IMPACTS OF ROUND GOBY INVASION IN THE ST. LAWRENCE RIVER
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.

EFFECTS OF INTRODUCED PREDATORS ON LITTORAL FISH COMMUNITIES
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.

WHY WE SHOULD MANAGE INVASIONS AS NATURAL DISASTERS
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.

NEW FUNDING FOR THE CANADIAN AQUATIC INVASIVE SPECIES NETWORK
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]

DIFFERENTIAL FUNCTIONAL RESPONSES EXPLAIN COEXISTENCE OF INVASIVE AND NATIVE CRUSTACEANS
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.

THE ASIAN CLAM AND OTHER EXOTIC INVERTEBRATES IN A THERMAL PLUME IN THE ST. LAWRENCE RIVER
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.

TWO NEW PHDs LEAVE THE RICCIARDI LAB
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. 

A CRYPTOGENIC PARASITE DIFFERENTIALLY INFECTS EXOTIC AMPHIPODS IN THE ST. LAWRENCE RIVER
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.

A NATIVE HABITAT-MODIFYING SPECIES OBSCURES THE EFFECTS OF INVASIVE MUSSELS
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.
Right: Half-cover shell treatment colonized by Cladophora and invertebrates at the end of the experiment. Photos by J. Ward.

INTERCONTINENTAL DISPERSAL OF FRESHWATER BRYOZOANS BY SHIPS
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.

WATER CHEMISTRY MEDIATES PREDATORY INTERACTIONS BETWEEN INVASIVE AND NATIVE CRUSTACEANS
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.

A CRITIQUE OF 'ASSISTED COLONIZATION'
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.

INVASIVE PONTO-CASPIAN SHRIMP FOUND IN THE ST. LAWRENCE RIVER
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.

REPLACEMENT OF NATIVE AQUATIC PLANTS BY INVASIVE PLANTS ALTERS INVERTEBRATE COMMUNITIES
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).

INVASIVE ECOSYSTEM ENGINEERS
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).

PREDICTING IMPACTS OF ZEBRA MUSSELS FROM PHYSICAL HABITAT VARIABLES
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.

ELTON'S LEGACY: THE BOOK THAT FOUNDED INVASION ECOLOGY
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]

GREAT LAKES' DATABASE LAUNCHED
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]

OUR RESEARCH ON FILM
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]

CONTRADICTING DARWIN
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]

INCREASING RATE OF DISCOVERY OF GREAT LAKES' INVADERS
The composition and number of invaders discovered in the Great Lakes at different periods during the past two centuries are correlated to changes in vector activity, particularly overseas shipping. Ballast water management has not halted the influx of ship-vectored invaders into the Great Lakes, but rather has altered the composition of invaders to favor benthic invertebrates with broad salinity tolerance. The rate of species discovery suggests that the Great Lakes basin is among the most highly invaded aquatic systems in the world.  Read the published study here. [July 2006]