairwise niche overlaps between non-indigenous and native consumers in the winter food web of Lake Trasimeno (central Italy). Overall, a relatively low pairwise overlap of isotopic niches was observed between NIS and native species. The only exception was the Louisiana crayfish Procambarus clarkii, which showed a relatively high and diffuse overlap with other native invertebrates. Our findings highlighted a high niche divergence between non-indigenous and native species in Lake Trasimeno, suggesting a potentially low degree of interspecific competition that may facilitate coexistence and, in turn, limit the strength of impacts. The divergent results obtained for the Louisiana crayfish indicate that additional control measures for this invasive species are needed to mitigate its impact on the Lake Trasimeno system.

Invasive carp alter trophic niches of consumers and basal resources in African reservoirs

Environmental pollution and biological invasions are key drivers of biodiversity change. However, the effects of invasion and pollution on food webs remain largely unexplored. Here, we used stable isotopes to examine the effects of common carp Cyprinus carpio and pollution on trophic dynamics in six small reservoirs. Our results revealed that the trophic niche widths of invertebrates, vertebrates, and invasive carp did not significantly differ among reservoirs with different pollution statuses. However, we found low niche conservatism among reservoirs, suggesting that while niche width may remain consistent, there is a shift in the position of the niches in isotopic space under both pollution and invasion scenarios. Niche conservatism among reservoirs was generally higher in invertebrates, but this was also regardless of reservoir condition (i.e. presence or absence of pollution and invasion). These results suggest that invasion by species coupled with organic pollution may cause subtle yet differing effects on components of a food web (basal end-members, invertebrates and vertebrates). Our findings provide a baseline measure of the potential in the development of detection and response strategies for carp invasions and organic pollution.