Invasive Species and Global Climate Change

By Samantha Ahdoot, Charles T. Bargeron, Jinyoung Barnaby and 54 more

This book addresses topics related to the impact of invasive species including biosecurity, demographics, species diversity and food security. It is meant for researchers, upper-level students, and policy makers and provides a factual basis for the underlying science and a discussion of that information with respect to current and future impacts and possible solutions. This book explores the nexus of climate change and biological invasions, resulting impacts (biological and economic) and assesses ways to reduce vulnerability and increase the resiliency and sustainability of managed and unmanaged ecosystems. The book has three parts, focusing on: (1) the dimensions of the problem; background and science; (2) case studies; (3) Management: detection, prevention, control and adaptation. This revised edition examines a wide range of topics and region, the underlying science, examples (case studies) from around the world, and ways and means to recognize, manage and control the consequences. It includes new cases and new threats; for example, a chapter summarizing case studies regarding climate change and invasive species that are also disease carriers (e.g. ticks and Lyme disease).

- Covers a wide range of topics and areas
- Examines the synergy between invasive species and climate change
- Explains options to control and mitigate effects

This book is of interest to academics, researchers and students studying climate change and invasive species. Those interested in the environment and ecology, land managers, policy makers, agronomists, federal and state departments of natural resources, climate change activists, public health professionals.

• Language: English
• Publisher: CAB International
• Release date: Dec 19, 2022
• ISBN: 9781800621459

Book preview

1 Introduction

Lewis H. Ziska

Environmental Health Sciences, Mailman School of Public Health, Columbia University, New York, USA

© CAB International 2023. Invasive Species and Global Climate Change, 2nd Edition (ed. Lewis H. Ziska)

DOI: 10.1079/9781800621459.0001

We do not inherit the earth from our ancestors; we borrow it from our children.

Native American Proverb

As I write this, the global population is close to 8 billion. At present rates, approximately 5 million new individuals will be added each month, every month, for the near future (United States Census Bureau, n.d.).

Ultimately, it is our rapidly increasing population and our need to increase production of food, feed, fiber and fuel from a finite set of natural resources that are driving the environmental issues in this book, and that give these issues urgency. We need to transition to a sustainable society if we are to provide for this population (or even a smaller one) into the future. Such sustainability is necessary if we are to preserve our planet’s ecosystem services, maintain its capacity to produce food and protect its biodiversity.

However, at present, the needs of our population, and the unprecedented transportation of biota into new regions to meet these needs, are occurring on a scale that threatens the planet’s natural resource capacity. This book is a collective attempt from a wide swath of experts, including medical doctors, ecologists and environmentalists from around the world, to describe the interaction between two of the resulting consequences. Specifically, to examine the nexus of climate change and biological invasions, the resulting impacts, and to identify means to reduce the vulnerability and increase the resiliency of managed and unmanaged ecosystems.

Such a complex, global topic is best addressed from a variety of perspectives. I thank the many people who have contributed to and commented on the chapters in this book. The individual chapter authors and the reviewers of those chapters are world experts, and very busy people. I appreciate their willingness to commit to this project, and their faith that a contribution to this book would be a worthy use of their time. There is no question that their contributions have enabled this book to convey a detailed, globally relevant and sometimes provocative portrait of what is known – and what is unknown – regarding climate change and invasive species. In addition, their contributions present a valuable overview of strategies for managing natural and agricultural systems on a rapidly changing planet.

In examining a complex set of issues, it helps to have common definitions. This book considers ‘climate change’ in a broad sense, that is, both the disruption of Earth’s relatively stable recent climate and the ongoing increase in atmospheric carbon dioxide (CO2) concentrations that are largely responsible for that disruption. The book also considers biological invasions broadly, including many taxa. We recognize that the term invasive species can have a variety of meanings (and these, in turn, can be complicated by climate change; Webber and Scott, 2012). Biologists may refer to these species as ‘biological invaders’, ‘alien species’, ‘alien exotics’ or simply as ‘invasives’. Regardless of the term, biologists are characterizing species that have crossed a major biogeographic barrier (e.g. an ocean) usually with the assistance of humans, and whose introduction has resulted or will result in significant negative economic or environmental damage.

Given the complexity of the subject matter, I recognize that not all chapters will appeal to an individual reader; rather, the book is intended to be accessible to a range of interested parties, not just the academic specialist. We do hope the book can educate broadly and provide a means for understanding the consequences of invasive species and climate change, not in isolation (such efforts are already well documented), but in a synergistic context. Still, for most readers, to understand the synergism, it is important to appreciate the components of the problem, and we attempt to provide some background here.

The Problem and Its Components

The desire for food and fuel has been endemic since the dawn of human civilization and the commencement of cultivated agriculture. As populations grew, and land/energy needs increased, the incorporation of fossil fuels, or energy captured from sunlight over millennia by plants, became an integral part of the Industrial Revolution, a revolution that, for billions of people, has provided ample food, water and an improved standard of living.

Use of natural resources to meet these human needs has, since its inception, impacted climate. For example, by removing forests and native plants, early agriculture altered hydrologic cycles and changed surface albedo, with consequences for regional climate (Pielke et al., 2007). In addition to regional climatic effects, burning of fossil fuels has jolted Earth’s atmosphere with a 40+% increase in CO2 since the onset of the Industrial Revolution with most of that increase occurring since the 1970s (+30%).

That CO2 generated by human society could influence climate is not a new concept. In the 19th century two scientists, Joseph Fourier and Svante Arrhenius, suggested that industrial pollutants, notably carbon dioxide, were building up in Earth’s atmosphere and could, potentially, result in increased surface temperatures (Fourier, 1827; Arrhenius, 1896). Quantitative measurements by Keeling in the 1950s confirmed that CO2 was, in fact, increasing globally (Revelle and Suess, 1957).

One of the properties of the CO2 molecule is that it absorbs energy in the infrared portion of the electromagnetic spectrum (making it a ‘greenhouse gas’). Adding CO2 to the air causes the atmosphere to trap more of the heat radiated up from Earth’s surface that would otherwise escape to space. The atmosphere warms up more, the rest of the planet heats up a bit to follow, and more water evaporates from the warmer seas into the warmer skies. Water vapor itself traps heat and further warms the planet in what is known as a positive feedback loop.

Overall, model projections based on future emissions of greenhouse gases suggest a marked warming of Earth’s surface and changes in precipitation patterns in many regions. Model projections also clearly indicate that the rate and degree of climate disruption over the coming decades will depend on how quickly we continue to release heat-trapping gases into the atmosphere (Solomon, 2007). It is worth acknowledging that, given the life span and ongoing release of CO2 and other greenhouse gases, there is sufficient momentum at present for a significant change in Earth’s climate to be essentially guaranteed. Therefore, as we prepare for warmer, uncertain climatic conditions, it is important to consider the consequences of these conditions for the utility and health of managed and unmanaged ecosystems.

In considering the importance of the effect of CO2 and climatic change on ecosystems, it is also important to consider CO2 as an essential substrate in plant biology, providing a primary building block for photosynthesis. The recent, rapid increase in atmospheric CO2 has been felt directly by plants, some of which are growing faster, with lower water consumption, in response (e.g. Keenan et al., 2013). Reports on climate change in the media only infrequently discuss direct effects of this CO2 increase, but this increase has been much larger, with a much stronger effect on plants, than any changes in climate experienced to date (and indeed, this may remain the case for many decades).

From an ecological perspective, such direct effects of CO2 on plant photosynthesis and growth may be seen as beneficial, the ‘green’ in the ‘greenhouse effect’. However, CO2 is indiscriminate with respect to which plant species may be favored. For example, there is substantial evidence that pest species such as yellow starthistle and poison ivy can have very strong responses to rising CO2 levels (Ziska, 2003; Mohan et al., 2006; Dukes et al., 2011). Clearly, how plant species and ecosystems respond, not only to climate but to rising CO2 directly, will also have significant biological consequences. Several of the chapters in this book help to examine these consequences in the context of invasive species biology.

In addition to the build-up of greenhouse gases, other human activities associated with the need for increased feed and fuel have contributed to large-scale environmental perturbation. Especially relevant has been the transportation, on a massive scale, of organisms that had been restricted to certain biogeographic zones, but are now distributed globally. Many of these species, such as soybean, are important for human welfare and a strong economy, but forced relocation of thousands of species outside of their native habitats can also result in the distribution of extraordinarily aggressive species with severe economic and environmental consequences.

Invasive species come in many shapes and sizes; they can be hard to recognize since their only common feature is biological domination outside of their native range. This book includes discussion of invasive weeds, insects and pathogens in many disparate taxa, from the poles to the tropics. These species disrupt a wide variety of ecosystem processes (Dukes and Mooney, 2004; Vilà et al., 2011), threaten biodiversity (Powell et al., 2013), the provision of ecosystem services (Charles and Dukes, 2007; Pejchar and Mooney, 2009) and food (Oerke, 2006), and cause damage estimated to cost around US$120 billion per year in the United States alone (Pimentel et al., 2005).

Why This Book?

In addition to affecting basic aspects of biology on a global scale, both climate change and invasive species pose existential threats to the basic ecosystem services necessary for human life. Furthermore, it should not be assumed that each threat acts independently of the other. The synergy between these issues is becoming increasingly evident. For example, changing climatic conditions (e.g. polar melting and the opening of new trade routes) will alter global commerce in the near future, with the subsequent introduction of unwanted species into new geographical regions (Hellmann et al., 2008; Bradley et al., 2012). Once they are introduced, climate change – either through changes in means or extremes – may then facilitate the establishment and spread of such species; or alternatively, may allow other species that are currently established to become invasive as environmental constraints (e.g. cold winters) are eased (Dukes and Mooney, 1999; Walther et al., 2009; Bradley et al., 2010; Diez et al., 2012). Research also suggests that invasive species management, particularly chemical applications, may further exacerbate greenhouse gas emissions (Heimpel et al., 2013).

While there have been many separate books documenting climate change or invasive species, only one has broadly linked these aspects of environmental transformation. In 2000, when Hal Mooney and Richard Hobbs published Invasive Species in a Changing World, very few researchers had thought about the combined implications of these two environmental changes. Since then, the field has grown rapidly, but has not been comprehensively reviewed.

Here we take a global look at what is currently known about the synergistic nature of these environmental changes in the book’s three sections. These parts, in turn, provide an overview of the current state of understanding in this field, the tools available to manage the problem and the challenges for future research. The first part of the book outlines the dimensions of the problem. In Chapter 1, I present a brief overview of the science of climate change and invasion biology, but also examine how we can communicate the science more effectively to policy makers. The next three chapters lay out the science with respect to three classes of invasive species in the context of changing climate and CO2 levels: Karen A. Garrett and colleagues discuss pathogens; Andrew Paul Gutierrez and Luigi Ponti examine insects; and Dana M. Blumenthal and Julie A. Kray look at plants.

The second part of the book highlights the global synergy between climate change and invasive species with case studies from around the world. We begin in Antarctica where Kevin A. Hughes and Peter Convey provide an overview of climate and invasives; we segue to aquatic environments where Cascade J.B. Sorte and colleagues appraise how invasives respond; then to Eastern Europe for a more specific examination of the implications of changing CO2 and temperature for ragweed by László Makra et al., followed by a consideration of climate and invasives in South Africa by Nicola J. van Wilgen and colleagues. Catherine Jarnevich and colleagues from the US Geological Survey then scrutinize invasives in National Parks in the United States, and Cecilia Sorensen examines invasive species from a public health perspective.

In Part III we turn to the issue of detecting, preventing, controlling and adapting to new invasive threats in a changing climate. We begin by emphasizing that early detection, which has the best hope for allowing problem species to be stopped in their tracks, is critical, in a chapter by Rebekah D. Wallace and Charles T. Bargeron focusing on tools for early detection and mapping. Hilda Diaz-Soltero presents information about the new CABI compendium on invasive species; Bethany A. Bradley examines approaches to modeling the current and future distributions of invasives; and Andrew Paul Gutierrez and Luigi Ponti present a new approach for modeling the impact of climate change on an invasive species, yellow starthistle. We then segue to the issue of what can be done when invasive species do show up on your doorstep. Anna S. Westbrook et al. examine how to model and manage invasive weeds impacted by climate and CO2. Randy G. Westbrooks and colleagues examine whether the Early Detection and Rapid Response paradigm can be configured to cope with climate change. Finally, if all else fails, Matthew A. Barnes et al. ask whether invasive species can potentially serve as an economic resource (i.e. it’s not Asian carp, it’s Kentucky tuna!).

What Do We Hope to Accomplish?

In the Twitter/TikTok/Instagram age, when visual overload can occur every time you stare at a flat screen, books may feel like an anachronism. But books – this one included – are not designed to provide you with information in five-minute increments. Rather, books function as a period in a long stream of text messaging – a chance to stop, reread and reassess what we currently know.

And, as it turns out, we know quite a bit. We know the climate is changing, and that this change is due primarily to human activity. We also know that the extent of this change is likely to alter further the transport and biology of invasive species – species whose introduction, establishment and spread are likely to disrupt the world’s ecosystems in unpredictable and undesirable ways. We know that this disruption, in turn, will almost certainly alter human welfare, with consequences for areas as diverse as food security, ocean ecology and forest dynamics.

But while the general outline is known, the details remain elusive. Sadly, part of this is because climate change is still viewed through a political and not a scientific lens. Consequently, the resources (students, scientists, equipment, labs, etc.) needed to address key questions are lacking. But scientists themselves also shoulder some responsibility. All too often we revel in the technical and ignore the pragmatic.

While the details may be complex and nuanced, our goal in assembling this book is not. We want to draw attention to the ‘big picture’: that global increases in CO2 mean more than a warm summer day; that abrupt climatic change is likely to act synergistically with other ongoing changes, most notably invasive species biology; and that the subsequent degradation in natural and managed ecosystems should be an area of increased scientific and policy concern.

The challenge now is what to do with this knowledge – how to provide for the desires of a society of 8 – soon to be 9 – billion, while protecting the biodiversity and ecosystem processes that ensure the planet’s capacity to continue to provide for us into the future. Minimizing the degradation of ecosystem services and biodiversity by invasive species is already a challenge; climate change is likely to heighten it. Continued, global investment in societal awareness of the problem, the tools to combat it (both scientific and legal) and the active management of invasive species will be critical if we are to minimize irreversible environmental impacts and maintain the ecosystem services needed to satisfy our growing population’s needs.

References

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Bradley, B.A., Blumenthal, D.M., Wilcove, D.S. and Ziska, L.H. (2010) Predicting plant invasions in an era of global change. Trends in Ecology & Evolution 25(5), 310–318. DOI: 10.1016/j.tree.2009.12.003.

Bradley, B.A., Blumenthal, D.M., Early, R., Grosholz, E.D., Lawler, J.J. et al. (2012) Global change, global trade, and the next wave of plant invasions. Frontiers in Ecology and the Environment 10(1), 20–28. DOI: 10.1890/110145.

Charles, H. and Dukes, J.S. (2007) Impacts of invasive species on ecosystem services. In: Nentwig, W. (ed.) Biological Invasions. Springer, Heidelberg, Germany, pp. 217–237.

Diez, J.M., D’Antonio, C.M., Dukes, J.S., Grosholz, E.D., Olden, J.D. et al. (2012) Will extreme climatic events facilitate biological invasions? Frontiers in Ecology and the Environment 10(5), 249–257. DOI: 10.1890/110137.

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Dukes, J.S. and Mooney, H.A. (2004) Disruption of ecosystem processes in western North America by invasive species. Revista Chilena de Historia Natural 77(3), 411–437. DOI: 10.4067/S0716-078X2004000300003.

Dukes, J.S., Chiariello, N.R., Loarie, S.R. and Field, C.B. (2011) Strong response of an invasive plant species (Centaurea solstitialis L.) to global environmental changes. Ecological Applications 21(6), 1887–1894. DOI: 10.1890/11-0111.1.

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Part I. Dimensions of the Problem: Background

2 Communicating the Dynamic Complexities of Climate, Ecology and Invasive Species

Lewis H. Ziska*

Environmental Health Sciences, Mailman School of Public Health, Columbia University, New York, USA

*[e-mail]

© CAB International 2023. Invasive Species and Global Climate Change, 2nd Edition (ed. Lewis H. Ziska)

DOI: 10.1079/9781800621459.0001

Abstract

Humans derive their resources from the natural world. As the human population continues to expand – with projections of up to 10 billion by mid-century – there is overwhelming evidence that global biological systems will deteriorate, threatening the complex biological interactions necessary for life support. Foremost among these disruptions is climatic change – the build-up of heat-absorbing gases from the burning of fossil fuels. The increase in surface temperature is altering climatic processes from precipitation to temperature to extreme weather events. In addition, these changes may be facilitating the spread, establishment and success of non-indigenous species. Such facilitation can overwhelm biological processes that range from biodiversity to biological constraints on agricultural productivity. This synergy represents a fundamental and ongoing threat to natural (forests, streams) and managed (agriculture) systems. But how to communicate such a threat? How can scientists and other specialists describe it in terms that non-specialists can understand? In particular, how can we provide a means to convince policy makers that it is crucial to understand the synergy between climate change and invasives, and to do something about it? Overall, science and societal awareness will be necessary to provide the global solutions essential to address the dynamic challenges of a changing climate, invasive species and resource needs.

On a global basis, the two great destroyers of biodiversity are, first, habitat destruction and, second, invasion by exotic species.

E.O. Wilson

Background

The needs of humanity are inexorably linked to the resources of the Earth. These resources, in turn, are provided by biological and physical systems that include food, feed, fiber and fuel. Any disruption of these systems will have strong ramifications for human existence.

Among the systems that are indispensable for humans are plants. Plants are the only organisms that are autotrophs – capable of producing their own energy from physical inputs such as light, water, nutrients and carbon dioxide (CO2). Consequently, they are fundamental not only to humans, but also all living systems – they are necessary for animal existence and ecosystem function. Plants are the core of all living systems, and it is our relationships and interactions with them that define human populations and civilization. Simply put, without agriculture, without plants, 8 billion people, the current global population, could not exist.

How then could a synergy between climate change and invasive species impact these systems? What are the likely consequences? Before we can answer these questions, we need to make sure we understand the context. It is necessary then to take a deeper dive into what is meant by climate change and invasive species, and then to undertake an overview of their potential interactions.

Invasive Species

A basic characteristic of humans is that we trade. And a large percentage of what we trade is living organisms, from fish to trees. But what began as local, regionalized trading has grown with the global population and the needs of that population – a population of 1.6 billion at the beginning of the 20th century is now ~8 billion at the beginning of the 21st. And we haven’t stopped trading. Biological trade is not inherently bad, but it represents a historically unprecedented global movement of DNA across continents, across countries, regions, towns, cities and ecosystems. In short, increases in international trade, travel, transport and tourism have created new and unique pathways for the intentional and unintentional introduction of invasive organisms. Such activity is only expected to intensify as the global population increases.

Not surprisingly, the widespread introduction and distribution of economically desired plants and animals, so necessary to meet the needs of 8 billion, has also resulted in the introduction of some species that do great harm to the environment, the economy and human health. Such species are referred to as invasive, or at times, ‘exotic’, ‘alien’ or ‘non-indigenous’. Officially, the United States defines ‘invasive species’ in Executive Order 13112 as ‘An alien species whose introduction does or is likely to cause economic or environmental harm or harm to human health’. The term invasive species is further clarified and defined as ‘a species that is non-native to the ecosystem under consideration and whose introduction causes or is likely to cause economic or environmental harm or harm to human health’ (Beck et al., 2008).

A strict definition of ‘invasive’ can be elusive. For example, if we were to focus on North America, we would find that common lambs quarters (Chenopodium album L.) is from Eurasia, but is considered a native weed; kudzu (Pueraria montana (Lour.) Merr. var. lobata (Willd.) Maesen & S.M. Almeida ex Sanjappa & Predeep) is from East Asia and is generally deemed invasive; whereas native weeds such as ragweed (Ambrosia spp.) are common, but if found beyond their geographic ranges, could be considered invasive. For the United States, a reported 50,000 invasive species have been introduced (accidentally or intentionally) in the past century (Pimentel, 2011).

What is the extent of the damage imposed by invasives? With 50,000 to choose from we don’t have to go far. Let’s take two plant species, kudzu (Pueraria lobata) and cheatgrass (Bromus tectorum), introduced into North America.

Kudzu

If you are from the South, you are already familiar with kudzu – an invasive vine that is one of the fastest-growing plants on Earth. There are various related epithets to describe kudzu: ‘a vegetative form of cancer’, the ‘plant that ate the South’, to give two of them. Some people think of it as that monster plant in old black-and-white science fiction movies of the 1950s whose tendrils snake in through your open window and abduct you.

How fast does kudzu grow? About 0.3 m (~1 ft.) per day. It is able to rapidly expand root production, and as a legume it fixes its own nitrogen. Estimates vary, but at present it is thought to cover about 2.9 million hectares (ha) (about 11,000 square miles) in the United States, and may colonize up to 50,000 ha (200 square miles) each year (Forseth and Innis, 2004). As with any invasive, it can dominate the landscape and reduce plant biodiversity to one (itself).

There’s more. Botanists recognize that when insects land and start attacking a plant, plants can release a chemical defense – volatile organic compounds or VOCs. These compounds are detected by other plants, which ramp up their own production of VOCs to stave off insect attacks. Kudzu produces two key VOCs (Wiberley et al., 2005): isoprene, which is released from kudzu leaves, and nitric oxide, which is released from the roots. Other plants produce VOCs, but kudzu can produce more, and once it achieves dominant status in nature (Fig. 2.1), a whole lot more.

A vine bush with green leaves.

Fig. 2.1. Kudzu, an invasive vine found throughout the southeastern United States. (United States Department of Agriculture–Agricultural Research Service (USDA-ARS); photo by Leslie J. Mehrhoff, University of Connecticut, Bugwood.org.)

Why does this matter? Well, if sunlight is present, isoprene and nitric oxide mix together to make ozone (nitric oxide is also a powerful greenhouse gas). Ozone is a ground-level pollutant, with subsequent health effects on humans, including damage to respiratory tissue. And for metropolitan regions around the southeastern United States, such as Atlanta, Georgia, you don’t have to go far to find kudzu.

Cheatgrass

How else can invasives alter the landscape? Well, one of the most egregious is cheatgrass (Bromus tectorum). Native to Eurasia and the eastern Mediterranean, it appears to have been introduced into the United States multiple times, probably as a contaminant in wheat seed, and was found – initially – in scattered patches throughout the west. It is a winter annual grass and can grow and set seed quickly following seasonal rains. From its humble beginnings cheatgrass has grown – and grown. Today it occupies about 41 million ha (158,000 square miles), or, in relative terms, an area roughly the size of California has become a monoculture of cheatgrass (https://www.eddmaps.org/distribution/usstate.cfm?sub=5214, accessed 2 July 2022) (Fig. 2.2).

A plantation engulfed in flames.

Fig. 2.2. Chaparral in Colorado prior to and following cheatgrass invasion. (United States Geological Survey, public domain.)

Cheatgrass likes fire – it is the plant version of an arsonist. It grows quickly, sets seed and dies, and all that dead plant material is – as you can imagine – fuel for the fire. In fact, the US Fire Service has a nickname for cheatgrass: ‘Grassoline’. Once cheatgrass comes to your neighborhood, the frequency and intensity of fires spike. And there are consequences. You see, the shrub-dominated landscape of the western United States (e.g. sagebrush, pinon, juniper) can take a brush fire and be fine, as long as there is only a fire every 40–50 years. But when cheatgrass enters the neighborhood, fire can occur every couple of years.

The result? Native shrubs disappear. Cheatgrass dominates the landscape from scrub to shining shrub – a quintessential example of how invasive species introduction can significantly alter plant ecosystems over large geographical areas. Fire costs go up, rangeland values go down. It has been estimated that cheatgrass infests up to 46 million acres (18.6 million ha) of winter wheat costing growers about US$300 million annually in lost crop yield, and another US$50–100 million annually in fire damage (Young and Evans, 1978).

These US examples are only meant to be illustrative – they do not do justice to invasive harm on a global scale. We rely on the natural world for human resources, and there is no question that these resources are becoming increasing limited by invasive species. How can you grow crops or livestock in South Africa once parthenium weed (Parthenium hysterophorus) is introduced? The weed is toxic to domestic animals and, if eaten, results in tainted meat (Adkins et al., 2018). It generates allelopathic effects in soils and outcompetes agronomic crops for available nutrients and moisture. Parthenium hysterophorus can cover crops with its pollen, which prevents seed set with productivity losses of up to 40% (Khosla and Sobti, 1979). How will the Middle East maintain wheat production if UG99, a new invasive wheat rust pathogen, becomes established in the region? In Australia the introduction of European rabbits threatens over 300 native species. How will this impact Australia’s economy? What about other countries and other invasives? European starlings? Cane toads? Boa constrictors?

Perhaps a single invasive species is incapable of harm on a global scale, but their collective impact, given a global population near 8 billion, may have reached a threshold whereby the current and future resource needs of the global human community, from shelter to food, as well as the increased need for ecosystem functionality, are at extreme risk.

Climate Change

Carbon sources represent energy captured by plants from sunlight over millennia. Geological forces pushed, squeezed and converted this carbon into large pools of fossil fuels that vary in consistency (coal, oil, gas) and location (Saudi Arabia to Canada). And these fuels can perform an amazing amount of work. A gallon of gas is roughly equivalent to 500 man-hours; a barrel of oil, over 23,000 man-hours (FatCatWatch, 2011). You can see the appeal of burning fossil fuels as an energy source.

But burning, or combusting, requires oxygen; oxygen and carbon produces carbon products, most notably carbon dioxide (CO2); and CO2 has a couple of unique properties. The first is that it absorbs energy in the heat (or infrared) portion of the spectrum. By absorbing heat, it acts like glass in a glasshouse, or closed car windows on a hot day – light can get in, but heat has a hard time getting out. This makes CO2 a ‘greenhouse’ gas. Other greenhouse gases rising in levels owing to human activity include methane (CH4) and nitrous oxide (N2O).

Any climatologist will tell you that these greenhouse gases are essential to maintaining biological existence. Sunlight alone would result in an average surface temperature of −18°C (0°F); sunlight and a blanket of greenhouse gases provides an average surface temperature of 15°C (59°F). It is the natural greenhouse effect produced by these gases that allows life to exist as we know it. But, as with most things, balance is important. The surface temperature of Venus, with an atmosphere of 99% CO2 is hot enough to melt lead. Mars, which has a thin atmosphere and less CO2, is much too cold. Earth, fortunately for us, is ‘just right’. For now, anyway.

In the 19th century two scientists, Joseph Fourier and Svante Arrhenius, suggested that CO2 was being generated in large quantities as a result of the Industrial Revolution and could result in increased surface temperatures (Fourier, 1827). By the late 1930s, Guy Callendar, the English engineer and inventor, was arguing that the level of CO2 in the atmosphere was increasing and raising global temperatures (Callendar, 1938). Finally, quantitative measurements in the 1950s through the 1970s confirmed that CO2 was, in fact, increasing globally (Revelle and Suess, 1957; Keeling et al., 1976). To sum up: the fact that adding more CO2 to the atmosphere traps heat – the greenhouse effect – had been discovered by 1824, shown in a lab by 1859 and quantified by the 1950s. Since 1960, background atmospheric CO2 has risen 32%, from 315 to 418 parts per million (ppm). At present, there is a ~98% consensus among climatologists and agreement among the national academies of 32 countries as to a clear human influence on global climate (Oreskes, 2004; Anderegg et al., 2010). Such a concurrence among experts should provide, at least from a scientific viewpoint, the functional equivalent of ‘the sun rises in the east’.

What are the consequences? Theoretical predictions and actual consequences are now starting to emerge; that is, more frequent and intense droughts, storms, heatwaves, rising sea levels, melting glaciers and warming oceans. These consequences relate to changes in the physical environment, but, of course, they will portend biological changes, ones that can destroy or affect animals and plants, as well as human livelihoods and communities. Such recognition and outcomes are often described in daily headlines.

The second unique property of CO2 is that it is the source of carbon for all plants. As plants evolved at a time when the world was warmer and wetter, the current level of CO2, about 415 ppm, is not sufficient to maximize photosynthesis or growth for about 95% of all plant species. An increase in CO2, as with an increase in nutrients, light and water, will therefore stimulate plant growth.

This stimulation has been shown in many studies (Kimball et al., 2002; Kimball, 2016), and it is tempting to view it as positive relative to plant biology per se (often expressed in the meme ‘CO2 is plant food’). Yet it is important to remember that when you change a resource plants need, like CO2 (or light, water or nutrients), not all plants respond in the same way. That is, when we change a resource, we change the growth differently within a community. For example, agriculture represents a plant community that consists of a crop (desired plant species) and weeds (undesired plant species). On average, any agricultural field has one crop, but 8–10 weed species (Bridges, 1992). At present there are a number of studies indicating that more CO2 will increase the growth of weeds over crops (e.g. Ziska et al., 2018; Manisankar and Ramesh, 2019) with negative consequences for crop yields (Ziska, 2011). Such consequences may also relate to invasive plants that show a larger than expected increase in response to recent and future CO2 increases (Ziska, 2003).

Synergy

Globally, climate change and invasive species are widely acknowledged as significant threats to our natural resource base, and the challenges of adapting to each are recognized. Less appreciated is that the twin threats to resource sustainability are not isolated from each other. That is, climate change and rising CO2 levels will significantly alter the biology of invasive species, with subsequent changes in both resource availability and invasive species management.

Writing more than two decades ago, Mooney and Hobbs (2000) recognized the importance of these two anthropogenic factors and noted that human-induced rates of change for both climate and biomes are unprecedented in geological history. Each aspect of change is, by itself, capable of significant disruption in global ecosystem functioning. But we now recognize – with great clarity – that it is their combined effects that are of key concern in the context of availability, development and allocation of the natural resources necessary for the growth and sustainability of human systems (Mooney and Hobbs, 2000).

The synergistic linkage between climate change and invasive species biology is likely to be a new, fundamental threat to system integrity and sustainability; from forests to wheat fields. Indigenous species that have evolved together to form elaborate, complex patterns of strong and weak interactions that are temporally stable, now compete not only with rapidly changing climatic conditions, but also with recently introduced species, some of which are more competitive, in part, because of those conditions. Should those conditions result in an overwhelming competitive advantage for one species, complex adaptive, biological systems are likely to be reduced to simple systems where one or two species dominate (e.g. kudzu). Loss of complexity, in turn, is likely to result in loss of diversity and ecosystem resiliency to abrupt climatic change (Reich et al., 2001).

The extent and consequences of such synergy are difficult to precisely characterize. Climate change is not happening uniformly and neither is global trade and the associated threat of exotic species becoming dominant outside their native region. But are there common factors associated with abrupt changes in climate and the potential success of biological invaders for a given system? Specific outcomes will be difficult to predict since climate change and invasive species are likely to interact both spatially and temporally to alter functional ecosystem integrity. That is, any characterizations of likely impacts would a have spatial reference (thousands of square kilometers vs. small environmental niches), the degree of species diversity (i.e. rare species vs. globally distributed), the rate of climatic change (i.e. gradual vs. rapid, extreme changes in temperature and precipitation) and, finally, human activity (i.e. extent of global trade, adoption and implementation of pest management). That all these actions can occur singly and in combination illustrates the dynamic complexity and uncertainty associated with predicting climate and invasive impacts on system function. Nevertheless, there are some general, empirical effects that are likely to occur.

Spatially, there is a range of likely biological responses associated with specific native species, including physiological, phenological and distributional changes (Minteer and Collins, 2010). Climatic change would not only affect the composition of native species within ecosystems, but could also affect key aspects of invasive species biology, including introduction, establishment, demography and distribution (Bardsley and Edwards-Jones, 2007). For example, plant pathologists recognize that climate change, specifically warmer temperatures, can lower humidity and induce turbulent winds, which can significantly increase the ability of Puccinia graminis f. sp. tritici, a form of wheat stem rust that is highly invasive, to spread its spores over greater distances (Prank et al., 2019).

Temporal aspects of climatic change are also likely to alter invasive establishment. The ability of an invasive species to disperse successfully is dependent on transfer to an environment similar to its origin. For example, over time, as the extent of the arctic ice cap is reduced, and trade is expanded via the Northwest Passage, it is likely that invasive species in ballast water that had previously found this environment too extreme could become established (Nong et al., 2019).

Although synergism related to the impact of invasives is expected with climate change, it is not certain. Vulnerability of invasive introductions with climate change can vary. Some introduced invasive species in certain ecosystems such as the Argentine ant, Linepithema humile Mayr, may be negatively impacted by changes in climatic patterns (Cooling et al., 2012), while others, such as buffelgrass, Pennisetum ciliare L., may continue to increase their range replacing current ecosystem communities (Franklin et al., 2006). The sensitivity of species is not limited to temperature and/or precipitation, but is also affected by changes in atmospheric CO2 such as those now altering the grasslands (savannahs) of Africa, which, within a century, may be transformed into forests (Higgins and Scheiter, 2012). Overall, while synergism cannot always be assumed, there are, and continue to be, ongoing global examples where climatic change already appears to be amplifying invasive species biology, specifically their establishment, and the spread and extent of ecosystem damage.

Europe and mycotoxins

An extensive survey by the European Food Safety Authority (EFSA, 2007) confirmed that as southern Europe has become more subtropical, there is a growing issue of potential mycotoxin contamination of corn, almonds and pistachios – specifically aflatoxin in maize. However, deoxynivalenol contamination in wheat is now viewed as an emerging problem in northern Europe (Battilani et al., 2016; Moretti et al., 2019). Overall, mycotoxins are globally recognized as a significant threat to human and animal health; aflatoxins in particular can be life-threatening, and are recognized as a carcinogen (Sarma et al., 2017).

California grasslands

A study by Sandel and Dangremond (2012) used a trait-climate relationship between invasive and native grasses to demonstrate that warmer areas contained a higher proportion of invasive species. The observable pattern was consistent with a simple model that was able to predict invasion severity. The study provided an in situ evaluation regarding rising temperatures and species invasions over a broad geographic and taxonomic scale.

Invasive insects in China

In recent decades, evidence of greater invasive species establishment has been observed for China (Lin et al., 2007). Although increases in global trade are thought to be associated with increased dispersion of such species, Chinese and USDA researchers have also examined the role of warming temperatures from 1900 through 2005 on invasive alien insect establishment (Huang et al., 2011). Their findings indicated that for every 1°C increase in average annual surface temperature for mainland China, there was an increase in the establishment rate of invasive alien insects of approximately 0.5 species per year. This relationship was still significant even after accounting for increases in global trade from 1950 to 2005 (Huang et al., 2011).

Canada and pine bark beetle

Tree-killing bark beetles are among the most destructive forest pests and their impacts have increased in recent decades. Although the mountain pine bark beetle (MPB; Dendrotonus ponderosae) is native to North America, human-induced changes in climate may also result in dispersion of the beetle into new geographical regions where it would be considered invasive. In recent decades a widespread increase in tree mortality has been observed in western North America (Bentz et al., 2010; Hlásny et al., 2021). The expansion of the MPB is directly related to climate change: infestations expand rapidly under prolonged dry spells and with warmer temperatures, with subsequent consequences for tree mortality (Mitton and Ferrenberg, 2012; Sambaraju et al., 2019). These consequences are not trivial. Over 13 million ha of trees have been killed in British Columbia alone (Kurz et al., 2008). If this dieback continues, conifer forests may change from regional carbon sinks to carbon sources, providing positive feedback for anthropogenic climate change (Anderegg et al., 2020).

If you are curious about the two examples of invasive plants used previously, kudzu appears to be migrating northwards with warmer winters, and additional CO2 can increase the flammability of cheatgrass (Ziska et al., 2005, 2011)

Communicating Science

Overall, the scope and impact of these current changes stress the need for a ‘deeper dive’, a thorough scientific evaluation of probable links, potential outcomes, and economic and environmental consequences to assess global risks. There is an equally critical need to begin assessments of how land managers, consumers, policy makers, etc. can mitigate (long-term) and adapt (short-term) to the twin challenges of climate change and invasive species.

At present there is a recognition among the general public that climate change and invasive species are disruptive, and consequential. By themselves. What is lacking is a recognition and acknowledgment that the combination is likely to offer the worst of both worlds. The examples of synergy provided here, while incomplete, should still be of sufficient magnitude to stimulate additional research, address key unknowns, provide a better assessment of the combined risk and, perhaps most importantly, arrange and prepare appropriate, cogent responses to adapt or reduce the vulnerability of human and natural systems to these changes.

Yet, overall, we seem inactive, static. Indeed, there are indications that we have regressed in recent years (Simberloff et al., 2020). Policy makers, business leaders, even environmental groups in the United States and elsewhere, are doing little to address potential synergy. For example, at present, at the federal level in the United States, funding is available to study climate change and invasives, but there is no funding directed toward studying their interaction. While scientists, by large margins, recognize the severity of interactive changes, they cannot motivate populations to take action unless they achieve the support of global leaders in industry, policy, etc.

So how can we, as scientists, translate and communicate the severity of the threat in a way that achieves understanding and support from the lay public? Science can offer evidence and assessments as to the seriousness of the potential interactions, but as is clear in the case of climate change per se, policy will lag behind science.

Part of the issue is uncertainty. If a threat is certain, and recognized as such, then action is forthcoming. Part of the uncertainty in communicating is language and framework. Abstract and theoretical constructs resonate little outside of scientific audiences. Admitted uncertainty is quintessential to science: we use statistics for a reason. Every published scientific paper contains data analysis and interpretation, providing ‘confidence’ regarding the probability of an outcome. But we cannot assign numbers to all climate/invasive interactions. The persistent, chronic disturbance of the land, the reshaping of landscapes and the externalization of economic development into the soil, air and water, are altering the complex adaptive systems that support life as we know it. For example, some models of future plant distributions show that a temperature rise of 2–3°C over the next 100 years could result in half the world’s plant species being threatened with extinction (Bramwell, 2007). Such a rate of extinction will be unfamiliar to human experience and the effect on higher animal species will be beyond calculation. It will be difficult to quantify, to be mathematically precise, when such events – in their scale – fall far outside a typical scientific, reductionist approach. It is analogous to not knowing the exact value of Pi, but knowing it well enough to calculate anything we might need to know. In regard to climate change or invasion biology, we need much more rigorous and absolute answers before any action is supported. Unlike mathematical solutions, rapid changes in plant communities resulting from climatic patterns and invasive species will produce outcomes that are not entirely predictable, and are likely to reinforce uncertainty at the policy maker level. Fostering such uncertainty has been the modus operandi of groups who oppose climate change, using the tobacco industry prototype of fostering doubt to push back against the scientific consensus that smoking is bad for your health (Oreskes, 2020).

Fortunately, the ability to effectively communicate large, complex questions has become a focus of social scientists who are interested in understanding what does and doesn’t work for science communication. As described by Yale law professor Dan Kahan, who has extensively studied science communication, ‘Never have human societies known so much about mitigating the dangers they face, but agreed so little about what they collectively know’ (Kahan, 2015, p.1).

Providing evidence and the statistics that back it up will not work with a lay audience. It is not a question of providing facts; rather, it needs to be contextual. A drowning polar bear on ice, often a favorite meme for conveying climate change, works for some, but for many people it lacks resonance. Few people can relate to polar bears. Conveying climate science, not as science per se but as outcomes – ones that are familiar, common, that affect individuals through personal experience – is a means to relate and motivate the non-scientist.

For scientists, such transitions can be difficult. ‘Just the facts’ is a scientific maxim – to generate evidence that can withstand logical scrutiny – but each individual listening to the same set of facts is almost certain to come up with different interpretations. Those subjective understandings will, ultimately, determine a person’s interest and concerns regarding the intersection of climate change and invasive species.

Rather than lament any deviation from ‘Just the facts’, it is essential to communicate the evidence while recognizing these subjective interpretations, and that a ‘one size fits all’ presentation will fall short. The evidence must be detailed relative to the listener’s ethnic and/or cultural background, to their experience as policy makers or business leaders, or to their environmental interest as farmers or conservationists.

To accommodate societal interpretations, it may be essential to relate issues on climate and invasives in the form of a narrative – a story that can provide sufficient granularity so as to resonate for a given audience. For example, if talking to apple farmers in Maine regarding climate and invasives, the issue of warmer winter temperatures and the northward migration of spotted lanternfly, an invasive insect that can damage apple orchards, is sure to draw interest. If presenting before a group of medical doctors in Hungary, the issue of rising CO2 and warmer temperatures can be related directly to invasions of common ragweed, increased pollen and allergic rhinitis; or if talking to coffee growers in Guatemala, the role of climate in the spread of coffee rust, a fungus endemic to East Africa, but which has now invaded all of Central America, will capture their attention. Such narratives provide cultural and business context; they enable audiences to make sense of complex issues and to understand how the interaction of climate and invasives affects them directly. As such they can also be fundamental in deriving appropriate solutions.

Communication to government and policy makers has its own set of challenges. Such challenges reflect inherent differences in thought and approach between science and policy especially with regard to time and certainty (Table 2.1). Scientific research can be complex and ongoing, with sufficient time needed to ensure that evidence is accurate and value-free. Evidence can also be reductionist and specific in nature, making larger, more general relationships difficult to interpret. In contrast, a governmental interpretation is situational, temporal and not value-free. Such a misalignment means that communication on scientific issues and impact may lack relevance to policy decisions. In this context, using a narrative can still be effectual, if linked to constituent or business interests. For example, if a large business within a district reflects agricultural interests, then changes in climate and invasive species that threaten those interests will be relevant; in addition, responses to such threats may be more likely to achieve policy responses. Or if warmer temperatures result in an increase in toxic blooms of exotic algae, threatening water supplies for a given region, then officials are likely to take interest.

Table 2.1. Behavioral and attribution characteristics of science and government. Differences in problem perceptions can contribute to how environmental challenges are translated into policy decisions. (Adapted from Bradshaw and Borchers, 2000.)

Overall, with respect to the science of climate change and invasive interactions, it is important to emphasize that future interactions with decision makers are likely to be ongoing and dynamic, and not a one-time problem–solution response (Webster, 2003). There is an inherent complexity to any problem that is global; as such, scientists need to inform policy makers of our current understanding – and uncertainty – in order to provide them with information as to costs, benefits and options. And such communication should be continuous. Uncertainty defines climate change and invasive species interactions; consequently, decisions made today must be continually revised as our understanding of the science evolves and the extent of the consequences becomes apparent. For scientists, documenting and communicating these risks are critical as a means to provide scientific authentication for decision makers.

Next Steps

In viewing climate change, the physical consequences, from heatwaves to rising sea level, are often emphasized. But it is the biological consequences, especially with regard to invasive or exotic species, that pose a fundamental and existential threat to the sustainability of natural and human systems, from forests to agricultural fields.

Humankind, along with all the flora and fauna of the world, is living through a period of rapid ecological change. Global trade will continue to expand, and with it the introduction of invasive species, and the amount of environmental and economic damage (Leung et al., 2002; Hellmann et al., 2008). If the numbers of invasive species are added to the predictions regarding climate change thresholds, cascades and tipping points (e.g. Lenton et al., 2008), societies may have very little time to prepare for implementing, let alone understanding, the needed adaptations and adjustments to agricultural practices, technologies and ecosystem service expectations. Yet furthering our understanding, especially in regard to providing up-to-date movement of new species introductions, and their ecological or environmental damage relative to climate change in real time, is imperative – a fundamental next step; as is the need to communicate the risks and vulnerabilities associated with climate disruptions and invasive species to a lay public. Bridging that communication gap through narratives that are relatable to the background and experiences of a given audience is essential to highlight impacts – and potential solutions – to a non-scientific audience. Such narratives can be enhanced by those researchers, environmentalists, land managers, ecologists and concerned citizens who are already familiar with and working on adaptation and mitigation efforts related to climate change and invasive species.

Finally, in regard to law makers or governmental leaders, relating the science of climate change in the context of invasive species poses a unique set of challenges. Such challenges are not insurmountable: constituents and interests within districts at state or national levels are almost certain to experience climate change and invasive biological interactions; it is helpful for science to point out where and how these may be occurring, and to convey the science to interested officials in a straightforward and understandable way. Especially relevant is to stress the chaotic nature of these changes, and that decisions will be necessitated even if uncertainty continues, and that it is in their political interest to assist scientists and others in determining the degree of uncertainty and the relative risks and benefits of a given approach.

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