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

Intraspecific variation in crayfish behavioral traits affects leaf litter breakdown in streams

Although intraspecific trait variation is increasingly recognized as affecting ecosystem processes, few studies have examined the ecological significance of among-population variation in behavioral traits in natural ecosystems. In freshwater habitats, crayfish are consumers that can influence ecosystem structure (e.g., macroinvertebrate communities) and function (e.g., leaf litter breakdown). To test whether crayfish behavioral traits (activity, boldness, and foraging voracity) are major contributors of leaf litter breakdown rates in the field, we collected rusty crayfish (Faxonius rusticus) from eight streams across the midwestern USA and measured behaviors using laboratory assays. At the same streams, we measured breakdown rates of leaf packs that were accessible or inaccessible to crayfish. Our results provide evidence that among-population variation in crayfish boldness and foraging voracity was a strong predictor of leaf litter breakdown rates, even after accounting for commonly appreciated environmental drivers (water temperature and human land use). Our results suggest that less bold rusty populations (i.e., emerged from shelter more slowly) had greater direct impacts on leaf litter breakdown than bold populations (P = 0.001, r2 = 0.85), potentially because leaf packs can be both a shelter and food resource to crayfish. Additionally, we found that foraging voracity was negatively related to breakdown rates in leaf packs that were inaccessible to crayfish (P = 0.025, r2 = 0.60), potentially due to a trophic cascade from crayfish preying on other invertebrates that consume leaf litter. Overall, our results add to the growing evidence that trait variation in animals may be important for understanding freshwater ecosystem functioning.

North American crayfish harbour diverse members of the Nudiviridae

Three novel crayfish-infecting nudiviruses from crayfish in North America represent the first genomic confirmation of nudiviruses in crayfish: Faxonius propinquus nudivirus (FpNV), Faxonius rusticus nudivirus (FrNV), and Faxonius virilis nudivirus (FvNV). Histopathology and electron microscopy revealed nuclear infections, including nuclear hypertrophy in hepatopancreatic epithelial cells and the presence of membrane-bound bacilliform virions. Metagenomic sequencing resulted in complete circular genome assembly, and phylogenetic analyses (based on nudivirus core genes) placed these viruses within the unofficial Epsilonnudivirus genus. One of the nudiviruses was detected in the antennal gland of its host, and another is correlated with invasive crayfish decline in one infected lake ecosystem - suggesting a potential route for viral transmission through water, and possible population level impact. This study highlights the importance of genomic and ecological data in elucidating the diversity and evolutionary relationships of the Nudiviridae, while expanding their known diversity and range of host species.

Alternosema astaquatica n. sp. (Microsporidia: Enterocytozoonida), a systemic parasite of the crayfish Faxonius virilis

Crayfish have strong ecological impacts in freshwater systems, yet our knowledge of their parasites is limited. This study describes the first systemic microsporidium (infects multiple tissue types) Alternosema astaquatica n. sp. (Enterocytozoonida) isolated from a crayfish host, Faxonius virilis, using histopathology, transmission electron microscopy, gene sequencing, and phylogenetics. The parasite develops in direct contact with the host cell cytoplasm producing mature spores that are monokaryotic and ellipsoid in shape. Spores have 9-10 coils of the polar filament and measure 3.07 ± 0.26 µm (SD) in length and 0.93 ± 0.08 µm (SD) in width. Our novel isolate has high genetic similarity to Alternosema bostrichidis isolated from terrestrial beetles; however, genetic data from this parasite is restricted to a small fragment (396bp) of the SSU gene. Additional data related to spore morphology and development, host, environment, and ecology indicate that our novel isolate is distinct from A. bostrichidis, which supports a new species description. Alternosema astaquatica n. sp. represents a novel member of the Orthosomella-like group which appears to be a set of opportunists within the Enterocytozoonida. The presence of this microsporidium in F. virilis could be relevant for freshwater ecosystems across this crayfish's broad geographic range in North America and may affect interactions between F. virilis and invasive rusty crayfish Faxonius rusticus in the Midwest USA.

Cambaraspora faxoni n. sp. (Microsporidia: Glugeida) from native and invasive crayfish in the USA and a novel host of Cambaraspora floridanus

Crayfishes are among the most widely introduced freshwater taxa and can have extensive ecological impacts. Knowledge of the parasites crayfish harbor is limited, yet co-invasion of parasites is a significant risk associated with invasions. In this study, we describe a novel microsporidium, Cambaraspora faxoni n. sp. (Glugeida: Tuzetiidae), from two crayfish hosts in the Midwest USA, Faxonius virilis and Faxonius rusticus. We also expand the known host range of Cambaraspora floridanus to include Procambarus spiculifer. Cambaraspora faxoni infects muscle and heart tissue of F. rusticus and develops within a sporophorous vesicle. The mature spore measures 3.22 ± 0.14 μm in length and 1.45 ± 0.13 μm in width, with 8-9 turns of the polar filament. SSU sequencing indicates the isolates from F. virilis and F. rusticus were identical (100%) and 93.49% similar to C. floridanus, supporting the erection of a new species within the Cambaraspora genus. The novel parasite was discovered within the native range of F. rusticus (Ohio, USA) and within a native congeneric (F. virilis) in the invasive range of F. rusticus (Wisconsin, USA). Faxonius virilis is invasive in other regions. This new parasite could have been introduced to Wisconsin with F. rusticus or it may be a generalist species with a broad distribution. In either case, this parasite infects two crayfish species that have been widely introduced to new drainages throughout North America and could have future effects on invasion dynamics or impacts.

Evidence of a compensatory response in invasive Rusty Crayfish (Faxonius rusticus) following intensive harvest removal from northern Lake Michigan fish spawning reefs

The goal of most invasive species suppression programs is to achieve long-term sustained reductions in population abundance, yet removal programs can be stymied by density-dependent population responses. We tested a harvest removal strategy for invasive Rusty Crayfish (Faxonius rusticus) at two nearshore native fish spawning habitats in northern Lake Michigan. Changes in average Rusty Crayfish densities were evaluated with a before-after reference-impact study design. We removed 3182 Rusty Crayfish, primarily adults (> 20 mm carapace length), at two sites over two harvest seasons, expending 17,825 trap days in effort. Generalized linear modeling results suggested a statistically significant reduction in Rusty Crayfish densities was achieved at one reef, Little Traverse Bay (LTB Crib). Reduced densities were sustained over the egg maturation period for native fish and into the following year after removal ceased. By late summer/early fall, between consecutive suppression efforts in 2018 and 2019, we observed a threefold increase in pre-removal densities. Size-frequency histograms from diver quadrat surveys showed higher abundances of juvenile (< 20 mm carapace length) size classes the following spring and summer at LTB Crib compared to its paired reference site. Stock-recruit curves fit to count data, pooled across all sites, provided further evidence of density-dependence. With a proviso that we only conducted two seasons of consecutive suppression, this study highlights an important aspect of invasive species management and raises questions about the efficacy of adult-only crayfish removal strategies.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10530-023-03076-6.

Long-term trends in crayfish invasions across European rivers

Europe has experienced a substantial increase in non-indigenous crayfish species (NICS) since the mid-20th century due to their extensive use in fisheries, aquaculture and, more recently, pet trade. Despite relatively long invasion histories of some NICS and negative impacts on biodiversity and ecosystem functioning, large spatio-temporal analyses of their occurrences are lacking. Here, we used a large freshwater macroinvertebrate database to evaluate what information on NICS can be obtained from widely applied biomonitoring approaches and how usable such data is for descriptions of trends in identified NICS species. We found 160 time-series containing NICS between 1983 and 2019, to infer temporal patterns and environmental drivers of species and region-specific trends. Using a combination of meta-regression and generalized linear models, we found no significant temporal trend for the abundance of any species (Procambarus clarkii, Pacifastacus leniusculus or Faxonius limosus) at the European scale, but identified species-specific predictors of abundances. While analysis of the spatial range expansion of NICS was positive (i.e. increasing spread) in England and negative (significant retreat) in northern Spain, no trend was detected in Hungary and the Dutch-German-Luxembourg region. The average invasion velocity varied among countries, ranging from 30 km/year in England to 90 km/year in Hungary. The average invasion velocity gradually decreased over time in the long term, with declines being fastest in the Dutch-German-Luxembourg region, and much slower in England. Considering that NICS pose a substantial threat to aquatic biodiversity across Europe, our study highlights the utility and importance of collecting high resolution (i.e. annual) biomonitoring data using a sampling protocol that is able to estimate crayfish abundance, enabling a more profound understanding of NICS impacts on biodiversity.

Long-term macrophyte and snail community responses to population declines of invasive rusty crayfish (Faxonius rusticus)

A central focus of invasive species research has been on human efforts to eradicate invaders or reduce their abundance to mitigate the worst of their impacts. In some cases, however, populations of invasive species decline without human intervention, which may inform management responses to these invaders. Such is the case of the invasive rusty crayfish (Faxonius rusticus) in northern Wisconsin, USA, where systematic population monitoring since 1975 has revealed population declines in approximately half of the lakes surveyed. Population declines of invasive species without human intervention remain understudied, but there is even less research on how communities respond following such declines. Using 10 lakes in Vilas County, Wisconsin, we investigated community recovery of habitat (macrophytes) and prey (freshwater snails) of F. rusticus following up to 33 years of declines of this invader in some lakes using a dataset with a rare, long-term span over which consistent data were collected (1987, 2002, 2011, and 2020). We compared community responses in lakes where F. rusticus populations reached a peak and subsequently declined (boom-bust lakes) and lakes where our dataset only captured the decline of F. rusticus (bust lakes) to reference lakes with consistently high or low crayfish abundance over time. We found partial recovery of macrophytes and snails in the bust and boom-bust lakes where F. rusticus has declined, with recovery of macrophyte abundance and richness in the boom-bust lakes achieving levels observed in the low-crayfish reference lakes. Snail abundance and richness increased after declines of F. rusticus, though not to the level of the low-crayfish reference lakes, suggesting that snail recovery may lag macrophyte recovery because snails are dependent on macrophytes and associated periphyton for habitat. The recovery we document potentially represents long-term ecosystem resilience of lakes to biological invasions. Our results suggest that lake communities may recover without active restoration interventions after invasive crayfish population declines, although identifying which lakes experience these natural declines remains a priority for future research and management.

Mitigation translocation as a management tool

Mitigation translocation is a subgroup of conservation translocation, categorized by a crisis-responsive time frame and the immediate goal of relocating individuals threatened with death. However, the relative successes of conservation translocations with longer time frames and broader metapopulation- and ecosystem-level considerations have been used to justify the continued implementation of mitigation translocations without adequate post hoc monitoring to confirm their effectiveness as a conservation tool. Mitigation translocations now outnumber other conservation translocations, and understanding the effectiveness of mitigation translocations is critical given limited global conservation funding especially if the mitigation translocations undermine biodiversity conservation by failing to save individuals. We assessed the effectiveness of mitigation translocations by conducting a quantitative review of the global literature. A total of 59 mitigation translocations were reviewed for their adherence to the adaptive scientific approach expected of other conservation translocations and for the testing of management options to continue improving techniques for the future. We found that mitigation translocations have not achieved their potential as an effective applied science. Most translocations focused predominantly on population establishment- and persistence-level questions, as is often seen in translocations more broadly, and less on metapopulation and ecosystem outcomes. Questions regarding the long-term impacts to the recipient ecosystem (12% of articles) and the carrying capacity of translocation sites (24% of articles) were addressed least often, despite these factors being more likely to influence ultimate success. Less than half (47%) of studies included comparison of different management techniques to facilitate practitioners selecting the most effective management actions for the future. To align mitigation translocations with the relative success of other conservation translocations, it is critical that future mitigation translocations conform to an established experimental approach to improve their effectiveness. Effective mitigation translocations will require significantly greater investment of time, expertise, and resources in the future.

The update and optimization of an eDNA assay to detect the invasive rusty crayfish (Faxonius rusticus)

Environmental DNA (eDNA) is nuclear or mitochondrial DNA shed into the environment, and amplifying this DNA can serve as a reliable, noninvasive way to monitor aquatic systems for the presence of an invasive species. Assays based on the collection of eDNA are becoming increasingly popular, and, when optimized, can aid in effectively and efficiently tracking invasion fronts. We set out to update an eDNA assay to detect the invasive rusty crayfish, Faxonius rusticus. We tested for species specificity compared to other stream crayfish and field tested the assay at sites with known presence (N = 3) and absence (N = 4) in the Juniata River watershed in central Pennsylvania, USA. To maximize sensitivity, we field tested different storage buffers (Longmire's buffer and ethanol), DNA extraction methods (Qiagen's DNEasy and PowerWater kits), and quantitative polymerase chain reaction (qPCR) chemistries (TaqMan and SYBR green). Our assay confirmed the presence data and performed optimally when filter samples were stored in Longmire's buffer, DNA was extracted with DNeasy Blood and Tissue Kit, and TaqMan qPCR chemistry was utilized. With proper sample processing, our assay allows for accurate, noninvasive detection of F. rusticus in streams.

Testing the relationship between intraspecific competition and individual specialization across both behavior and diet

Individual specialization within populations is increasingly recognized as important in both ecology and evolution, but researchers working on intraspecific variation in behavior and diet infrequently interact. This may be because individual specialization on diet and behavior was historically difficult to investigate simultaneously on the same individuals. However, approaches like stable isotope analysis that allow hindcasting past field diets for laboratory organisms may provide opportunities to unite these areas of inquiry. Here, we tested the role of intraspecific competition on individual specialization through analysis of both behavior and diet simultaneously. We focused on intraspecific competition as a mechanism that might drive individual specialization of both diet and behavior. We conducted this study in Vilas County, Wisconsin, United States (US), using rusty crayfish Faxonius rusticus from six lakes across a relative abundance gradient. We conducted six assays to measure individual specialization of behavior and used stable isotope analysis to measure individual specialization of diet. We then related both measures of individual specialization to relative abundance of F. rusticus using linear and quadratic models. We found a unimodal relationship between intraspecific competition and individual specialization of diet for F. rusticus, likely because some preferred resources are unavailable to specialize on at the highest densities of this well-studied crayfish invader. Conversely, we found greater support for a linear relationship between individual specialization of behavior and intraspecific competition, perhaps because specialization by behavior is not inherently resource-limited. Our results show that dietary and behavioral specialization may exhibit different responses to increased intraspecific competition, and demonstrate a potential technique that can be used to investigate individual specialization of diet and behavior simultaneously for the same individuals and populations.

Density-Dependent Escapement of Rusty Crayfish from Modified Minnow Traps with Varying Throat Configurations

Modified minnow traps are the most widely used gear for collecting tertiary burrowing crayfishes. The throats of modified minnow traps are often widened more than 60% to accommodate the capture of larger crayfish. However, widening this crucial chokepoint into the trap may facilitate easier escape of captured individuals, especially as density in the trap increases. Increased escapement rates may reduce catch rates and corresponding estimates of relative abundance and lower detection probability. Incorporating a design feature, that is, throat restriction, that allows entry of all sizes of crayfish while reducing escapement would be an improvement over current designs. Here, I present the results of a paired field and laboratory experiment comparing the effectiveness of modified minnow traps with a throat restriction (restricted) and without such a feature (unrestricted) under varying crayfish densities. I chose rusty crayfish Faxonius rusticus as a study organism because it is widespread and abundant in the Laurentian Great Lakes region and commonly the focus of research and removal efforts. Rusty crayfish capture and escapement were strongly influenced by throat design and crayfish densities. The field component demonstrated that both traps performed similarly under low-to-moderate densities; however, at high densities catch in unrestricted traps plateaued at approximately 50 crayfish/trap, while restricted traps kept accumulating catch up to 155 crayfish/trap. Laboratory trials demonstrated that escapement for both trap types was negligible at low density and slightly higher at medium density: 0.8% for restricted and 11.5% for unrestricted. However, at high density, escapement from restricted traps was 8.8 vs. 45.3% for unrestricted traps. Our findings suggest that inclusion of a throat restriction may increase catch of rusty crayfish by reducing escapement and may be of particular use in removal projects or when sampling in high-density populations.

Habitat explains patterns of population decline for an invasive crayfish

Invasive nonindigenous species are defined by their impacts: they substantially change native communities or ecosystems. Accordingly, invasive species might transform their habitats in ways that eventually become unfavorable to them, causing population declines or even extirpations. Here we use over 40 yr of systematically collected data on the abundance of the invasive rusty crayfish Faxonius rusticus from 17 lakes in northern Wisconsin, USA to explore whether population declines of this invader are related to the prevalence of rocky habitat, which shelters crayfish from predators and is unchanged by crayfish. We predicted that lakes with rock-dominated substrates would be resistant to F. rusticus population declines, whereas lakes lacking rock-dominated substrates would experience F. rusticus declines due to crayfish destruction of shelter-providing macrophytes. We found that in nearly one-half (47%) of the study lakes, F. rusticus experienced population declines over the study time period, and these lakes had significantly lower proportions of rock substrate than lakes that did not experience population declines. We recommend that more studies should investigate the potential for invasive species-mediated community or ecosystem feedbacks to eventually contribute to their own population declines.

Environmental DNA (eDNA) detects the invasive rusty crayfish Orconectes rusticus at low abundances

Early detection is invaluable for the cost‐effective control and eradication of invasive species, yet many traditional sampling techniques are ineffective at the low population abundances found at the onset of the invasion process. Environmental DNA (eDNA) is a promising and sensitive tool for early detection of some invasive species, but its efficacy has not yet been evaluated for many taxonomic groups and habitat types.

We evaluated the ability of eDNA to detect the invasive rusty crayfish Orconectes rusticus and to reflect patterns of its relative abundance, in upper Midwest, USA, inland lakes. We paired conventional baited trapping as a measure of crayfish relative abundance with water samples for eDNA, which were analysed in the laboratory with a qPCR assay. We modelled detection probability for O. rusticus eDNA using relative abundance and site characteristics as covariates and also tested the relationship between eDNA copy number and O. rusticus relative abundance.

We detected O. rusticus eDNA in all lakes where this species was collected by trapping, down to low relative abundances, as well as in two lakes where trap catch was zero. Detection probability of O. rusticus eDNA was well predicted by relative abundance of this species and lake water clarity. However, there was poor correspondence between eDNA copy number and O. rusticus relative abundance estimated by trap catches.

Synthesis and applications. Our study demonstrates a field and laboratory protocol for eDNA monitoring of crayfish invasions, with results of statistical models that provide guidance of sampling effort and detection probabilities for researchers in other regions and systems. We propose eDNA be included as a tool in surveillance for invasive or imperilled crayfishes and other benthic arthropods.

Our study demonstrates a field and laboratory protocol for eDNA monitoring of crayfish invasions, with results of statistical models that provide guidance of sampling effort and detection probabilities for researchers in other regions and systems. We propose eDNA be included as a tool in surveillance for invasive or imperilled crayfishes and other benthic arthropods.

The Effects of Biodiesel and Crude Oil on the Foraging Behavior of Rusty Crayfish, Orconectes rusticus

Environmental pollutants, such as crude oil and other petroleum-based fuels, inhibit and limit an organism's ability to perceive a chemical stimulus. Despite the increased use of alternative fuels, such as biodiesel, there have been few studies investigating the impact of these chemicals on the behavior of aquatic organisms. The purpose of this study was to compare the sublethal effects of biodiesel and crude oil exposure on chemically mediated behaviors in a freshwater keystone species. Crayfish (Orconectes rusticus) were tested on their ability to respond appropriately to a positive chemical stimulus within a Y-maze choice paradigm. Behavior was quantified by measuring time spent finding an odor source, duration of time spent at the odor source, percentage of crayfish that found the odor source, and percentage of crayfish that chose the correct arm of the arena. Results indicated negative impacts of both biodiesel and crude oil on the ability of crayfish to locate the food source. However, there were no significant differences between behavioral performances when crayfish were exposed to crude oil compared with biodiesel. Thus, biodiesel and crude oil have equally negative effects on the chemosensory behavior of crayfish. These findings indicate that biodiesel has the potential to have similar negative ecological impacts as other fuel source toxins.

Evolution of invasive traits in nonindigenous species: increased survival and faster growth in invasive populations of rusty crayfish (Orconectes rusticus)

The importance of evolution in enhancing the invasiveness of species is not well understood, especially in animals. To evaluate evolution in crayfish invasions, we tested for differences in growth rate, survival, and response to predators between native and invaded range populations of rusty crayfish (Orconectes rusticus). We hypothesized that low conspecific densities during introductions into lakes would select for increased investment in growth and reproduction in invasive populations. We reared crayfish from both ranges in common garden experiments in lakes and mesocosms, the latter in which we also included treatments of predatory fish presence and food quality. In both lake and mesocosm experiments, O. rusticus from invasive populations had significantly faster growth rates and higher survival than individuals from the native range, especially in mesocosms where fish were present. There was no influence of within-range collection location on growth rate. Egg size was similar between ranges and did not affect crayfish growth. Our results, therefore, suggest that growth rate, which previous work has shown contributes to strong community-level impacts of this invasive species, has diverged since O. rusticus was introduced to the invaded range. This result highlights the need to consider evolutionary dynamics in invasive species mitigation strategies.

The effects of bt corn on rusty crayfish (Orconectes rusticus) growth and survival

Bt crops are one of the most commonly used genetically modified crops worldwide. Bt crops contain a gene that is derived from the bacteria Bacillus thuringiensis, which produces the Cry1Ab toxin. Bt corn that contains the Cry1Ab toxin is used throughout the Midwest United States to control crop pests such as the European corn borer (Ostrinia nubilalis). Headwater streams in regions known for intensive agriculture receive Bt corn detritus after the fall harvest, which is then consumed by a diverse community of stream invertebrates. The rusty crayfish (Orconectes rusticus) is a common invertebrate detritivore in these headwater streams. Both isogenic and Bt corn were grown under the controlled environmental conditions of a greenhouse and, after senescence, were tested for nutritional equality. Rusty crayfish were exposed to one of several detrital treatments composed of Bt corn, Bt corn plus American sycamore (Platanus occidentalis), isogenic corn alone, isogenic corn plus P. occidentalis, or P. occidentalis alone for 8 weeks. Both strains of corn were grown under the controlled environmental conditions in a greenhouse and were tested for nutritional equality after senescence. Crayfish were housed in live streams with a water temperature of 12.8 °C and a 12:12 h light-to-dark photoperiod. Survival and growth of animals within each experimental treatment were monitored each week. After 8 weeks of exposure, there was no statistically significant difference in growth between crayfish in Bt and isogenic treatments. However, survivorship was 31 % lower in the Bt treatment compared with the isogenic treatment. These results suggest that the Bt corn and isogenic corn were of equivalent nutritional value but that Bt corn does have a toxic effect on rusty crayfish during long-term exposure.

A trematode parasite alters growth, feeding behavior, and demographic success of invasive rusty crayfish (Orconectes rusticus)

Nonindigenous species can cause major changes to community interactions and ecosystem processes. The strong impacts of these species are often attributed to their high demographic success. While the importance of enemy release in facilitating invasions has often been emphasized, few studies have addressed the role of parasites in the invasive range in controlling demographic success of potential invaders. Here we examine whether a trematode parasite (Microphallus spp.) can contribute to previously documented alternate states in the abundance of invasive rusty crayfish (Orconectes rusticus) in north temperate lakes in Wisconsin, USA. Microphallus infect O. rusticus after emerging from their first intermediate host, a hydrobiid snail. As previously documented, O. rusticus reduce densities of hydrobiid snails through direct predation and destruction of macrophyte habitat. Therefore, if Microphallus substantially reduce O. rusticus fitness, these parasites may reinforce a state of low crayfish abundance, and, at the other extreme, abundant crayfish may repress these parasites, reinforcing a state of high crayfish abundance. From samples collected from 109 sites in 16 lakes, we discovered (1) a positive relationship between crayfish infection intensity and hydrobiid snail abundance, (2) a negative relationship between parasite prevalence and crayfish abundance, and (3) a negative relationship between parasite prevalence and crayfish population growth. With experiments, we found that infection with Microphallus reduced foraging behavior and growth in O. rusticus, which may be the mechanisms responsible for the population reductions we observed. Overall results are consistent with the hypothesis that Microphallus contributes to alternate states in the abundance and impacts of O. rusticus.

Integrated assessment of biological invasions

As the main witnesses of the ecological and economic impacts of invasions on ecosystems around the world, ecologists seek to provide the relevant science that informs managers about the potential for invasion of specific organisms in their region(s) of interest. Yet, the assorted literature that could inform such forecasts is rarely integrated to do so, and further, the diverse nature of the data available complicates synthesis and quantitative prediction. Here we present a set of analytical tools for synthesizing different levels of distributional and/or demographic data to produce meaningful assessments of invasion potential that can guide management at multiple phases of ongoing invasions, from dispersal to colonization to proliferation. We illustrate the utility of data-synthesis and data-model assimilation approaches with case studies of three well-known invasive species--a vine, a marine mussel, and a freshwater crayfish--under current and projected future climatic conditions. Results from the integrated assessments reflect the complexity of the invasion process and show that the most relevant climatic variables can have contrasting effects or operate at different intensities across habitat types. As a consequence, for two of the study species climate trends will increase the likelihood of invasion in some habitats and decrease it in others. Our results identified and quantified both bottlenecks and windows of opportunity for invasion, mainly related to the role of human uses of the landscape or to disruption of the flow of resources. The approach we describe has a high potential to enhance model realism, explanatory insight, and predictive capability, generating information that can inform management decisions and optimize phase-specific prevention and control efforts for a wide range of biological invasions.

Interactive effects of calcium decline and predation risk on the potential for a continuing northward range expansion of the rusty crayfish (Orconectes rusticus)

Over the last three decades, the rusty crayfish (Orconectes rusticus (Girard, 1852)) has been expanding its range northward via human-mediated dispersal. If this species is to continue expanding its range northward, it will move onto the Canadian Shield, where calcium (Ca) availability is low and is predicted to decline further in the future. Dissolved Ca is a vital functional component of mollusc and crustacean physiology, important for exoskeletal condition and strength, as well as metabolic activity. However, some organisms are able to compensate for reduced structural integrity by modifying their behaviour. In this study, we asked if the invasive O. rusticus can survive low levels of ambient Ca and, if it can, whether it exhibits modified antipredator behavior in response to the physiological limitations imposed by low [Ca]. We found that, under reduced Ca levels, O. rusticus reduced the frequency of standard activities (such as grooming and foraging) and was more likely to engage in vigilance and (or) escape behavior. We also found that some individuals, in extremely low [Ca], died while molting. This study suggests that Ca limitation on the Shield, especially where predators are present, may limit the northward expansion of O. rusticus beyond their current range limit.

Translocation of imperiled species under changing climates

Conservation translocation of species varies from restoring historic populations to managing the relocation of imperiled species to new locations. We review the literature in three areas--translocation, managed relocation, and conservation decision making--to inform conservation translocation under changing climates. First, climate change increases the potential for conflict over both the efficacy and the acceptability of conservation translocation. The emerging literature on managed relocation highlights this discourse. Second, conservation translocation works in concert with other strategies. The emerging literature in structured decision making provides a framework for prioritizing conservation actions--considering many possible alternatives that are evaluated based on expected benefit, risk, and social-political feasibility. Finally, the translocation literature has historically been primarily concerned with risks associated with the target species. In contrast, the managed relocation literature raises concerns about the ecological risk to the recipient ecosystem. Engaging in a structured decision process that explicitly focuses on stakeholder engagement, problem definition and specification of goals from the outset will allow creative solutions to be developed and evaluated based on their expected effectiveness.

Associative learning in male rusty crayfish (Orconectes rusticus): conditioned behavioural response to an egg cue from walleye (Sander vitreus)

Chemical communication governs a diversity of life processes in aquatic organisms. Crayfish use chemoreception during reproduction, social hierarchy formation, predation avoidance, and resource localization. Fish eggs release recognizable chemoattractants for vertebrate predators of eggs that can motivate crayfish to engage in egg predation as well. We hypothesized that male rusty crayfish ( Orconectes rusticus (Girard, 1852)) from a lake free of walleye ( Sander vitreus (Mitchill, 1818)) would not possess an innate recognition of a walleye egg cue. However, if conditioned by employing a single 2 h paired stimulus exposure (known food cue + egg cue), then male rusty crayfish would be attracted to the same egg cue upon subsequent exposure. Using a Y-maze behavioural arena we discovered that once conditioned, crayfish took significantly less time to choose the arm containing the egg cue alone relative to a control. Our study suggests that male rusty crayfish exhibit second-order conditioning through associative learning, allowing them to quickly and easily learn to identify novel odour stimuli from fish eggs under laboratory conditions.

Assessing ecosystem vulnerability to invasive rusty crayfish (Orconectes rusticus)

Despite the widespread introduction of nonnative species and the heterogeneity of ecosystems in their sensitivity to ecological impacts, few studies have assessed ecosystem vulnerability to the entire invasion process, from arrival to establishment and impacts. Our study addresses this challenge by presenting a probabilistic, spatially explicit approach to predicting ecosystem vulnerability to species invasions. Using the freshwater-rich landscapes of Wisconsin, USA, we model invasive rusty crayfish (Orconectes rusticus) as a function of exposure risk (i.e., likelihood of introduction and establishment of O. rusticus based on a species distribution model) and the sensitivity of the recipient community (i.e., likelihood of impacts on native O. virilis and O. propinquus based on a retrospective analysis of population changes). Artificial neural networks predicted that approximately 10% of 4200 surveyed lakes (n = 388) and approximately 25% of mapped streams (23 523 km total length) are suitable for O. rusticus introduction and establishment. A comparison of repeated surveys before vs. post-1985 revealed that O. virilis was six times as likely and O. propinquus was twice as likely to be extirpated in streams invaded by O. rusticus, compared to streams that were not invaded. Similarly, O. virilis was extirpated in over three-quarters of lakes invaded by O. rusticus compared to half of the uninvaded lakes, whereas no difference was observed for O. propinquus. We identified 115 lakes (approximately 3% of lakes) and approximately 5000 km of streams (approximately 6% of streams) with a 25% chance of introduction, establishment, and extirpation by O. rusticus of either native congener. By identifying highly vulnerable ecosystems, our study offers an effective strategy for prioritizing on-the-ground management action and informing decisions about the most efficient allocation of resources. Moreover, our results provide the flexibility for stakeholders to identify priority sites for prevention efforts given a maximum level of acceptable risk or based on budgetary or time restrictions. To this end, we incorporate the model predictions into a new online mapping tool with the intention of closing the communication gap between academic research and stakeholders that requires information on the prospects of future invasions.

Species replacement by a nonnative salmonid alters ecosystem function by reducing prey subsidies that support riparian spiders

Replacement of a native species by a nonnative can have strong effects on ecosystem function, such as altering nutrient cycling or disturbance frequency. Replacements may cause shifts in ecosystem function because nonnatives establish at different biomass, or because they differ from native species in traits like foraging behavior. However, no studies have compared effects of wholesale replacement of a native by a nonnative species on subsidies that support consumers in adjacent habitats, nor quantified the magnitude of these effects. We examined whether streams invaded by nonnative brook trout (Salvelinus fontinalis) in two regions of the Rocky Mountains, USA, produced fewer emerging adult aquatic insects compared to paired streams with native cutthroat trout (Oncorhynchus clarkii), and whether riparian spiders that depend on these prey were less abundant along streams with lower total insect emergence. As predicted, emergence density was 36% lower from streams with the nonnative fish. Biomass of brook trout was higher than the cutthroat trout they replaced, but even after accounting for this difference, emergence was 24% lower from brook trout streams. More riparian spiders were counted along streams with greater total emergence across the water surface. Based on these results, we predicted that brook trout replacement would result in 6-20% fewer spiders in the two regions. When brook trout replace cutthroat trout, they reduce cross-habitat resource subsidies and alter ecosystem function in stream-riparian food webs, not only owing to increased biomass but also because traits apparently differ from native cutthroat trout.

Preventing the spread of invasive species: economic benefits of intervention guided by ecological predictions

Preventing the invasion of freshwater aquatic species is the surest way to reduce their impacts, but it is also often expensive. Hence, the most efficient prevention programs will rely on accurate predictions of sites most at risk of becoming invaded and concentrate resources at those sites. Using data from Vilas County, Wisconsin (U.S.A.), collected in the 1970s, we constructed a predictive occurrence model for rusty crayfish (Orconectes rusticus) and applied it to an independent data set of 48 Vilas County lakes to predict which of these were most likely to become invaded between 1975 and 2005. We nested this invasion model within an economic framework to determine whether targeted management, derived from our quantitative predictions of likely invasion sites, would increase the economic value of lakes in the independent data set. Although the optimum expenditure on lake protection was high, protecting lakes at this level would have produced net economic benefits of at least $6 million over the last 30 years. We did not attempt to determine the value of nonmarket benefits of protection; thus, our results are likely to underestimate the total benefits from preventing invasions. Our results demonstrate that although few data are available early in an invasion, these data may be sufficient to support targeted, effective, and economically rational management. In addition, our results show that ecological predictions are becoming sufficiently accurate that their application in management can produce net economic benefits.