Month: July 2021

On the Rise: US Imports & the Risks of Tree-killing Pests

containers at Port of Long Beach; photo courtesy of Bob Kanter, Port of Long Beach

Here I update information on two of the major pathways by which tree-killing pests enter the United States: wood packaging and living plants (plant for planting).

Wood Packaging

Looking at wood packaging material, we find rising volumes for both shipping containers – and their accompanying crates and pallets; and dunnage.

Crates and pallets – Angell (2021; full citation at the end of the blog) provides data on North American maritime imports in 2020. The total number of TEUs [a standardized measure for containerized shipment; defined as the equivalent of a 20-foot long container] entering North America was 30,778,446.U.S. ports received 79.6% of these incoming containers, or 24,510,990 TEUs. Four Canadian ports handled 11.4% of the total volume (3,517,464 TEUs; four Mexican ports 8.9% (2,749, 992 TEU). Angell provides data for each of the top 25 ports, including those in Canada and Mexico.

To evaluate the pest risk associated with the containerized cargo, I rely on a pair of two decade-old studies.  Haack et al. (2014) determined that approximately 0.1% (one out of a thousand) shipments with wood packaging probably harbor a tree-killing pest. Meissner et al. (2009) found that about 75% of maritime shipments contain wood packaging. Applying these calculations, we estimate that 21,000 of the containers arriving at U.S. and Canadian ports in 2020 might have harbored tree-killing pests.

While the opportunity for pests to arrive is obviously greatest at the ports receiving the highest volumes of containers with wood packaging, the ranking (below) does not tell the full story. The type of import is significant. For example, while Houston ranks sixth for containerized imports, it ranks first for imports of break-bulk (non-containerized) cargo that is often braced by wooden dunnage (see below). Consequently, Houston poses a higher risk than its ranking by containerized shipment might indicate.

Also, Halifax Nova Scotia ranks 22nd for the number of incoming containerized shipments (258,185 containers arriving). However, three tree-killing pests are known to have been introduced there: beech bark disease (in the 1890s), brown spruce longhorned beetle (in the 1990s), and European leaf-mining weevil (before 2012) [Sweeney, Annapolis 2018]

The top ten ports receiving containerized cargo in 2020 were

Port                                         2020 market share                2020 TEU volume

Los Angeles                           15.6%                                      4,652,549

Long Beach                            13%                                         3,986,991

New York/New Jersey         12.8%                                      3,925,469

Savannah                             7.5%                                        2,294,392

Vancouver BC                        5.8%                                        1,797,582

Houston                                   4.2%                                        1,288,128

Manzanillo, MX                      4.1%                                        1,275,409

Seattle/Tacoma    4.1%                                        1,266,839

Virginia ports                        4.1%                                        1,246,609

Charleston                             3.3%                                        1,024,059

Import volumes continue to increase since these imports were recorded. U.S. imports rose substantially in the first half of 2021, especially from Asia. Imports from that content reached 9,523,959 TEUs, up 24.5% from the 7,649,095 TEUs imported in the first half of 2019. The number of containers imported in June was the highest number ever (Mongelluzzo July 12, 2021).

Applying the calculations from Haack et al. (2014) and Meissner et al. (2009) to the 2021 import data, we find that approximately 7,100 containers from Asia probably harbored tree-killing pests in the first six months of the year. (The article unfortunately reports data only for Asia.) Industry representatives quoted by Mongelluzzo expect high import volumes to continue through the summer. This figure also does not consider shipments from other source regions.

Dunnage on the pier at Port of Houston; photo by Port of Houston

Infested dunnage – Looking at dunnage, imports of break-bulk (non-containerized) cargo to Houston – the U.S. port which receives the most – are also on the upswing. Imports in April were up 21% above the pandemic-repressed 2020 levels.

Importers at the port complain that too often the wooden dunnage is infested by pests, despite having been stamped as in compliance with ISPM#15. CBP spokesman John Sagle confirms that CBP inspectors at Houston and other ports are finding higher numbers of infested shipments. CBP does not release those data, so we cannot provide exact numbers (Nodar, July 19, 2021).

The Houston importers’ suspicion has been confirmed by data previously provided by CBP to the Continental Dialogue on Non-Native Insects and Diseases. From Fiscal Year 2010 through Fiscal Year 2015, on average 97% of the wood packaging (all types) found to be infested bore the stamp. CBP no longer provides data that touch on this issue.

Detection of pests in the dunnage leads to severe problems. Importers can face fines up to the full value of the associated cargo. Often, the cargo is re-exported, causing disruption of supply chains and additional financial losses (Nodar, July 19, 2021).

In 2019 importers and shippers from the Houston area formed an informal coalition with the Cary Institute of Ecosystem Studies to try to find a solution to this problem. The chosen approach is for company employees to be trained in CBP’s inspection techniques, then apply those methods at the source of shipments to identify – and reject – suspect dunnage before the shipment is loaded.  In addition, the coalition hopes that international inspection companies, which already inspect cargo for other reasons at the loading port will also be trained to inspect for pests.  Steps to set up such a training program were interrupted by the COVID-19 pandemic, but are expected to resume soon (Nodar, July 19, 2021).

Meanwhile, the persistence of pests in “treated” wood demands answers to the question of “why”. Is the cause fraud – deliberate misrepresentations that the wood has been treated when it has not? Or is the cause a failure of the treatments – either because they were applied incorrectly or they are inadequate per se?

ISPM#15 is not working adequately. I have said so.  Gary Lovett of the Cary Institute has said so (Nodar July 19, 2021). Neither importers nor regulators can rely on the mark to separate pest-free wood packaging from packaging that is infested.

APHIS is the agency responsible for determining U.S. phytosanitary policies. APHIS has so far not accepted its responsibility for determining the cause of this continuing issue and acting to resolve it. Preferably, such detection efforts should be carried out in cooperation with other countries and such international entities as the International Plant Protection Convention (IPPC) and International Union of Forest Research Organizations (IUFRO). However, APHIS should undertake such studies alone, if necessary.

In the meantime, APHIS and CBP should assist importers who are trying to comply by facilitating access to information about which suppliers often supply wood packaging infested by pests. The marks on the wood packaging includes a code identifying the facility that carried out the treatment, so this information is readily available to U.S. authorities.

Plants for Planting

A second major pathway of pest introduction is imports of plants for planting. Data on this pathway are too poor to assess the risk – although a decade ago it was found that the percentage of incoming shipments of plants infested by a pest was 12% – more than ten times higher than the proportion for wood packaging (Liebhold et al. 2012).

According to APHIS’ annual report, in 2020 APHIS and its foreign collaborators inspected 1.05 billion plants in the 23 countries where APHIS has a pre-clearance program. In other words, these plants were inspected before they were shipped to the U.S.  At U.S. borders, APHIS inspected and cleared another 1.8 billion “plant units” (cuttings, rooted plants, tissue culture, etc.) and nearly 723,000 kilograms of seeds. Obviously, the various plant types carry very different risks of pest introduction, so lumping them together obscures the pathway’s risk. The report does not indicate whether the total volume of plant imports rose or fell in 2020 compared to earlier years.

SOURCES

Angell, M. 2021. JOC Rankings: Largest North American ports gained marke share in 2020. June 18, 2021. https://www.joc.com/port-news/us-ports/joc-rankings-largest-north-american-ports-gained-market-share-2020_20210618.html?utm_campaign=CL_JOC%20Port%206%2F23%2F21%20%20_PC00000_e-production_E-103506_TF_0623_0900&utm_medium=email&utm_source=Eloqua

Haack R.A., Britton K.O., Brockerhoff, E.G., Cavey, J.F., Garrett, L.J., et al. (2014) Effectiveness of the International Phytosanitary Standard ISPM No. 15 on Reducing Wood Borer Infestation Rates in Wood Packaging Material Entering the United States. PLoS ONE 9(5): e96611. doi:10.1371/journal.pone.0096611

Liebhold, A.M., E.G. Brockerhoff, L.J. Garrett, J.L. Parke, and K.O. Britton. 2012. Live Plant Imports: the Major Pathway for Forest Insect and Pathogen Invasions of the US. www.frontiersinecology.org

Meissner, H., A. Lemay, C. Bertone, K. Schwartzburg, L. Ferguson, L. Newton. 2009. Evaluation of Pathways for Exotic Plant Pest Movement into and within the Greater Caribbean Region. A slightly different version of this report is posted at 45th Annual Meeting of the Caribbean Food Crops Society https://econpapers.repec.org/paper/agscfcs09/256354.htm

Mogelluzzo, B. July 12, 2021. Strong US imports from Asia in June point to a larger summer surge.

Nodar, J. July 19, 2021. https:www.joc.com/breakbulk/project-cargo/breakbult-volume-recovery-triggers-cbp-invasive-pest-violations_20210719.htm 

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

USFS report: treatment of introduced forest pests

still-healthy hemlocks in Cook Forest State Park, PA; photo by F.T. Campbell

In February the USFS published a lengthy analysis of invasive species: Invasive Species in Forests and Rangelands of the United States. A Comprehensive Science Synthesis for the US Forest Sector (Poland et al. 2021; full citation at the end of the blog). The book is available for download at no cost here.

In a separate blog, I evaluated several aspects of the report as they apply to invasive species generally. Here I focus on invasive insects and pathogens that attack North American tree species (that is, forest pests).

As I said in the separate blog, I doubt that the book will stimulate policy-makers to increase Forest Service resources allocated to invasive species research, much less management. Sections 14.5 and 16.5 of the report state that the continued absence of a comprehensive investigation of the impacts of invasive species, especially the full value of ecosystem services lost, is a barrier to policymakers seeking to develop priorities and realistic management strategies.

I think the book’s editors tried to provide as much information about impacts as possible given existing knowledge. But the book’s length, comprehensive inclusion of all bioinvaders, organizational structure, and the detailed discussions of theories and models reduce the contribution the book might make to management decisions. I did not find “lessons learned” that could be applied in the policy realm. 

Chapters address impacts in terrestrial and aquatic systems; impacts on ecosystem processes; impacts on various sectors of the economy and cultural resources; interactions with climate change and other disturbances; management strategies for species and landscapes; tools for inventory and management. Each chapter evaluates the current status of knowledge about the topic and suggests research needs. There are also summaries of the invasive species situation in eight regions.

The choice to organize the book around the chapters listed above means that some information one might expect to find in a book about invasive species is scattered or even absent. This is not a good resource for concise descriptions of individual invasive species and their impacts. That information is scattered among the chapters depending on whether some aspect of the species was chosen to illustrate a scientific challenge or success. The regional summaries partially remedy this problem – but they do not provide perspective on organisms that have invaded more than one region, e.g., emerald ash borer or white pine blister rust. To some extent, information about individual species is provided in the several subchapters on forest insects and pathogens. Or the reader of the PDF version can use the word search function!

Of course, information on several individual high-profile bioinvaders can be found in other publications; see the species write-ups and references posted at www.dontmovefirewood.org. Under these circumstances, a description of invasive species impacts from the ecosystem perspective is a welcome addition. I have long wished for a “crown to root zone” description of invasive species’ impacts.

HWA-killed hemlocks in Linville Gorge, NC; photo by Steven Norman, USFS

In this blog, I will focus on issues that the report raises that I found most interesting.

Information in the Report on Invasive Insects & Pathogens that Attack North American Trees

At several places the report states that non-native pests that have the potential to threaten the survival of an entire tree genus should be a high priority (p. 136) (what actions should be prioritized are not specified). They name the emerald ash borer (EAB) and Dutch elm disease. Elsewhere, EAB and hemlock woolly adelgid (HWA) are described as among the most significant threats to forests in the Eastern U.S. While EAB and HWA have certainly received considerable attention from the Forest Service, threats to elm have not. (I regret that the timing of the report precluded reference to Kevin Potter’s priority-setting publication. Potter is not listed as a co-author of the book.)

Hemlock woolly adelgid, emerald ash borer, chestnut blight, white pine blister rust (WPBR), and laurel wilt are cited as examples of highly virulent, host-specific agents that kill dominant, abundant, and ecologically unique hosts (p.18), resulting in exceptionally severe long-term impacts. WPBR and HWA specifically can have profound and far-reaching negative effects on ecosystem structure and function. These can rise to the level of an irreversible change of ecological state (p. 97). Of this list, no federal agency has allocated many resources to efforts to slow the spread of laurel wilt. The Forest Service is certainly tracking its spread and impacts.

Exaggerations or Errors

I think the report exaggerates the level of resources allocated to host resistance breeding. The report mentions programs targetting Dutch elm disease, beech bark disease, EAB, HWA and laurel wilt. It describes programs for white pines and Port-Orford cedar as examples of success. However, I would say that all the programs, except American chestnut, are starved for funds and other resources. The report’s authors concede this on p. 195.

TACF American chestnut in field trial; photo by F.T. Campbell

I think the report is too optimistic about the efficacy – so far – of biocontrol agents targeting HWA & EAB. On the other hand, I appreciate the report’s recognition that application of augmentative biocontrol of the Sirex woodwasp is more complicated in North America than in Southern Hemisphere countries (p. 162).

I am concerned about the statement that many plant pathogens are transported, but few have major impact. Examples in the U.S. are said to be WPBR, chestnut blight, and Phytophthora ramorum (p. 97). However, the report does not mention laurel wilt – which has a broad host range; nor rapid ‘ōhi‘a death — which threatens the most widespread tree species on the Hawaiian Islands. Nor does it mention several pathogens attacking single tree species, including beech bark disease, Port-Orford cedar root disease, and butternut canker. The report was written before recognition of beech leaf disease. The report notes that the three diseases it did mention have huge impacts. I am greatly disappointed that the report does not address how scientists and managers should deal with this “black swan” problem other than long discussions of data gaps, and ways to improve models of introduction and spread.

In addition, I am concerned that the discussion of economic factors that influence trade flows and accompanying invasive species (p. 308) focusses too narrowly on inspection alone, rather than other strategies for curtailing introduction. This section also shortens a description of the point made in Lovett et al. (2016). The report notes that Lovett et al. (2016) say that rates of introduction of wood-boring species decreased after ISPM#15 was implemented. However, the report leaves out the major caveat in that paper and the studies by Haack et al. (2014) and Leung et al. (2014) on which it is based: the reduction was insufficient to protect America from damaging introductions! [A further error has crept in: the Haack study explicitly excluded imports from China from their calculations. The Lovett paraphrase is not really clear on this matter.]

Curiosities/Concerns Re: Regional Write-Ups

I wish the sections on the Northwest and Southwest region discussed why areas with so many characteristics favoring introduction of plant pests – major ports, extensive transportation networks, major horticultural industry, extensive agriculture, and abundant urban and native forests – have so few damaging forest pests. (Admittedly, those present are highly damaging: white pine blister rust, sudden oak death, Port-Orford cedar root disease, pitch canker, balsam woolly adelgid, larch casebearer, polyphagous shot hole borer (I add Kuroshio shot hole borer), and banded elm bark beetle). The report does mention the constant threat of introduction of the European and Asian gypsy moths. (The Entomological Society of America has decided to coin a new common name for these insects; they currently to be called by the Latin binomial Lymatria dispar). The report notes that 22 species of non-native bark and ambrosia beetles have recently been introduced in the Southwest.

The report cites a decade-old estimate by Aukema et al. (2010) in saying that a small proportion of introduced pest species has killed millions of trees or pushed ecologically foundational species toward functional extinction. The figure was 14% of the more than 450 non-native forest insect species. I greatly regret that overlapping preparation and publication periods precluded inclusion of data from studies by Potter, Guo, and Fei.  http://nivemnic.us/what-fia-data-tell-us-about-non-native-pests-of-americas-forests/

Section 7.3 of the report discusses frameworks for setting priorities. It identifies five factors: 1) pest species having the greatest negative impacts; 2) uniqueness of the affected ecosystem or community; 3) state of the invasion in space and time; 4) management goals; 5) availability of effective tools. Examples of species meeting these criteria include EAB and Dutch elm disease (pest threatens entire host genus); white pine blister rust on whitebark pine (key species in a system with low arboreal diversity).

The report notes increasing understanding of critical aspects of several important pests’ biology and host interactions – but it does not sufficiently acknowledge the decades of effort required to achieve this knowledge. The time required for additional scientific advances will probably be equal or greater, given falling number of “ologists” in government and academia.

I appreciate inclusion of a discussion (Sections 8.3.1 and 8.3.2) on breeding trees resistant to introduced pests

dead Port-Orford cedar in Redwoods National Park; photo by Richard Sniezko, USFS

This section states that host resistance, forest genetics, and tree improvement might be the most effective approaches to managing many established pests. The section says such breeding does not require the use of genetically modified organisms, although transgenic or gene editing technologies can provide useful tools. I appreciate the report conceding that necessary infrastructure and expertise has been declining for two decades (p. 195).

In discussing international cooperation to reduce transport of invasive species, the report refers to increasing availability of data allowing identification of potentially damaging species in their regions of origin. Again, since this chapter was written, the Forest Service has increased its engagement on this approach: the USFS International Program is supporting sentinel plantings managed by the International Plant Sentinel Network (http://www.plantsentinel.org) … see my recent blog here.

SOURCE

Poland, T.M., P. Patel-Weynand, D.M Finch, C.F. Miniat, D.C. Hayes, V.M Lopez, editors. 2021. Invasive Species in Forests and Rangelands of the United States. A Comprehensive Science Synthesis for the US Forest Sector. Springer

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

USFS invasive species report: Herculean effort that could have had greater impact

In February the USFS published a lengthy analysis of invasive species: Invasive Species in Forests and Rangelands of the United States. A Comprehensive Science Synthesis for the US Forest Sector (Poland et al. 2021; full citation at the end of the blog). More than 100 people contributed to the book; I helped write the chapters on legislation and regulations and international cooperation. The book is available for download at no cost here.

Chapters address impacts in terrestrial and aquatic systems; impacts on ecosystem processes; impacts on various sectors of the economy and cultural resources; interactions with climate change and other disturbances; management strategies for species and landscapes; tools for inventory and management. Each chapter evaluates the current status of knowledge about the topic and suggests research needs. There are also summaries of the invasive species situation in eight regions.

Miconia – one of many invasive plants damaging ecosystems in Hawai`i

I greatly appreciate the effort. Authors first met in 2015, and most chapters were essentially written in 2016. The long delay in its appearance came largely from negotiations with the publisher. The delay means some of the information is out of date. I am particularly aware that several experts – e.g., Potter, Guo, and Fei – have published about forest pests since the Aukema source cited. I wonder whether inclusion of their findings might change some of the conclusions about the proportion of introduced pests that cause noticeable impacts.

Since the report’s publication in February I have struggled with how to describe and evaluate this book. What is its purpose? Who is its audience? The Executive Summary says the report is a sector-wide scientific assessment of the current state of invasive species science and research in the U.S.

However, the Introduction states a somewhat different purpose. It says the report documents invasive species impacts that affect ecosystem processes and a wide range of economic sectors. This would imply an intention to enhance efforts to counter such effects– not just to shape research but also to change management. Indeed, the Conclusion of the Executive Summary (pp. xvi-xvii) is titled “An Imperative for Action”.

Tom Vilsack, Secretary of Agriculture

I am not the author to evaluate how effectively the book sets out research agendas. Regarding its usefulness in prompting policy-makers to do more, I regretfully conclude that it falls short.

Getting the balance right between an issue’s status and what needs to be done is difficult, perhaps impossible. I appreciate that the report makes clear how complex bioinvasion and ecosystem management and restoration are. Its length and density highlight the difficulty of making progress. This daunting complexity might well discourage agency leadership from prioritizing invasive species management.

On the other hand, summary sections sometimes oversimplify or bury important subtleties and caveats. The question of whether some key questions can ever be resolved by science is hinted at – but in detailed sections that few will read. The same is true regarding the restrictions imposed by funding shortfalls.

The Report Would Have Benefitted from Another Round of Editing

Editing this tome was a Herculean task. I feel like a curmudgeon suggesting that the editors do more! Nevertheless, I think the report would have been improved by the effort. One more round of editing – perhaps involving a wider range of authors – could have pulled together the most vital points to make them more accessible to policymakers. It could also have tightened the ecosystem-based descriptions of impacts, which are currently overwhelmed by too much information.

A precis for policymakers

A precis focused on information pertinent to policymakers (which the current Executive Summary does not) should contain the statement that the continued absence of a comprehensive investigation of invasive species’ impacts hampers research, management, and policy (mentioned only in §16.5, on p. 332). It should note situations in which insufficient funding is blocking recommended action. I note three examples: programs aimed at breeding trees resistant to non-native pests (resource issues discussed only in §§8.3.1 and 8.3.2, p. 195); sustaining “rapid response” programs (§6.4.3, p. 125); costs of ecosystem restoration, especially for landscape-level restoration (§16.4). I am sure there are additional under-funded activities that should be included!

cross-bred ash seedlings being tested for vulnerability to EAB; photo courtesy of Jennifer Koch

 Other important information that should be highlighted in such a precis includes the statement that many ecosystems have already reached a point where healthy functions are in a more tenuous balance due to invasive species (p. 51). Effective carbon storage and maintaining sustainable nutrient and water balance are at risk. Second, costs and losses caused by invasive forest pests generally fall disproportionately on a few economic sectors and households. They cannot be equated to governmental expenditures alone (p. 305).  Third, even a brief estimate of overall numbers of invasive species appears only in §7.4. Information about individual species is scattered because it is used as example of particular topic (e.g., impacts on forest or grassland ecosystems, or on ecosystem services, or on cultural values).

Ecosystem Impacts Overwhelmed

As noted above, the report laments the absence of a comprehensive investigation of invasive species’ impacts. Perhaps the editors intended this report to partially fill this gap. To be fair, I have long wished for a “crown to root zone” description of invasive species’ impacts at a site or in a biome. Concise descriptions of individual invasive species and their impacts are not provided by this report, but they can be found elsewhere. (The regional summaries partially address the problem of too much information – but they do not provide perspective on organisms that have invaded more than one region, e.g., emerald ash borer or white pine blister rust.) Another round of editing might have resulted in a more focused presentation that would be more easily applied by policymakers.

Welcome Straightforward Discussion of Conceptual Difficulties

I applaud the report’s openness about some important overarching concepts that science cannot yet formulate.  If supportable theories could be conceived, they would assist in the development of policies:

  • Despite decades of effort, scientists have not established a clear paradigm to explain an ecosystem’s susceptibility to invasion (p. 85). Invasibility is complex: it results from a dynamic interplay between ecosystem condition and ecological properties of the potential invader, especially local propagule pressure.
  • Scientists cannot predict how climate warming will change distributions of invasive species [see Chapter 4] and alter pathways. This inability hampers efforts to develop effective prevention, control, and restoration strategies (p. xi). Climate change and invasive species need to be studied together as interactive drivers of global environmental change with evolutionary consequences.

The Report’s Recommendations

Policy-oriented recommendations are scattered throughout the report. I note here some I find particularly important:

  • Measures of progress should be based on the degree to which people, cultures, and natural resources are protected from the harmful effects of invasive species.
  • Managers should assess the efficacy of all prevention, control, and management activities and their effect upon the environment. Such an evaluation should be based on a clear statement of the goals of the policy or action. [I wish the report explicitly recognized that both setting goals and measuring efficacy are difficult when contemplating action against a new invader that is new to science or when the impacts are poorly understood. Early detection / rapid response efforts are already undermined by an insistence on gathering information on possible impacts before acting; that delay can doom prospects for success.]
    • Risk assessment should both better incorporate uncertainty and evaluate the interactions among multiple taxa. Risk assessment tools should be used to evaluate and prioritize management efforts and strategies beyond prevention and early detection/rapid response.
    • Economic analyses aimed at exploring tradeoffs need better tools for measuring returns on invasive species management investments (§16.5).
  • Actions that might be understood as “restoration” aim at a range of goals along the gradient between being restored to a known historic state and being rehabilitated to a defined desired state. The report stresses building ecosystem resilience to create resistance to future invasions, but I am skeptical that this will work re: forest insects and disease pathogens.
  • Propagule pressure is a key determinant of invasion success. Devising methods to reduce propagule pressure is the most promising to approach to prevent future invasions (p. 115). This includes investing in quarantine capacity building in other countries can contribute significantly to preventing new invasions to the US.
  • Resource managers need additional studies of how invasive species spread through domestic trade, and how policies may differ between foreign and domestic sources of risk.

I appreciate the report’s attention to such often-ignored aspects as non-native earthworms and soil chemistry. I also praise the report’s emphasis on social aspects of bioinvasion and the essential role of engaging the public. However, I think the authors could have made greater use of surveys conducted by the Wisconsin Department of Natural Resources and The Nature Conservancy’s Don’t Move Firewood program.

Lost Opportunities

I am glad that the report makes reference to the “rule of 25” rather than “rule of 10s”. I would have appreciated a discussion of this topic, which is a current issue in bioinvasion theory. As noted at the beginning of this blog, the long time between when the report was written and when it was published might have hampered such a discussion

Also, I wish the report had explored how scientists and managers should deal with the “black swan” problem of infrequent introductions that have extremely high impacts. The report addresses this issue only through long discussions of data gaps, and ways to improve models of introduction and spread.

I wish the section on the Northwest Region included a discussion of why an area with so many characteristics favoring bioinvasion has so few damaging forest pests. Admittedly, those present are highly damaging: white pine blister rust, sudden oak death, Port-Orford cedar root disease, balsam woolly adelgid, and larch casebearer. The report also notes the constant threat that Asian and European gypsy moths will be introduced. (The Entomological Society of America has decided to coin a new common name for these insects; they currently to be called by the Latin binomial Lymatria dispar).

And I wish the section on the Southeast and Caribbean discussed introduced forest pests on the Caribbean islands. I suspect this reflects a dearth of research effort rather than the biological situation. I indulge my disagreement with the conclusion that introduced tree species have “enriched” the islands’ flora.

SOURCE

Poland, T.M., P. Patel-Weynand, D.M Finch, C.F. Miniat, D.C. Hayes, V.M Lopez, editors. 2021. Invasive species in Forests and Rangelands of the United States. A Comprehensive Science Synthesis for the US Forest Sector. Springer

Posted by Faith Campbell

We welcome comments that supplement or correct factual information, suggest new approaches, or promote thoughtful consideration. We post comments that disagree with us — but not those we judge to be not civil or inflammatory.

For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at http://treeimprovement.utk.edu/FadingForests.htm

“Rule of Tens” – Time to Refine It

are wood-borers examples of species more likely to “proceed through the steps of invasion” than the theory suggests?

Much of the literature about biological invasion has relied on the “tens rule”. First enunciated in the mid-1990s by Williamson and Fitter (1996), it was actually conceived a decade earlier by Williamson and Brown (1986).

The “tens rule” hypothesizes that about 10% of all species transported to a new environment will be released or escape and become introduced species. Subsequently, 10% of those introduced species establish viable populations in the wild. Finally, about 10% of the established species become highly damaging. That is, 1% of the number originally transported to the new environment is a highly damaging invader.

Is the “tens rule” supported by evidence?

Empirical support for the hypothesis has been mixed; the number of studies questioning it has increased over the decades (Jeschke and Pyšek 2018). So Jeschke and Pyšek (2018) decided to evaluate the basis for the hypothesis. First, they divided the hypothesis into two sub-hypotheses so they could separate the concept of impact from the process of introduction, establishment, and spread. They justified this separation by noting that novel species can have an impact at any stage. The two sub-hypotheses:

1st sub-hypothesis: At each of the three transitions between the invasion stages listed here the number of species completing the transition is reduced by 90% (invasion tens rule).

  • transport to exotic range

transition

  • introduction (release or escape into the environment)

transition

  • establishment of a least one self-sustaining population

transition

  • spread

2nd sub-hypothesis: about 10% of established non-indigenous species cause a significant detrimental impact. This sub-hypothesis applies to the transition from establishment (iii, above) to significant impact (iv). Stepping back to the earlier introduction, so as to consider the situation overall, about 1% of all introduced non-native species cause a significant detrimental impact; this sub-hypothesis thus relates to the transition from introduction (ii) to significant impact (iv).

Jeschke and Pyšek carried out a quantitative meta-analysis of 102 empirical tests of the tens rule drawn from 65 publications. They found no support for the “invasion tens rule”. Indeed, their analysis found that about 24% of non-native plant and 23% of non-native invertebrate species are successful in taking consecutive steps of the invasion process. Among non-native vertebrates, about 51% are successful in taking consecutive steps of the invasion process.

The “impact tens rule” is also not supported by currently available evidence. However, Jeschke and Pyšek decided that more data are needed before a reasonable alternative hypothesis can be formulated.

Findings

Jeschke and Pyšek state that the “tens rule” is not based on a model or other defensible concept. It is also hampered by confusion of terms. Thus, different authors define the invasion process differently. Particularly confounding is the mixing of “impact” with steps in the invasion process. At the same time, there have been few studies of the “impact tens rule” hypothesis.

Finally, the “tens rule’s” predictions are not adjusted to consider changes in temporal and spatial scales. That is, it does not recognize that more invaders will be detected in any given place during more recent times than in the past. Furthermore, more invaders will find suitable niches in large areas than small.

The note that analysis is hampered by the paucity of reliable data about establishment success – especially for taxa other than mammals and birds. They do not discuss how this lack might affect efforts to analyze proportions of entering species that succeed in becoming invasive, especially among the small and inconspicuous taxa such as insects and fungal organisms that concern thus of us that focus on threats to forests. This same data gap has limited other studies as well; see, for example, Aukema et al. (2010) – who restricted their discussion of pathogens to “high impact” species.

Although Jeschke and Pyšek (2018) do not specify which studies they relied on to determine the proportion of successful invaders among species belonging to particular taxa, it seems likely that they relied principally on Vila et al. (2010) in determining that on average 25% invertebrates that are introduced (that is, proceed to the second stage in the process given above) become invasive. Vila et al. analyze introductions to Europe. They found that 24.2% of terrestrial invertebrates caused recognized economic impacts.

Jeschke and Pyšek (2018) Results and Discussion

Considering the “invasion tens rule”, two-thirds of the empirical tests in the dataset focused on the “invasion tens rule”. The majority of these focused on the transition from introduction to establishment (the transition from (ii) to (iii). The observed average percentage of species making this transition is more than 40% – or greater than four times larger than the “tens rule’s” prediction.

At the next transition, from establishment to spread (from iii to iv), the observed percentage of species making the transition is  greater than 30% – or greater than three times the predicted value under the “tens rule”.

Considering the “impact tens rule”, on average a quarter of established non-indigenous species have a significant detrimental impact, which is again significantly more than the 1 out of 10 species predicted by the rule. Specifically by taxon, 18% of established plants have shown detrimental impacts. Among invertebrates and vertebrates that estimate is greater than 30%. All these observations are higher than predicted by the rule. However, sample sizes are low so more studies are needed to test whether these values hold true.

Regarding the fullest possible extent of the invasion process, 16 out of 100 species that were introduced (stage ii) had a significant impact. This is 16 times greater than the 1% predicted by the “tens rule”. Considering specific taxa, 6% of established plants and 15% of established invertebrates had a significant impact. Data were too poor to support an evaluation for vertebrates.

I note that the alarmingly high “impact” estimates for invertebrates are probably biased by scientists’ and funding entities’ lack of interest in species that don’t cause noticeable impacts.

Poor data preclude an analysis of the transition from transport (i) to introduced (ii).

Strengthening The Estimates

Might these introduction and impact estimates be tightened by analysis of additional sources, such as the studies led Seebens, forest pest impact analyses by Potter et al. (2019) and Fei (2019) and reviews of pest introduction numbers by Haack and Rabaglia (2013)? 

Is it worth pursuing efforts to refine the Jeschke and Pyšek (2018) estimates? I think it is. An underestimation of the risk of introduction might lead decision-makers to downplay the need for a response.

Some scientists have accepted the new “rule of 25” (Schulz, Lucardi, and Marsico. 2021. Full citation at end of blog; also cited by USFS report – Poland et al. 2021). Others have not. Venette and Hutchison (2021; full reference at end of blog) continue to cite the estimate of approximately one “invasion success” for every 1,000 attempts – that is, a low-probability, high-consequence event. This challenges those responsible for managing invasive species.

Or are there other conundrums of introduction, establishment, and predicting impacts that have more direct relationship to improving programs? I note that the recent Forest Service report on invasive species (Poland et al. 2021) does not address the “rule of tens”.

Other Reasons Why Bioinvasion Damage is Underestimated

Jaric´ and G. Cvijanovic´ (2012) note that scientists lack a full understanding of ecosystem functioning, so they probably often miss more subtle – but still important – impacts.

Jeschke and Pyšek (2018) note that the percentage of introduced or established species with a quantifiable impact is not always the most important information. A single introduced species can have devastating impact by itself. They cite the amphibian disease chytrid (Batrachochytrium dendrobatidis) and such mammals as rats and cats.

SOURCES

Aukema, J.E., D.G. McCullough, B. Von Holle, A.M. Liebhold, K. Britton, & S.J. Frankel. 2010. Historical Accumulation of Nonindigenous Forest Pests in the Continental United States. Bioscience. December 2010 / Vol. 60 No. 11

Brockerhoff, E.G. and A. M. Liebhold. 2017. Ecology of forest insect invasions. Biol Invasions (2017) 19:3141–3159

Fei, S., R.S. Morin, C.M. Oswalt, and A.M. Liebhold. 2019. Biomass losses resulting from insect and disease invasions in United States forests. Proceedings of the National Academy of Sciences of the United States of America, 12 Aug 2019, 116(35):17371-17376

Haack, R.A. and R.A. Rabaglia. 2013 Exotic Bark and Ambrosia Beetles in the USA: Potential and Current Invaders. CAB International. 2013. Potential Invasive Pests of Agricultural Crops (ed. J. Pena)

Jaric´, I. and G. Cvijanovic´. 2012. The Tens Rule in Invasion Biology: Measure of a True Impact or Our Lack of Knowledge and Understanding? Environmental Management (2012) 50:979–981 DOI 10.1007/s00267-012-9951-1

Jeschke J.M. and P. Pyšek. 2018. Tens Rule. Chapter 13 of book by CABI posted at http://www.ibot.cas.cz/personal/pysek/pdf/Jeschke,%20Pysek-Tens%20rule_CABI%202018.pdf

Poland, T.M., Patel-Weynand, T., Finch, D., Miniat, C. F., and Lopez, V. (Eds) (2019), Invasive Species in Forests and Grasslands of the United States: A Comprehensive Science Synthesis for the United States Forest Sector. Springer Verlag. (in press).

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Schulz, A.N., R.D. Lucardi, and T.D. Marsico. 2021. Strengthening the Ties That Bind: An Evaluation of Cross-disciplinary Communication Between Invasion Ecologists and Biological Control Researchers in Entomology. Annals of the Entomological Society of America · January 2021

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