Biotic‐environment interactions have long been considered an important factor in functional phenotype differentiation in organisms. The differentiation processes determining functional phenotypes can reveal important mechanisms yielding differences in specific functions of animal traits in the ecosystem. In the present study, we examined functional morphological variations in relation to increasing geographic altitude. Six fish species were examined for how environment factors affect intra‐specific functional morphology in the subtropical Pearl River in southern China. Functional morphology traits revealed variable effects due to geographic elevation, although spatial autocorrelation existed among the species tested. The results showed that high‐elevation individuals had a more narrow‐bodied morphology, with more flexible maneuvrability when swimming, and more evenly distributed musculature than low‐elevation individuals. Low‐elevation individuals preyed upon larger food sources than high‐elevation individuals in some species. Fish functional morphology was strongly affected by regional environmental factors (such as elevation and water temperature) and physical characteristics of local rivers (such as flow velocity, river fractals, and coefficients of fluvial facies). In addition, the effects of the regional factors were stronger than those of the local factors in the Pearl River. Furthermore, it was found that morphological traits associated with locomotion were primarily effected by the river's physical characteristics. While morphological traits associated with food acquisition were primarily affected by water chemical factors (such as DO, water clarity, NH₄‐N concentration, and TDS). These results demonstrated that habitat has an influence on the biological morphology of fish species, which further affects the functioning of the organism within the ecosystem.

Widespread introductions of non-native species, including aquaculture and ornamental species, threaten biodiversity and ecosystem functioning by modifying the trophic structure of communities. In this study, we quantified the multiple facets of trophic disruption in freshwater communities invaded by Nile tilapia, by comparing uninvaded and invaded rivers downstream of the Pearl River, China. Nile tilapia invasion reduced the trophic status of native fish species by forcing native herbivores and planktivores to seek new food sources. The food chain was also shortened by decreasing the trophic levels of native invertivores, omnivores, and piscivores, while the total isotopic niche area (TA) of native invertivores, omnivores, piscivores, and planktivores species also decreased. Simultaneously, Nile tilapia invasion affected the isotopic diversity of the fish community. Decreasing isotopic richness (IRic), isotopic evenness (IEve), and increasing isotopic uniqueness (IUni) indicated that Nile tilapia had a high trophic niche overlap with native species and competed with native species for food resources, and even caused the compression of the trophic niche of native species. Understanding the process described in this study is essential to conserve the stability of freshwater ecosystems, and improve the control strategy of alien aquatic organisms in south China.

Background: Although freshwater ecosystems cover less than 1% of the earth's surface, they support extremely high levels of biodiversity and provide vital ecosystem services. However, due to the introduction of non-native fishes, aquatic ecosystem functioning has been altered, and in some cases, declined sharply. Quantifying the impacts of invasive species has proven problematic. In this study, we examined the relative trophic position of native piscivorous fishes to estimate the effects of invasive Nile tilapia on food webs in the downstream sections of an invaded large subtropical river, the Pearl River, China. Furthermore, we quantified how native piscivorous fish diets changed as the Nile tilapia invasion progressed. Results: The trophic position of the widely distributed and locally important economically harvested piscivorous culter fish (Culterrecurviceps), mandarinfish (Sinipercakneri), and catfish (Pelteobagrusfulvidraco) lowered significantly in the invaded Dongjiang River compared to an uninvaded reference Beijiang River. The lower trophic position of these piscivorous fishes was reflected by a major reduction in the proportion of prey fish biomass in their diets following the Nile tilapia invasion. Small fishes in the diet of culter fish from the reference river (33% small fishes, 17% zooplankton) shifted to lower trophic level zooplankton prey in the invaded river (36% zooplankton, 25% small fish), possibly due to the presence of Nile tilapia. Additionally, small fishes in the diet of mandarinfish in the reference river (46% small fishes, 11% aquatic insects) declined in the invaded river (20% aquatic insects, 30% small fishes). Similarly, the diet of catfish from the reference river shifted from fish eggs (25% fish eggs, 25% aquatic insects) to aquatic insects in the invaded river (44% aquatic insects, 5% fish eggs). Conclusions: The results of this study contributed to a growing body of evidence, suggesting that Nile tilapia can modify trophic interactions in invaded ecosystems. It is crucial to understand the processes outlined in this study in order to better assess non-native aquatic species, conserve the stability of freshwater ecosystems, and improve current conservation strategies in reaches of the Pearl River and other similar rivers that have experienced invasions of non-native species.