J. Robert Britton

Introduced parasites in food webs: new species, shifting structures?

Introduction of free-living species also results in co-introduction of their parasites. Since recent advances have shown that native parasites dramatically alter food web structure, I evaluate here how introduced parasites might reorganise food webs. Empirical evidence suggests that introduced parasites alter food webs qualitatively through topological changes and quantitatively through shifts in trophic relationships arising from modified host phenotypic traits. I argue that predicting the extent of food web reorganisation is, however, difficult due to underlying ecological and evolutionary processes that could provide contrasting food web outcomes, including enemy release, biotic resistance, and parasite spillover and spillback. Nevertheless, I suggest these food web reorganisations represent a further aspect of human-mediated global change resulting in irreversible consequences across multiple trophic levels.

Patterns of trophic niche divergence between invasive and native fishes in wild communities are predictable from mesocosm studies.

Summary Ecological theory attempts to predict how impacts for native species arise from biological invasions. A fundamental question centres on the feeding interactions of invasive and native species: whether invasion will result in increased interspecific competition, which would result in negative consequences for the competing species, or trophic niche divergence, which would facilitate the invaders integration into the community and their coexistence with native species. Here, the feeding interactions of a highly invasive fish, topmouth gudgeon Pseudorasbora parva, with three native and functionally similar fishes were studied to determine whether patterns of either niche overlap or divergence detected in mesocosm experiments were apparent between the species at larger spatial scales. Using stable isotope analysis, their feeding relationships were assessed initially in the mesocosms (1000 L) and then in small ponds (<400 m2) and large ponds (>600 m2). In the mesocosms, a consistent pattern of trophic niche divergence was evident between the sympatric fishes, with niches shifting further apart in isotopic space than suggested in allopatry, revealing that sharing of food resources was limited. Sympatric P. parva also had a smaller niche than their allopatric populations. In eight small ponds where P. parva had coexisted for several years with at least one of the fish species used in the mesocosms, strong patterns of niche differentiation were also apparent, with P. parva always at a lower trophic position than the other fishes, as also occurred in the mesocosms. Where these fishes were sympatric within more complex fish communities in the large ponds, similar patterns were also apparent, with strong evidence of trophic niche differentiation. Aspects of the ecological impacts of P. parva invasion for native communities in larger ponds were consistent with those in the mesocosm experiments. Their invasion resulted in divergence in trophic niches, partly due to their reduced niche widths when in sympatry with other species, facilitating their coexistence in invaded ecosystems. Our study highlights the utility of controlled mesocosm studies for predicting the trophic relationships that can develop from introductions of non‐native species into more complex ecosystems and at larger spatial scales.

Understanding the threats posed by non-native species: public vs. conservation managers.

Public perception is a key factor influencing current conservation policy. Therefore, it is important to determine the influence of the public, end-users and scientists on the prioritisation of conservation issues and the direct implications for policy makers. Here, we assessed public attitudes and the perception of conservation managers to five non-native species in the UK, with these supplemented by those of an ecosystem user, freshwater anglers. We found that threat perception was not influenced by the volume of scientific research or by the actual threats posed by the specific non-native species. Media interest also reflected public perception and vice versa. Anglers were most concerned with perceived threats to their recreational activities but their concerns did not correspond to the greatest demonstrated ecological threat. The perception of conservation managers was an amalgamation of public and angler opinions but was mismatched to quantified ecological risks of the species. As this suggests that invasive species management in the UK is vulnerable to a knowledge gap, researchers must consider the intrinsic characteristics of their study species to determine whether raising public perception will be effective. The case study of the topmouth gudgeon Pseudorasbora parva reveals that media pressure and political debate has greater capacity to ignite policy changes and impact studies on non-native species than scientific evidence alone.

A modular assessment tool for managing introduced fishes according to risks of species and their populations, and impacts of management actions

A modular assessment scheme for assisting the risk management of introduced fishes is described, with its recent application to England and Wales demonstrated. The initial module prioritises the introduced fishes in the risk assessment area according to their potential invasiveness and current distribution. The second module then assesses populations of the prioritised species in relation to the character of their receiving waters and the potential risks posed by their population in that circumstance; the output is a suggested management action for each population. The third module evaluates the suggested management action in relation to its potential impacts in the environment and how these impacts may be mitigated. The final module assesses the estimated cumulative cost of the selected management action relative to an alternative action. To demonstrate its potential value for managing extant populations of introduced fish, three eradication case-studies from England were assessed retrospectively using the scheme. This revealed eradication of two topmouth gudgeon Pseudorasbora parva populations was commensurate with their levels of ecological risk in the environment. By contrast, initial assessment of the eradication of a feral population of fathead minnow Pimephales promelas suggested control and containment was the commensurate management action due to a relatively low risk of natural dispersal. Application of the scheme elsewhere in the world and to other faunal groups should enable more objective decision-making in management programmes and enhance conservation outcomes.

Pathological and Ecological Host Consequences of Infection by an Introduced Fish Parasite

The infection consequences of the introduced cestode fish parasite Bothriocephalus acheilognathi were studied in a cohort of wild, young-of-the-year common carp Cyprinus carpio that lacked co-evolution with the parasite. Within the cohort, parasite prevalence was 42% and parasite burdens were up to 12% body weight. Pathological changes within the intestinal tract of parasitized carp included distension of the gut wall, epithelial compression and degeneration, pressure necrosis and varied inflammatory changes. These were most pronounced in regions containing the largest proportion of mature proglottids. Although the body lengths of parasitized and non-parasitized fish were not significantly different, parasitized fish were of lower body condition and reduced weight compared to non-parasitized conspecifics. Stable isotope analysis (δ15N and δ13C) revealed trophic impacts associated with infection, particularly for δ15N where values for parasitized fish were significantly reduced as their parasite burden increased. In a controlled aquarium environment where the fish were fed ad libitum on an identical food source, there was no significant difference in values of δ15N and δ13C between parasitized and non-parasitized fish. The growth consequences remained, however, with parasitized fish growing significantly slower than non-parasitized fish, with their feeding rate (items s−1) also significantly lower. Thus, infection by an introduced parasite had multiple pathological, ecological and trophic impacts on a host with no experience of the parasite.

Temperature-dependent feeding interactions between two invasive fishes competing through interference and exploitation

Context-dependent ecological interactions between invasive species are important in determining the outcomes of their introductions. The consequences of competitive interactions between the Nile tilapia Oreochromis niloticus (interference competitor) and common carp Cyprinus carpio (exploitative competitor) were investigated here across a temperature gradient (20–28°C). These highly invasive fish are now present in many regions where populations increasingly coexist, inducing trophic interactions and niche overlaps. Experimental feeding and growth trials revealed the feeding rate (items s−1) and specific growth rate (% day−1) of these fishes were not significantly different at 24°C, but were significantly higher for C. carpio at 20°C and significantly higher for O. niloticus at 28°C. An additional experiment completed at 24°C revealed that O. niloticus rapidly form hierarchies, where dominant fish monopolise food resources through interference, resulting in their faster growth. Introductions of 1 and 3 C. carpio (an exploitative competitor) into the hierarchy had no effect on this food monopolisation as carp were excluded through aggression. The addition of 6 C. carpio did, however, significantly reduce the food intake of the dominant tilapia. This was due to increased exploitative competition rather than breaking of the hierarchy. The effect of adding 3 O. niloticus was similar to 6 C. carpio, suggesting inter- and intra-specific competitive strength was similar. These findings suggest when populations co-exist, temperature-dependent feeding interactions may result in the competitive exclusion of C. carpio through the aggressive interference by O. niloticus, potentially influencing invasion outcomes.

Dietary niche constriction when invaders meet natives: evidence from freshwater decapods

Invasive species are a key driver of global environmental change, with frequently strong negative consequences for native biodiversity and ecosystem processes. Understanding competitive interactions between invaders and functionally similar native species provides an important benchmark for predicting the consequences of invasion. However, even though having a broad dietary niche is widely considered a key factor determining invasion success, little is known about the effects of competition with functionally similar native competitors on the dietary niche breadths of invasive species. We used a combination of field experiments and field surveys to examine the impacts of competition with a functionally similar native crab species on the population densities, growth rates and diet of the globally widespread invasive red swamp crayfish in an African river ecosystem. The presence of native crabs triggered significant dietary niche constriction within the invasive crayfish population. Further, growth rates of both species were reduced significantly, and by a similar extent, in the presence of one another. In spite of this, crayfish maintained positive growth rates in the presence of crabs, whereas crabs lost mass in the presence of crayfish. Consequently, over the 3-year duration of the study, crab abundance declined at those sites invaded by the crayfish, becoming locally extinct at one. The invasive crayfish had a dramatic effect on ecosystem structure and functioning, halving benthic invertebrate densities and increasing decomposition rates fourfold compared to the crabs. This indicates that replacement of native crabs by invasive crayfish likely alters the structure and functioning of African river ecosystems significantly. This study provides a novel example of the constriction of the dietary niche of a successful invasive population in the presence of competition from a functionally similar native species. This finding highlights the importance of considering both environmental and ecological contexts in order to predict and manage the impacts of invasive species on ecosystems.

Characterizing the trophic niches of stocked and resident cyprinid fishes: consistency in partitioning over time, space and body sizes

Abstract Hatchery‐reared fish are commonly stocked into freshwaters to enhance recreational angling. As these fishes are often of high trophic position and attain relatively large sizes, they potentially interact with functionally similar resident fishes and modify food‐web structure. Hatchery‐reared barbel Barbus barbus are frequently stocked to enhance riverine cyprinid fish communities in Europe; these fish can survive for over 20 years and exceed 8 kg. Here, their trophic consequences for resident fish communities were tested using cohabitation studies, mainly involving chub Squalius cephalus, a similarly large‐bodied, omnivorous and long‐lived species. These studies were completed over three spatial scales: pond mesocosms, two streams and three lowland rivers, and used stable isotope analysis. Experiments in mesocosms over 100 days revealed rapid formation of dietary specializations and discrete trophic niches in juvenile B. barbus and S. cephalus. This niche partitioning between the species was also apparent in the streams over 2 years. In the lowland rivers, where fish were mature individuals within established populations, this pattern was also generally apparent in fishes of much larger body sizes. Thus, the stocking of these hatchery‐reared fish only incurred minor consequences for the trophic ecology of resident fish, with strong patterns of trophic niche partitioning and diet specialization. Application of these results to decision‐making frameworks should enable managers to make objective decisions on whether cyprinid fish should be stocked into lowland rivers according to ecological risk.

Trans-national horizon scanning for invasive non-native species: a case study in western Europe

Horizon scanning for high-risk invasive non-native species (INNS) is crucial in preparing and implementing measures to prevent introductions, as well as to focus efforts in the control of species already present. We initiated a trans-national horizon-scanning exercise focused on four countries in western Europe: Great Britain, France, Belgium and The Netherlands, which share similar environmental and socio-economic characteristics. We followed a structured four-step approach combining existing knowledge about INNS, with a collaborative identification of priorities for research and management: (1) systematic review of potential INNS of concern, (2) discrimination of INNS into an Alert and Black List depending on their absence or presence in the study area respectively, (3) risk analysis of the Alert List, and (4) expert ranking of species in the Black List. Amongst species not yet present in the four countries (i.e. Alert List), assessors reliably pointed to the Emerald ash-borer (Agrilus planipennis) and Sosnowski’s hogweed (Heracleum sosnowskyi) as those INNS with the highest risk of invasion and impact. The Japanese knotweed (Fallopia japonica), Himalayan balsam (Impatiens glandulifera), zebra mussel (Dreissena polymorpha) and killer shrimp (Dikerogammarus villosus) were consistently highlighted as some of the most problematic INNS already present in the study area (i.e. Black List). The advantages of our combined approach include that it is inclusive of all-taxa, prioritizes both established and emerging biological threats across trans-national scales, and considers not only the ecological impact, but also potential direct economic consequences as well as the manageability of invasive species.

Stable isotope signatures and trophic‐step fractionation factors of fish tissues collected as non‐lethal surrogates of dorsal muscle

RATIONALE Dorsal white muscle is the standard tissue analysed in fish trophic studies using stable isotope analyses. As muscle is usually collected destructively, fin tissues and scales are often used as non-lethal surrogates; we examined the utility of scales and fin tissue as muscle surrogates. METHODS The muscle, fin and scale δ(15) N and δ(13) C values from 10 cyprinid fish species determined with an elemental analyser coupled with an isotope ratio mass spectrometer were compared. The fish comprised (1) samples from the wild, and (2) samples from tank aquaria, using six species held for 120 days and fed a single food resource. Relationships between muscle, fin and scale isotope ratios were examined for each species and for the entire dataset, with the efficacy of four methods of predicting muscle isotope ratios from fin and scale values being tested. The fractionation factors between the three tissues of the laboratory fishes and their food resource were then calculated and applied to Bayesian mixing models to assess their effect on fish diet predictions. RESULTS The isotopic data of the three tissues per species were distinct, but were significantly related, enabling estimations of muscle values from the two surrogates. Species-specific equations provided the least erroneous corrections of scale and fin isotope ratios (errors < 0.6‰). The fractionation factors for δ(15) N values were in the range obtained for other species, but were often higher for δ(13) C values. Their application to data from two fish populations in the mixing models resulted in significant alterations in diet predictions. CONCLUSIONS Scales and fin tissue are strong surrogates of dorsal muscle in food web studies as they can provide estimates of muscle values within an acceptable level of error when species-specific methods are used. Their derived fractionation factors can also be applied to models predicting fish diet composition from δ(15) N and δ(13) C values.