Establishment Of Non-indigenous Species Research Articles

The future of the Mediterranean Sea Ecosystem: towards a different tomorrow

The structure and function of the Mediterranean Sea Ecosystems (MSE) are rapidly changing. Global warming is the main driver of change, and the MSE responses are dramatic. The establishment of non-indigenous species, heavily influenced ecosystem functioning; human pressures such as overfishing, triggered regime shifts such as that from fish to jellyfish; the Eastern Mediterranean Transient changed the circulation patterns of the Eastern Mediterranean basin. Models did not predict these changes. Science, so far, focused much on the constraints that lead to regular sequences of events, allowing for predictions. In periods of rapid change, though, the historical nature of ecology becomes prominent, and contingencies acquire an overwhelming importance. This calls for new approaches to the study of complex systems, for instance with a shift from monitoring to observation, with a revival of natural history. The identification of cells of ecosystem functioning, based on oceanographic processes that enhance production at different levels of marine food webs in specific areas, is a challenge for future studies that will eventually lead to better management and protection of the marine natural heritage. Even if historical systems do not allow for predictions, some trends are clear and a set of possible scenarios of what will happen in the future Mediterranean Sea can be proposed.

Ecosystem functioning impacts of the invasive seaweed Sargassum muticum (Fucales, Phaeophyceae).

Ongoing changes in natural diversity due to anthropogenic activities can alter ecosystem functioning. Particular attention has been given to research on biodiversity loss and how those changes can affect the functioning of ecosystems, and, by extension, human welfare. Few studies, however, have addressed how increased diversity due to establishment of nonindigenous species (NIS) may affect ecosystem function in the recipient communities. Marine algae have a highly important role in sustaining nearshore marine ecosystems and are considered a significant component of marine bioinvasions. Here, we examined the patterns of respiration and light-use efficiency across macroalgal assemblages with different levels of species richness and evenness. Additionally, we compared our results between native and invaded macroalgal assemblages, using the invasive brown macroalga Sargassum muticum (Yendo) Fensholt as a model species. Results showed that the presence of the invader increased the rates of respiration and production, most likely as a result of the high biomass of the invader. This effect disappeared when S.muticum lost most of its biomass after senescence. Moreover, predictability-diversity relationshipsof macroalgal assemblages varied between native and invaded assemblages. Hence, the introduction of high-impact invasive species may trigger major changes in ecosystem functioning. The impact of S.muticum may be related to its greater biomass in the invaded assemblages, although species interactions and seasonality influenced the magnitude of the impact.

Managing Multiple Vectors for Marine Invasions in an Increasingly Connected World

Invasive species remain a major environmental problem in the world's oceans. Managing the vectors of introduction is the most effective means of mitigating this problem, but current risk assessments and management strategies are largely focused on species, not on vectors and certainly not on multiple simultaneous vectors. To highlight the issue that multiple vectors contribute to invasions, we analyzed the historical and contemporary contributions of eight maritime vectors to the establishment of nonindigenous species in California, where most species were associated with two to six vectors. Vessel biofouling looms larger than ballast water as a major vector and a management opportunity, but aquaculture risk appears reduced from historic levels. Standardized data on species abundances in each vector are lacking for a robust cross-vector assessment, which could be obtained in a proof-of-concept “vector blitz.” Management must shift away from one or two target vectors to coordination across multiple vectors.

Ecosystem impacts of feral rabbits on World Heritage sub-Antarctic Macquarie Island: A palaeoecological perspective

The introduction and establishment of non-indigenous species through human activities often poses a major threat to natural biodiversity. In many parts of the world management efforts are therefore focused on their eradication. The environment of World Heritage sub-Antarctic Macquarie Island has been severely damaged by non-indigenous species including rabbits, rats and mice, introduced from the late AD 1800s. An extensive eradication programme is now underway which aims to remove all rabbits and rodents. To provide a long-term context for assessing the Island's pre-invasion state, invasion impacts, and to provide a baseline for monitoring its recovery, we undertook a palaeoecological study using proxies in a lake sediment core. Sedimentological and diatom analyses revealed an unproductive catchment and lake environment persisted for ca. 7100 years prior to the introduction of the invasive species. After ca. AD 1898, unprecedented and statistically significant environmental changes occurred. Lake sediment accumulation rates increased >100 times due to enhanced catchment inputs and within-lake production. Total carbon and total nitrogen contents of the sediments increased by a factor of four. The diatom flora became dominated by two previously rare species. The results strongly suggest a causal link between the anthropogenic introduction of rabbits and the changes identified in the lake sediments. This study provides an example of how palaeoecology may be used to determine baseline conditions prior to the introduction of non-indigenous species, quantify the timing and extent of changes, and help monitor the recovery of the ecosystem and natural biodiversity following successful non-indigenous species eradication programmes.

Mixed but not admixed: a spatial analysis of genetic variation of an invasive ascidian on natural and artificial substrates

Following the introduction to a new area (pre-border dispersal), post-border processes determine the success in the establishment of non-indigenous species (NIS). However, little is known on how these post-border processes shape the genetic composition of NIS at regional scales. Here, we analyse genetic variation in introduced populations along impacted coastlines to infer demographic and kinship dynamics at the post-border stage. We used as a model system the ascidian species Microcosmus squamiger that has been introduced worldwide. This species can colonize and grow fast on man-made artificial structures, impacting activities such as mariculture. However, it can also establish itself on natural substrates, thus altering natural communities and becoming an ecological problem. We genotyped 302 individuals from eight populations established on natural and artificial substrates in the north-western Mediterranean Sea, using six microsatellite loci. We then compared the resulting genotypes with those found within the native range of the species. We found high levels of genetic diversity and allelic richness in all populations, with an overall deficit of heterozygotes. Autocorrelation analyses showed that there was no within-population genetic structure (at a scale of tens of metres); likewise, no significant differentiation in pairwise comparisons between populations (tens of kilometres apart) and no isolation-by-distance pattern was found. The results suggest that M. squamiger has a natural capacity for high dispersal from one patch of hard substrate to another and no differences whatsoever could be substantiated between natural and artificial substrates. Interestingly, two groups of genetically differentiated individuals were detected that were associated with the two ancestral source areas of the worldwide expansion of the species. Individual assignment tests showed the coexistence of individuals of these two clusters in all populations but with little interbreeding among them as the frequency of admixed individuals was only 15 %. The mechanism responsible for maintaining these genetic pools unmixed is unknown, but it does not appear to compromise post-border colonization of introduced populations.

Contrasting patterns of genome‐wide polymorphism in the native and invasive range of the marine mollusc Crepidula fornicata

Selection processes are believed to be an important evolutionary driver behind the successful establishment of nonindigenous species, for instance through adaptation for invasiveness (e.g. dispersal mechanisms and reproductive allocation). However, evidence supporting this assumption is still scarce. Genome scans have often identified loci with atypical patterns of genetic differentiation (i.e. outliers) indicative of selection processes. Using microsatellite- and AFLP-based genome scans, we looked for evidence of selection following the introduction of the mollusc Crepidula fornicata. Native to the northwestern Atlantic, this gastropod has become an emblematic invader since its introduction during the 19th and 20th centuries in the northeastern Atlantic and northeastern Pacific. We examined 683 individuals from seven native and 15 introduced populations spanning the latitudinal introduction and native ranges of the species. Our results confirmed the previously documented high genetic diversity in native and introduced populations with little genetic structure between the two ranges, a pattern typical of marine invaders. Analysing 344 loci, no outliers were detected between the introduced and native populations or in the introduced range. The genomic sampling may have been insufficient to reveal selection especially if it acts on traits determined by a few genes. Eight outliers were, however, identified within the native range, underlining a genetic singularity congruent with a well-known biogeographical break along the Florida. Our results call into question the relevance of AFLP genome scans in detecting adaptation on the timescale of biological invasions: genome scans often reveal long-term adaptation involving numerous genes throughout the genome but seem less effective in detecting recent adaptation from pre-existing variation on polygenic traits. This study advocates other methods to detect selection effects during biological invasions-for example on phenotypic traits, although genome scans may remain useful for elucidating introduction histories.

Continent-wide risk assessment for the establishment of nonindigenous species in Antarctica

Invasive alien species are among the primary causes of biodiversity change globally, with the risks thereof broadly understood for most regions of the world. They are similarly thought to be among the most significant conservation threats to Antarctica, especially as climate change proceeds in the region. However, no comprehensive, continent-wide evaluation of the risks to Antarctica posed by such species has been undertaken. Here we do so by sampling, identifying, and mapping the vascular plant propagules carried by all categories of visitors to Antarctica during the International Polar Year's first season (2007-2008) and assessing propagule establishment likelihood based on their identity and origins and on spatial variation in Antarctica's climate. For an evaluation of the situation in 2100, we use modeled climates based on the Intergovernmental Panel on Climate Change's Special Report on Emissions Scenarios Scenario A1B [Nakićenović N, Swart R, eds (2000) Special Report on Emissions Scenarios: A Special Report of Working Group III of the Intergovernmental Panel on Climate Change (Cambridge University Press, Cambridge, UK)]. Visitors carrying seeds average 9.5 seeds per person, although as vectors, scientists carry greater propagule loads than tourists. Annual tourist numbers (∼33,054) are higher than those of scientists (∼7,085), thus tempering these differences in propagule load. Alien species establishment is currently most likely for the Western Antarctic Peninsula. Recent founder populations of several alien species in this area corroborate these findings. With climate change, risks will grow in the Antarctic Peninsula, Ross Sea, and East Antarctic coastal regions. Our evidence-based assessment demonstrates which parts of Antarctica are at growing risk from alien species that may become invasive and provides the means to mitigate this threat now and into the future as the continent's climate changes.

Evaluation of stochastic gravity model selection for use in estimating non-indigenous species dispersal and establishment

Predicting dispersal of nonindigenous species (NIS) is an essential component of risk analysis and management, as preventative measures are most readily applied at this stage of the invasion sequence. Gravity models provide one of the most useful techniques available to model dispersal of nonindigenous invasive species (NIS) across heterogeneous landscapes, as these models are able to capture transport patterns of recreational boaters who are dominant vectors responsible for aquatic NIS dispersal. Despite the widespread use of gravity models in forecasting biological invasions, different classes of gravity models have not been evaluated regarding their comparative ability to capture recreational transport patterns and subsequent use in predicting NIS establishment. Here we evaluate model selection between unconstrained, total-flow-constrained, production-constrained and doubly-constrained stochastic gravity models to assess dispersal of the spiny waterflea Bythotrephes between Ontario lakes. Differences between the models relate to the amount of data required and constraints under which calculations of source/destination interactions are made. For each class of gravity model, we then estimated the probability of a lake having established Bythotrephes populations by modeling the relationship between empirical presence/absence data and inbound recreational traffic (i.e. propagule pressure) via boosted regression. The unconstrained gravity model provided the best fit to observed traffic patterns of recreational boaters. However, when output from the gravity models was used to predict Bythotrephes establishment, the doubly-constrained gravity model provided the strongest relationship between inbound recreational traffic and observed Bythotrephes presence/absence, followed by the production-constrained model. Our results indicate production-constrained gravity models offer an acceptable balance between modeling recreational boater traffic and their utility to estimate establishment probabilities.

The value of nonindigenous species risk assessment in international trade

Managing the introduction of nonindigenous species is becoming a major goal of policy-makers at regional, national and international scales. Here we investigate, at the national level, the ideal design and expected net benefits of a risk assessment program for evaluating the desirability of nonindigenous species imports. We show how to enhance the statistical rigor of such a system by correcting a common non-random sampling problem encountered in the data. This correction enables model output to be interpreted in an economically relevant way and facilitates a theoretically rigorous characterization of the balance between trade and nonindigenous species establishment risk. Using reptiles and amphibians imported to the U.S. as a case study, we characterize economic outcomes over a range of cases and demonstrate substantial expected returns to such a screening program, relative to the current effectively open-door policy. Our results are informative for the current debate in the U.S. about whether to require federal agencies to apply risk assessment before allowing a species for import. The framework presented decomposes a complex argument about risk management into component economic and statistical parts, allowing for debate and improved understanding over each element to inform the overall program in a transparent fashion.

Can biotic resistance be utilized to reduce establishment rates of non-indigenous species in constructed waters?

Understanding the mechanisms that facilitate establishment of non-indigenous species is imperative for devising techniques to reduce invasion rates. Passively dispersing non-indigenous organisms, including zooplankton, seemingly invade constructed waters (e.g., ornamental ponds, dams and reservoirs) at faster rates than natural lakes. A common attribute of these invaded water bodies is their relatively young age, leading to the assertion that low biotic resistance may lead to their higher vulnerability. Our aim was to determine if seeding of young water bodies with sediments containing diapausing stages of native zooplankton could accelerate community development, leading to greater biotic resistance to the establishment of new species. Twenty outdoor tanks were filled with water (1,400 L) and nutrients added to attain eutrophic conditions. Ten treatment tanks had sediments added, sourced from local water bodies. In the remaining ten, sediments were autoclaved, and received zooplankton via natural dispersal only. In an initial 12 month monitoring period, species richness increased at a greater rate in the treatment tanks (at 12 months average standing richness per tank = 3.8, accumulated richness = 8.2) than control tanks (2.6 and 5.0, P < 0.05). Treatment tanks developed assemblages with greater proportions of species adapted to pelagic conditions, such as planktonic cladocerans and copepods, while control tanks generally comprised of smaller, littoral dwelling, rotifers. Analysis of similarities indicated community composition differed between the control and treatment groups at 12 months (P < 0.01). Two copepod, four rotifer and one cladoceran species were intentionally added to tanks at 12 months. In the 3 month post-introduction period, five of these species established populations in the control tanks, while only two species established in the treatment tanks. The calanoid copepod Skistodiaptomus pallidus, for example, a non-indigenous species confined to constructed waters in New Zealand, established exclusively in tanks where native calanoid copepod species were absent (primarily control tanks). Our study suggests that biotic resistance could play an important role in reducing the establishment rate of non-indigenous zooplankton. It also provides evidence that seeding constructed water bodies with sediments containing diapausing eggs of native species may provide an effective management tool to reduce establishment rates of non-indigenous zooplankton.

Development and non-indigenous species at the global scale

This paper investigates the explanatory power of development, prosperity and regulatory capability as proximate causes of non-indigenous species (NIS) occurrence in different countries, doing this by statistically analysing a global cross-sectional data set. Since the quantification of development is subject to much discussion, two different indicators are tested: gross domestic product (GDP) per capita and the United Nations human development index (HDI). A corruption index is used as an indicator of regulatory capacity. In addition, variables capturing country openness, which facilitates NIS introduction, and habitat conditions, which determine NIS establishment, are included as explanatory variables. The GDP indicator together with the corruption index fits the NIS data best, where the number of NIS is higher in countries with larger incomes. However, countries with relatively high income but low institutional capacity show a larger number of NIS than countries with a similar level of income but with higher institutional capacity. The results also point to the significant contributions of openness and habitat fragmentation to NIS occurrences in the countries.

Factors affecting survivorship of defouled communities and the effect of fragmentation on establishment success

The environmental risks associated with the defouling of artificial structures (e.g., vessels, oil rigs, marina pontoons, aquaculture structures) in the marine environment are gaining international attention. This paper presents a series of laboratory- and field-based experiments that collectively aimed to elucidate biotic and abiotic factors that influence re-establishment success of biofouling organisms and fragmented colonial organisms defouled to the seabed. Reattachment success of colonial organisms experimentally fragmented was found to be species specific and dependent on fragment size. In both laboratory and field trials, some colonial ascidians had consistently greater reattachment success for larger size classes of fragments, while other encrusting and erect taxa showed poor reattachment capabilities. This study also revealed that sedimentation and turbidity are likely to have a strong influence on the survivorship of defouled material. Furthermore, for both high and low sedimentary environments, survivorship was found to be greater where predators were excluded. Despite risks posed by non-indigenous species (NIS), it is proposed that in-water defouling may be an appropriate management response in situations where a “do nothing” approach is potentially more detrimental. Moreover, results from this study suggest that environmental risks associated with defouling may be mitigated through appropriate defouling strategies (e.g., defouling location, frequency and method). In order to increase our predictive abilities for NIS establishment success resulting from in-water defouling, future studies should aim to further elucidate the relative importance of factors affecting survivorship of defouled material at locations where defouling is routinely undertaken.

Substrate type as a selective tool against colonization by non-native sessile invertebrates

Different substrates of varying composition, color, texture and orientation may selectively influence recruitment of sessile invertebrates and thereby influence the resultant community. Thus substrates may act as a barrier to the establishment of non-indigenous species (NIS). In southern Brazil, granite is the main rock forming natural rocky walls that are available for encrusting organisms. In this study we tested whether granite selectively influences recruitment and impedes colonization by introduced and cryptogenic species that are already established on artificial substrates within the region. Plates of rough cut granite and of polyethylene were made available each month under a pier at a yacht club in Paranaguá Bay. A community is already established on concrete columns and fiber glass floats on the piers. After one, two and twelve months, the faunal composition of the plates was compared between the two treatments and other artificial substrates. Granite was recruited by all the seven introduced species found in the Bay and by 18 of 26 cryptogenic species and therefore is ineffective as a barrier to NIS colonization.

Non indigenous ascidians in port and natural environments in a tropical Brazilian bay

Despite limited natural dispersal, some species of ascidians can be transported vast distances via oceanic petroleum platforms, ship hulls and ballast water and therefore may be good indicators of bioinvasion. Usually non indigenous species (NIS) are abundant in harbors. This is caused in part because of the higher propagule delivery rate in these areas. An alternative explanation of why invasion is enhanced in harbor and marinas is that environmental degradation commonly found in these habitats favors the establishment of NIS. Most surveys for introduced species were not comprehensive and targeted mainly ports and marinas. Angra dos Reis is an excellent system that provides an opportunity to compare the potential distribution of introduced and native species of Ascidiacea between port and natural environments. Here, we compared the colonization of experimental subtidal plates placed in harbors and marinas with the colonization of plates placed in nearby natural areas. With 27 taxa (15 identified to species), species richness was greater in port environments (25 versus 8). Six taxa were common to both environments while 19 taxa were exclusively found in ports. Among the identified species in ports, three were introduced, five were cryptogenic and only one was native. Only three species were found exclusively in the natural sites and all were cryptogenic. The presence of introduced species only in the port areas of Angra dos Reis reinforces the need for continued, periodic monitoring in the region for early detection of new, potentially invasive, species as well as for better understanding of abnormal population growth of the already known species. Management to reduce the transfer of exotics to natural habitats must be implemented.

Does larval supply explain the low proliferation of the invasive gastropod Crepidula fornicata in a tidal estuary?

Human-mediated transport and aquaculture have promoted the establishment of non-indigenous species in many estuaries around the world over the last century. This phenomenon has been demonstrated as a major cause of biodiversity alterations, which has prompted scientists to provide explanations for the success or failure of biological invasions. Crepidula fornicata is a gastropod native from the East coast of North America which has successfully invaded many European bays and estuaries since the 19th century, with some noticeable exceptions. Its spread over Europe has been explained by a combination of human-mediated transport and natural dispersal through its long-lived planktonic larva. We here investigated whether larval supply may explain the failure in the proliferation of this species within a particular bay, the Bay of Morlaix (France). Patterns of larval distribution and larval size structure were analysed over ten sites sampled three times (20 July, 4 August and 21 August 2006), regarding characteristics of the adult population and environmental features. Our results evidenced a strong spatial structure in both larval abundance and size at the bay scale, even if larval abundances were low. In this scheme, the location of spawning adults played a critical role, with high numbers of early larvae above the main spawning location. The larval size structure further showed that settlement-stage larvae were rare, which suggested that released larvae might have been exported out of the bay. The use of an analytical model aimed to study the effect of tidal currents on the potential for larval exportation confirmed that larval retention within the bay might be low. The limitation in larval supply resulting from the interactions between spawning location and local hydrodynamics may thus impede the proliferation of this species which is well established for more than 50 years. This study provided an example of factors which may explain the failure of the transition between two major steps of biological invasions, i.e. sustainable establishment and proliferation.

Relative contributions of domestic and foreign sourced ballast water to propagule pressure in Puget Sound, Washington, USA

The movement and release of non-indigenous species (NIS) in ship ballast water is a global threat to the conservation of native aquatic species and habitats. One key to successful NIS establishment in coastal waters is propagule pressure – the size and frequency of NIS inoculations. We estimated propagule pressure of high-risk coastal zooplankton delivered in ballast water to Puget Sound, Washington, USA, which receives 7.5 × 10 6 m 3 of ballast water annually. We weighed the relative propagule pressure from domestic and foreign ballast water, in terms of propagule size (number of individuals), frequency of NIS occurrences, and diversity (number of known NIS species). Ship discharge volume was not a good predictor of propagule pressure. Instead route type (domestic or foreign) and ballast water exchange status (exchanged in mid-ocean vs. unexchanged) were much better predictors of propagule supply. Overall, while the diversity of non-indigenous zooplankton was higher in trans-oceanic ballast, the annual discharge of coastal zooplankton propagules to Puget Sound was much greater for ships conducting domestic voyages. These results suggest that intra-coastal ballast water must be further scrutinized as a pathway for NIS introduction, and calls into question regional “common waters” agreements that allow vessels to move ballast without conducting ballast water treatment.

Invasibility of reservoirs in the Paraná Basin, Brazil, to Cichla kelberi Kullander and Ferreira, 2006

The invasion process comprises not only the characteristics of nonindigenous species but also the attributes of the invaded environment which make it susceptible to the establishment of nonindigenous species. Habitat attributes operate like filters in determining the establishment of introduced species and the invasibility of a region. In the Upper Parana River Basin, Brazil, the practice of introducing species was quite common and frequently carried out by hydroelectric companies. The target species of the present study, the peacock-bass Cichla kelberi, is native to the Amazon Basin. This species was introduced into several reservoirs of the Upper Parana River Basin and is dispersing rapidly throughout the system. This study shows which characteristics of the reservoirs facilitate their invasibility after testing for the effect of propagule pressure. We conclude that a set of abiotic factors favors the invasibility of these reservoirs. To be more precise, the largest, deepest, most transparent and warmest reservoirs are the most likely to be colonized by Cichla kelberi. It is possible that other environments with similar characteristics to these reservoirs, such as the lagoons from the Upper Parana River Basin floodplain, can also be colonized by Cichla kelberi.

Potential impact of genome exclusion by alien species in the hybridogenetic water frogs (Pelophylax esculentus complex)

Globalization and increasing human impact on natural aquatic systems have facilitated the movement of species and the establishment of nonindigenous species enhancing hybridisation opportunities between naturally allopatric species. In this review, we focus on a special case of natural hybrid speciation and the consequences of recent anthropogenic hybridisation in the water frog complex (Pelophylax esculentus complex), which consists of two parental species, Pelophylax lessonae and Pelophylax ridibundus and a hybrid taxon. The hybrid water frogs reproduce hybridogenetically and eliminate the genome of the syntopic water frog species. Although the actual cause triggering chromosome exclusion remains elusive, it has been proposed that chromosome elimination takes place prior to meiosis and may involve enzymatic degradation of the discarded genome. Translocations of water frogs in Western Europe have become frequent the last decade leading to rapid expansion of the range of the marsh frog P. ridibundus. Subsequent hybridisation of the exotic P. ridibundus may dramatically affect the viability and maintenance of hybrid water frog populations throughout Europe. Interestingly, the impact of this introduced species may differ depending on their geographic origin, which defines the ability to induce genome elimination. This may result in fertile or sterile hybrids, making global conservation guidelines challenging. We predict a severe genetic and ecological impact of nonindigenous P. ridibundus prompting for strict conservation measures to reduce species translocations and for studies on the geographic origin of exotic frog species.

The Influence of Anthropogenic Disturbance and Environmental Suitability on the Distribution of the Nonindigenous Amphipod, Echinogammarus ischnus, at Laurentian Great Lakes Coastal Margins

Invasion ecology offers a unique opportunity to examine drivers of ecological processes that regulate communities. Biotic resistance to nonindigenous species establishment is thought to be greater in communities that have not been disturbed by human activities. Alternatively, invasion may occur wherever environmental conditions are appropriate for the colonist, regardless of the composition of the existing community and the level of disturbance. We tested these hypotheses by investigating distribution of the nonindigenous amphipod, Echinogammarus ischnus Stebbing, 1899, in co-occurrence with a widespread amphipod, Gammarus fasciatus Say, 1818, at 97 sites across the Laurentian Great Lakes coastal margins influenced by varying types and levels of anthropogenic stress. E. Ischnus was distributed independently of disturbance gradients related to six anthropogenic disturbance variables that summarized overall nutrient input, nitrogen, and phosphorus load carried from the adjacent coastal watershed, agricultural land area, human population density, overall pollution loading, and the site-specific dominant stressor, consistent with the expectations of regulation by general environmental characteristics. Our results support the view that the biotic facilitation by dreissenid mussels and distribution of suitable habitats better explain E. ischnus’ distribution at Laurentian Great Lakes coastal margins than anthropogenic disturbance.

Waiting for Invasions: A Framework for the Arrival of Nonindigenous Species

The process of nonindigenous species (NIS) arrival has received limited theoretical consideration despite importance in predicting and preventing the establishment of NIS. We formulate a mechanistically based hierarchical model of NIS arrival and demonstrate simplifications leading to a marginal distribution of the number of surviving introduced individuals from parameters of survival probability and propagule pressure. The marginal distribution is extended as a stochastic process from which establishment emerges with a waiting time distribution. This provides a probability of NIS establishment within a specified period and may be useful for identifying patterns of successful invaders. However, estimates of both the propagule pressure and the individual survival probability are rarely available for NIS, making estimates of the probability of establishment difficult. Alternatively, researchers are able to measure proportional estimates of propagule pressure through models of NIS transport, such as gravity models, or of survival probability through habitat-matching indexes measuring the similarity between potentially occupied and native NIS ranges. Therefore, we formulate the relative waiting time between two locations and the probability of one location being invaded before the other.