Invasive Species Part I: What They Are and Why We Care
Invasive species are the largest ecological issue we face in our restoration work. The vast majority of our projects involve invasive species removal or management to some degree. The presence of invasive species is one of the first things I look for when I enter a site, and identifying the presence and density of invasive species is a key component of every site assessment and consultation. Since they are such a core component of our ecological restoration work, we are going to explore what exactly are invasive species, and why we are so concerned about them.
What Are Invasive Species?
Definitions of invasive species can vary a little bit depending on who you ask—they are non-native, they are problematic, they are aggressive, they cause damage to human and environmental health, etc. You might also hear various names for invasive species such as alien species, noxious weeds, or aggressive exotics. In fact, there are even some philosophical debates among ecology circles concerning which of these names and descriptions are most adequate. We’re going to skip the philosophy for today though, and just stick to the basics.
According to most government agencies (including the US Forest Service and the Minnesota Dept. of Natural Resources), an invasive species is any living organism that has two characteristics:
It is non-native to the ecosystem or area it occupies, and
It causes economic harm, environmental harm, and/or (usually and) harm to human health.
Let’s break down these two key components. For starters, if something is non-native to an area, it means it didn’t initially come from there. Living organisms such as plants, animals, fungi, insects, and even diseases all have regions of origin. Essentially, over time, various organisms evolved and adapted to grow and live in specific areas. Natural barriers such as mountains, oceans, temperature, humidity, or food availability kept different species within certain geographic areas. Those species evolved to function within those specific areas, and the rest of the surrounding ecosystem evolved to control those species’ populations through natural checks such as predators or food scarcity. Furthermore, when a species is in its native area, the rest of the surrounding organisms are used to that species being there. They have natural adaptations that allow them to coexist with that species in a balanced way.
When you remove a species from its native habitat and place it somewhere else, three things typically happen. Sometimes the species can’t handle the conditions of the new location. For instance, it might get too cold in the winter for the species to persist, and so it dies out without ever establishing reproducing populations. In very (very!) rare occasions, the species adapts to living in the area without causing any major disruptions of the ecosystem. More often, however, the species becomes aggressive and damaging. Either there are no natural predators to keep the species in check, or the new species causes extensive damage to the surrounding ecosystem by disrupting other species not used to its presence there, or outcompeting others for available resources. Species that are brought into new areas where they are not native are called introduced species, and are considered non-native. If their presence starts causing problems by disrupting ecosystem function or displacing native species, or causes other harm, we move to the next component of our invasive species definition.
Economic harm, environmental harm, and human harm are the three levels of harm that a species may cause to be considered invasive in the United States. A species only needs to cause one of these kinds of harm, though often invasive species cause at least two, if not all three.
Causing economic harm usually indicates that the species is disrupting an ecosystem service of some kind. Ecosystem services are those goods and services we receive from the natural environment. For example, Eurasian watermilfoil (Myriophyllum spicatum) is an invasive aquatic plant that creates dense mats on the surface of lakes. In some parts of Wisconsin, it has reduced lakefront property values by 13%. Additionally, it clogs boat motors, inhibiting boat activity and water recreation. By decreasing the economic value of lakefront property and inhibiting monetary gain from water recreation, Eurasian watermilfoil causes economic harm.
Environmental harm is often tied to economic harm, as ecosystem services are inherently connected to ecosystem health and function. When an ecosystem isn’t functioning properly, the number of resources it produces also suffers. Overall, though, environmental harm is focused on the effects non-native species have on the natural environment. This effect can take many forms. A species might outcompete native species for resources, leading to a decline in native populations. Alternatively, a non-native might affect another species’ habitat, either indirectly or directly. For instance, zebra mussels (Dreissena polymorpha), a common problem here in Minnesota, are prolific consumers of plankton. The amount of plankton they consume can double water clarity in lakes. More water clarity sounds nice, but it provides better conditions for algae growth which leads to algae blooms which kill fish. Additionally, there is less plankton available for other aquatic creatures to eat, which can cause native species to starve. By increasing water clarity, zebra mussels are indirectly changing the habitat conditions, and by eating plankton, they are directly competing with other species for food. Lastly, a non-native species might also cause environmental harm by directly killing native species. In Guam, for example, the invasive brown tree snake (Boiga irregularis) preys on native birds, and has decimated bird populations since its introduction to the island in the 1950s. These non-native snakes are believed to be partially responsible for the extinction of at least 10 different species.
The final indicator of harm, human harm, is not as common as the first two. While you can argue that lowering economic gain and disrupting ecosystems are harmful to humans, this level of harm is more focused on harm to physical human health. For a species to classify as invasive under this level of harm, it must cause a threat to human health. In the southern United States, the imported red fire ant (Solenopsis invicta) can sting humans, causing red welts and burns. In Minnesota lakes, swimmers badly cut themselves on the sharp edges of zebra mussels that coat lake floors and shorelines. Off the coast of Florida, divers in the Gulf of Mexico can sting themselves on the venomous spines of the invasive Lionfish (Pterois). These are all examples of non-native species that pose a direct threat to human health, and therefore classify as invasive. In all of these examples, the species also cause substantial environmental damage and subsequent economic damage. This is one of the reasons invasive species can be so devastating—many cause multiple levels and scales of harm that compound for disastrous results.
Case Study: To tie together our discussion of the two components of the invasive species definition, I want to describe a brief case study. For this example, we will be traveling south to the Florida Everglades. In 1979, staff at Everglades National Park discovered the first known case of a non-native constrictor snake called the Burmese python (Python bivittatus). By 2000, the state of Florida was receiving wide spread reports of python populations. Originally introduced to the state as part of the exotic pet trade, the Burmese python became established in the wild, likely after being released by overwhelmed pet owners. This constrictor can grow over 20 feet long, and has no known natural predators on the continent, meaning there is no natural check on its population. Burmese pythons aggressively compete with other native wildlife for food, and since native prey in the Everglades are not adapted to this particular predator, there have been significant declines in mammal species throughout the Everglades. Raccoon populations have decreased by 99%, opossums by 98%, bobcats by 87%, and small mammals such as marsh rabbits and foxes have virtually disappeared. The greatest levels of decline are in areas where this snake has been established the longest. At this point, there are an estimated 10,000+ invasive Burmese pythons in the Everglades, making their eradication virtually impossible. Overall, these constrictor snakes are considered one of the worst invasive species in the United States. They are a good example of a classic invasive species case—a species that does not naturally occur in Florida (or anywhere in the country for that matter), but was introduced and able to take advantage of the resources to reproduce and flourish. Since then, it has caused devastating environmental harm, and the cost of controlling populations is significant, therefore checking the harmful box of our invasive species definition.
Why Are We Concerned?
By now, the reason for concern about invasive species should be fairly clear. The examples provided above are just a few of the many reasons invasive species warrant our concern and attention. Unfortunately, there are so many others. Invasive feral swine in the south destroy crops and uproot tree seedlings. The Asian citrus psyllid, an invasive insect in Florida transmits a disease carrying bacteria that kills orange tree crops. European starlings, an invasive bird first released in New York, swarm in massive flocks that steal food and threaten air travel. Emerald ash borer has devastated ash populations across the eastern and central United States. Asian carp threaten native fisheries across major U.S. waterways. Brown tree snakes cause power outages in Guam by climbing power lines. Kudzu, known as “the vine that ate the south” grows at rates up to one foot per day until it blankets everything, killing off native plant species. The list goes on and on.
It’s estimated that the damages caused by invasive species in the United States cost more that $120 billion per year, and that only includes the quantifiable costs. The cost of losses in biodiversity, aesthetics, cultural resources, historical landscapes, and other non-monetary resources are difficult to expense, but very real nonetheless. Additionally, invasive species have been recognized worldwide as one of the greatest threats to biodiversity, endangered species, and overall environmental health. They now rank alongside water/air pollution, habitat loss, and resource exploitation as a major source of environmental concern.
Even though not all the invasive species that we encounter here in Minnesota are quite as devastating as some of the examples listed above, we still focus efforts on controlling and removing their presence as part of our restoration activities. By managing their growth and spread, we can help restore some balance within the ecosystem, and give native species a chance to re-establish healthy, resilient populations. Doing so also encourages growth of biodiversity and return of important ecological services and function. These are all important aspects of why we do the kind of ecological work that we do. At NRP, our focus is the health of the land and the ecosystem, and removing invasive species is one major way we promote that health.
We’re always happy to discuss invasive species management with anyone interested in learning more. Stay tuned for the next series of blog posts where we will dig into a snap shot of major invasive species in Minnesota, and ways you can help!