Intra- and inter-continental variation in the functional responses of a high impact alien invasive fish

Recently, a body of literature has indicated the utility of comparisons among introduced and native species of their functional responses, that is, the relationship between resource use (e.g. predator consumption rate) and resource availability (e.g. prey density) to predict their impact. However, a key feature of this methodology, that has not yet been examined, is the degree to which the functional response curves of an introduced species differ within and between its native and introduced geographical ranges. Information on the variation in functional responses is key to make robust assessments on the ecological impact and to assess possible differences between native and invasive species. Here, we examine the predatory functional responses in multiple native and introduced populations of a globally high impact alien invasive fish, the topmouth gudgeon (Pseudorasbora parva). In standardised aquaria and with two prey species, significant differences in the functional responses among and between different populations occurring in the native and the invaded range were found. Among populations in the native range, the functional response indicated little variation, and fish always showed a Type II response, irrespective of the type of prey used. In the introduced range, populations showed a Type II response when fed chironomid larvae as prey, while a Type III response was observed when feeding Daphnia magna. Populations in the invaded range consumed overall more prey when fed D. magna compared to the populations in the native range. When feeding chironomid larvae, no consistent trend was observed. Context dependencies as well as species-specific traits and fish density most likely play an important role when comparing the functional response between populations occurring in their native and invaded ranges

Shell shocked: high potential impacts on native prey by non-native turtles irrespective of benthic habitat context

The poorly-regulated pet trade is a major source of potential invasive species, with deliberate release often resulting in introductions of non-native species without invasion histories. Predicting the potential impacts of species with no invasion history is particularly difficult. Functional responses (FRs; resource use as a function of resource density) have proven useful in the quantification of ecological impacts of invasive species, and may be used to screen likely impacts of species which lack invasion histories. Here, we used laboratory experiments to quantify the FRs of four freshwater turtles commonly traded as household pets: Trachemys scripta scripta, the yellow-bellied slider; T. s. troostii, the Cumberland slider; Sternotherus odoratus, the common musk turtle; and Kinosternon subrubrum, the Eastern mud turtle, towards representative chironomid and gammarid prey under eight densities in the presence and absence of substrate. All turtles exhibited potentially population destabilising Type II (hyperbolic) FRs towards each prey type, irrespective of the presence of substrate, characterised by high predation rates at low prey densities. Magnitudes of FRs were generally higher for T. s. scripta and T. s. troostii compared to S. odoratus and K. subrubrum. The presence of substrate reduced the magnitude of FRs towards both prey types overall, however, these effects were modest and most pronounced for the two. T. scripta subspecies. We demonstrate marked and sustained predatory impacts of non-native turtles on native prey, irrespective of benthic habitat contexts. We conclude that commonly traded turtles that lack invasion histories could precipitate substantial ecological impact, particularly in freshwater ecosystems where there are no native analogues

Assessing the ecological impacts of invasive species based on their functional responses and abundances

Invasive species management requires allocation of limited resources towards the proactive mitigation of those species that could elicit the highest ecological impacts. However, we lack predictive capacity with respect to the identities and degree of ecological impacts of invasive species. Here, we combine the relative per capita effects and relative field abundances of invader as compared to native species into a new metric, “Relative Impact Potential” (RIP), and test whether this metric can reliably predict high impact invaders. This metric tests the impact of invaders relative to the baseline impacts of natives on the broader ecological community. We first derived the functional responses (i.e. per capita effects) of two ecologically damaging invasive fish species in Europe, the Ponto-Caspian round goby (Neogobius melanostomus) and Asian topmouth gudgeon (Pseudorasbora parva), and their native trophic analogues, the bullhead (Cottus gobio; also C. bairdi) and bitterling (Rhodeus amarus), towards several prey species. This establishes the existence and relative strengths of the predator-prey relationships. Then, we derived ecologically comparable field abundance estimates of the invader and native fish from surveys and literature. This establishes the multipliers for the above per capita effects. Despite both predators having known severe detrimental field impacts, their functional responses alone were of modest predictive power in this regard; however, incorporation of their abundances relative to natives into the RIP metric gave high predictive power. We present invader/native RIP biplots that provide an intuitive visualisation of comparisons among the invasive and native species, reflecting the known broad ecological impacts of the invaders. Thus, we provide a mechanistic understanding of invasive species impacts and a predictive tool for use by practitioners, for example, in risk assessments

Alien aquatics in Europe: assessing the relative environmental and socio-economic impacts of invasive aquatic macroinvertebrates and other taxa

Invasive alien aquatic species, including marine and freshwater macroinvertebrates, have become increasingly important in terms of both\ud environmental and socio-economic impacts. In order to assess their environmental and economic costs, we applied the Generic Impact Scoring\ud System (GISS) and performed a comparison with other taxa of invaders in Europe. Impacts were scored into six environmental and six socioeconomic\ud categories, with each category containing five impact levels. Among 49 aquatic macroinvertebrates, the most impacting species were the\ud Chinese mitten crab, Eriocheir sinensis (Milne-Edwards, 1853) and the zebra mussel, Dreissena polymorpha (Pallas, 1771). The highest impacts\ud found per GISS impact category were, separately; on ecosystems, through predation, as competitors, and on animal production. Eleven species have\ud an impact score > 10 (high impact) and seven reach impact level 5 in at least one impact category (EU blacklist candidates), the maximum score\ud that can be given is 60 impact points. Comparisons were drawn between aquatic macroinvertebrates and vertebrate invaders such as fish, mammals\ud and birds, as well as terrestrial arthropods, revealing invasive freshwater macroinvertebrates to be voracious predators of native prey and damaging\ud to native ecosystems compared with other taxa. GISS can be used to compare these taxa and will aid policy making and targeting of invasive\ud species for management by relevant agencies, or to assist in producing species blacklist candidates

Ecological impacts of an invasive predator are mediated by the reproductive cycle

Predicting the ecological impacts of invasive species is impeded by context-dependencies that can mediate the strength of trophic interactions. Reproductive status is a pervasive context across trophic and taxonomic groups, yet has mostly been neglected in studies of invader impact. The present study examines the influence of the reproductive cycle on predatory impacts of a known damaging invasive freshwater crustacean, Gammarus pulex (Amphipoda), using a comparative functional response approach. Across females that were non-ovigerous, or had immature- or mature-stage embryonic broods, all G. pulex exhibited potentially prey destabilising Type II functional responses towards chironomid larvae. Attack rates were highest by immature embryonic brood stages, and significantly lower by mature embryonic brood stages. Conversely, handling times were consistently lower, and hence maximum feeding rates higher, where broods were present as compared to absent. These predatory patterns may reflect changing resource demands for progeny development across the reproductive cycle, the influence of brood bulk, female moult stage and/or motivational aspects of feeding, such as avoidance of filial cannibalism. Accordingly, many aspects of reproduction may alter the intensity of predatory impacts by invasive species. Future studies considering invader impacts should consider the influence of sex and reproductive status to more holistically quantify and predict population-level invader impacts

Prey switching data from Resistance is futile: lack of predator switching and a preference for native prey predict the success of an invasive prey species

Underlying raw consumption data for Experiment

Prey size and predator density modify impacts by natural enemies towards mosquitoes

1. Interactions between multiple predators can modify prey risk and profoundly alter ecological community dynamics. Further, ontogenic prey size changes are known to mediate prey risk through refuge effects. Understandings of these biotic factors is important for robust quantifications of natural enemy effects on target species, yet their combined influence lacks investigation. 2. Functional responses were used to quantify the predatory impacts of Notonecta glauca (water boatman; Ng) and Gammarus pulex (river shrimp; Gp) towards four different larval instars of Culex pipiens in container‐style habitats. Using conspecific pairs of predators, multiple predator effects (MPEs) of both predator species were examined across larval prey sizes, and prey preference tests were applied to examine prey selectivity across predator–prey body size ratios. 3. Both predators were able to feed on C. pipiens across their larval ontogeny; however, Ng consumed significantly more larvae than Gp. Functional responses of Ng were typically Type IIs, whereas Gp trended towards sigmoidal Type IIIs. Predation by pairs of Ng and Gp showed independent MPEs towards first‐, third‐, and fourth‐instar stages (except predation by Gp at higher densities of fourth‐instar) stages, whereas, for second‐instar stages, Ng showed synergistic MPEs and Gp showed antagonistic MPEs. Both predators preferred late instar mosquitoes (Ng: fourth instar; Gp: third instar). These preferences reflected predator:prey weight–length ratios, showing that relative sizes of predators and prey are important factors in prey selectivity. 4. The results obtained in the present study demonstrate that MPEs, combined with intraspecific prey preferences, may modulate trophic interactions within ecosystems. Therefore, such effects should be increasingly considered to further the understanding of agent efficacies