Natural born killers: an invasive amphipod is predatory throughout its life-history

Ecological impacts of an invasive top predator fish across South America

Peacock bass Cichla ocellaris is a piscivorous cichlid native from the Amazon and Orinoco river basins, which has been broadly introduced into tropical areas worldwide, leading to several adverse local effects. However, predictors of its invasibility and assessments of its ecological impacts over large spatial scales are still lacking. The importance of different environmental factors in explaining the relative abundance of peacock bass in 62 sites across South America (30 native and 32 invaded systems) was investigated. The impacts of peacock bass on fish assemblages were appraised, using years since introduction as a proxy of its cumulative impacts and modern statistical techniques, such as random forests, and negative binomial regression models. Random forests highlighted maximum depth, introduced status, and ecosystem type as the best predictors of the peacock bass relative abundance, which ranged 0.01-26.0%, increased with maximum depth, was highest in invaded reservoirs but decreased with depth in native riverine populations. Other factors such as climate or limnological features were less important in explaining C. ocellaris abundance, which did not vary markedly with years since introduction. Introduction year was not related to latitude but varied among hydrographic regions, indicating invasion pathways not linked to geographical proximity. Variation partitioning of different fish assemblage metrics showed that hydrographic region followed by limnological and reservoir features accounted for most explained variation, indicating a strong historical and local influence. Introduction time accounted for 5-8% of variation in species composition and diversity, independently of limnological features. Our results suggest that the ecological effects of introduced C. ocellaris on native fish fauna are likely but small compared to large geographical and environmental gradients. Although experiments and before-after designs are probably more sensitive in detecting the ecological impacts of invasive species, large-scale compilations of available data are more feasible and can provide invaluable information, especially for large-sized invaders that are often illegally introduced.

Ontogenetic shift in the trophic role of the invasive killer shrimp Dikerogammarus villosus: a stable isotope study

The introduction of the amphipod Dikerogammarus villosus in European fresh waters is to date recognized as a threat to the integrity of invaded communities. Predation by D. villosus on native benthic invertebrates is assumed as the key determinant of its ecological impact, yet available information describe the species as a primary consumer as well as a carnivore depending on local conditions. Here, we assessed the trophic position (TP) of D. villosus in Lake Trasimeno, a recently invaded lentic system in central Italy, using the CN isotopic signatures of individuals captured in winter spanning two orders of magnitude in body size. TP estimations were compared with those characterizing the native amphipod Echinogammarus veneris and other representative invertebrate predators. On average, D. villosus showed a trophic position higher than E. veneris, and comparable with that of odonate nymphs. An in-depth analysis revealed that large-sized individuals had a trophic position of 3.07, higher than odonates and close to that of the hirudinean predator Erpobdella octoculata, while small-sized specimens had a trophic position of 2.57, similar to that of E. veneris (2.41). These findings indicate that size-related ontogenetic shifts in dietary habits may per se vary the nature of the interaction between Dikerogammarus villosus and native invertebrates from competition to predation. Information collated from published isotopic studies corroborated the generality of our results. We conclude that intra-specific trophic flexibility may potentially amplify and make more multifaceted the impact of the species on other invertebrate species in invaded food webs.