LITERATURE REVIEW

An Introduction to Invasive Species and Their Impacts in The United States

Erin Brennan

Western Kentucky University

BIOL 516 Research - Plant Ecology

4 February 2022

Introduction

Invasive species are a rapidly growing problem in the United States. Over the last 500 years, invasive species have spread across at least 3% of the ice-free surface of the Earth (Mooney & Cleland 2001). New biological invasions have increased by orders of magnitude in the last 200 years due to the expansion of human migration around the world and the increase in global commerce (Mack et al. 2000). Vast tracts of terrestrial, aquatic, and marine ecosystems have been overcome by invaders (Mooney & Cleland 2001). Invasive species are one of the most important ecological impacts of colonization, human transportation, and global commerce. In an increasingly globalized world, very few areas remain immune from biological invasions (Mack et al. 2000). The ecological impacts of these invasions are significant and far-reaching.

Biological invasions cause a wide range of issues, from biodiversity loss and extinction to changes in ecosystem functions and ecosystem services. The consequences of biological invasions vary immensely. In some cases, simply the presence of a non-native species in a conservation reserve could be regarded as deleterious (Mack et al. 2000). In other cases, invasions can fundamentally change ecosystems by altering the dominant species in a community, the physical features of an ecosystem, the cycling of nutrients, or the productivity of the plant community (Mack et al. 2000).

The spread of invasive species is not simply an environmental problem. Biological invasions also pose a significant threat to human health, disrupt food security, and have a significant economic impact. Economic impacts include enormous costs and losses in the areas of agriculture, forestry, fisheries, recreation, power production, and international trade (Lovell et al. 2006). In many cases, the impacts of these invasions are too significant to be ignored.

The widespread effects of human-caused biological invasions threaten efforts to conserve and protect biodiversity, support natural ecosystems, sustain productive agricultural systems and food security, and protect human health (Mack et al. 2000). The number of invasive species in a particular area directly correlates to the levels of trade, so increased globalization thereby increases the threat of new invasions (Paini et al. 2016).

The recognition of the impacts of invasive species has grown over the last couple of decades (Lockwood et al. 2013). This increased acknowledgment is due to several factors. The first of which is that the problem of biological invasion has grown significantly (Lockwood et al. 2013). The number of species that have been moved out of their native range and into a new location is growing, therefore, the problems caused by these invaders are growing in tandem (Lockwood et al. 2013).

In this paper, I will review the current literature regarding invasive species, our understanding of how biological invasions occur, and the terminology used to describe such invasions. In addition to an overview of invasive species, I will examine the impacts of biological invasions on four critical areas - ecological, human health, agricultural, and economic impacts.

Invasive Species

According to Executive Order 13112 signed by President Bill Clinton in 1999, which established the National Invasive Species Council (NISC), an invasive species is 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" (“Executive Order 13112 - Invasive Species” n.d.). Invasive species are those which survive long enough within an introduced range to establish themselves, reproduce, spread, and cause a detrimental impact to the environment (Mack et al. 2000). For this paper, I will utilize the definition of invasive species established by Executive Order 13112 and used by the United States National Invasive Species Council. Additional terminology related to invasive species will also be introduced in this section.

Biological Invasions

All biological invasions begin with an individual or set of individuals being taken from their native range, transported to a novel location, and released into the wild (Lockwood et al. 2013). It is important to note that not all introductions of non-native species (also called non-indigenous, immigrants, alien, exotic, or introduced) result in an invasion (Lovell et al. 2006, Mack et al. 2000). The Tens Rule, described by Williamson and Fitter, states that approximately 5-20% of species successfully transit the stages from introduction to invasion, with the average being 10% (Williamson & Fitter 1996). The taxing structure of the invasion process ensures that just a small fraction of the species introduced outside of their native range will ultimately become invasive species and cause ecological or economic harm (Lockwood et al. 2013). Biological invasions do occur when organisms are transported to new ranges that are compatible with their survival and proliferation, allowing their offspring to survive, spread, and form an established population (Mack et al. 2000).

When individuals of a species are transported from their native range to new locations, a multistage process begins, with biological invasions being the least likely outcome (Mack et al. 2000). This is due in part, to the fact that most organisms die en route to a new location (Mack et al. 2000). The vast majority of introduced species are unable to survive in their new location; if an organism does succeed in reaching a new site, it is likely to be quickly killed by any number of biotic or abiotic factors. (Mack et al. 2000, Lovell et al. 2006). Of those which do survive to reproduce, most do so without causing a decline in native species or dramatically changing the ecosystem (Lovell et al. 2006). In many cases, descendants of introduced species may survive for only a few generations before going extinct locally, but a small fraction will become naturalized (Mack et al. 2000). Upon becoming naturalized, the persistence of a species no longer depends on the arrival of individuals from the native range, although a greater frequency and number of new arrivals raises the probability of a species becoming permanently established (Mack et al. 2000). There are multiple stages that a non-native species must pass through before being considered an invasive species (Lockwood et al. 2013).

The Invasion Process

The process of invasion occurs along a continuum, in which a species must overcome a sequence of barriers to become naturalized or invasive (Pyšek & Richardson 2010). Few non-native species survive introduction and colonization, only a small fraction become naturalized, and even fewer become invasive (Mack et al. 2000). However, all non-native species start this process as individuals are transported from their native range into a new area where they are released into the wild (Lockwood et al. 2013).

Introduction means that an organism has been transported by humans across a major geographic barrier, such as over a mountain range or across an ocean (Richardson et al. 2000). An introduction occurs when a species is moved from a region where it is native to a new range by human action, either deliberately or unintentionally (Pyšek & Richardson 2010).

The first phase, colonization, starts when the non-native individuals establish a self-sustainable population within the new range (Lockwood et al. 2013). The introduced individuals form a colony by reproducing and increasing in number (Richardson et al. 2000). Only a small fraction of introduced species can successfully establish a colony in a new region (Pyšek & Richardson 2010).

The second phase, naturalization, starts when abiotic and biotic barriers to survival and regular reproduction are overcome, allowing a population to grow and expand (Richardson et al. 2000). An established population may then expand its geographic range (Lockwood et al. 2013). Through naturalization, a species establishes new self-sustaining populations, becomes widely dispersed, and incorporates into native populations (Richardson et al. 2000). Of the species that persist through the process of naturalization, only very few will continue to the invasion phase (Mack et al. 2000).

The third phase, invasion, requires that introduced species reproduce in great abundance in areas distant from the original site of introduction (Richardson et al. 2000). It is only when the non-native population becomes abundant enough that it causes ecological or economic harm that it earns the name “invasive” and passes through the third stage (Lockwood et al. 2013).

The vast majority of introduced species do not survive this entire process. The majority of species that can survive some stages are relatively benign and cause no ecological harm. Therefore, most introduced non-native species do not meet the standards to be identified as “invasive” defined in Executive Order 13112 (“Executive Order 13112 - Invasive Species” n.d.). However, the species that successfully transition through each of the stages of invasion have the ability to cause harm to the ecosystem, the economy, or human health (Lovell et al. 2006). Examining these stages of the invasion process allows humans to recognize our role as either facilitators or inhibitors of invasions (Lockwood et al. 2013).

The Role of Humans

Humans play a major role in the introduction and spread of invasive species. Humans often begin the invasion process by either purposefully or inadvertently transporting individuals outside of their natural range and then releasing them into the wild (Lockwood et al. 2013). Since humans started exploring the globe, the biogeographic barriers that had previously isolated life for millions of years have been breached in great numbers (Mooney & Cleland 2001). As long as humans have been able to move across the oceans and from continent to continent, they have broken through these geographic boundaries and facilitated the movement of species from place to place (Lockwood et al. 2013). As human settlers moved into novel territories, they have also brought with them domesticated plants and animals (Crosby 1986).

Biological invasions are not driven exclusively by humans, however, the global scale, scope, and frequency of the introductions of invasive species have grown in correlation with the expansion of human transport and commerce (Mack et al. 2000). There are now very few habitats left on earth that are free from invasive species introduced by humans, and even fewer that are so unique or remote that they are immune from biological invasions (Mack et al. 2000).

Humans are responsible for both accidental and deliberate biological invasions. The dramatic rise in plant, animal, and microbial invasions globally correlates to the rise in human transport and commerce (Mack et al. 2000). Global commerce has grown tremendously over the last 500 years and this growth has provided an opportunity for biological invasions to take place (Mack et al. 2000).

Invasions most often happen accidentally, however, many of the invasive species we fight today were purposely spread by humans as they moved from place to place. The proportion of organisms that have become invasive as the result of accidental vs. deliberate action varies considerably along taxonomic divisions (Mack et al. 2000). Very few invasive microorganisms have been deliberately introduced. Deliberate microorganism introductions are limited to yeasts used in fermentation and mutualists introduced along with other organisms, such as mycorrhizal fungi introduced with plants (Mack et al. 2000). The majority of invasive insects have been accidentally introduced. Notable exceptions include the deliberate introduction of bee species, such as honey bees being introduced in the Americas and bumblebees being (disastrously) introduced in New Zealand (Mack et al. 2000). Introductions of marine invertebrates are similar to that of insects. A few oyster species have been introduced deliberately, such as the Pacific Oyster that was imported from Japan to Washington, however, most arrived accidentally, such as the zebra mussel arrival in a ship ballast (Mack et al. 2000).

While invasions most often happen accidentally, many of the invasive species we fight today were purposely spread by humans as they moved from place to place. Most invasive vertebrates, including mammals, birds, and fish, have been deliberately introduced, usually for the purposes of creating food stock (Mack et al. 2000). Some of the invasive plants we fight today were accidentally introduced via seed crop or cargo, however, most invasive plants in the United States have been deliberately introduced (Mack et al. 2000). Some of these deliberate introductions have developed into the worst weedy plant invaders that threaten agricultural crop yields. Still more species are intentionally released into the wild, often starting as pets and outgrowing their aquariums or cages, and being released into a novel habitat with no natural predators and little competition.

Humans play a role in biological invasions by facilitating the expansion of the geographic range of non-native populations via travel or global commerce, but that is not the extent of human influence in biological invasions (Lockwood et al. 2013, Mack et al. 2000). Humans also impact invasions through our myriad environmental effects and impacts on the ecosystem in which an invader is attempting to establish itself (Lockwood et al. 2013). Ecosystems under stress from human activity are more susceptible to biological invasions.

While humans play an obvious role in the facilitation of biological invasions, humans also play a major role in the eradication of invasive species. These eradications can happen through direct and indirect actions. Human actions can strengthen the barriers between the invasion stages through the use of direct eradication techniques, such as the physical removal of invasive species from an environment (Lockwood et al. 2013). Indirect actions, such as public policy, legislation, and public education can slow or stop the transportation of non-native species, thereby halting new invasions (Lockwood et al. 2013).

Factors Contributing to the Spread of Invasive Species

Non-native populations face a variety of forces that determine whether they will persist into the future including competition, predation, parasitism, and other interactions between individuals and species (Lockwood et al. 2013). They also include physical forces such as fire, freezing, and flooding (Lockwood et al. 2013). The response of non-native populations to these forces depends greatly upon their life history and adaptations (Lockwood et al. 2013). The properties of the environment, community, and species often also play a role in the transition of non-native populations across all of the stages of invasion (Lockwood et al. 2013).

To pass through the stages of invasion, an introduced species invading a new region must either be able to tolerate a myriad of physical and environmental stressors, or it must undergo genetic differentiation to reach the necessary level of fitness to survive. Evidence suggests that some invaders are “born”–they are born with invasive traits and become so once released from the constraints of their native environment–and some are “made”–they evolve invasiveness after colonization of a new environment (Pyšek & Richardson 2010). The balance of ecological forces and evolutionary pressures are unique to each invasion (Pyšek & Richardson 2010).

Genetic Factors

Some species are predisposed to becoming invasive when introduced to new areas. Individuals of a species share a set of common traits that may allow non-native populations of this species to become invasive in a favorable environment (Lockwood et al. 2013). However, traits that contribute to the success of invasive species are not universal to taxa and are often related to external factors such as the geographic conditions and features and pressures of the community being invaded (Pyšek & Richardson 2010). Attempts have been made to create a list of common traits shared by successful invaders to identify and predict potential invaders from major taxonomic groups.

Many of the worst invasive species come from relatively few families and genera. Most of the world’s invasive plants come from the Asteraceae, Poaceae, Acacia, Mimosa, and Cyperus families (Mack et al. 2000). Rejmanek and Richardson (1996) have been able to successfully predict which pines introduced to South Africa are most invasive, based on a list of morphological and ecological characteristics observed in populations that have become invasive elsewhere (Mack et al. 2000). There are examples in the animal kingdom as well. Starlings and Crows have several naturalized and invasive species that share common traits (Mack et al. 2000). However, most biological invaders have very few invasive relatives (Mack et al. 2000). This does not necessarily reflect a lack of traits or attributes suitable for invasion, but rather a lack of opportunity for introduction in new environments (Mack et al. 2000, Simberloff 2010).

Two variables seem to be consistent in invasive species - short juvenile period and short interval between reproduction (Rejmánek & Richardson 1996). These factors contribute to early and consistent reproduction, which leads to the rapid growth of a population (Rejmánek & Richardson 1996). A third predictor of invasiveness found in plants is small seed mass. Small seed mass is beneficial because it generally also leads to a larger number of seeds produced, better seed dispersal, high germinability, shorter dormancy period, and a higher rate of growth of seedlings (Rejmánek & Richardson 1996).

Hybridization is an important mechanism of evolution in invasive species. Many widespread invasive plants owe their success to hybridizing with closely related native species (Pyšek & Richardson 2010). While hybridization with invasive species can be a threat to the genetic integrity of native species, it can also create new variations and even cause the development of new species (Mooney & Cleland 2001). In addition to the evolution of traits to adapt to new environments and to invaders, behavioral shifts have been observed in invasive species and in native species in response to invaders (Mooney & Cleland 2001).

While some species do appear to have common traits that make them more suitable to survive the stages of invasion, a universal list of traits common to all invaders across taxa does not exist (Mack et al. 2000). Whether an introduced species succeeds in becoming invasive depends not only on how well their biological traits allow them to thrive in their new environment but whether they are able to reproduce and spread, compete with native species in the community, depends upon how well the environment and climate match their own and depends upon the susceptibility of the new environment to invasion (Pyšek & Richardson 2010).

Environmental Factors

When a species is introduced to a new region, interactions may occur that accelerate the invasion process and amplify the effects of invaders on native communities (Pyšek & Richardson 2010). However, invasion of a mature and undisturbed community is quite difficult and requires that introduced species overcome significant barriers by passing through a series of stages to reach invasion (Richardson et al. 2000). One of the major environmental factors that determine whether an introduced species will reach the invasion stage is the similarity of their introduced habitat to that of their native range.

For herbaceous plants, being introduced to a new area in their native latitudinal range is the best predictor of invasiveness (Rejmánek & Richardson 1996). If the latitudinal range is the same or similar, the climate will likely be similar, and the traits that allowed the plants to spread across their native range will allow them to invade new areas (Rejmánek & Richardson 1996).

The ability of an introduced species to overcome barriers to invasiveness in their new environment can be positively or negatively impacted by native and non-native species already living in the area (Pyšek & Richardson 2010). Interactions between and among species can overcome even inherent traits. Some communities are more easily invaded than others. Invasibility depends upon the disturbance level of the environment, the availability of resources, the presence of herbivores and predators, and the level of pathogens and disease; all of these factors can serve as barriers to the establishment of a new species and prevent invasion (Pyšek & Richardson 2010).

Some additional factors can impact the invasibility of an area and the ability of introduced species to become invasive. The first is the escape from biotic constraints. When an introduced species arrives in a new area, the biotic constraints which kept it in check in its native range are no longer present (Richardson et al. 2000). Environmental pressures such as predators, competitors, parasites, and disease are suddenly lifted allowing the introduced species to reproduce without the normal constraints (Pyšek & Richardson 2010). Community species richness is another constraint that plays a major role in invasivity (Richardson et al. 2000). The resistance of a community to invasiveness increases proportionally to the number of species in the community. The level of disturbance of a community also plays a role in its invasibility (Richardson et al. 2000). Undisturbed habitats are much harder to invade than those already under pressure from disturbances (Pyšek & Richardson 2010). Humans, along with the domesticated plants and animals we keep can cause significant environmental disturbance (Pyšek & Richardson 2010). A factor that leaves communities particularly susceptible to invasion is vacant (or underutilized) niches (Richardson et al. 2000). This is most often seen in islands that have gaps in their communities which leave them susceptible to invasion (Richardson et al. 2000). Finally, culture, history, and social and economic activity can be factors in invasibility (Pyšek & Richardson 2010). Trade and tourism increase the chances that new species are introduced to an area (Pyšek & Richardson 2010).

The critical factor in determining the invasibility of an area is the survival rate of non-native species introduced to the area. The higher the pressures a community is facing from invasions by non-native species, the less resistant it will become to subsequent invasions (Pyšek & Richardson 2010). Even resistant communities can become invaded if the pressures they face are high enough (Pyšek & Richardson 2010).

Impacts of Invasive Species

Invasive species are one of the major causes of global change accelerated by humans. Invasive species threaten biodiversity, change ecosystem structure and function, diminish ecosystem services, carry substantial economic costs, and cause serious human health issues (Mazza et al. 2014). The impacts of invasive species are classified as economic, environmental, or social (Charles & Dukes 2007). Economic impacts typically lead to increased costs and monetary losses (Charles & Dukes 2007). Environmental impacts affect ecosystem structure and function and lead to the loss of biodiversity and unique habitats (Charles & Dukes 2007). Social impacts include human health and safety but can also include recreation, cultural heritage, quality of life, and other elements of social structure (Charles & Dukes 2007).

Due to the concept of ecosystem services, some impacts will fall into all three categories (Charles & Dukes 2007, Mazza et al. 2014). All three areas of impact are useful in determining the effects of invasive species on ecosystem services (Charles & Dukes 2007). Species extinctions and biodiversity loss due to invasive species threaten the quality and delivery of many ecosystem services. The impacts of invasive species on ecosystem services in agriculture, industry, and human health are substantial and carry well-quantified costs (Charles & Dukes 2007). These impacts affect our ability to obtain food, freshwater, and fiber, and diminish ecosystem services we depend upon, such as water purification, natural pest control, disease regulation, pollination, soil fertility, and nutrient and water cycling (Charles & Dukes 2007). Invasive species also impact cultural services including aesthetic values, recreation, and tourism, however, these may be more challenging to quantify (Charles & Dukes 2007).

Ecosystem Impacts of Invasive Species

Invasive species can impact the interactions which occur among the species living within an ecosystem as well as the physical environment and processes of the ecosystem itself. Invasive species are a significant factor influencing global change, and contribute not only to biodiversity loss but also cause ecosystem degradation and loss of ecosystem services (Pyšek & Richardson 2010).

Many of the limiting factors that invasive species face in their native range, such as competitors, pests, diseases, and preditors, are absent in their new range, allowing them to multiply rapidly causing harm to native species (Wisconsin Department of Natural Resources n.d.). The populations of native species living within an ecosystem can be directly affected by the introduction of an invasive species through predation or herbivory, or disease (U.S. Department of Agriculture n.d.). Invasive species may indirectly cause the decline of native species through competition for food and other resources and the alteration of habitat (U.S. Department of Agriculture n.d.).

The introduction of invasive species disturbs natural communities and disrupts ecological processes (Wisconsin Department of Natural Resources n.d.). This disruption changes the way organisms compete and alters the role of predation, exacting negative impacts on the food web (Wisconsin Department of Natural Resources n.d.). Other negative impacts include the displacement of native species, the loss of species diversity, and the degradation of habitats (Wisconsin Department of Natural Resources n.d.). Invasive plants and animals can also be vectors for diseases that threaten the health of the ecosystem (Wisconsin Department of Natural Resources n.d.).

The rate at which biological invasions are occurring has increased rapidly over the last century. Along with habitat change, resource exploitation, pollution, climate change, loss of keystone species, and altered ecosystem functioning, biological invasions contribute to a decline of biodiversity and a rise in extinctions (Pyšek & Richardson 2010). There has been a rapid increase in the number of biological invasions worldwide, as well as the massive escalation of the implications of these invasions (Pyšek & Richardson 2010). There are very few ecosystems left that are free from biological invasions, and an increasing number of ecosystems are becoming dominated by invasive species (Pyšek & Richardson 2010).

Biodiversity Changes

Invasive species can lead to changes in biodiversity, by out-competing native species, putting stress on food webs, changing competition within a community, reducing native populations, and driving other species to extinction locally. The Millennium Ecosystem Assessment acknowledged biological invasions as one of the five main causes of declining biodiversity, which causes reduced ecosystem services (Pyšek & Richardson 2010). Invasive species are the second greatest danger posed to threatened and endangered species in the United States (Mooney & Cleland 2001). Biological invasions drive local native species to extinction, especially on islands or when involving predators (Mooney & Cleland 2001). Predation and grazing by invasive species can decimate the population of native species (Mack et al. 2000). Invasive species are a major cause of animal extinctions and increase the likelihood of the extinction of native plant populations (Richardson & Riccardi 2013).

Invasive species are often the cause of lethal stressors that diminish biodiversity, and especially in circumstances when other stressors have diminished native populations, biological invasions can hasten population declines (Richardson & Riccardi 2013). Disruptions to ecological processes can also impact biodiversity, albeit often in subtle ways that might take decades to manifest implications, such as in the case of mutualisms between plants and animals (Richardson & Riccardi 2013). In some cases, the rapid evolution of native species will occur in response to the introduction of an invasive species (Mooney & Cleland 2001). Invasive species alter the evolution of native species due to predation, competition, and niche displacement, but also through hybridization, introgression, and eventually, extinction (Mooney & Cleland 2001).

Invasions by disease-causing organisms can severely impact the biodiversity of native species and lead to extinctions. The American chestnut was a dominant tree species in the forests of the eastern United States until the arrival of the Asian chestnut blight fungus (Mack et al. 2000). Within the span of just a few decades, the invasive blight rapidly spread and killed almost all native American chestnuts (Mack et al. 2000). The invasive southern house mosquito is a carrier of the avian malaria parasite. The mosquito was inadvertently introduced to the Hawaiian Islands in the early 1800s along with invasive Eurasian birds (Mack et al. 2000). As avian malaria spread rapidly through the Hawaiian islands, the malaria-resistant Eurasian birds survived at the expense of native Hawaiian birds, which were faced with stressors from multiple biological invasions (Mack et al. 2000). There are countless examples of disease-causing biological invasions wreaking havoc on native species.

Ecosystem Changes

Invasive species are “key drivers” of ecosystem-level changes (Lockwood et al. 2013). Non-native and invasive species compete with native species for resources (Mack et al. 2000). Invasive species may alter community structure through indirect competition, such as resource use, and direct competition, such as allelopathy in plants (Lockwood et al. 2013). Invasive species impact other species interactions, including predation, herbivory, parasitism, and mutualisms, and can alter the populations of species with key traits that can influence critical ecosystem processes (Charles & Dukes 2007). Invasive plant, animal, and pathogens have also been implicated directly in the extinction of native species (Charles & Dukes 2007).

The greatest threat posed by invasive species is the disruption of entire ecosystems. Invasive species can cause massive environmental damage in a couple of ways. First, animal invaders can cause the extinction of vulnerable native species through competition, predation, grazing, and the physical alteration of the habitat (Mack et al. 2000). Second, plant invaders can alter ecosystem processes, such as fire regime and hydrology (Mack et al. 2000). The impacts of invasive species occur at the ecosystem level through the alteration of natural cycles, such as nutrient cycles (Charles & Dukes 2007). Energy flows can be changed by alterations in trophic interactions, food webs, and keystone species (Charles & Dukes 2007). These ecosystem changes can lower the chances of survival for native species (Mack et al. 2000).

One of the most common and dramatic impacts of invasive species is predation on native species (Simberloff 2010). Introduced herbivores can decimate the native flora of areas, especially on islands with limited resources (Simberloff 2010). If invasive species are closely related to native species, they may be able to mate with them. Introgression can change the genetic makeup of the native population enough that the original species experiences a genetic extinction (Simberloff 2010). This can happen when the invasive species outnumbers the native species to the point that an individual is more likely to come across an invasive individual than a native individual as a potential mate (Simberloff 2010).

The productivity of an ecosystem can be altered by invasive species that use resources more efficiently than native species, or those that eliminate a prominent species (Charles & Dukes 2007). Natural cycles within ecosystems can be altered by the presence of invasive species. Changes in decomposition rate can occur if invasive species alter litter chemistry, which can affect nutrient cycling (Charles & Dukes 2007). Nutrient cycling can also be changed by invasive nitrogen-fixing plants or by invasive plants that leach chemicals inhibiting nitrogen fixation by other species. Some invasive species release compounds that alter the availability or retention of nutrients such as nitrogen and phosphorus (Charles & Dukes 2007). Invasive plant species can also change hydrological cycles by changing evaporation and transpiration rates, the amount of runoff, and the level of the water table (Charles & Dukes 2007).

Some invasive species so alter the ecosystem that they change the disturbance regimes including fire, erosion, and flooding (Charles & Dukes 2007). Fire regimes can be altered depending upon the type of invasive species introduced. For example, when grasses invade shrublands, they increase fire frequency and intensity, and when trees invade grassland, fire suppression occurs (Charles & Dukes 2007). Additional ecosystem services affected by invasive species include air purification, atmospheric composition, and water regulation (Simberloff 2010)). Invasive woody plants can affect water regulation by causing flooding and aquatic sedimentation (Charles & Dukes 2007). Some invasive species, such as invasive mammals, increase erosion and soil disturbance (Charles & Dukes 2007). Impacts like these are significant because they can cause a dramatic shift in ecosystem type and species populations (Charles & Dukes 2007).

Invasive species can also alter the physical habitat of an environment. The greatest impact an invasive species can have on its new environment is modifying the entire ecosystem because such ecosystem-level changes are likely to affect most of the native species (Simberloff 2010). Both plant and animal invaders can outcompete native species and take over habitat, and some can even make the habitat less appropriate for other species by changing the physical structure of the habitat (Charles & Dukes 2007, Simberloff 2010). This often happens when invasive plants replace native plants that an entire ecosystem is built upon (Mack et al. 2000). Invasive plants can modify an entire ecosystem by outcompeting and shading out native species (Simberloff 2010). Invasive pathogens that eliminate a dominant native plant can impact an entire ecosystem as well. Many introduced and invasive plant pathogens have modified entire ecosystems by eliminating the dominant and foundational plants (Simberloff 2010).

Some ecosystem transformations due to invasions have been so extreme that even the landscape has been altered. This is the case in the “Bluegrass State” of Kentucky. Bluegrass, Poa pratensis, is an invasive grass native to Eurasia (Pyšek & Richardson 2010). Its introduction to Kentucky pushed out foundational native vegetation and transformed much of the state from open forest and savannah to bluegrass pasture (Pyšek & Richardson 2010). Because of this invasion, Kentucky lost a substantial amount of biodiversity with the loss of its native forest and diverse grassland ecosystems and gained a substantially less diverse non-native bluegrass pasture. Invasions such as this are a fundamental cause of ecosystem change and degradation (Pyšek & Richardson 2010).

Human Health Impacts of Invasive Species

Biological invasions pose serious implications for human health and well-being (Pyšek & Richardson 2010). Invasive species can, directly and indirectly, impact human health. Invasive species negatively impact human health by serving as vectors for existing diseases, spreading new diseases, and causing wounds through allergens, toxins, and bites or stings (U.S. Department of Agriculture n.d.). Invasive species can bring with them novel pathogens or parasites (Mazza et al. 2014). Some invasive species produce toxins, as is the case with poisonous plant invaders (Mazza et al. 2014).

Invasive species indirectly affect human health by impacting crucial ecosystem services upon which humans rely, such as agriculture and fisheries (Mazza et al. 2014). Invasive species can also negatively impact human livelihood by affecting our sources of food or otherwise causing cause denutrition/malnutrition (Mazza et al. 2014). The direct and indirect impacts of invasive species on human health are expected to grow in the future due to the increase of climate change-induced invasions (Mazza et al. 2014).

Poisonous Plants

Many invasive plants also happen to be poisonous to humans. These plants are not only spreading into natural areas but are increasingly more abundant in suburban and urban areas. While most poisonous plants must be ingested to cause harm, some can cause harm just by touch. Simply rubbing against wild parsnip or water hemlock with bare skin can result in blistered arms and legs and chemical burns (Wisconsin Department of Natural Resources n.d.). Poisonous invasive species are spreading rapidly through grasslands and along roadsides, but few people are aware of their dangerous impacts (Wisconsin Department of Natural Resources n.d.).

Vector-Borne Disease

Mosquitos, ticks, and other insects are effective vectors of diseases that cause great harm to humans. As these invaders spread, they bring with them diseases they carry. Many are increasing their ranges as climate changes makes greater tracks of land warmer, wetter, and more hospitable

Insects and other invertebrates are increasingly becoming vectors for serious diseases impacting humans, along with livestock and wildlife (Wisconsin Department of Natural Resources n.d.). The range of many disease vectors are also expanding (Centers for Disease Control and Prevention 2019). The lone star tick, which causes AlphaGal Syndrome was previously found only in the southeastern United States but has spread throughout the midwest and northeast (Wisconsin Department of Natural Resources n.d.). With the increasing range of ticks and the introduction of invasive ticks has come an increase in serious tickborne diseases infecting thousands of people per year (Wisconsin Department of Natural Resources n.d.). Lyme Disease, also spread by ticks, and West Nile Virus, spread by invasive mosquitoes, are vector-borne diseases of great concern in the United States (Centers for Disease Control and Prevention 2019).

Other Zoonotic Diseases

Invasive species can bring with them zoonotic diseases. Zoonotic diseases are illnesses that can spread between animals and people (Centers for Disease Control and Prevention 2019). Six out of every 10 infectious diseases in people are zoonotic diseases which infect tens of thousands of people in the United States every year (Centers for Disease Control and Prevention 2019).

Common diseases that can cross from wildlife to humans include avian flu, swine flu, salmonella, plague, rabies, and brucellosis (Centers for Disease Control and Prevention 2019). Coronaviruses are also zoonotic diseases of major concern. Severe Acute Respiratory Syndrome (SARS), Middle East Respiratory Syndrome (MERS), and COVID-19 are all zoonotic coronaviruses (Centers for Disease Control and Prevention 2019). Often overlooked, the current scale and cost of virulent diseases caused by parasites is incalculable (Mack et al. 2000).

Humans are increasingly coming into greater contact with wildlife as we spread into natural areas. As humans extend our range into natural areas, we cause the fragmentation of these natural areas and displace wildlife, causing them to, in turn, extend their range. Climate change is also contributing to the range expansion of biological invaders and vectors of disease. The more we are in contact with wildlife, the greater our risk of contracting zoonotic diseases. I will discuss this more in a later section of this paper.

Agricultural Impacts of Invasive Species

The impacts of invasive species on agriculture are enormous and, unlike other areas of impact, the associated costs tend to be easier to quantify and are well-documented. In agriculture, the majority of “pest” species and agricultural weeds that damage crops are non-native invasive species (Mack et al. 2000)(Wisconsin Department of Natural Resources n.d.). The costs tied to the impacts of invasive species on agricultural sectors include the expense of pest control in addition to crop losses (Mack et al. 2000). These costs constitute a “tax” on the production of forage, fiber, and food products (Mack et al. 2000).

A study examining the economic impacts of 1,297 different invasive species around the world found that large agricultural producers, like the United States and China, suffer the highest potential cost due to biological invasions (Paini et al. 2016). From 1960 to 2017, invasive species cost the agricultural sector over $527 billion in the United States alone (Crystal-Ornelas et al. 2021). Invasive weeds cost the agricultural sector in the United States $27 billion per year (Mack et al. 2000). The loss of forage for livestock and the cost of herbicides applied to rangelands and pastures due to invasions cause an additional loss of $6 billion per year (Mack et al. 2000). Not only do invasive plants impact crops and forage for livestock, but they also prevent the growth of young trees, negatively impacting forestry, a multibillion-dollar-per-year industry employing tens of thousands of people (Wisconsin Department of Natural Resources n.d.). The impacts of biological invasions on agriculture are significant and continue to grow, affecting crops, livestock, and threatening food security.

Invasive species are a major cause of crop loss, therefore adversely impacting food security (Paini et al. 2016). In the United States alone, crop losses due to invasive insects and pathogens cost approximately $40 billion per year (Paini et al. 2016). Vector-borne disease and invasive poisonous plant species also negatively impact agriculture and livestock. In addition to the monetary cost of biological invasions in agriculture, invasive species contribute to lower quality agricultural land, threaten agricultural productivity, bring with them disease, and contribute to additional environmental issues, such as pollution, through increased use of insecticides and herbicides (Lockwood et al. 2013).

Economic Impacts of Invasive Species

Ecosystem services are the benefits provided to humans by natural ecosystems. They are the ecosystem processes that maintain the survival of human life (Charles & Dukes 2007). Efforts have been made to quantify the economic value of ecosystem services, but the costs associated with their loss or a reduction in services due to invasive species are likely underestimated (Charles & Dukes 2007). Although evidence exists that biological invasions impact ecosystem services, the direct connection between ecosystem services and invasive species has rarely been addressed in research or public policy (Charles & Dukes 2007). Past attempts to gain public and governmental support for the prevention and control of invasive species have failed due to a lack of acknowledgment of the link between nature and the economy (Charles & Dukes 2007). Humans often fail to grasp that, despite our beliefs to the contrary, we do not live separate from the natural world.

According to the Millennium Ecosystem Assessment completed in 2005, ecosystem services fall into four categories: provisioning services, regulating services, cultural services, and supporting services (Charles & Dukes 2007). Provisioning services include any products obtained from ecosystems, such as food, freshwater, fibers, fuel, biochemicals, pharmaceuticals, and medicines (Charles & Dukes 2007). Regulating services are those that result from the regulation of ecosystem processes, including water regulation and purification, air quality regulation, climate regulation, waste treatment, natural pest control, disease regulation, waste treatment, pollination, coastal storm protection, and erosion control (Charles & Dukes 2007). Cultural services are benefits that are non-material but improve society, such as recreation and tourism, spiritual and religious values, aesthetic values, educational and scientific values, and cultural heritage values (Charles & Dukes 2007). Supporting services are necessary for the maintenance of other services (Charles & Dukes 2007). They are indirect, occur on large temporal scales, and include processes necessary for the survival of life on Earth, including photosynthesis, primary production, nutrient and water cycling, soil formation, and regulation of atmospheric composition (Charles & Dukes 2007).

The threats that biological invasions pose to biodiversity and ecosystem-level processes, such as losses in crops, fisheries, forestry, and livestock grazing, translate directly into economic costs (Mack et al. 2000). Some of the industries negatively impacted by invasive species include commercial and sport fishing, forestry, agriculture, power companies, industrial water, and municipal water plants. Often, the additional expenses associated with biological invasions are passed on to consumers in the form of higher water and electric bills and higher food costs (Wisconsin Department of Natural Resources n.d.).

The economic impacts of invasive species include both direct effects of a species on agricultural productivity, forest resources, fisheries, public utilities, tourism, and outdoor recreation, as well as indirect costs, such as those associated with the control and/or eradication of invasive species (U.S. Department of Agriculture n.d.). Biological invasions cause two main types of economic impact. The first type of economic impact is the loss in potential economic output, such as lower crop production, reductions in domesticated animal and fisheries production, or lower forestry resource production. The second type of economic impact is the direct cost of fighting and preventing invasions, including quarantine, control methods, and eradication methods (Mack et al. 2000). A third type includes the costs of fighting invasive species that are threats to human health, either as vectors of disease directly or as vectors of disease-causing parasites (Mack et al. 2000). The economic costs of invasive species include the monetary losses associated with the loss of biodiversity and a reduction in ecosystem services, in addition to the costs of controlling and mitigating invasive species their impacts (Pyšek & Richardson 2010).

Economic impacts can be relatively straightforward to estimate. However, little is known about the economic impacts of even the most damaging invasive species (Courchamp et al. 2017). Recent research is illuminating the economic consequences of biological invasions. New approaches are for describing and evaluating the impacts of invasive species, and for translating these impacts into monetary terms have been developed (Pyšek & Richardson 2010). Using these methods, the total cost of all invasive species, including plants, animals, and microorganisms has been found to exceed $138 billion per year (Mack et al. 2000).

A 2021 study conducted by Crystal-Ornelas et al. estimated that invasive species cost the North American economy at least $1.26 trillion between 1960 and 2017. Invasive species cost North America approximately $2 billion per year beginning in the early 1960s (Crystal-Ornelas et al. 2021). That cost has risen to over $26 billion per year since 2010 (Crystal-Ornelas et al. 2021). Of the North American countries, the United States had the highest recorded costs, even when controlling for associated research efforts within each country (Crystal-Ornelas et al. 2021). Additionally, very few of the known invasive species in North America had reported economic costs, so these numbers are likely greatly underestimated (Crystal-Ornelas et al. 2021). While the total estimated costs of invasive species to the North American economy are massive, the $1.26 trillion estimate is likely to be very low (Crystal-Ornelas et al. 2021).

Discussion

Since humans developed the means to travel from continent to continent and across vast oceans, we have been moving organisms with us ever what were for millions of years of evolution insurmountable barriers to movement. By both purposefully and accidentally moving species, we have dramatically changed entire ecosystems and the evolutionary trajectory of countless species (Mooney & Cleland 2001). This movement of species allowed us to spread over most of the planet, taking with us the familiar - domesticated plants and animals, and also often unexpected tagalongs. In their new environment without predators and other environmental pressures they faced in their native range, many of these species have become invaders.

Invasive species are one of the most important and devastating ecological impacts of colonization, human transportation, and global commerce (Mooney & Cleland 2001). Because the number of invasive species in an area correlates directly with the level of trade, increased globalization leads to an increase in invasions (Paini et al. 2016). Invasive species are a rapidly growing problem in the United States and around the world with vast tracts of terrestrial, aquatic, and marine ecosystems being forever changed by invaders (Mooney & Cleland 2001).

Biological invasions lead to a wide range of issues ecological and economic problems ranging from extinction and loss of biodiversity to dramatic changes in ecosystem structure, function, and services. However, the spread of invasive species is not just an environmental problem we must contend with. Biological invasions represent a threat to food security, access to resources, and human health. Economic impacts due to biological invasions come at a massive cost and cause significant losses in agriculture, forestry, and fisheries. Even outdoor recreation, power production, and international trade are impacted by the spread of invasive species (Lovell et al. 2006).

In many cases, the widespread environmental, economic, and social impacts of these invasions have become too great to be ignored. The effects of human-caused biological invasions threaten conservation efforts and diminish our ability to protect biodiversity, support natural ecosystems, sustain productive agricultural systems and food security, and protect human health (Mack et al. 2000).