 |
| Asian Carp on the Mississippi. From the Illinois Extension website |
Every day we hear about a new "apocalypse species" taking over our backyards, usually brought in on ships and planes from faraway lands: Burmese pythons taking over the Everglades; giant Asian "murder hornets" slaughtering honeybees in Washington; Asian carp (several species!) conquering the Mississippi River and threatening the Great Lakes; jumping worms making cheesecake out of our beloved American soil. The list of invasive species to appear just in the last twenty or thirty years is absolutely enormous, and covers basically every Kingdom in the tree of life. And much as we worry about imports from Europe and Asia, our own exports - bullfrogs, for example, or boxelder trees - are running amok in other vulnerable environments of the world. Even as we continue to wring our hands, we simply cannot stop transporting these creatures all over the world, whether indirectly through hastily-checked organic goods (wood, minerals, soil) or directly through the exotic pet trade and harebrained ecological schemes. And this trend is only speeding up. Invasive species, abetted by our lazy and ignorant ways, are transforming the world as we know it.
But I want to pause and consider, beyond the hype, exactly what it means to be "invasive". We spend so much time fretting and ferreting out and quite frankly, genociding these nonnative species, we never stop to look in the mirror and contemplate the most invasive species on the planet: Homo sapiens.
Consider it for a moment! We came out of Africa, and proceeded to conquer - in waves - every corner of the globe. And even our hillbilly hominid cousins before us got in on the game, with Homo erectus, Neanderthals, and Denisovans providing some much-needed genetic material as we infested the coldest, hottest, wettest, and driest corners of the earth. And along the way we brought the seeds, plants and animals that we found most convenient to our survival...and in those root balls, sticks, and animal guts, were even more organisms hitching a ride, ready to escape and establish their own populations wherever we stopped. Wherever we landed, we altered the environment, planting new species while we cleared and hunted others, introducing fire or irrigation or draining or farming or enclosing or building. And then we were fruitful and multiplied and expanded, and continued chasing the herds over the horizon. On an ecological scale, we expanded at light-speed, taking advantage of the wild swings in climate that ruled the Pleistocene and our insatiable need to just travel, our driving curiosity to see what was over the next hill. And however long it took, we completely dominated environments that weren't prepared for us and didn't stand a chance.
Now don't get me wrong: this isn't a faux-nihilistic "Humanity is a virus" tirade (brought to you by that creepy kid in your class with a black trench coat who brags about his knife collection). I'm actually very pro-human, and not just because I'm a human myself. Of every organism on earth, no other species shows the endless variety and imagination and tenacity to survive. We're at once the toughest and most vulnerable of organisms, using the very existential anxiety of our squishy, flat-toothed, weaponless bodies to fuel our tenacity to survive and create culture out of practically nothing. The fact that we've survived everything nature could throw at us, and conquer the world to such an extent that we're trying to preserve the animals that might predate us (and in some cases forgive the ones that do!) is a testament to the astounding creatures we are.
But part of what makes us human is having the self-reflection to realize what we're doing to the environment, and how we got to this point. We can look at ourselves, as a species, with a critical eye. We can also understand that in many cases, the environmental havoc we've wrecked was not intentional, but part of our struggle to survive*.
And once we understand this, we can also find in our shriveled hearts a wee bit of empathy for invasive species - creatures dropped into an alien environment and obeying their genetic programming: go forth and increase in number; adapt and conquer. And understand to, that none of this was their fault - it was our fault. We fucked up, and these creatures are simply doing what they're supposed to.
Non-native: Invasive, or Naturalized?
So what makes a species "invasive", anyway? Why do some creatures manage to escape and wreak environmental havoc, while others are simply exotic curiosities? Why do South Carolinian zebras make the "fluff" segment of the news, while we devote front-page alarm headlines to murder hornets?
The first and most obvious requirement is that the species in question be non-native to the ecosystem they are "invading". And it's important to remember that species were moving and expanding into new areas long before we started transporting cute but murderous pets. The main vector for this expansion was geological, aided by climate change. For instance, proboscideans (that's elephants, mammoths, gomphotheres, mastodons, et al.) evolved in an Africa that was cut off from Eurasia by the Tethys Sea. But 50 million years ago, in the Eocene, Africa crashed into Eurasia, creating land bridges into new proboscidean-free territory. Ancient elephant relatives exploded out into the new areas in waves, essentially invading ecosystems that were ill-equipped to deal with them. Before this time, it was the perissodactyls (rhino and horse relatives) that dominated Asia, including the gigantic, 6-meter-tall Paraceratherium. We don't know much about the habits of these gigantic rhinos - did they travel in herds? Did they knock over trees to create clearings? How much did they alter and interact with their environments? - but we know a lot about what elephants do. And what elephants do is destroy vast swathes of forest, knocking down trees and creating huge clearings, which are then colonized by light-loving organisms. Another thing that elephants are good at, is traveling - and carrying seeds from one area in their guts to deposit in another area. They essentially recreate their ideal environments by carrying their favorite foods with them...sound familiar? While it's tough to assign a particular set of extinctions to a single set of organisms, considering everything that was going on during this period (geological shuffling, the uplift of major mountain ranges, the cooling and drying of the climate), we do know that as gomphothere proboscideans expanded into Asia, giant rhinos started dying off.
But just being "non-native" - or even expanding into new ecosystems - does not necessarily make a species "invasive". The Midwest (I live in Michigan) is host to many nonnative species that aren't invasive. Some, like the ringneck pheasant, are Asian species stocked for hunting purposes; they manage to maintain a breeding population and slot into the ecosystem, but don't seem to be doing any harm (often their populations die off anyway and have to be re-stocked). The same goes for the red fox, which is native to the western half of North America but was introduced to New England for fox hunts (no, really) and expanded across the United States without putting too much pressure on native organisms. Even opossums are originally South American, but expanded northward naturally as the climate warmed; they just waddle around being 'possums, and not doing much harm (unless you have chickens). These species are called "naturalized", since they integrated into the local ecosystem, eating and being eaten and maintaining relatively stable populations. Some naturalized species never even established breeding populations: the Osage orange tree was brought from the Red River region of Texas and planted extensively as a farm hedge and animal deterrent, but with the advent of barbwire was simply neglected, leaving lines of thorny hundred-year-old-trees that don't seem to reproduce**.
In order to be considered Invasive, an organism has to overtake an ecosystem and displace the native species - to disrupt and overturn the natural order for its own benefit. And while we tend to think of Invasives as barbarian invaders coming out of nowhere to conquer our pristine lands, slaying and pillaging along the way, the truth is actually a lot more complicated - and humans have a much bigger hand in creating this mess than we like to admit.
So how do species become "invasive"?
1) The groundwork must be laid. We tend to believe that invasive species just swoop in to a perfectly happy habitat and wreck it, but the fact is that nine times out of ten, that habitat was already in trouble. In the past, natural climate change, geologic movement, and ecosystem turnover were the main drivers of this; if the habitat of Paraceratherium was healthy and could support a large breeding population of the mega-rhinos, they could essentially resist the incursion of other mega-herbivores - simply by taking up space and defending their individual territories. But mega-herbivores are extremely sensitive to climactic changes, as even a slight reduction in the enormous amount of forage they require can cause a ripple effect throughout their population: the more time they spend searching for food, the less energy they have to spend on their giant, hungry babies. Many simply stop having babies at all. And as the population slowly shrinks, more adaptable invaders can spread into the newly-opened areas, further reducing available space and forage. In ages past, the giant mammals might survive climate cycles, perhaps evolving to take advantage of other forage; but the arrival of non-native mega-herbivores was simply too much pressure, and Paraceratherium was overrun.
In the Human Era, this is happening much more quickly - partly because we're able to quickly visit vulnerable habitats (like the Mauritius Islands, where the hapless dodo lived), and partly because of the way we alter and disrupt ecosystems. If native amphibians have a strong and diverse population, it's less likely that introduced bullfrogs can take over, despite their aggression and adaptability; but local populations stressed by habitat loss and pollution (and especially the pandemic chytrid fungus) will be ripe for a froggy takeover.
2) The introduction must be a the right place, and the right time - and happen more than once.
One of the most egregious (and perhaps the stupidest) examples of species invasion is the European Starling. As the story goes, an eccentric New York City group in the 1890's wanted to populate Central Park with every bird mentioned in Shakespeare. An original population of 100 birds in one park has since expanded to every corner of North America, totaling at least 200 million, wreaking agricultural havoc and displacing native bird species.
The worst part of this - besides the extremely dumb reason they were introduced in the first place - is that the group tried to introduce the birds several times over. The first few birds brought over either didn't reproduce enough, were killed by tough winters, or predated by native raptors. The Shakespeare group was absolutely determined that, come hell or high water, the European Starling would live in Central Park. And while the science of invasive species and ecosystems was not common knowledge in the 1890's (hell, probably not common knowledge today), you'd have to expect that somebody might have asked, "Are we sure this is a good idea?"
But I guess I'm giving people too much credit.
The point here is, that invasive species rarely descend from one pregnant female or one mating pair. More likely, it's successive waves of invaders, sometimes retreating as many times as they expand. Human beings didn't just troop out of Africa like an invading army, they meandered out through the Middle East in several populations, some of which may have actually gone extinct before we established a foothold. It initially took us tens of thousands of years to reach the Americas, and at least 12,000 to reach the tip of South America - many thousands of generations. It took many, many tries before we finally "stuck" - but once we did, we made out in a big way.
3) The invading species begins to change. Something strange happens when a species establishes itself in an environment where it has no natural predators or niche: its genetic limits begin to expand. In a sense, the "lid comes off". Much like the starlings, it may take several introductions before the right mix of genetic material is present to allow the organism to take advantage of its new environment; but once this occurs, the organism begins to act differently from how it interacted with its original environment.
Consider the brown tree snake. In its native Australasia, this medium-sized rear-fanged colubrid feeds on birds, eggs, and small mammals; while adaptable, it has its own niche, where its population is kept in check by predators and other, competing organisms. But after being introduced to Guam, where it had no natural predators and an island full of ground-nesting birds, the brown tree snake went full Mutant: they began producing more young, became diurnal, grew to twice their normal size, and began eating bizarre items (for a snake), including roadkill - and once, even attempting to eat a human infant. It was as though the tree snake realized it had nothing to fear, and transformed into a far larger and more aggressive version of itself. Of course the reality is that the normal evolutionary pressures that kept the snake smaller, skittish, and noctural had simply lifted; the larger and bolder snakes were able to feed and breed more quickly than their cousins in the Old Country. The result was a kind of "Super Snake" that could potentially spread to other ecosystems much more quickly than its ancestors could ever have dreamed.
4) Invasion is a driver of evolutionary change.
So what is the future of the brown tree snake? If we look beyond its destruction of uniquely beautiful, near-helpless island birds in this herpetological apocalypse, the case of this Edenic serpent represents an interesting evolutionary case. Its newfound adaptability and boldness will allow it to expand to new territories, no doubt aided by us primates and our magical flying vehicles (it is believed the first brown tree snakes stowed away in the landing gear of an airliner). As the snake adapts to new habitats, especially isolated islands, each population will begin to evolve and change in isolation. After several hundred years, separate populations will become more and more distinct from their brethren, until each one becomes a completely separate species. Thus the "new" brown tree snake could become the Most Recent Common Ancestor of a whole slew of brown tree snakes, many of which will no longer be brown or live primarily in trees. Local climate, geography, prey availability, and predators will exert new evolutionary pressures on each unique species, further changing these new snake species.
If this sounds odd - even far-fetched - consider the studies that have looked at genetic uniqueness in invasive populations. Now that the evidence has been found for this phenomenon, we begin to understand that modern "invasions" are not something new under the sun, but simply a natural evolutionary process on hyperdrive. Our actions of shuffling species around the world, whether accidental or deliberate, is driving this change. The difference is in the time scale; and the danger is that these invasives will destroy other organisms much more quickly than they themselves give rise to new species.
5) The dilemma of "Invasives".
The problem here is, once again, that these creatures are simply doing what they are supposed to be doing all along - they are not evil. Even "murder hornets" can't be convicted of murder; they eat honeybees in their native habitat, so they're naturally going to take advantage of defenseless local populations. Any opportunity is taken in the bid to survive. And this gives rise to the question: is it ethical to eradicate such creatures? Whose needs are we actually serving? Should we spend millions of dollars shooting, poisoning, chopping and digging out these creatures in a singleminded murder-spree, or are there other ways of curbing and nativizing such organisms? In a world where invasives aren't going anywhere, we need to figure out to live with them. Some invasives are filling niches left open by locally extinct species, thus balancing their adopted ecosystem. There are even cases where animals that are nearly extinct in their native habitats have become invasive in others.
And there's another case I'd like to make, perhaps an odd one: habitats with high invasion loads become powerful evolutionary laboratories. Consider the Everglades. Due to decades of lax exotic-pet laws and poor water management, Florida has become an environmental disaster area. Between the Melaleuca trees planted to suck up thousands of gallons of water, the Burmese pythons and Argentinian tegu lizards from the exotic pet trade, and the South American nutria intended as beaver-fur alternatives, this wide North American river is inundated with invasive species. But where else in North America can you observe giant constrictors and American alligators vying for apex predator status? Or parrots reintroduced a hundred years after they were extirpated from the continent? The bizarre accumulation of tropical and subtropical species is creating a brand-new biotia, one never seen before on the face of the earth. How long will it take to "settle down"? How will all these species interact with one another? How will they evolve to take advantage of their new habitat? And is this just a weird ecological corner of the United States, destined to remain isolated with its invasive species, or will it provide a foothold for new species to expand across the continent with rising waters and temperatures? Will we begin to see, for instance, pythons evolving to survive the extreme temperature swings of the central US?*** While it may seem far-fetched, the genetic explosions created when evolutionary limits are removed means many "impossible" scenarios may become possible, especially as the earth continues to warm. Are we dead-set on continuing the Sisyphean task of exterminating these wily creatures, or can we learn to live with and manage these amazing organisms?
Update: Not long after I wrote this, I realized that there are cases where eradication makes absolute sense. There is a real difference between invasion scenarios on islands, versus invasive species on the mainland. On an island, especially an isolated, small landmass, the endemic species are absolutely unprepared for the onslaught of mainland creatures; in this case, the eradication of invaders is both necessary and - a key point - possible.
Consider the famous case of the Santiago Island goats. In the early 20th century, goats were introduced to the middle island of the Galapagos archipelago - known, of course, for its giant tortoises. The goats proceeded to eat and breed their way to dominance, directly outcompeting the native (reptilian) herbivores and destroying the environment. In 2001, an eradication campaign was launched; within 8 years, there were no more goats on the island. The native ecosystem, no longer suppressed by the ravenous mammals, bounced back with gusto. In this case, I fully support eradication efforts.
My neck of the woods
I'm walking through a woodlot in a Michigan state game area in mid-October, the wet leaves muffling my footsteps. It's warm, about mid-50's, and most the trees still have their leaves, although some are beginning to turn colors. I'm edging my way through dense thickets of Amur honeysuckle, a woody shrub from the far southeast of Siberia planted for landscaping and erosion control; at this point their leaves are still healthy and green, and will stay green well into November. As I break into a clearing down by the creek, my clothes catch on the thorny branches of black locust, a tree from southern Appalachia brought north for its amazing lumber qualities but now displacing native species in thick, ineradicable stands. Across the stream are even denser thickets of Russian olive, another thorny shrub that produce nearly a hundred pounds of edible, juicy berries. These are just three of the most obvious invasive species in this northern lowland ecosystem. Their heavy foliage and monoculture crowd and shade out native seedlings, spreading faster than the hardwoods they displace. And yet I'm hesitant to say, "burn it all out", to go in with axe and spade and "save the habitat" from these invaders. Honeysuckle, with their long-staying leaves, provide browse and cover for whitetail deer. Honeysuckle and Russian olive feed and provide cover for birds (depending on the bird species). And both of these species, as well as black locust, fix nitrogen in the soil, providing much-needed nutrients to wet, acidic habitats. Obviously these plants are altering their ecosystems; but how dangerous they are to native species is up for debate. This is no virgin woodlot, but an area reclaimed from drained farmland - an ecosystem already altered beyond its original state - and what is to say these invasive species aren't providing some benefit? Especially since I hunt the whitetail deer in this area, it seems the thickets are just the ticket for highly-pressured ungulates on state land, and could further provide feed and cover for threatened warblers and other birds.
I love the native Michigan woods; there are few places of more peaceful greenery on a soft June morning, or more colorful and shifting on an afternoon in late October. But I also know that they are living ecosystems, constantly changing and evolving, and I wonder if it's beneficial to simply preserve them like museums. Amur honeysuckle, Russian olive, and black locust aren't going anywhere; while it's necessary to monitor and check their spread, I don't think it's helpful to tear up the soil trying to dig them out - eradication would simply disrupt the habitat further, and provide even more opportunities for the thickets to regrow and spread. These plants are survivors, and able to outwit us if we approach them with hostile intent. It's much better to study their affect on the habitat, and manage them just like any other species in our northern forests. Perhaps management, rather than extermination, is the best solution to the continuing saga of invasive species.
Rick Out.
*I need to insert some caveats and nuance here: I'm not excusing greedy Capitalistic industry-at-all-costs environmental degradation. I'm talking about hunter-gatherers overhunting in a cold snap, or farmers faced with wolves and bears slaughtering their flocks, or the Amazonian native who logs endangered trees to support his family - all the while facing ostracism and possible murder from his own people. Hell, why get dramatic? What about the average American who wants to recycle but can't because their municipality doesn't provide the service, meaning that tons of plastic gets tossed in a landfill (that's me)? Or can't afford electric vehicles because they're too goddamn expensive (also me)? I'm talking about the powerless people who, even if they wanted to preserve the environment, are at the mercy of institutions that either can't or won't, who are stuck in their old wasteful ways - if they aren't actively exploiting and destroying the earth for their own short-term gain. While I don't think we, the people, should just throw up our hands and say "What can you do?" I believe the greatest polluters and exploiters should bear the greatest burden.
**Apparently they evolved in conjunction with megafauna like giant sloths and - you guessed it - proboscideans; only these giant fermentation tanks could digest their pulpy, bitter fruit and deposit their seeds. There used to be four species of Osage orange spread across the continent; now that their seed-spreaders are gone, only one species remains. Humans were the only creatures that can help the tree expand their range, and now that we have no practical use for them (and cattle choke on the fruit) they are more often ripped up as a nuisance than planted. (On a side note, the Osage nation of Indians originally used the Osage orange for bow-wood, hence the French name, Bois d'arc (colloquialized as "Bodark" in the South). I include this little anecdote simply because the Osage Orange has such an interesting story, and also because I'm a huge tree geek).
***Burmese pythons have been known to survive freezing temperatures in their native habitat, hibernating when temperatures dip below 60 F. Although constrictor snakes are associated with tropical and subtropical climates, I'm wondering if Burmese pythons in particular might have the genetic plasticity to adapt and spread beyond their normal climactic range. Even Michigan (my state) now experiences summer temperatures well above 90 F for extended periods, with high humidity and relatively heavy rainfall; while I don't expect pythons to slither through our northern forests (we still experience subzero winters, especially now that the polar vortex regularly wanders south), I wouldn't be surprised if they are sighted in the Ohio River Valley within the next 50 years.
Comments