Environmental Journeys 18: Invasive species

I’m a snowbird, spending most of my time in Florida and summers in New York. Year-round and long-term residents of Florida likely consider snowbirds a type of invasive species. And when Spring Break comes, the cities brace for the wild influx of college students. Florida is considered the poster child for non-human invasive species. (The Nature Conservancy provides a nice overview.) I’ve encountered peacock bass, green iguanas, feral pigs, cane toads, Jesus lizards, lion fish, Asian jumping worms, Brazilian pepper-trees, and more. Because they can capture and eat almost every walking or swimming animal in the Everglades, Burmese pythons have gotten particular attention and there is an ongoing capture program with rewards. Python hunts are frequently covered by the press, TV stations, and on social media.

Five men capture ~200 pound python in Florida. Photo credit: Mike Elfenbein / Facebook

While large pythons might capture the press, one of the most impactful invasive species from Florida is the tiny mite Varroa destructor. I became aware of this species when my Exponent colleagues and I organized an experts workshop to identify causes of honey bee declines and the loss of commercial hives-referred to as Colony Collapse Disorder (CCD)-in the United States. The mite comes from Asia and was accidentally introduced to Florida in the 1980s. Because hives are transported to orchards and other agricultural areas and because bees fly, the infestation spread throughout the country. The mites eat larvae, pupae, and adult honeybees. The following picture shows the size scale of these parasites relative to their host.

Varrora mite on the side of a bee. Source: Varrora Mite Treatment Service.

There are numerous contributing causes to CCD and honeybee declines. The mite is certainly one of them. It is bad enough that the mite munches down on bees. But we learned that even worse is that mites can be infected with various lethal viruses that are injected into the bees when the mites are feasting. Experts at our workshop felt that the mites infected with the virus were particularly harmful. We published the expert groups’ causal analysis in this paper:

• Staveley J, Law S, Fairbrother A, Menzie C. A causal analysis of observed declines in managed honeybees (Apis mellifera). Human and Ecological Risk Assessment 2014; 20:566–591.

The expert workgroup convened to evaluate the causes of bee decline. Jane Staveley of Exponent who led the effort is in the center front. Susan Cormier — one of EPA’s leading experts on causal analysis — is to her immediate right. Sheryl Law of Exponent is fifth from the left in the front row.

Invasive species arrive by being transported into an area from another country or through migration. In either case multiple factors are involved leading to their population growth and associated impacts on environments and humans. Some introductions are intentional acts such as the placement of snakehead fish in freshwater areas to provide a food source, or the disposal/release of pets into the wild. Some invasive species such as the bark beetle hitch a ride on imported products or are inadvertently spread like the Varrora mite described above. Regarding migration, the warming of the planet is enabling movement of species from warmer latitudes into more northern areas as for example is occurring for jellyfish.

I will describe two additional projects that illustrate applications of ecological risk assessment to questions regarding invasive species. The first involves marine aquaculture of shrimp and the potential for pathogen infections of native shrimp populations and the second involves a proposal to introduce a new oyster species into Chesapeake Bay.

Marine Shrimp Farms and Potential Threats to Native Shrimp

In the late 1990s I was asked by EPA (through Eastern Research Group) to chair a series of stakeholder meetings in several states from South Carolina to Texas. Stakeholders included shrimpers, shrimp farmers, local and state regulatory agencies, politicians, environmental and conservation groups, and other members of the public. The media were always present at these meetings. The main tension was between wild shrimp fishing that relied on native stocks of shrimp in the Atlantic Ocean and Gulf of Mexico and the shrimp farms which raised non-native shrimp. Coastal shrimp farms had become established in the southeast especially in Texas beginning in the 1960s and increasing in acreage through the 1990s. The concern on the part of the wild shrimp fishing community was that the wild native shrimp were at risk of being harmed by diseases associated with the farmed shrimp. The minutes for these stakeholder meetings set the stage for the EPA project involving the Shrimp Virus Work Group:

• New, highly virulent viruses have been documented in foreign shrimp aquaculture. Evidence suggests that these viruses could cause widespread disease in shrimp raised in aquaculture and in wild shrimp in the Gulf of Mexico and southeastern Atlantic coastal regions. The Joint Subcommittee on Aquaculture (JSA), which is under the auspices of the President’s Office of Science and Technology Policy, formed the interagency Shrimp Virus Work Group to assess the risks associated with these emerging viral pathogens. Four Federal agencies are represented on the Work Group: the National Marine Fisheries Service (NMFS), the U.S. Environmental Protection Agency (EPA), the U.S. Fish and Wildlife Service (USFWS), and the U.S. Animal and Plant Health Inspection Service (APHIS).

The concern regarding the farms stemmed in part from the fact that saline waters from coastal areas were pumped to the farm ponds and effluents from the ponds were reaching the coastal waters. Thus, the possibility existed that the farms were a source of new pathogenic shrimp viruses such as White Spot Syndrome Virus. Releases of this virus were known to have occurred in South Carolina and large impacts of the virus have been observed in some Latin America countries where shrimp farming was especially prone to releases of viruses and shrimp into adjacent marine waters.

White spot virus disease in farmed Pacific white shrimp was the focus of our stakeholder meetings and workshop. There are other viruses as well. And there were also concerns abut the possibility of foreign shrimp such as the tiger shrimp being farmed in Latin America invading Gulf waters.

Following the meetings with stakeholders, an experts meeting was held to address questions related to risks. This took place over two days and led to the Report On The Shrimp Virus Peer Review And Risk Assessment Workshop: Developing A Qualitative Ecological Risk Assessment (Workshop Report, 1998). As stated therein:

• The workshop focused on the scientific and technical aspects of the likelihood that nonindigenous viruses will become established in wild shrimp populations in the Gulf of Mexico and southeastern Atlantic coastal regions and on the potential ecological consequences of establishment. The workshop included 22 experts with varied backgrounds, including shrimp biology, toxicology, virology, marine ecology, ecological risk assessment, and shrimp aquaculture and processing.

We reached the following conclusions:

• There is a high likelihood that viruses are present in the aquaculture pathway, shrimp processing pathway, and some of the other potential pathways.
• Potential for entry of viruses from sources to coastal waters ranged from low to high depending on source and associated sub-pathways for aquaculture, shrimp processing, and other sources. Our level of certainty regarding these evaluations was quite variable.
• There were a number of uncertainties associated with colonization of the shrimp viruses in native shrimp populations. There is either a low or medium likelihood that, once introduced, viruses would be able to colonize native shrimp at a local level (i.e., within specific estuaries or embayments).
• Research was needed to address a number of questions raised by the report.

With the benefit of time, we can look back from 2024 at what took place in the Gulf of Mexico as potential problems were recognized in the 1990s and the report was issued in 1998. We find that:

• Many older shrimp farms along the southern U.S. coast have shut down because of water pollution and economic concerns.
• Technological advancements led to farm systems using pathogen-free stocks and increasing reliance on recirculating water systems.
• Recent studies have not revealed an elevated prevalence of white spot virus in the northern Gulf of Mexico. But, there is concern regarding potential for future impacts associated with transport of infected shrimp from the southern Gulf.
• Another invasive species issue has arisen involving the Black Tiger Prawn. Farming of this species is prohibited in United States coastal areas. But the animals have been farmed in Latin America countries and it is thought that “escapees” have made their way northward through the Gulf of Mexico. Research scientists at NOAA are concerned; Gulf of Mexico shrimp fishermen are on watch; and the public is being alerted through news reports.

A news report about the potential threat of the Black Tiger Prawn. Read the news story here.

The Asian Tiger Shrimp is easily recognized. Shrimp fishermen participate in helping eradicate this species. One suggested method of disposal is to eat lots of them (i.e., those caught in offshore waters). However, Eradication won’t be possible, and harvests aren’t a silver bullet. For more on that see this paper: Developing a recipe for success: commentary on Seaman et al. (2021) “Eating invasives: chefs as an avenue to control through consumption” by my colleague Andy Deines and others.

In closing I commend the work of Alice Cascorbi — a Fisheries Research Analyst with the Monterey Bay Aquarium — who provides a nice history of marine shrimp farms that I found helpful for my story. I also commend Bill van der Schalie who was with EPA’s Risk Assessment Forum and led the development of Ecological Risk Assessment (ERA) approaches including the application to biological stressors such as the shrimp viruses.

Dr. William van der Schalie was EPA’s lead for the Shrimp Virus project. I met him when he was with EPA’s Risk Assessment Forum. He went on to do incredible research with the Army becoming the Science and Technology Director for the U.S. Army Center for Environmental Health Research. He was a brilliant scientist who developed a novel approach involving fish to detect toxicants harmful to humans in surface waters.

Proposal to Introduce a New Oyster Species into Chesapeake Bay

I enjoy eating oysters. What do P.E.I., Wellfleet, Blue Point, Chesapeake Bay, Apalachicola Bay, and Galveston Bay Oysters have in common?

Varieties of oysters commonly found on menus and favored by oyster aficionados for distinctive tastes.

They are all the same species — Crassostrea virginica — commonly known as the Eastern or Virginian Oyster. Variations in their size, shape, saltiness, and overall taste reflect geographic differences in environmental factors along the east coast of North and South America, a range of almost 5,000 miles. What would be the effects on the Eastern Oyster, the ecosystem, people, culture, and socioeconomics if a new oyster species was introduced into Chesapeake Bay, famous as a source of oysters? Why would such an introduction be proposed and given serious consideration by the states bordering the Bay and by the national government?

The “why” has to do with the dramatic decline in oyster landings from Cheapeake Bay in the 20th century. The decline reflects a combination of overharvesting, pollution, two oyster diseases, and sedimentation from silt which covers the hard substrates (e.g., oyster shells and rocks) this species needs for the settlement of oyster spat.

The native oyster Crassostrea virginica has declined substantially in Cheapeake Bay

The decline in oysters was not only a blow to the socioeconomics and reputation of the Bay, it had huge ramifications for the Bay’s ecology and environmental health. A major issue in the Bay involves the input of nutrients from the watersheds that drain to it. High inputs of nutrients to the Bay has resulted in dense blooms of phytoplankton, microscopic plants that are suspended in the water.

Watersheds draining to the Chesapeake Bay bring nutrients from multiple sources (USGS, 1999). A few examples of Harmful Algal Blooms (HABs) in the Bay are shown on the right: Photo credits to: Virginia Department of Health, Stewards of Water, and VIMS/Professor Wolfgang Vogelbein.

While moderate levels of phytoplankton sustain planktonic food webs, frequent heavy blooms of phytoplankton and shifts to undesirable species can lead to environmental problems. These problems include shading of submerged aquatic plants and excessive oxygen consumption due to respiration and decomposition of the tiny plants. The loss of oxygen in the water has created “dead zones” in the Bay where animals cannot live. Oysters along with filter-feeding Menhaden fish feed on the phytoplankton, limiting the potential for such blooms.

As both oyster and Menhaden stocks declined, natural biological checks on phytoplankton diminished even as nutrient inputs to the Bay increased, stimulating more phytoplankton growth. The result was an undesirable shift in ecological and economic conditions. Environmental managers and many stakeholders felt that corrective action was needed, one that would revitalize oyster populations and reduce the impact of phytoplankton and nutrients.

Among these options was a proposal to introduce and support the development of a reproducing population of a new oyster species in the Bay, the Suminoe Oyster (aka Asian Oyster) Crassostrea ariakensis. This species was deemed hardier than the Eastern Oyster with respect to the diseases and the effects of sedimentation. But there were questions around disease tolerance and growth in new habitats. Dr. Mark Luckenbach of the Virginia Institute of Marine Science (VIMS) nicely summarized some of the issues regarding introducing the Suminoe Oyster to the bay in an infographic, two panels of which are shown below.

Perceived potential gains from introducing the Suminoe oyster to Chesapeake Bay. Note there are numerous other documents on the subject but this gives the gist of the matter for consideration.

Recognized ecological uncertainties that existed at the beginning of the Programmatic Environmental Impact Statement (PEIS) undertaken to evaluate the proposal.

The matter of introducing a new oyster species was taken up by the National Academy of Sciences National Research Council.

The National Research Council published its report in 2003.

The 2003 report begins with a framing question that lays out the range of viewpoints around the proposed introduction:

The body of the 2003 report contains numerous questions to be considered regarding potential benefits and consequences. In 2004 the states of Maryland and Virginia agreed to undertake a Programmatic Environmental Impact Statement (PEIS) with oversight and ultimate decision making in the hands of the U.S. Army Corps of Engineers. The PEIS included an Ecological Risk Assessment (ERA) for Oyster Restoration Alternatives. Eight alternatives were considered. Several involved ways to enhance the population of the Eastern Oyster, one involved aquaculture with triploid (presumably non-reproducing) aquaculture of the Suminoe Oyster, and one involved establishing a reproductive population of the Suminoe Oyster.

The assessment of ecological risks was a collaborative effort between my company Exponent and Versar. We benefitted from advice and peer review by the Ecological Risk Assessment Advisory Group, consisting of Dr. Todd Bridges (USACE), Mr. Chris Guy (FWS), Simeon Hahn (NOAA), and Ms. Barbara Okorn (EPA). My team at Exponent included Johanna Salatas and Ted Wickwire. Lisa Methratta of Versar was lead author on Section 4.3 of the ERA investigating the probability of a diploid Suminoe Oyster introduction resulting from triploid aquaculture. And Bill Richkus of Versar contributed to editing and writing of the document.

The ERA addressed eight risk questions five of which were derived from the NRC report.

The 8 questions addressed in the ERA. Those associated with the NRC 2003 report are shown with an asterisk.

We began by working from the various reports and research papers that had been developed and a conceptual model that reflected interactions among and between the two oysters species and the ecological components of the Chesapeake Bay.

Conceptual model of mechanisms of effects through which oysters and receptors interact. Solid lines indicate predator-prey relationships. Dashed, numbered lines indicate the following other types of relationships: (1) competition for space, (2) change in the amount of oyster-reef habitat, (3) change in habitat/cover, (4) change in nutrients or other water chemistry because of changes in oyster abundance, (5) change in production of phytoplankton or submerged aquatic vegetation because of changes total dissolved solids, light attenuation, or dissolved oxygen.

A key consideration involved competition between the two oyster species. This was informed by research. A conceptual view of the nature of competition is shown below. We detailed the available research and knowledge in our report.

Figure depicting the nature of competition between oyster species is taken from our report.

A unique feature of the ERA was the use of the Relative Risk Model (RRM) to evaluate influences of alternatives on the ecological components and associated ecosystem services within the Bay. The RRM methodology was introduced by Wayne Landis and colleagues and facilitates evaluations of multiple stressors and the use of quantitative and qualitative information at regional scales. It involves a scalar approach to capture the relative dimensions of each stressor or outcome in relation to its magnitude and extent. And it permits comparative evaluations which was ideal for our evaluation of alternatives for oyster restoration. I modified the RRM method for use in our ERA for Chesapeake Bay by normalizing the scales so that they were easier to understand and to also to facilitate comparison of oyster restoration options. (I’ve since used that method in evaluations of multiple stressors and for holistic cumulative risk/impact assessments involving both human health and ecological risk.)

Wayne Landis is a visionary ecological risk assessor and scientist. He developed the Relative Risk Model (RRM) which has been applied to a broad range of regional risk matters around the globe.

The RRM was applied to alternatives involving restoration of the Eastern oyster only, as there was too much uncertainty regarding future population growth of the Suminoe oyster. Projected future biomass estimates for the Eastern oyster were obtained using population models. Examples of RRM results are shown below for two different regions in the Bay. These regions differ in environmental conditions that affect oyster growth and that translates to different responses in ecosystem components.

In Maryland oligohaline (low salinity) waters

In Virginia mesohaline (moderate salinity) water

The RRM results together with other information on competition and assessments of reproductive success, potential for unintended consequences such as migration of the Suminoe oyster beyond the Bay were collectively used to answer the risk questions using a weight-of-evidence approach. The report is filled with much technical detail. But, we also used the following relative likelihood descriptors to communicate our findings to decision makers in plain language:

• Negligible — If assumptions are met, there is no chance or virtually no chance that this outcome will occur.
• Low — The chance that the outcome will occur is very small.
• Moderate — An outcome is possible.
• High — The chance that an outcome will occur is large.

To communicate the degree of uncertainty we had with the estimates of likelihood and magnitude we used the following terms:

• Low — The mechanisms of action are well understood and available information is sufficient to support a conclusion
• Moderate — We are reasonably sure of the conclusion, but some aspects of the mechanism of action or consequences are not well understood, and its magnitude may be uncertain.
• High — Available information (e.g. data from laboratory studies) provides some insight into mechanisms of action, but we are not sure about how interactions would be manifested in the environment (i.e., regarding either likelihood or magnitude).

The PEIS was produced over a 4-year period and involved research by universities to address information gaps. The ERA can be accessed at Ecological Risk Assessment (ERA) for Oyster Restoration Alternatives. We later published our findings and conclusions as a series in the journal Human and Ecological Risk Assessment (HERA):

• Richkus WA Menzie CA. Application of an ecological risk assessment for evaluation of alternatives considered for restoration of oysters in Chesapeake Bay: background and approach. Human and Ecological Risk Assessment 2013; 19(5):1172–1186.
• Weber ED, Vølstad EH, Christman MC, Lewis D, Dew-Baxter JR. Application of a Demographic Model for Evaluating Proposed Oyster-Restoration Actions in Chesapeake Bay Human and Ecological Risk Assessment 2013; 19(5):1187–1203
• Menzie CA, Salatas JH, Wickwire WT. Ecological risks associated with oyster restoration options for Chesapeake Bay. Human and Ecological Risk Assessment 2013; 19(5):1204–1233.
• Methratta ET, Menzie CA, Wickwire WT, Richkus WA. Evaluating the Risk of Establishing a Self-Sustaining Population of Non-Native Oysters Through Large-Scale Aquaculture in Chesapeake Bay. Human and Ecological Risk Assessment 2013; 19(5): 1234–1252
• Richkus WA. Role of Ecological Risk Assessment Findings in Agency Decision-Making Regarding Oyster Restoration in Chesapeake Bay. Human and Ecological Risk Assessment 2013; 19(5): 1253–1263

The Menzie et al. paper directly addresses the ERA for the introduction and development of reproducing population of the Suminoe Oyster. We communicated the following:

• There was low risk that the Suminoe oyster would not provide ecosystem services similar to those afforded by the Eastern Oyster.
• There is moderate to high risk that the Suminoe Oyster would interact with and compete with the Eastern Oyster.
• The potential for introduction and spread of diseases from the Suminoe Oyster to other species in the Bay is considered negligible.
• There is high risk that the Suminoe Oyster would disperse outside of the Bay.

Based on the PEIS which includes our ERA, the ACOE reached a decision in 2009 not to introduce non-native oysters due in large part to ecological risks. They recommended continuing oyster restoration approaches involving the native Eastern Oyster.

Conclusion of ACOE Record of Decision in 2009.

With the benefit of time, we can look into signs of progress for the oysters of Chesapeake Bay. Many restoration projects are underway in coastal areas and tributaries. This 2024 news piece from the Maryland Department of Natural Resources sounds promising and nicely lays out the history since the ACOE decision and the efforts to restore the oyster.

That positive tone is carried by the Chesapeake Bay Foundation, one of the leading stewards and science education centers for the Bay.

Access this report here: Chesapeake Bay Foundation. And the Nature Conservancy has provided positive news for oysters in Virginia waters.

Following the dismissal of the non-native oyster as a cure for the Bay, the efforts to restore the native Eastern oyster have involved many organizations and collaborations. It appears that steady effort will be required to ensure that the Bay’s oyster populations and the health or the Bay continue to move in the right direction. Certainly initiatives to manage nutrient and solids input to the Bay will be essential to lighten those burdens.

I’d like to acknowledge two colleagues who worked with me at Exponent and did much of the heavy lifting on the oyster project. Johanna Salatas and Ted Wickwire. They have gone on to successful careers tackling other challenging environmental matters.

Johanna (Jo) Salatas led a number of initiatives as a biologist at Exponent. I was fortunate to work with her on projects involving both risk assessment and causal analysis. We teamed together on endangered species in the Unite States and a case before the World Court. She is now the Environmental Risk Management Coordinator, Spill Prevention Officer, and Sustainability Project Manager for Teck Resources Limited.

Ted Wickwire is one of the most capable biologists I know. He was instrumental in advancing and testing spatially-explicit exposure and risk models for wildlife. He was an early leader in causal analysis work including an application in the Yemen desert. Ted is now a Senior Environmental Scientist at the Woods Hole Group.

I thank Sue Kane Driscoll, Andy Deines, and Jane Staveley for their review of this story.