Sandip Tapkir

Anthropogenic eutrophication poses a significant threat to freshwater environments globally. It also influences the population dynamics of invasive and native species, yet the cumulative effects of eutrophication and invasive species on native organisms are not well understood. We used invasive gibel carp (Carassius gibelio Bloch, 1782) and native crucian carp (Carassius carassius Linnaeus, 1758) as model taxa to investigate how eutrophication influences their interspecific competition. Carassius carassius is being outcompeted by invasive C. gibelio across most of its native range, and eutrophication may play a role in the competitive displacement of C. carassius. We explored how varying eutrophication levels influence growth and survival of both species in laboratory and mesocosm experiments, where we exposed them to different feeding rations and nutrient levels, respectively. We hypothesized that (1) C. gibelio benefits more from increased nutrient levels than C. carassius—which favors invasive C. gibelio over native C. carassius in interspecific competition—and that (2) the growth and survival of C. carassius deteriorate under interspecific competition with C. gibelio compared to intraspecific competition. Our experiments revealed that excess nutrient availability was more beneficial to the invasive C. gibelio. Overall, C. gibelio individuals grew more than C. carassius, and the difference between the two species increased with nutrient levels in both experiments, especially in the weight of smaller individuals. Moreover, interspecific competition led to lower survival in C. carassius but not in C. gibelio in the mesocosm experiment. This study shows that eutrophication may modulate interspecific competition, and cultural eutrophication of freshwater habitats may enhance the success of invasive species. Graphical abstract

Invasive species pose a major threat to natural ecosystems and directly outcompete many native species, placing them at imminent threat of extinction. The topmouth gudgeon (Pseudorasbora parva) is on the EU’s blacklist of invasive freshwater species and threatens biodiversity, especially in wetland and floodplain ecosystems, aquacultures and village ponds. The crucian carp (Carassius carassius) is native to Europe and its populations have declined in large part of its native range, with invasive gibel carp (C. gibelio) suspected as a major cause of its decline. Invasions by topmouth gudgeon have been implicated in the decline of crucian carp populations but this still needs to be verified. The aim of this study was to evaluate by the experimental approach the competitive interaction between the two species, topmouth gudgeon and crucian carp, focusing on isotopic niche sizes and their overlap in syntopy. A four-month mesocosm experiment was performed to determine the isotopic niche of crucian carp and topmouth gudgeon living alone and in syntopy. Additionally, stable isotope data were collected at the sites where the two species co-occur to compare niche sizes and overlaps. Experimental data showed that the isotopic space of topmouth gudgeon responded more flexibly (reducing niche size at syntopy) than that of the crucian carp and confirmed a high isotopic niche overlap between the species. Field studies have shown that topmouth gudgeon has invaded the isotopic niche of the crucian carp, especially when another invasive species, the gibel carp, lived in the community (25% at 40% ellipse area and 50% at 95% ellipse area). When only the topmouth gudgeon and crucian carp were present in the field, the overlap was lower (3% and 48%, respectively) and directional overlap modelling showed that the crucian carp was more likely to invade the isotopic niche of topmouth gudgeon than vice versa. The data indicated that competition between crucian carp and topmouth gudgeon is likely, especially in syntopy with other invasive species. This study shows that the feeding plasticity of topmouth gudgeon likely facilitates its establishment outside its native range and, due to high isotopic niche overlap, threatens native fish with similar feeding ecology with competitive displacement.

The introduction of invasive species leading to decline of freshwater fauna is a major concern for conservation biologists. In this study we examined the effect of introduced Tilapia on the survival of the loach Lepidocephalichthys thermalis via predation experiments with Tilapia and a native predator, the Snakehead fish Channa gachua. Examining the chemoecology of prey-predator interactions, we found that L. thermalis failed to detect water-borne cues from Tilapia but did recognize cues from C. gachua, indicating innate predator recognition. We also observed that L. thermalis can learn to associate kairomones with Tilapia when conditioned with kairomones and injured conspecific cues. Trained L. thermalis showed higher survival during Tilapia predation trials. Thus under experimental conditions the vulnerability of L. thermalis to Tilapia predation due to failure to detect chemical cues can be reduced via associative training. It remains to be determined how useful this behavioral plasticity can be in wild L. thermalis populations exposed to introduced Tilapia.