The exotic epidemic
I was caught off guard recently when someone assured me that invasive species are no big deal; that, when it comes to the mass extinction we humans are driving forth, taking a couple thousand species from one unique ecosystem and plunking them in another won’t matter much in the long run.
“Nature moves species around all the time,” they said. “Some might start as invasive species, but after a while, they become native. The red fox is technically invasive, you know.”
Yes, I do know. They were introduced to North America and elsewhere in the 1700s by Europeans eager to shoot them, however, in those days of ecological ignorance, a few escaped and began their adorable, if problematic, conquest of North America. In the words of Canadian journalist and author J.B. Mackinnon:
“[Red foxes] can do serious harm when they move into a natural system they were not a part of before. (They) threaten some two dozen rare animals in California, [for example], including such federally endangered species as the Santa Cruz longtoed salamander, least Bell’s vireo, blunt-nosed leopard lizard and [the] giant kangaroo rat ... they can spread diseases such as rabies, distemper and mange. Not every introduced species is a problem, but the red fox makes the top 100 list of the world’s worst, as compiled by the Global Invasive Species Database.”
What’s more is that the invasive red fox has, and continues, to displace species of native fox which were evolutionarily unprepared for this sort of competition. To be clear, I don’t blame anyone for not adequately understanding invasive species and the crisis they represent. It’s complicated, as I myself am still learning. So may the following be a friendly introduction to this most serious of issues.
Firstly, does nature move species around, peppering some ecosystems with the denizens of another? Yes, but the process is exceedingly slow and tends to occur between ecosystems which share borders, perhaps separated by a mountain range, water body or a significant shift in climate. The odd land bridge might allow caribou to populate Prince Edward Island for example, as happened at the end of the last ice age, but not so easily can an Asian insect reach the Maritimes without human intervention.
Consider Hawaii, U.S., the extinction capital of the world. Long before the arrival of human beings, it was colonized, incrementally, by birds, insects and plant life carried to it by ocean currents, great updrafts of wind or tremendous storms at a rate of perhaps one new species every thousand years. This can be considered a natural speed for this remote archipelago, and when each species arrived, it faced immediate and ruthless tests, such as the finding of food, shelter, mates and general belonging in this increasingly diverse assemblage of castaways.
I’m certain that some of these species were problematic when they first arrived, but their influence was tempered by surrounding diversity and the obstacles which faced them. In time they were shaped by the island, becoming unique, endemic and perfectly native. Give this process several million years and you’re left with a kingdom of biodiversity unlike any that evolved before or since and is strange, magnificent, balanced and nearly immune to the destructive powers of surrounding continents. The great Pacific killed or slowed down all would-be invaders, except notably, humanity.
When we arrived, Hawaii was entirely unprepared to us, its ecological defences inadequate to hold back our axes and agriculture, even less so our livestock and the species we brought by accident, such as insects, rodents, large mammals, diseases and fungi. In time, exotic species were conveyed here on purpose, as pets or lively reminders of home.
The result? Of the 125 to145 birds native to Hawaii and nowhere else, only 35 endured as of 2001, with a staggering 24 of whom endangered. The majority of modern Hawaii’s bird life: the warblers, doves, robins and mockingbirds, were introduced. Of its 2,000 some flowering plant species, almost half are exotic. Its forests, once free of mammalian predators, is prowled by rats, mongoose, cats and pigs. Hawaii is a graveyard of biodiversity, with the majority of its landmass dominated by aliens; its native glories reduced to rarities or shoved into the annals of extinction.
Nature does move life around and those transplanted organisms do have the opportunity to become native, but this process is slow, on the scale of millennia and much more judicious than us homo sapiens. We have enabled the catastrophic conquest of several thousand non-native species across North America in a few short centuries, some of whom have cost colossal sums of money and biodiversity.
Consider now the forests of eastern North America. At one time, New England, U.S., was dominated by the American chestnut, whose nutty bounties sustained innumerable species and whose lumber was incredibly valuable to historic foresters. But in 1904, a Japanese chestnut was planted in the Bronx Zoo in New York, U.S., carrying with it a fungus known in the Old World as “chestnut blight.” Chestnut species in Europe and Asia had evolved alongside this ailment and possessed a natural resistance to it, but the American chestnut had never known this affliction and was wholly unprepared.
Upwards of five billion American chestnuts across a significant swath of North America were struck down in a mere half century, pushing this species to the brink of extinction. The only true survivors, by virtue of their isolation, were those planted outside their natural range where the blight could not reach them, like in Nova Scotia. In our province stands the ashdale tree, believed by some to be the largest Americ- an chestnut left in Canada, perhaps even North America.
And if this blight were the only invasive to reach our shores in a thousand years, our forests would recover. Perhaps they would have taken on a new shape in the absence of the American chestnut, or perhaps a few chestnuts would have developed a natural immunity to this fungus and slowly repopulated their empty kingdom. But this blight wasn’t the only invasive to strike our forests.
In 1890, the beech scale, an insect endemic to the Old World, was accidentally introduced to the Halifax Public Gardens where it began assaulting American beech across the Maritimes. It gave rise to beech bark disease, killing or disfiguring a once proud hardwood across its range. The spread continues to this day and we are all hard-pressed to find a beech without grotesque rot compromising its health and productivity.
At this very moment, the hemlock woolly adelgid is raging north across Nova Scotia; promising to eradicate our eastern hemlocks as it did across the eastern United States, while the emerald ash borer advances east through New Brunswick; its appetite for our native ash trees deadly and insatiable. Recently it was discovered in Bedford, N.S. Together, they are reshaping our eastern forests at a heavy cost to regional biodiversity.
The rampant introduction of invasive species by human beings is considered the second most potent driver of mass extinction, outpaced only by habitat lose from our clear-cutting of forests, draining of wetlands and plowing of prairies. The difference is that we can halt our destruction of habitat with sustainable harvesting practises, legislation and simple land protection, but solutions for invasives must be specific to each and every species. Neither you nor I will ever live in a country free from the beech scale, emerald ash borer or hemlock woolly adelgid or for that matter the European starling, red fox or purple loosestrife. It is essential that we take these invaders seriously, lest we surrender our native biodiversity to someone else’s.