Anthony Ricciardi
Associate Professor, Invasion Ecology

Redpath Museum & McGill School of Environment
McGill University

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The Great Lakes are the most invaded freshwater ecosystem in the world. We have examined the history of invasions in the basin, and demonstrated a strong correlation between vector activity and the occurrence of non-native species (Ricciardi & MacIsaac 2000, Ricciardi 2001, Ricciardi 2006). In spite of increasing efforts over the past two decades to halt the influx of invaders, and the apparent successful implementation of ballast water regulation in the mid-2000s, the Great Lakes remain vulnerable to future disruptions. New threats are emerging because of risks associated with trade in live organisms and climate change; invasion risks are dynamic and management responses must evolve to cope with them. In a new analysis, we present three future scenarios (optimistic, pessimistic and status quo) that depend upon the development and effectiveness of regulations aimed at blocking current means of entry for invasive species. The paper will be published in a special issue of the Journal of Great Lakes Research. See the McGill press release here. [February 2015].

The northern snakehead (Channa argus) is a potential future invader of the Great Lakes. Photo by USGS.

Students from my lab discovered microplastics (in the form of polyethylene ‘microbeads’, <2 mm diameter) widely distributed across the bottom of the St. Lawrence River, the first time such pollutants have been found in freshwater sediments. The microbeads likely originate from cosmetics (e.g. exfoliants) and industrial applications. Microplastics are a global contaminant in the world’s oceans, but have only recently been detected in the surface waters of lakes and rivers. At some locations, we measured thousands of microbeads per liter of sediment (and hundreds of thousands per square meter), a magnitude that rivals the world’s most contaminated ocean sediments. Contrary to the assumption that floating microplastics are flushed through rivers to the sea, our study shows that rivers can act as a sink for this pollution. The article will be published in the Canadian Journal of Fisheries and Aquatic Sciences. September 2014].

Microbeads collected from St. Lawrence River sediments, summer 2013.

In July 2013, I was fortunate to join several European and South African colleagues in Leipzig for an invited workshop that examined the science and management of impacts of non-native species. The creativity and energy within that gathering was abundant, and a series of papers have begun to emerge from our efforts. One of these, led by Tim Blackburn, outlines a logical and flexible method for classifying invasion threats according to the magnitude and mechanism of their environmental impacts. The paper has appeared in PLoS Biology recently and is the focus of a review article in Conservation. Another paper, led by Jonathan Jeschke and appeared in Conservation Biology, addresses the issue of properly defining 'impact' for the purposes of non-native species research and risk assessment. [June 2014].

In recent years, there has been a rise in opinion articles in which authors have claimed that non-native species are no more likely than natives to cause ecological or economic damage. Our lab has tested this claim in a series of recent and ongoing studies. In an analysis of data for freshwater plants and animals, MSc graduate Ahdia Hassan found that non-native species comprise the majority (60%) of aquatic pests in North America and Europe, and are six times more likely than native species to become a pest (i.e. cause socioeconomic damage). Moreover, the pest status of a species is dependent on the scale of its introduction: species moved between continents have a greater likelihood than intra-continentally transplanted species of being a pest. Our paper has been published in the journal Frontiers in Ecology and the Environment. [May 2014].

Some freshwater pest species in North America: (a) The quagga mussel (Dreissena rostriformis bugensis);(b) Water hyacinth (Eichhornia crassipes); (c) Silver carp (Hypophthalmichthys molitrix), a hazard to recreational boaters; and (d) the bryozoan Plumatella rugosa, a native fouling pest, shown encrusting a water pump. From Hassan & Ricciardi (2014).

Collaborations with Jaimie Dick's lab (Queen's University Belfast) have involved multiple graduate students working on both sides of the Atlantic and have led to rapid progress in the use of functional responses for understanding and predicting invasion success and impact. Previous work in our lab by Dr Åsa Kestrup (e.g. Kestrup et al. 2011) and ongoing work by PhD candidate Josie Iacarella have shown the value of this approach in explaining context-dependent variation in invader impact. Åsa used functional response comparisons to shed light on asymmetrical intraguild predation among native and non-native crustaceans, whereas Josie is using this method to test novel hypotheses - including the Environmental Matching Hypothesis (Ricciardi et al. 2013 Ecol. Monogr.).

Now, PhD candidate Danny Barrios-O'Neill (supervised by Jaimie Dick, in collaboration with me and Hugh MacIsaac) has demonstrated its value in quantifying and predicting the impact of invasive consumers within an elegant multi-species experimental system. Danny compared native and non-native mesopredators (mysid shrimp) in the presence and absence of a higher predator a stickleback), and showed that the native mysid was more inhibited by the higher predator - and thus had consumed fewer basal prey (cladocerans) than the more bolder non-native mysid. His results recent are published in the Journal of Animal Ecology and is highlighted in an "In focus" article in the same issue. [April 2014].

Functional responses of native mysids (in blue) and invasive mysids (in red) alone (b) and in the presence of a stickleback (c). Shaded areas are bootstrapped 95% confidence intervals. After O'Gorman (2014), adapted from Barrios-O'Neill et al. (2014).

Last year, I had the privilege of participating in the Freshwater Invasives – Networking for Strategy (FINS) conference in Galway, Ireland, where managers, academics, policy makers and international experts met to collectively identify the key issues relating to invasive alien species in Europe. These 10 key issues were determined in a horizon scanning workshop, and are relevant to invasion threats in terrestrial and marine systems as well. Our findings have now been published in the journal Management of Biological Invasions. Governmental support for this endeavour was strong; the Irish Minister of Fisheries was in attendance. I thank Joe Caffrey and Frances Lucy for inviting me to take part in this important workshop. [April 2014].

For the past decade, the science of biological invasions (invasion ecology, invasion biology) has been subject to frequent attacks by a small but vocal minority of critics who claim that invasion impacts are exaggerated, that the research is biased, that non-native species are no more likely than natives to cause ecological or economic disruptions, and that the native/non-native dichotomy has no scientific merit.  Although these criticisms have been refuted, they are often repeated and given media attention. My colleague Dave Richardson and I present a comprehensive summary of these misleading criticisms, and referenced rebuttals to each of them, in an editorial published in the latest issue of Diversity and Distributions [November 2013].

There has been a recent surge in top-tier journal articles that aim to synthesize theories of invasion biology, but each of these articles focuses primarily on the process of colonization rather than on the ecological impacts of non-native species.  In a new paper published in Ecological Monographs, my colleagues (Martha Hoopes, Michael Marchetti and Julie Lockwood) and I identify and assess 19 testable hypotheses that explain mechanistically the impacts of non-native plants, animals, or both. Our review also reveals taxonomic biases in the empirical testing of many of these hypotheses.  Certain hypotheses are broad in scope and suggest key elements of a general theoretical framework for understanding and predicting impact. [July 2013].

Congratulations to four students who graduated from McGill this spring: Lisa Jones (PhD), Sunci Avlijas (MSc), Rowshyra Castaneda (MSc), and Andrea Reid (MSc). Each of these graduates has a bright future ahead of them.

Several students from our lab presented their research at the International Conference on Aquatic Invasive Species, held at Niagara Falls in April, and at the annual general meeting of the Canadian Aquatic Invasive Species Network (CAISN), held at Kananaskis in May. At the Kananaskis meeting, which was attended by over thirty CAISN students and postdocs, MSc candidate Ahdia Hassan received an award (2nd place) for an excellent presentation about her work comparing the socioeconomic impacts of native versus non-native freshwater species. MSc candidate Jordan Ouellette-Plante's presentation on phenotypic plasticity in dreissenid mussels also received recognition. A total of 13 student presentations were made by our lab at these two conferences.

Finally, former MSc student Rebekah Kipp's article concerning the impacts of the round goby on molluscs in the St. Lawrence River has been chosen as the Elsevier Best Paper by a Young Scientist for the 2012 edition of the Journal of Great Lakes Research. [May 2013].

Ahdia Hassan accepting her award from Dr. Hugh MacIsaac, Director of CAISN. Photo by Matthew Daley.

It has been claimed (Davis et al. 2011) that non-native species are no more likely to cause ecological disruptions than are native species.  Using data from a broad range of ecosystems, Esteban Paolucci, Hugh MacIsaac and I have conducted a meta-analysis that found that alien consumers (predators and herbivores) cause more than twice the amount of damage to native prey populations than do native consumers.  Moreover, compared with native consumers, alien consumers are associated with more negative and fewer positive effects on prey populations. This disproportionate impact might be a consequence of prey naïveté to alien consumers. Our results counter the assertion that the biogeographic origin of species has no bearing on their ecological impact. The paper appears in the journal Diversity and Distributions [February 2013].

Example of an alien predator at work: a Eurasian black rat attacking chicks in a fantail bird nest in New Zealand. Photo courtesy of Nga Manu Images.

Congratulations to Lisa Jones for successfully defending her doctoral dissertation on Tuesday October 30, 2012.  Lisa's thesis examined the phenomenon of exotic species replacement, specifically the shift in dominance between zebra mussels and quagga mussels in the St. Lawrence River.  Her field experiments provided unique insight into the abiotic factors and species trait differences that mediate these dominance shifts. Her work was featured in the documentary film Invasive Species by the American Museum of Natural History. We can expect a series of publications of her research findings to appear in the coming months. She has currently doing postdoctoral work as a Visiting Fellow at Fisheries and Oceans Canada, Burlington.

Dr. Jones examines an invasive mussel population in the Soulanges Canal.

Andrea Reid, MSc candidate co-supervised by me and Dr. Lauren Chapman, recently presented the results of her work at a student conference held at the American Museum of Natural History. A once hyper-diverse assemblage of cichlid fishes was decimated in Lake Victoria, largely as a result of predation by the introduced Nile perch. Working in Lake Nabugabo, a satellite of Lake Victoria, Andrea showed that several species escaped this fate by using the edges of wetlands as refugia. Her study, which will be published in the journal Aquatic Conservation, has been featured in a New York Times article [October 2012].

Photo of a cichlid by Andrea Reid. More of Andrea's photography can be found here.

We have been collaborating with Prof. Jaimie Dick (Queen’s University Belfast) to test an innovative method for assessing the risk posed by non-native predators to their invaded communities. The method involves a comparative analysis of the predator’s functional response – i.e. the relationship between its predation rate and prey density. If harmful invaders display consistently greater functional responses than closely-related native species, then the impacts of new invaders may be predictable.  For example, we have found that the invasive freshwater shrimp Hemimysis anomala has functional responses towards prey species that are consistently higher than those of native shrimp species, both in Ireland and Canada. Moreover, the prey species that are associated with the greatest differential functional responses in the lab are the same ones that are most impacted by Hemimysis in the field (see our recent paper in Biological Invasions). Students in both our labs are testing this methodology using a broad range of taxa, in a concerted effort to determine the extent to which functional responses will allow us to predict the population-level impacts of invaders [September 2012].

Functional responses of the invasive mysid shrimp Hemimysis anomala and the native Mysis diluviana feeding on cladocerans in a 12-hr laboratory trial.

On April 25, I gave evidence to the House of Commons Standing Committee on Fisheries concerning the threat of invasive species to native fish populations in the Great Lakes. In my statement, I emphasized the role of live trade as an unregulated series of vectors (e.g. involving live bait, ornamental plants, aquarium pets, and species sold for human consumption) that deliver thousands of aquatic species into Canada each year. Unpublished work by my students suggests that a non-negligible proportion of these species can become established and exert negative impacts in Canadian lakes and rivers. Some of these species (e.g. Asian carp) are sold in fish markets in major Canadian cities. [April 2012].

Asian bighead carp sold alive at a fish market in Montreal. Photo by A. Ricciardi.

A Ponto-Caspian shrimp, the 'bloody red mysid' (Hemimysis anomala), has rapidly colonized the lower Great Lakes and the St. Lawrence River in recent years, prompting concern over the potential consequences of this invasion to native communities and food webs. To forecast these consequences, we reviewed the impacts of invasive freshwater mysids worldwide. Surprisingly few impact studies exist for H. anomala, but data from invaded Dutch reservoirs suggest that it can depress zooplankton populations. The impact histories of other freshwater mysids (Mysis spp.) provide potentially valuable predictive information for H. anomala. Mysids have been introduced into many lakes in the United States and Scandinavia, where they have frequently caused declines in the abundance of zooplankton - especially cladocerans. Some of these declines were drastic enough to cause a reduction in the abundance and growth of pelagic fishes. It is not clear whether H. anomala will have similarly strong effects, but it can achieve higher densities locally and invade a broader range of aquatic habitats than other mysids. A major question is whether planktivorous fishes can regulate mysid populations and limit their impact on Great Lakes' food webs. Nonetheless, predation on H. anomala could increase the biomagnification of contaminants in fishes. See our article in the Journal of Great Lakes Research [February 2012].

Photo of bloody red mysids by S. Pothoven (NOAA/GLERL).

The Eurasian round goby (Neogobius melanostomus) has spread from the Great Lakes into the St. Lawrence River, where it has become increasingly abundant in recent years. Studies by Rebekah Kipp (MSc, 2010) and former NSERC USRA students Issac Hébert and Myriam Lacharité have revealed the potential ecosystem impact of this dominant benthivorous predator. Stomach content analysis showed that gastropods and certain insect larvae were a preferred prey item, whereas zebra and quagga mussels were generally avoided by gobies in the river. A concomitant field survey showed that the biomass, diversity and median body size of gastropods were negatively correlated with goby density across all sites, and declined at sites sampled before and after invasion. Contrary to studies in the Great Lakes, there were no consistent effects of goby density on the size structure of zebra and quagga mussels in the river, although gobies exhibited an ontogenetic diet shift toward mussels. Benthic algal biomass increased with goby density across sites, suggesting a trophic cascade driven by the impacts of gobies on gastropods and other algal grazers. See our papers in the Canadian Journal of Fisheries and Aquatic Sciences and the Journal of Great Lakes Research. [February 2012].

Round gobies amongst zebra mussel-covered rocks in the St. Lawrence River. Photo by Sandra Warren.

Former MSc student Justin Trumpickas examined the community-level impacts of predatory fishes introduced to lakes in Algonquin Park mostly within the past few decades. After controlling for other environmental variables, his analysis showed that lakes with introduced littoral predators (e.g., northern pike, smallmouth bass, rock bass, walleye) contain fish assemblages that differ conspicuously from those in lakes without introduced predators. In lakes with these predators, small-bodied species (e.g. minnows, sticklebacks, dace) tend to be absent or severely reduced in abundance. Surprisingly, introductions of multiple species of predatory fishes to a lake seem to have no additional effect on fish assemblages compared with single predators. Therefore, if conservation resources are limited, efforts to prevent predatory fish introductions should be focused on lakes with no littoral predatory fish already present. An unanswered question is whether the addition of predators to lakes that already contain introduced predatory fish would have effects on other aquatic organisms, such as amphibians and invertebrates. See our article in Aquatic Conservation: Marine and Freshwater Ecosystems. [December 2011].

Justin holding specimens of northern pike and smallmouth bass collected from a seine at Farm Lake, Algonquin Park, Ontario. Photo courtesy of J. Trumpickas.

In the April 2011 issue of BioScience, my colleagues and I describe fundamental similarities between biological invasions and natural disasters that suggest these events should be managed with similar precautions. Like natural disasters, biological invasions are almost impossible to predict and difficult to control once they occur. Invasions can have more persistent impacts and a greater capacity for ecological and economic damage; and yet, systems of preparedness for invasions (except for infectious diseases) are lacking in most countries. Hazard-reduction plans, similar to those in place for natural disasters, could minimize the impacts of invasive species at a cost that is quite low relative to the cost of an uncontrolled invasion [April 2011].

Environmental stressors vary with respect to the degree to which they are understood and controlled. As a result of a strong societal commitment to address them, several stressors (e.g., acid rain, DDT, heavy metals) are now well controlled. The effects of genetically modified organisms (GMOs) are poorly understood but potentially controllable through enforceable legislation. Natural disasters and invasions are both well understood, but generally defy control.

Colleagues Brian Leung (McGill), Ladd Johnson (Univ. Laval) and I are members of a team of Canadian scientists that has received $6.5 million from the Natural Sciences and Engineering Research Council to study aquatic invasions in Canada's lakes, rivers and coastal marine habitats. This will launch the second phase of the Canadian Aquatic Invasive Species Network (CAISN), which began in 2006. The network consists of 30 members from 12 universities. A cohort of new graduate students funded by CAISN-II will begin work in my lab in the Fall 2011. Some of these students will be
working on projects co-supervised by Brian and Ladd. [February 2011]

A new study by Åsa Kestrup compares intraguild predation (IGP) by invasive and native amphipods (Echinogammarus ischnus and Gammarus fasciatus, respectively) on each other's juveniles in high- and low- conductivity water. The native amphipod had a higher type-II functional response towards the invasive amphipod's juveniles than vice versa. Contrary to expectations and our previous work examining IGP amongst the adults, conductivity did not influence the predation rate on juveniles of either species. Previous work in our lab (Kestrup & Ricciardi 2009) showed the invader to be the dominant predator in high-conductivity water and the native to be dominant in low-conductivity water. The results of this new study reveal that the native amphipod's advantage at lower conductivities is compounded by asymmetric predation on juveniles, which also counteracts the invasive amphipod's advantage in high-conductivity waters, helping to explain observed patterns of co-existence. This study was done in collaboration with Dr. Jaimie Dick of Queen's University Belfast. See our article in Biological Invasions. [January 2011]

Functional responses of Gammarus fasciatus (native amphipod, in blue) and Echinogammarus ischnus (invasive amphipod, in red) after 40h. Solid lines = low conductivity; dashed lines = high conductivity.

MSc candidate Rowshyra Castaneda has begun investigating the distribution and abundance of exotic molluscs and crustaceans in a section of the St. Lawrence River affected by thermal discharge from a nuclear power plant at Bécancour, Quebec. In 2009, the Asian clam Corbicula fluminea – one of the world's most invasive aquatic invertebrates – was discovered in the thermal plume by Dr. Anouk Simard and colleagues at Quebec's Ministère des Ressources naturelles et de la Faune. The species has not been previously reported from the river. We are collaborating with Dr. Simard to examine the occurrence, condition and abundance of this species and other invaders along the temperature gradient created by the power plant. [October 2010]

Two specimens of Corbicula collected by Rowshyra Castaneda from the St. Lawrence River at Bécancour, Quebec. Photo by A. Ricciardi.

Congratulations to Åsa Kestrup and Jessica Ward for successfully defending their doctoral dissertations, on January 19 and April 1, 2010, respectively. Åsa's thesis examined the effects of interspecific interactions and environmental heterogeneity on the dominance of a Ponto-Caspian crustacean (Echinogammarus ischnus) in the St. Lawrence River. She subsequently completed a contract with the Quebec Centre for Biodiversity Science, and has been hired as a science advisor in the Department of Fisheries and Oceans. Jessica's thesis investigated the community-level impacts of nonindigenous ecosystem engineers in freshwater and marine habitats.  She has been hired as an ecologist at AECOM, an international consulting firm. Based on their accomplishments to date, we anticipate that both Åsa and Jess will have productive careers. 

While sampling amphipod populations in the St. Lawrence River, Åsa Kestrup observed mysterious mass die-offs of amphipods, particularly involving the exotic species Echinogammarus ischnus. Amphipods that had been brought back from the field were also rapidly dying in the lab. The culprit appears to be a parasitic water mold of unknown origin. Samples were sent to Dr. Meghan Duffy's lab (Georgia Tech.) for genetic analysis, and the parasite was identified as a species closely related to Saprolegnia, but we cannot determine whether it is native or introduced to the St. Lawrence. In laboratory experiments, infection prevalence was found to be higher in live E. ischnus than in native amphipods; furthermore, dead E. ischnus individuals exhibited more intense infections than the natives. Therefore, this pathogenic parasite could be facilitating the co-existence of the two amphipod species in the river, by reducing the exotic species in ion-rich waters where it would otherwise be dominant. Our findings will be published in the journal Biological Invasions. [August 2010]

(a) Infected live female (top) and male (bottom) E. ischnus with melanized spots; (b) newly dead E. ischnus; (c) hyphae growing out from a gill; and (d) a dead E. ischnus overgrown with hyphae. Photos by Å Kestrup.

Introduced ecosystem engineers can substantially alter native communities by transforming the physical structure of habitats. In the Great Lakes and St. Lawrence River, invasive dreissenid (zebra and quagga) mussels commonly occur with the native benthic macroalga Cladophora, and both act as ecosystem engineers by increasing substratum complexity and providing interstitial habitat for benthic macroinvertebrates. In a series of experiments in the St. Lawrence River, PhD student Jessica Ward manipulated the topography of patches of dreissenid mussel shells on artificial substrates that were deployed to be colonized by other organisms. She found that the patchiness of dreissenid mussels is an important driver of variation in benthic invertebrate diversity at small spatial scales, but that colonization by Cladophora modifies the impacts of the mussels on other invertebrates, leading to community responses that differ markedly from those observed in the absence of the macroalga. Her findings thus demonstrate that interactions between habitat-modifying species can complicate efforts to predict the community-level effects of an invasion. Her article will appear in an upcoming issue of the journal Freshwater Biology. [February 2010]

Left: Experimental substrates: Half-cover, checkered, and full-cover shell treatments.
: Half-cover shell treatment colonized by
Cladophora and invertebrates at the end of the experiment. Photos by J. Ward.

The transport of organisms in ships’ ballast tanks is a dominant vector of aquatic invasions worldwide. Until recently, efforts to manage this vector have overlooked the potential transport of invertebrate resting stages in the residual waters and sediments within emptied ballast tanks, i.e. NOBOB ('No Ballast On Board') tanks (Ricciardi & MacIsaac 2008). For example, freshwater bryozoans have resting stages (statoblasts) that are often buoyant and locally abundant, and thus can be taken up easily during ballasting operations. Statoblasts are also resistant to extreme environmental conditions and can generate a new bryozoan colony even after being dormant for decades. Therefore, they would likely remain viable propagules after lengthy transport in ship ballast tanks.

In collaboration with Drs. Hugh MacIsaac and Sarah Bailey (University of Windsor), MSc student Rebekah Kipp and I quantified the diversity and abundance of freshwater bryozoan statoblasts in NOBOB tanks of transoceanic ships visiting the Great Lakes. We identified 11 species, which represent an astonishing fraction (12%) of the number of freshwater bryozoans known worldwide. These include two exotic species unrecorded in the Great Lakes (Fredericella sultana and Lophopus crystallinus). We also found an exotic species already established in the region (Lophopodella carteri), and three cosmopolitan species (Plumatella casmiana, P. fungosa and P. repens), which indicates the potential for cryptic invasions via the introduction of exotic genotypes. Our estimates suggest that a ship with NOBOB tanks may carry up to 106 statoblasts, posing a significant risk of new species introductions.  The study has been published in the journal Diversity and Distributions. [January 2010]

A bryozoan (Plumatella casmiana), newly hatched in the laboratory from a statoblast retrieved from ballast tank sediments. Photo by S. Bailey.

PhD student Åsa Kestrup has discovered a previously undocumented predatory interaction between invasive and native crustaceans in the St. Lawrence River.  Ten years after it began colonizing the river, an invasive freshwater amphipod Echinogammarus ischnus has replaced a native amphipod Gammarus fasciatus at some sites, but not at others. The mechanism and pattern of replacement are poorly understood. Recent experiments in the laboratory and at field sites in the river have revealed that the two species are mutual predators, with the males of one species attacking and killing the females of the other species (see photo). What is particularly intriguing is that the direction and magnitude of this interaction vary with the conductivity of the water. The invasive species is the superior predator in ion-rich waters, while the native species is superior at lower conductivities at sites near the Island of Montreal. Thus, mixed flows caused by the convergence of the ion-rich St. Lawrence River with the ion-poor Ottawa River create a chemically heterogeneous system that may be responsible for spatial variation in the relative dominance of the two species. The article appears in the journal Biological Invasions. [October 2009]

A female invasive amphipod (E. ischnus), killed by a native male amphipod (G. fasciatus). The arrow points to predation damage. Photo by Å Kestrup.

In recent years, several biologists have proposed intentional large-scale translocations of species as a strategy to conserve threatened species or to enhance the biodiversity of a target region. They assert that carefully planned translocations (termed 'assisted colonization', 'assisted migration', or 'managed relocation') guided by decision frameworks and risk assessments would reduce the potential hazards associated with introductions of nonindigenous species. In an article in Trends in Ecology and Evolution, Dan Simberloff and I argue that conservation biologists have not yet developed a sufficient understanding of the impacts of introduced species to make safe decisions regarding their translocation. The literature documents myriad examples of planned invasions that have produced unintended and unpredictable consequences. Until we develop more accurate and general methods of predicting impact, risk assessments could be dangerously misleading and assisted colonization strategies amount to ecological gambling.

Our paper was selected as a "must read" article by the Faculty of 1000 Biology, and has provoked an exchange of letters in the September issue of TREE. [August 2009]

One serious ecological risk posed by assisted colonization is increased hybridization. Many species of birds, fishes, mammals and plants became genetically diluted or completely assimilated when they were brought into contact with relatives from which they were previously isolated. For example, an endemic North American fish, the Amistad gambusia Gambusia amistadensis, was hybridized to extinction when it came into contact with another North American species, the western mosquitofish G. affinis (shown above). Hybridization with introduced relatives is assumed to be at least partially responsible for over 30% of North American freshwater fish extinctions. Photo by C. Appleby, USGS.

PhD student Åsa Kestrup has made another discovery: the invasive Ponto-Caspian mysid shrimp, Hemimysis anomala, is recorded for the first time in the St. Lawrence River. The mysids were discovered in sampling trays of cobble that were deployed on two occasions at a site near Montreal this past summer in order to collect amphipods. We did not detect their presence until preserved samples were examined a few months later. Specimens included gravid females and juveniles, which suggest that the species is reproducing in the river. Colonization of the river is likely being driven by downstream dispersal of individuals from Lake Ontario but, given the 250 km distance from the outflow of the lake, it seems probable that cryptic riverine populations exist upstream of our site. This species has apparently spread widely since being found in Lake Michigan and Lake Ontario in 2006. In the past two years, H. anomala has been recorded in all large waterbodies within the Great Lakes-St. Lawrence basin, except for Lake Superior. The St. Lawrence is the first North American river to be invaded by the mysid. See our article in the journal Aquatic Invasions. [December 2008]

Top: A preserved adult female H. anomala (7.5 mm in length) found in the St. Lawrence River.
Bottom: Dorsal view of the truncated telson, a distinguishing feature of the species. Photos by Guy L'Heureux.

Aquatic plants serve as habitat for dense communities of invertebrates, and thus provide important feeding grounds for fish in lakes and rivers. By sampling native and exotic milfoils (Myriophyllum spp.) at various sites, MSc graduate Sarah Wilson examined whether the replacement of native plants by closely-related exotic plants results in changes in plant-associated invertebrate communities. She found higher invertebrate diversity, higher invertebrate biomass, and greater gastropod abundance on native milfoils than on Eurasian milfoil. Her results demonstrate that Eurasian milfoil supports macroinvertebrate communities that differ from those on closely-related and morphologically similar native plants. Therefore, the ongoing replacement of native milfoils by Eurasian milfoil may have indirect effects on biodiversity and food webs in invaded waterbodies. See our article in the Canadian Journal of Fisheries and Aquatic Sciences. [August 2008]

Rarefaction curves for invertebrate taxa on native and exotic milfoils in Saranac Lake (Wilson & Ricciardi 2009).

Exotic species can dominate communities and replace native species that should be better adapted to their local environment, a paradox that is usually explained by the absence of their natural enemies and the effects of anthropogenic disturbance. An alternative explanation is that some exotic species can enhance their invasion success and impact on native species by altering the environment through ecosystem engineering. A modelling study in collaboration with Dr. Andy Gonzalez (McGill) and Dr. Amaury Lambert (Ecole Normale Supérieure, Paris) explored competition for habitat between a native non-engineering species and an exotic engineering species.  A key factor was the invader's density dependence (how its rate of engineering varied with its abundance), which determined whether the invader co-existed with the native species or drove it to extinction.  Another intriguing result is that a series of failed invasions can successively reduce environmental resistance to subsequent invasion, through a cumulative effect of ecosystem engineering.  See our recent article in Oikos. [July 2008]

A nonlinear result of our modelling study. Change in abundance for a resident species (thick solid line) and its natural habitat (thin solid line) in response to an invasive engineer (dashed line) and its engineered habitat (dotted line) (Gonzalez et al. 2008).

The impacts of invaders vary across space and time, posing a formidable challenge to risk assessment. However, sometimes only a few key environmental variables may be important for predicting impact. This may be the case for the zebra mussel's impact on native bivalve populations, which is largely related to the level of fouling by zebra mussels attached to the shells of native species. Anneli Jokela, a former MSc student, has shown that the fouling intensity of zebra mussels is positively correlated with calcium concentration and negatively correlated to the mean particle size of surrounding sediments.  This finding suggests that habitats whose native populations are most vulnerable to zebra mussel impacts can be identified in advance of invasion.  See our article in Freshwater Biology. [April 2008]

A native unionid bivalve, Lampsilis radiata, fouled by zebra mussels. It was collected from the upper St. Lawrence River in 1993. Photo by A. Ricciardi.

This year is the 50th anniversary of the publication of Charles Elton's classic text The Ecology of Invasions by Animals and Plants (Methuen, 1958), the book that pioneered an entire field of research.  Over the past half century, its influence has grown exponentially (like an invasive species population) and is cited now more than ever before.  Read our retrospective article in Nature.  [March 2008]

NOAA's Great Lakes Environmental Research Laboratory has created the most comprehensive database available for Great Lakes invaders. The Great Lakes Aquatic Nonindigenous Species Information System (GLANSIS) provides information on the distribution, life history and known impacts of most of the invaders reported in the basin. A major contributor to the database was Rebekah Kipp, a research assistant in our lab (and soon to be a graduate student in January 2008) who compiled information for over 80 exotic species of invertebrates, fishes, plants and algae.  An example of one of these factsheets can be found here for the VHS virus - an invasive pathogen of major concern for Great Lakes fisheries.  Search GLANSIS for your invader of interest. [December 2007]

The American Museum of Natural History has produced a short documentary film called Invasive Species, which features research by graduate students in our lab (as well as students in Dr. David Lodge's lab at the University of Notre Dame). This past summer, the filmmakers followed a team of divers led by my PhD students Lisa Jones and Åsa Kestrup to a field site where we conduct work on zebra mussels, quagga mussels, round gobies and Ponto-Caspian amphipods. The film is one of the "Bio Features" that can be viewed on the AMNH website.  [November 2007]

Through a meta-analysis of published data, PhD student Jessica Ward and I tested the popular assumption that native herbivores are more likely to suppress plant invaders that are closely-related to native species – an idea originally proposed by Darwin. Contrary to Darwin's hypothesis, the impact of native herbivores is several times stronger on introduced plants that are exotic genera in the invaded region than on introduced plants that share genera with native plants in the region! Exotic plants may be pre-adapted to the conditions of herbivory experienced by native species of the same genus. Our finding highlights the potential importance of evolutionary naivete in mediating the interactions between exotic and native species.  See our article in Science and a commentary [July 2006