Comparison of trophic function between the globally invasive crayfishes <Emphasis Type="Italic">Pacifastacus leniusculus</Emphasis> and <Emphasis Type="Italic">Procambarus clarkii</Emphasis>

Impacts of invasive species may manifest most strongly if these organisms are highly distinct functionally from the native species they often replace. Yet, should we expect functional differences between native and invasive species of generalist organisms like freshwater crayfish? Some existing evidence has pointed to native and invasive crayfish species as ecologically equivalent. We contribute to this literature by comparing the trophic niches of the globally invasive crayfishes Pacifastacus leniusculus and Procambarus clarkii, by applying carbon and nitrogen stable isotope analyses to replicated allopatric (alone) and sympatric (together) lake populations in western Washington State, USA, where P. clarkii has been recently introduced and P. leniusculus is presumed native. Our study corrected for potential inherent differences in lake food webs as a consequence of lake abiotic or biotic characteristics using random effects in linear mixed effects models. We found that although overall trophic niche size or area of these species was not significantly different, P. leniusculus was significantly higher in trophic position than P. clarkii when also accounting for the effects of body size, sex, and lakes as random effects. This pattern of increased trophic position of P. leniusculus over P. clarkii was conserved over time in one sympatric lake for which we had data over multiple years. Cumulatively, our findings point to trophic differences between the globally cosmopolitan crayfishes P. leniusculus and P. clarkii, particularly when accounting for the ways that ecosystem context can affect food web structure of communities and the trophic resources available to these consumers.     

Effects of crayfish on leaf litter breakdown and shredder prey: are native and introduced species functionally redundant?

Recent increases in biological invasions frequency may have important consequences on native communities. However, functional redundancy between invasive and native species could reduce non-native species effects on native ecosystems. Despite this, even small differences in functional traits between these species may still have unpredictable effects on colonized ecosystems. Invasive crayfish, as ecosystem engineers, potentially have wide and complex effects on recipient ecosystems, even when replacing a native counterpart. We used laboratory microcosms to test whether native (Astacus astacus) and invasive crayfish species (Orconectes limosus, Pacifastacus leniusculus and Procambarus clarkii) are actually functionally redundant in their effects on prey/shredder density and leaf litter breakdown. Results show that crayfish strongly influenced macroinvertebrate numbers and leaf litter breakdown and indicate that differences in direct (prey and leaf litter consumption) and indirect (prey habitat use and leaf litter breakdown) effects between crayfish species do exist. While the replacement of A. astacus by O. limosus may have induced only minor changes in freshwater ecosystems, invasions by the larger and more aggressive P. clarkii and P. leniusculus will likely have strong effects on invaded ecosystem. Overall, there seems to be no functional redundancy between these four species and outcomes of crayfish invasion will likely be species specific.     

Macro-invertebrate predators of freshwater pulmonate snails in Africa,with particular reference to Appasus grassei (Heteroptera) and Procambarus clarkii (Decapoda)

A range of African and alien macro-invertebrates has been reported preying on freshwater pulmonate snails, including those that serve as intermediate hosts for bloodflukes of the genus Schistosoma. Predation by five molluscivorous taxa is reviewed here: indigenous leeches (Glossiphoniidae), marsh fly larvae (Sciomyzidae), waterbugs (Belostomatidae), crabs (Potamonautidae) and invasive crayfish (Astacidae). Common features are a lack of prey specificity but clear prey-size specificity. Attention is drawn to the ability of invasive snail species (Physidae and Lymnaeidae) to avoid predation by several of these taxa. Evidence suggests that only the alien invasive crayfish Procambarus clarkii has potential as a snail biocontrol agent, but that its use should not be encouraged.     

Integration of an invasive consumer into an estuarine food web: direct and indirect effects of the New Zealand mud snail

Introduced species interact both directly and indirectly with native species. We examine interactions between the introduced New Zealand mud snail (Potamopyrgus antipodarum) and native estuarine invertebrates and predators through experiments and field studies. A widely held management concern is that when P. antipodarum, which has low nutritional value, becomes abundant, it replaces nutritious prey in fish diets. We tested two key components of this view: (1) that fish consume, but get little direct nutritional value from P. antipodarum; and (2) that P. antipodarum has an indirect negative effect on fish by reducing the energy derived from native prey. We also examined predation by the native signal crayfish, Pacifastacus leniusculus. Laboratory feeding trials showed that both crayfish and fish consume P. antipodarum, a direct effect. Crayfish consumed and successfully digested higher numbers of snails than did fish [Pacific staghorn sculpin (Leptocottus armatus), three spine stickleback (Gasterosteus aculeatus), and juvenile starry flounder (Platicthys stellatus)]. P. antipodarum occurred at low frequencies in the stomachs of wild-caught fish. More interesting were the indirect effects of this invader, which ran counter to predictions. P. antipodarum presence was associated with no change or an increase in the amount of energy derived from native prey by predators. The presence of P. antipodarum also led to increased consumption of and preference for the native amphipod Americorophium salmonis over the native isopod Gnorimosphaeroma insulare. This is an example of short-term, asymmetric, apparent competition, in which the presence of one prey species (snails) increases predation on another prey species (the amphipod).     

Invasion of the shelter snatchers: behavioural plasticity in invasive red swamp crayfish,Procambarus clarkii

1. In many freshwater systems, competition for shelter plays an important role in determining the persistence of both native and alien species. The red swamp crayfish, Procambarus clarkii, is currently invading the native habitat of the signal crayfish, Pacifastacus leniusculus, in southern Oregon, and interspecific competition for shelter may be driving the species replacement in this region. 2. We designed a 2 × 3 factorial mesocosm experiment, with shelter density and species combination as factors, to investigate shelter occupancy and resource competition. Contrary to our predictions, the two crayfish species are equal competitors for shelter. Further, the invasive P. clarkii modified its shelter occupancy behaviour in the presence of the native P. leniusculus and has broader microhabitat preferences. 3. Specifically, we found that P. clarkii alters shelter occupancy and space use patterns when the two species occurred together, such that shelter use was identical between P. clarkii and P. leniusculus in mixed‐species treatments. In such treatments, both species increased their use of shelters when shelter density increased. When P. clarkii was alone, however, individuals did not alter shelter use as a function of shelter density, whereas P. leniusculus exhibited similar density‐dependent behaviour in both mixed‐ and single‐species treatments. 4. In a complementary field survey, we employed an ‘epicentre‐based’ design to sample two field sites. We observed patterns of microhabitat use and breadth for each species similar to those in our mesocosm experiment: the invasive P. clarkii was more abundant across different habitats and used a broader range of microhabitats than the native P. leniusculus. As such, we found that P. clarkii was more abundant across both field sites than the P. leniusculus, occupying microhabitats within and beyond the preferred range of P. leniusculus. Both field sites were affected by urban development and agriculture. 5. The use of microhabitats by both species was similar in the laboratory and the field. This study confirms that P. clarkii individuals can, and do, successfully occupy microhabitats preferred by P. leniusculus in the Willamette Valley. The results from our study may be relevant to other freshwater systems inhabited by P. clarkii and contribute to the understanding of ‘niche opportunity’, a concept which defines the environmental conditions that promote biological invasions.     

Snail responses to cues produced by an invasive decapod predator

Abstract. Cues released by predators and injured prey often induce shifts in prey behavior that allow prey to evade predators, but also affect prey resource use. I investigated the effects of chemical and mechanical signals produced by injured snails (Physella gyrina) and predatory crayfish (Procambarus clarkii) on microdistributions of P. gyrina. In an initial experiment, I observed snail responses to the presence of a caged crayfish predator, to injured conspecifics, or to both. There were significant effects of time and the treatment × time interaction on the proportion of snails moving above the water line, with greater proportions of snails above the water line at night than during the day and with weak snail crawl‐out behavior being elicited by caged crayfish at night, but not during the day. In a second experiment, I examined snail microdistributions when exposed to crayfish confined to a small cage within each aquarium, crayfish confined to half of each aquarium, and crayfish ranging freely throughout each aquarium. Snails responded most strongly to free‐ranging crayfish by moving above the water line, but also demonstrated significant, but reduced, crawl‐out responses to crayfish confined to half of each aquarium; however, snails did not respond behaviorally to crayfish confined to small cages. In both experiments, there were marginally significant effects of unfed caged crayfish on the proportions of snail populations hiding under benthic shelters, with this response being the strongest at the start of the experiments but weak overall (with only 4–5% of P. gyrina responding in each experiment). These results indicate that cues (e.g., chemical or mechanical) produced by predators altered prey microdistributions, but that the exact prey responses (e.g., moving above the water line or into horizontal or benthic refugia) depended on the intensity and nature of cues.     

Using avatar species to model the potential distribution of emerging invaders

Aim Anticipating the potential distributions of emerging invasive species is complicated by the tendency for species distribution models to perform better when both native and invasive range data are available for model development. If invasive range data are lacking, species models are liable to under‐estimate distributions for emerging invaders, particularly for species that are not at equilibrium with their native range environment due to historical factors, dispersal limitation and/or ecological interactions. We demonstrate the potential to use well‐quantified niche shifts from established ‘avatar’ (i.e. the remote or virtual manifestation of an entity) invaders to develop plausible distributions for data‐poor emerging invaders contingent on niche shifts of similar magnitude or character. Location Global. Methods Using the globally invasive crayfishes Pacifastacus leniusculus and Procambarus clarkii as our avatar invaders, we quantify how niche position, size and structure differs between native and total ranges using Mahalanobis distance (a measure of multivariate similarity) and the climate predictors of annual minimum and maximum air temperature. We then generalize patterns of niche shift from these species to the emerging crayfish invader Cherax quadricarinatus. Results Some patterns of niche shifts were similar for Pacifastacus leniusculus and Procambarus clarkii, but niche shifts were of considerably greater magnitude for P. clarkii. When a native range model for C. quadricarinatus was modified with generalized niche shifts similar to Pacifastacus leniusculus and Procambarus clarkii, the potential global distribution for this species increased considerably, including many areas not identified by the native range model. Main conclusions We illustrate the potential to use avatar invaders to provide cautionary, niche shift‐assuming species distribution models for emerging invaders. Many theoretical and applied implications of the avatar species concept require additional investigation, including the development of frameworks to select appropriate avatar species and evaluate the performance of avatar‐derived models for emerging invaders. Despite these research needs, we believe this concept will have considerable utility for predicting vulnerability to invasion by data‐poor species; this is a critical management need because shifting pathways of introduction and climate change will produce many novel, emerging invasive species in the future.     

Over-invasion in a freshwater ecosystem: newly introduced virile crayfish (Orconectes virilis) outcompete established invasive signal crayfish (Pacifastacus leniusculus)

Biological invasions are a key threat to freshwater biodiversity, and identifying determinants of invasion success is a global conservation priority. The establishment of introduced species is predicted to be hindered by pre-existing, functionally similar invasive species. Over a five-year period we, however, find that in the River Lee (UK), recently introduced non-native virile crayfish (Orconectes virilis) increased in range and abundance, despite the presence of established alien signal crayfish (Pacifastacus leniusculus). In regions of sympatry, virile crayfish had a detrimental effect on signal crayfish abundance but not vice versa. Competition experiments revealed that virile crayfish were more aggressive than signal crayfish and outcompeted them for shelter. Together, these results provide early evidence for the potential over-invasion of signal crayfish by competitively dominant virile crayfish. Based on our results and the limited distribution of virile crayfish in Europe, we recommend that efforts to contain them within the Lee catchment be implemented immediately.     

The rapid spread of rusty crayfish (Orconectes rusticus) with observations on native crayfish declines in Wisconsin (U.S.A.) over the past 130 years

The rusty crayfish, Orconectes rusticus, is one of America’s best-known non-indigenous crayfishes, having been identified as extirpating native crayfishes and disrupting local aquatic ecosystems. Over the past 40–50 years, rusty crayfish have spread from its historical range in the Ohio River drainage (U.S.A.), to waters throughout much of Illinois, Michigan, Wisconsin, and Minnesota and parts of 11 other states, Ontario (Canada) and the Laurentian Great Lakes. Using a comprehensive dataset based on all known historical records and extensive present-day surveys (n = 2775) this study reports on the invasion history of rusty crayfish, with observations on concomitant declines of native crayfishes in Wisconsin over the past 130 years (1870–2004). We found that rusty crayfish occurrences have increased from 7% of all crayfish records collected during the first 20 years of their invasion (1965–1984) to 36% of all records during the last 20 years, and that rusty crayfish have replaced the northern clearwater crayfish (O. propinquus) and virile crayfish (O. virilis) as the most dominant member of the contemporary crayfish fauna. In light of our results we discuss the introduction, establishment and integration phases of the rusty crayfish invasion and provide preliminary predictions of the potential distribution of rusty crayfish in Wisconsin lakes based on critical environmental requirements.     

Predator–prey interactions of Procambarus clarkii with aquatic macroinvertebrates in single and multiple prey systems

Understanding the interspecific interactions of Procambarus clarkii with other aquatic macroinvertebrates will help to unveil the mechanisms and processes underlying biological invasiveness. The purpose of this study was to investigate predator–prey interactions of two ontogenic phases of P. clarkii with native and exotic species of aquatic macroinvertebrates at a single and multiple prey level. We performed laboratory experiments to determine the consumption and the behavioral responses of Chironomus riparius, Physa acuta and Corbicula fluminea to P. clarkii. The presence of P. clarkii significantly affected the abundance of C. riparius and P. acuta, but not of C. fluminea whether prey species were provided singly or simultaneously. The consumption of C. riparius by P. clarkii was higher than P. acuta for both crayfish sizes and situations (single/multiple prey systems) and C. fluminea was never consumed. Physa acuta was the only species that exhibited an anti-predator behavior to P. clarkii. Our results show that P. clarkii can have strong consumptive and trait effects on aquatic macroinvertebrate prey at a single and multiple prey level, resulting in differential impacts on different prey species. This study clarifies some aspects of the predator–prey interactions between P. clarkii and native as well as other exotic macroinvertebrate species that have invaded freshwater biocenosis worldwide.     

Interactions among invaders: community and ecosystem effects of multiple invasive species in an experimental aquatic system

With ecosystems increasingly supporting multiple invasive species, interactions among invaders could magnify or ameliorate the undesired consequences for native communities and ecosystems. We evaluated the individual and combined effects of rusty crayfish (Orconectes rusticus) and Chinese mystery snails [Bellamya (=Cipangopaludina) chinensis] on native snail communities (Physa, Helisoma and Lymnaea sp.) and ecosystem attributes (algal chlorophyll a and nutrient concentrations). Both invaders are widespread in the USA and commonly co-occur within northern temperate lakes, underscoring the importance of understanding their singular and joint effects. An outdoor mesocosm experiment revealed that while the two invaders had only weakly negative effects upon one another, both negatively affected the abundance and biomass of native snails, and their combined presence drove one native species to extinction and reduced a second by >95%. Owing to its larger size and thicker shell, adult Bellamya were protected from crayfish attack relative to native species (especially Physa and Lymnaea), suggesting the co-occurrence of these invaders in nature could have elevated consequences for native communities. The per capita impacts of Orconectes (a snail predator) on native snails were substantially greater than those of Bellamya (a snail competitor). Crayfish predation also had a cascading effect by reducing native snail biomass, leading to increased periphyton growth. Bellamya, in contrast, reduced periphyton biomass, likely causing a reduction in growth by native lymnaeid snails. Bellamya also increased water column N:P ratio, possibly because of a low P excretion rate relative to native snail species. Together, these findings highlight the importance of understanding interactions among invasive species, which can have significant community- and ecosystem-level effects.     

Distribution and community-level effects of the Chinese mystery snail (Bellamya chinensis) in northern Wisconsin lakes

Managing invasive species requires information about their distributions and potential effects, but community-level impacts of invasive animals remain poorly understood. The Chinese mystery snail (Bellamya chinensis) is a large invasive gastropod that achieves high densities in waters across North America, yet little is known about its ecological significance in invaded systems. We surveyed 44 lakes to describe the patterns and determinants of B. chinensis distributions in northern Wisconsin, USA, and to assess the likelihood of effects on native snail communities in the invaded systems. B. chinensis was widespread among surveyed lakes (21 of 42 lakes with snails) and its occurrence was correlated with indicators of lake productivity and anthropogenic dispersal vectors (boat landings, distance to population centers, shoreline housing density). Some native snail species tended not to occur at sites where B. chinensis was abundant; among these was Lymnaea stagnalis, which suffered reduced survival in the presence of B. chinensis in a recently published mesocosm study. However, there was no difference in overall snail assemblage structure at either the site or lake level as a function of B. chinensis presence or abundance. Lake occurrences of many snail species have apparently been lost over time, but a comparison to a 1930s survey showed that there was no increased likelihood of species loss in lakes invaded by B. chinensis (or by the invasive crayfish Orconectes rusticus). Although B. chinensis is widespread and sometimes abundant in northern Wisconsin lakes, it does not appear to have strong systematic impacts on native snail assemblages.     

Geographic expansion of the invasive red crayfish <Emphasis Type="Italic">Procambarus clarkii</Emphasis> (Girard, 1852) (Crustacea: Decapoda) in Mexico

The red crayfish Procambarus clarkii, which is native to southcentral USA and northeastern Mexico, has been successfully introduced into several countries around the world. This study documents the geographic expansion of the exotic red crayfish in Mexico and discusses the consequence of a greater propagation of this species in Mexican inland waters. New state records of this crayfish in the Baja California peninsula and in the states of Durango and Sinaloa indicate its progressive dispersion. The propagation of P. clarkii in Mexico has been caused mainly by human introduction, but it is also facilitated because of the species’ tolerance to an ample range of environmental conditions. Because of the invasive capability of P. clarkii, we suspect that this exotic species is competing for habitat and food with native freshwater shrimp of the genus Macrobrachium in many sites of northern Mexico.     

Enhanced ecological indication based on combined planktic and benthic functional approaches in large river phytoplankton ecology

Environmental DNA (eDNA) is a powerful method for assessing the presence and distribution of invasive aquatic species. We used this tool to detect and monitor several invasive crayfishes Procambarus clarkii, Orconectes limosus and Pacifastacus leniusculus present in, or likely to invade, the ponds of the Brenne Regional Natural Park. A previous study showed that the eDNA method was not very efficient in detecting P. clarkii. In the present study, we explored new improvements in the detection of invasive crayfish. We designed specific primers for each crayfish species, and set up an experimental mesocosm approach to confirm the specificity of the primers and the sampling protocol. We analysed samples taken from ponds in 2014 and 2015. We compared two qPCR protocols involving either SybrGreen or TaqMan assays. Using these same primers, we were able to detect crayfish eDNA with both assays during the mesocosm experiment. However, crayfish from field samples could only be detected by performing qPCR with a SybrGreen assay. We successfully monitored the presence of three invasive species of crayfish using eDNA. This method is a powerful tool for establishing the presence or absence of invasive species in various freshwater environments.     

Use of termites, Reticulitermes virginicus, as a springboard in the invasive success of a predatory ant, Pachycondyla (=Brachyponera) chinensis

Invasive ant species have general diet and nest requirements, which facilitate their establishment in novel habitats and their dominance over many native ants. The Asian needle ant, Pachycondyla chinensis, native throughout Australasia was introduced to the southeastern US where it has become established in woodland habitats, nests in close proximity to and consumes subterranean termites (Rhinotermitidae). P. chinensis do not occur in habitats lacking Rhinotermitidae. We suggest that subterranean termites are critical for P. chinensis success in new habitats. We demonstrate that P. chinensis is a general termite feeder, retrieving Reticulitermes virginicus five times more often than other potential prey near P. chinensis colonies. Odors produced by R. virginicus workers, as well as other potential prey, attract P. chinensis. Furthermore, P. chinensis occupy R. virginicus nests in the lab and field and display behaviors that facilitate capture of R. virginicus workers and soldiers. Termites are an abundant, high quality, renewable food supply, in many ways similar to the hemipteran honeydew exploited by most other invasive ant species. We conclude that the behavior of P. chinensis in the presence of termites increases their competitive abilities in natural areas where they have been introduced.     

Bivalve or gastropod? Using profitability estimates to predict prey choice by <Emphasis Type="Italic">P. clarkii</Emphasis>

Recently, a highly invasive alien species, the zebra mussel Dreissena polymorpha, has colonized Italian aquatic ecosystems that had been previously colonized by another highly invasive alien species—the North American crayfish Procambarus clarkii. This was the first time and world region where these two species met. To evaluate the relative importance of their interactions, we studied prey selection according to its length, as well as prey choice by P. clarkii preying on D. polymorpha in the presence and absence of alternative prey—the freshwater snail Physella acuta. We followed an optimability-based approach, by first estimating the most profitable length of each mollusc and then by conducting a prey choice experiment, on which both species were provided simultaneously to a crayfish with different size combinations. Prey selection was dependent on prey length, handling time and crayfish length for both molluscs. According to our profitability estimates, snails should be more profitable than mussels in the length range 7–10 mm, while for lengths over 11.0 mm, mussels should be more profitable. The results of the prey choice experiment indicated that D. polymorpha length, P. acuta length, and the difference in profitability between the offered individuals were all relevant for the choice of one species over the other by P. clarkii. However, the overall tendency was the choice of the smallest prey, regardless of species, and the estimates of prey profitability were not useful to predict prey choice. Our study shows that D. polymorpha represents a novel prey resource for P. clarkii, even in the presence of an alternative prey, and that zebra mussels may be a preferred prey, especially small-sized individuals (5–10 mm).     

An invaded invader: high prevalence of entocytherid ostracods on the red swamp crayfish <Emphasis Type="Italic">Procambarus clarkii</Emphasis> (Girard, 1852) in the Eastern Iberian Peninsula

The American red swamp crayfish Procambarus clarkii (Girard, 1852) was introduced in 1973 into the Iberian Peninsula for commercial purposes. As a result of both the expansion from the Iberian Peninsula and, probably, further introductions in other European countries, now it is widely distributed throughout much of Europe. The ecological impacts of this invading crayfish have received increasing attention, but nothing is known about its symbiotic entocytherid ostracods outside the American continent. The present survey has examined more than 200 crayfishes from 12 localities distributed over a wide area of Eastern Spain. Entocytherid ostracods were extracted from individual crayfishes and they were identified, counted, assigned to developmental instars and sexed. In all the study locations but one, we found at least one crayfish individual infected by entocytherid ostracods and the species determined was the same in all cases: Ankylocythere sinuosa (Rioja, 1942). The number of ostracods on individual P. clarkii varied notably in relation to crayfish size and also differed significantly among sampling sites. The crayfish size effects on ostracod densities might be related to the amount of resources and to the crayfish age and moulting frequency affecting ostracod distribution and population structure. In addition, the spatial variation in ostracod densities could also be related to site-specific habitat traits and the variability of crayfish population dynamics. Our study represents the first citation of an alien entocytherid species in Europe and demonstrates its wide distribution in the Iberian Peninsula. Further research is needed to know the potential effects of this ostracod species on the ecology of P. clarkii and of native species, with implications on the management of this aquatic invader.     

An invasive species may be better than none: invasive signal and native noble crayfish have similar community effects

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Species-specific differences in dynamics of agonistic interactions may contribute to the competitive advantage of the invasive signal crayfish (Pacifastacus leniusculus) over the native narrow-clawed crayfish (Astacus leptodactylus)

Species-specific differences in dynamics of agonistic interactions may influence the outcome of interspecific competition and potentially contribute to competitive advantage of one species over another. In this study, we compared the dynamics of agonistic interactions of one of the most successful crayfish invaders of European freshwater ecosystems, the signal crayfish (Pacifastacus leniusculus) and the widespread native European species currently undergoing range expansion in Croatia and Europe, the narrow-clawed crayfish (Astacus leptodactylus). Comparisons between P. leniusculus pairs and the A. leptodactylus pairs demonstrated significant differences in frequency and duration of agonistic encounters: P. leniusculus dyads engaged in fewer fights, but their duration was significantly longer. In staged interactions between size-matched interspecific pairs, agonistic behaviour of P. leniusculus individuals translated into dominance over their A. leptodactylus counterparts. This indicates that the success of P. leniusculus in agonistic encounters with the native competitor might stem from its readiness to continue fighting for a longer time period, and could lead to ecological advantages during niche competition even when facing a successful native crayfish species.     

Do anuran larvae respond behaviourally to chemical cues from an invasive crayfish predator? A community-wide study

Antipredator behaviour is an important fitness component in most animals. A co-evolutionary history between predator and prey is important for prey to respond adaptively to predation threats. When non-native predator species invade new areas, native prey may not recognise them or may lack effective antipredator defences. However, responses to novel predators can be facilitated by chemical cues from the predators’ diet. The red swamp crayfish Procambarus clarkii is a widespread invasive predator in the Southwest of the Iberian Peninsula, where it preys upon native anuran tadpoles. In a laboratory experiment we studied behavioural antipredator defences (alterations in activity level and spatial avoidance of predator) of nine anurans in response to P. clarkii chemical cues, and compared them with the defences towards a native predator, the larval dragonfly Aeshna sp. To investigate how chemical cues from consumed conspecifics shape the responses, we raised tadpoles with either a tadpole-fed or starved crayfish, or dragonfly larva, or in the absence of a predator. Five species significantly altered their behaviour in the presence of crayfish, and this was largely mediated by chemical cues from consumed conspecifics. In the presence of dragonflies, most species exhibited behavioural defences and often these did not require the presence of cues from predation events. Responding to cues from consumed conspecifics seems to be a critical factor in facilitating certain behavioural responses to novel exotic predators. This finding can be useful for predicting antipredator responses to invasive predators and help directing conservation efforts to the species at highest risk.