Killer bees. Kudzu. Glassy-winged sharpshooters. Zebra mussels. White-nose syndrome. Lionfish. Medusahead grass. This laundry-list of organisms is one small sample of a long and ever-growing compilation titled “invasive species”. It’s an ominous sounding term and for good reason. Invasive species can wreak havoc on an ecosystem, causing widespread disruption and even the extinction of other species. They can also wreak havoc on the economy, disrupting agriculture and industry and costing us humans a fortune.
According to the National Wildlife Federation, “approximately 42% of threatened or endangered species are at risk primarily due to invasive species.” Equally as scary, the US Fish and Wildlife Service puts the annual cost of invasive species to the United States at somewhere around $120 billion. Have I made my point? These things, whatever they are, are a big deal.
So then what exactly is an “invasive species”? Is it simply an organism from somewhere outside the state? The county? The neighborhood? How foreign does a species have to be to classify as invasive?
As it turns out, not all species that are alien to an ecosystem are commonly considered to be invasive in it, and in fact, the terminology might well be more political than it is ecological. Several factors have to come into play in order for a species to earn this dubious moniker. First, the species must be introduced to a novel environment (the alien component). Once there it has to survive, which is no guaranteed thing (imagine a crocodile introduced to Antarctica). After surviving in its new home, the alien species must then multiply and thrive, eventually out-competing one or more species native to the ecosystem. At this point, when the alien becomes damaging, that’s when we tend to term it invasive.
Let me give you an example. Recently, ornamental lionfish have become a bit of a news item. These striped, long-finned tropical fish are a favorite of aquaculturists and fish enthusiasts world wide. The slender, barbed, poisonous lobes that make up the lionfish’s “mane” are gorgeous and look amazing in a salt-water tank. This flamboyant species hails from the western Pacific where it occupies coral reefs and preys on smaller fish. Somehow in the mid-1980’s, the species was introduced to the waters off of Florida. In all likelihood, a pet-owner released his or her collection in a misguided attempt to liberate the captive pets. Florida is a long, long way from the western Pacific. Often in this situation, the newly freed pets are quickly killed. Either simple environmental factors do them in (again, crocs in Antarctica) or else they fall prey to predators and pathogens they’ve never experienced before. In this particular case, something close to the opposite occurred. The lionfish survived. The coastal waters off Miami are not all that different from the lionfish’s native habitat so factors like water temperature worked in their favor. Likewise, there are no predators in the Caribbean eager to eat lionfish and no diseases hanging about that proved fatal to them. In the absence of these population-limiting factors, the fish not only survived, they thrived. They quickly established a position as an apex-predator, feasting on reef fish who’d never seen a lionfish before and had no means of defending themselves. Those introductory lionfish grew healthy and strong and bred prodigiously. Before long, the coast of Florida became the site of a lionfish explosion.
For the first decade or so following introduction, the population remained restricted to the coastal waters off Miami. In 2000, lionfish sightings began to pop up further along the Atlantic seaboard. By 2007, the fish had become true colonists; expanding their range New York to Cuba and Haiti. According to the US Geological Survey, the fish can now be found in coastal water throughout the Caribbean, the Gulf of Mexico, and as far north as Rhode Island as indicated by this beautiful animated map. The lionfish is now firmly designated as an invasive species because as its range grows, the populations of other reef fish have declined. They are causing widespread shifts and changes in their ecosystem.
Florida faces a nearly identical invasive problem with Burmese pythons; another pet species released into the Everglades. As with the lionfish, the pythons find the environment comfortable and thrive in the absence of python-hungry predators or competitors for resources. Their population has exploded and hungry snakes are decimating once relatively stable populations of small mammals and birds. Florida is also exporting invasive of its own. As detailed in the month’s Safari Spotlight, the red-eared slider, a turtle native to Florida and other southern states, has invaded California. Fast-breeding and larger than our native turtles, they are dominating our local freshwater habitats and wreaking havoc on our local turtle populations. In all the cases outlined above, the designation of invasive was applied once it became clear that the successes of the alien species were coming at the expense of the natives.
Although it’s certainly a big contributor, the pet trade isn’t the only source for problematic invasive species. Glassy-winged sharpshooters, innocuous looking insects from the Southeastern US made it into Southern California in the late 80’s; likely by traveling on a shipment of nursery plants. The little insects feed on the vital fluids of vascular plants and carry something called Pierce’s Disease; a pathogen which decimates grape vines. Zebra mussels were introduced into the Great Lakes (again in the late 80’s) from the ballasts of Russian and eastern European tankers. The fingernail sized bivalves produce between 100,000 and 500,000 eggs per year and are spreading through American waterways at a terrifying rate. They frequently congregate in concrete-like masses which block plumbing and infrastructure, costing us millions in damages. The list goes on and on and though the individual species may differ, the pattern is the same. An alien species appears and changes things to the extent that it becomes ecologically or (and this is what tends to get our attention) economically damaging.
When framed like this, it becomes readily understandable why invasive species tend to be categorized as implicitly negative. Like acid rain or global warming, they are often thought of as a purely human-caused phenomenon having purely negative consequences. This is a gross oversimplification of the facts.
In truth, while human-caused invasions are almost universally the source of ecologic turmoil including dramatic population shifts and extinctions, what we have done is not to invent a new phenomenon but to rapidly accelerate an existing one. Since the dawn of life on this planet, invasions have been taking place. Species from the sea invaded the land, colonizing and claiming as they went. Since then species that have thrived in one location have spread to others, competing with and pushing out other species as they’ve moved.
Without invasions, the Hawaiian islands would likely be little more than sterile heaps of lava sprouting from the mid-Pacific. Throughout the history of the islands, birds, reptiles, plants, and the occasional mammal have found their way across thousands of miles of ocean to colonize those tropical shores. Prior to the human discovery of the island chain, the rate of invasion was approximately one novel species every 100,000 years. In spite of these ongoing invasions, the term “invasive species” hasn’t often been used in describing historic Hawaii. In part, this is because these invasions happened before humans were on the scene. We tend not to concern ourselves with the shifts and extinctions that led to what is. We’re far more worried about shifts and extinctions changing what exists now into something new. The other reason why we don’t often consider these ancient invasions in the same manner that we consider today’s is because they were less apparently damaging.The key word there is “apparent”. While the interspecies conflicts and occasional extinctions that likely occurred whenever a new plant or bird arrived on the islands are of no concern to us today, they would have been a very big concern for the species being edged out at the time.
While these natural invasions are most dramatic and obvious in island ecology, they happen across the planet in every ecosystem. From the intermixing of North and South American ecologies that occurred when the Isthmus of Panama rose above sea level, to the widespread changes that both led to and resulted from human beings crossing the Bering Strait from Eurasia, the globe has always been a melting pot of advancing and retreating populations. It’s not the act of invasion that’s the problem, it’s the frequency. They’re happening too fast for our comfort and population shifts that would normally have taken centuries or longer are happening on the scale of years or decades. While not unprecedented in geologic history, this is certainly unprecedented in human history.
The other conception about invasives that must be challenged is this idea that all of them are inherently detrimental and fundamentally negative. This is where the politics of terminology reveals itself. An invading species that is economically beneficial usually escapes the label whereas those that hit us in the wallet wind up on roadside billboards. The mistake made by the glassy-winged sharpshooter was to invade Napa Valley and destroy grapes rather than poison oak. An invasive insect that doesn’t impact our agriculture usually escapes our notice.
As proof of this, consider the ubiquitous earthworm. Prior to European colonization of North America, this continent had few earthworms. Worms had been largely extirpated from North America during the ice ages; carved away and crushed by the movement of glaciers across the terrain. These days, you can’t turn a spade without revealing at least a few of the wriggling annelids. It is common knowledge that these helpful creatures aerate our soil and aid in decomposition. We add them to our gardens to gain beautiful flowers and vegetables. We keep them in our compost bins to turn our trash into fertilizer They are an unexamined boon to our agriculture and productivity.
So most earthworms are definitely invasive though we rarely call them that, and though there is increasing scientific evidence that their presence in north-eastern forests is, in fact, detrimental, it’s not detrimental to us. Even though they are virtually everywhere, the wriggling masses of earthworms beneath our feet are viewed positively while the quiet colonies of zebra mussels in our waterways are viewed as the advancing enemy horde. Both species are virulently invasive and tend to dominate their respective ecosystems. So what’s the difference? Zebra mussels cost us money while earthworms help us make it.
The long and short of this whole discussion is this; invasive species are nothing new. From the dawn of time, ecosystems have had to deal with newcomers, whether it was a palm nut riding ocean currents to a far off island, or the opossum walking up the isthmus of Panama to a brave new North American world. In all cases, an established community has had to adapt to something novel. Now that we humans are part of the status quo, we’re very concerned. We tend not to see the long view. From our perspective, the current arrangement of species and habitats is the correct one and we shudder to see it change. We see news items about deserts expanding and forests shrinking. We wait in line at roadside border security checkpoints or agriculture screening stations in airports. There is increasing focus on gardening with native plants and removing invasive weeds from the neighborhood. We are trying to preserve what is and fend off what might be.
This concern is more than anything, a value judgment. Sure, invasives change systems and sure, the introduction of the lionfish to the Caribbean may cause widespread changes, but there’s a larger view to consider. Will lionfish end the Caribbean? Will sharpshooters turn the Napa Valley into a sterile moonscape? Not likely. The systems will adjust and adapt, transforming themselves into the newest version of that system. This is life in motion. This is how new species are born and old species are edged out. This is all part and parcel of the great process of life.
So how should we think about invasives? It’s a balancing act. Trying to maintain sterile, unchanging environments is not only impossible, it’s unwise. As we see in New Zealand, Hawaii, Madagascar, Australia, and numerous other isolated communities, strict separation tends to lead to ecological precariousness. Island communities typically have fewer species overall and much of what they do have are highly specialized. While these ecosystems do fine while isolated, exposure to novel species is often devastating. Consider that Madagascar lemurs and Australian koalas aren’t invading new territories. Rather, they are struggling to maintain position against an influx of rodents, cane toads, cats, and other novel species. The ebb and flow is important to ecological health.
What is equally important to recognize is that pacing matters. While some colonization and expansion is expected and healthy, the rampant transplanting of species brought about by human expansion is unprecedented. While the Hawaiian Islands used to experience a novel species once every 100,000 years or so, in the modern era, they’re being swamped with novelty. In recent history, not only did our species invade, but we have brought with us the great plethora of life from across the globe; rats and bees, snakes and lizards, cats and dogs. This is the very definition of “too much of a good thing”. We’ve accelerated the process beyond healthy standards and must do what we can to mitigate the damage.
Going forward, we will have to take a firm objective look at alien species that pop up in our neighborhoods. We’ll have to ask ourselves, is this change happening because life is doing what it does best? Or because somebody down the road didn’t wipe their boots before flying back from vacation? We’ll have to consider whether the shifts made by the ecosystem to accommodate the new arrival are fundamentally detrimental, or simply changes that don’t serve our self-interest. Conservation done right demands that we look beyond our own concerns and try to determine what works best for the world at large.
For now, continue wiping down your boat and draining the bilge to keep zebra mussels from spreading too quickly. Don’t release your exotic pets into the city park. Be aware of what you plant in your yard and whether or not it’ll soon be growing in your neighbor’s. Do what you can to limit invasions, but do so understanding that these species aren’t bad just because they don’t fit with our plan. They are remarkable examples of life doing what life does best. They’ve out-competed their rivals and found ways to piggy-back on the unprecedented mobility of humanity. While we want to limit their expansion and mitigate their invasions, we also want to recognize that we owe our lives to invasive predecessors who fought, scrabbled, and competed to forge the living world we take for granted today.