An excerpt from
Demons in Eden
The Paradox of Plant Diversity
The very name of the state of Florida evokes its botanical riches. Plants, like humans, revel in its subtropical climate. Florida's forests, swamps, and freshwaters harbor more than four thousand plant species, but nearly a third of them were introduced, not native. In 1920 a prophetic naturalist, Charles Torrey Simpson, foresaw the problems of introduced plants: "There are the adventive plants, the wanderers, of which we have, as yet, comparatively few species; but later, when the country is older and more generally cultivated, there will surely be an army of them." The army has arrived with a vengeance and is advancing mercilessly across Florida's natural ecosystems, taking no prisoners. In the United States, only Hawaii has a worse problem with invasives than Florida. The problem is caused by only a small minority of the twelve hundred nonindigenous plant species, but this minority contains some real demons.
The army of alien plants is made up mainly of conscripts, not volunteers. At least 90 percent of the nonnative species were deliberately introduced into the state as ornamental plants, as new crops, or for other purposes. The kudzu vine is an example that is well known because it is a problem throughout much of the South, where it is estimated to cover 2 million acres of forest land alone. The epithets by which it is commonly known in the region tell the story: "the vine that ate the South," "mile-a-minute-vine," and "foot-a-night vine." At the height of the growing season kudzu can actually grow a foot in twelve hours and southerner's joke that you must close your windows at night to stop the vine getting in. Abandoned rural buildings can quickly disappear under a blanket of kudzu, but much worse, so do whole forest stands. The U.S. Department of Agriculture declared the plant a weed in 1972, though it still has its defenders who use it for everything from brewing tea to making baskets. For these stalwarts, a real virtue of the plant is that supplies are free and inexhaustible. Kudzu evokes both loathing and affection in the South, though hardly in equal measure. The ecological damage caused by the other sixty-one plants on a list of the most unwanted compiled by the Florida Exotic Pest Plant Council can induce only loathing.
Let's begin our trip to Florida in pristine habitat at Archbold Biological Station, near the heart of the Florida peninsula, at a point roughly equidistant from the cities of Miami, Tampa, and Orlando, and about as far away as it is possible to get from the sprawling growth of urban Florida. It is a special place where some natural habitat still survives amidst the orange groves and ranchland, and where Archbold commands a two-thousand-hectare tract of land dedicated to research and conservation. We accompany Eric Menges, the station's resident plant ecologist, a few steps from his laboratory, across a forlorn railroad track, and into the plant community that he has studied for the last fifteen years—Florida scrub, a sandy plain filled with shrubs, knee-high palmettos, and scattered slash pines. The soil beneath our feet is a pure-white sand that gleams in the early morning light.
"This area burned last year," Eric says, "and it's looking real good. Most plants survived the burn." Fire is a natural element in this ecosystem and the plants are adapted to it. It's amazing how green everything looks, the fresh green leaves of the palmettos and the new leafy branches of the shrubs hiding the charred sticks that remain from last year's burn. Notice your clothes and shoes; a prankster seems to have taken a charcoal pencil to them, sketching random strokes from thigh to sole while your attention was diverted by the spectacle of this phoenix plant community.
"It takes only two years to get full leaf cover back," Eric continues, much to my amazement. I stoop to grab a handful of sand and let it fall through my fingers. It is flecked with charcoal, but otherwise it looks like pure silica. Where are the plants getting their nutrients?
"These are dunes, about a million years old," Eric says, as he kneels and gently prizes a hole in the sand surface. He points to a grey layer that lies about five millimeters beneath the top of the hole. "There's a whole community of organisms in this crust. It includes cyanobacteria that fix nitrogen from the air. We've used tracers and found that the nitrogen fixed in the crust gets into the vascular plants."
It seems that the sand transmits enough light, not only for the cyanobacteria, but also for small plants: "We've found that rosemary seedlings germinate below ground and spend their first year under there too." The Florida rosemary is an evergreen shrub with tiny, fragrant leaves that is characteristic of Florida scrubs. As we walk back to the station we see a rare sandhill crane probing the mud of a temporary marsh for food. Pausing beneath some slash pines, I ask Eric whether the scrub is threatened by any of Florida's army of alien plants. Perhaps detecting a note of disappointment in my voice at not seeing any of the notorious demons, he is almost apologetic in his reply.
"No, though there are some alien species around the station buildings. They can't survive out here, its too tough for them. Not enough nutrients, and they aren't adapted to the fire regime. Cogon grass can be a problem, but we've got that under control here."
At that moment, two scrub jays arrive, chattering in the branches over our heads. They pose and dip in our direction, evidently interested in our presence and quite unafraid. The birds sport leg bands and belong to a celebrated population that has a scientific monograph devoted to them, published by Princeton University Press. This Ivy League avifauna expects attention, and gets plenty. As we retrace our steps toward the field station, I notice some familiar plants growing by the railroad track: Lantana, a notorious invader from the West Indies that now occurs in every tropical region, and a briar rose from Europe. These plants lurking in the wings are a warning of the fate awaiting the last remnants of Florida scrub, if prescribed burning is not maintained or human activities enrich the soil. Not only the Florida scrub plants, but sand cranes, scrub jays, and the gopher tortoise (with the numerous other creatures that live in its sand burrows) would be lost. Fortunately, at Archbold, the future of the scrub looks secure. The picture is not so favorable for natural habitats farther south.
From Archbold, we take Route 27 and head south for the Everglades. We quickly leave behind the ridge of old Pliocene dunes and descend into geologically newer terrain. The "river of grass," as Marjory Stoneman Douglas evocatively described the Everglades in her poetic book of 1947, once stretched a hundred miles from Lake Okeechobee to Florida Bay. Water from the lake spilled southward toward the sea, nourishing a marshland prairie dominated by sawgrass. "For sixty miles or so south of Lake Okeechobee the river of saw grass sweeps wider than the horizon, nothing but saw grass utterly level to the eye, a vast unbroken monotony. The grass crowds all across the visible width and rondure of the earth, like close-fitting fur."
Sawgrass was a demon in its natural habitat, but now those sixty miles south of Lake Okeechobee are cultivated and settled, and the lake itself is little more than a reservoir supplying agriculture and the burgeoning population of southern Florida. The remains of the Everglades must make do with what water is left after Floridians have irrigated their fields, filled their pools, and flushed their toilets.
The water from Lake Okeechobee no longer finds its own course to the sea, but is canalized and controlled every step of the way. It is in these canals that we find the first signs of trouble with demon plants. The waterways are plagued by water hyacinth, a native of the Amazon; water lettuce that reached Florida as early as the eighteenth century, and hydrilla, a relatively recent arrival. All three species have rapid rates of growth and overwhelm native aquatic plants, entirely replacing them and reducing oxygen levels in the water with dire effects for fish and wildlife. Millions of dollars are spent annually on controlling them with herbicides and other measures.
Water hyacinth is truly one of the world's worst weeds, able to double in population size in only fourteen days and infesting sixteen states of the United States and fifty-six countries in the tropics and subtropics. The plant is free-floating and has air-filled bladders that make it extremely buoyant. New plants are produced by budding and by seed. This combination of traits allows the plant to multiply and spread with ease, so that it can blanket the surface of an entire lake in one season of growth. Water hyacinth is successfully managed with herbicides but hydrilla, which grows submerged, has not been brought under control.
It is a curious thing that all hydrilla plants in Florida are female and so, for want of mates, produce no seed. However, the plant has two other means of vegetative propagation that serve it very well. Established plants produce small tubers on the roots that can reach densities of six thousand per square meter. As if this were not enough, the plant also produces small fleshy buds called "turions," which drop off the plant and sprout to form new plants. Turions can reach densities of three thousand per square meter. Tubers, turions, rapid growth, and no natural enemies make hydrilla a demon among demons.
Natural enemies can be used to control invasive plants if suitable herbivores or diseases can be found in a plant's native range. Alligator weed from South America is another alien water plant that was once a problem in Florida, but it has been successfully controlled by the introduction of three insect species from indigenous populations in Argentina. This kind of biological control is the ideal way to manage invading organisms, but natural enemies must be tested before release to make sure that they do not attack nontarget species. There are cases where this precaution has been ignored and the introduced predator has attacked native species in preference to the alien target, driving the natives extinct. Where biological control succeeds, it restores the kind of balance between a plant and its natural enemies that is normal in indigenous species. Alligator weed now survives at about 1 percent of its abundance before its natural enemies caught up with it. This quantity is sufficient to maintain a stock of natural enemies ready to seek out and destroy any new infestations of the plant that may appear.
Back on the road, we head further south along Route 27 and in the town of Clewiston, on the southern shore of Lake Okeechobee, we have our first encounter with Casuarina, or Australian pine. These trees have been planted as wind breaks, but they spread and grow rapidly to giant proportions, shading out all other plants with their melancholy, evergreen foliage. Once past the town, the casuarinas disappear, but as soon we cross the county line into Broward, at the northern edge of the Everglades, another sinister Australian appears. Paperbark is only sporadic at first, appearing in stretches along the roadside, but as we turn off Route 27 onto the minor road to the Everglades National Park the road is suddenly engulfed on both sides by tall, crowded stands of the tree. I have to stop myself from braking hard, the impression is so sudden, intense, and oppressive.
Tim Low, an Australian ecologist writes in his book Feral Future of his reaction to seeing paperbark growing in Florida:
I drove past dark forests of paperbarks much vaster than any growing in Australia, lining highways for tens of miles. American paperbarks grow straighter and more crowded than ours, with up to 37,000 trees per hectare, forming forests so gloomy that nothing grows inside. It was amazing to see a familiar native tree playing the role of supreme villain. These were the worst weed invasions I had ever seen.
The route we are on could well be the same one Tim Low describes—the impression is the same for mile after mile.
Paperbark was deliberately introduced into Florida and broadcast-sown from aircraft over the Everglades in the mid-1930s in a misguided attempt to drain them. Natural areas were bombed with these demons that suck five times more water from the ground than native sawgrass. Back home in Australia paperbark is attacked by a whole fauna of insect enemies, but in Florida nothing will eat it and, perhaps for this reason, trees grow bigger and better than in their native range.
Growing up to two meters a year, trees can reach thirty-three meters and a large one can produce 20 million wind-borne seeds in a year. Trees start to produce seeds when only two years of age: precocity is another demon habit. The spread of paperbark has been accelerated by large-scale alterations to the hydrology of the Everglades, which have increased the frequency of fire. Paperbark is adapted to fire in its native Australia, so burns only encourage it. In these conditions the number of paperbark stems can increase tenfold each year. At the height of the problem, paperbark occupied nearly two hundred thousand hectares (half a million acres) in southern Florida, though some areas were cleared of seed-bearing individuals in the 1990s.
Eventually the paperbarks give way to cultivated fields and as we near the town of Homestead dozens of nurseries raising and selling garden plants line the roadside. Horticulture is responsible for introducing more than its fair share of Florida's catalogue of woe, including a contender for the title of "Worst Demon": Brazilian pepper. This evergreen shrub with bright-red berries forms huge monotonous stands that exclude all native plants. In the mid-1990s it occupied seven hundred thousand acres of central and southern Florida and was still spreading. Brazilian pepper has destroyed the habitats of rare and endangered plants such as beach jacquemontia and beach star, and threatens the nesting habitat of the gopher tortoise in the Everglades National Park. In addition to the usual demon tricks of copious seed production, good seed dispersal, and rapid growth, Brazilian pepper appears to be toxic to other plants.
We have arranged to rendezvouz with three graduate students from Florida International University who will meet us at the entrance to the Everglades National Park and show us around. John Geiger, Hong Liu, and Jed Redwine are waiting for us when we arrive. John is crazy about wild plants and has lived in Florida most of his life: "Brazilian pepper is a great climbing tree, with all those branches. I used to love climbing in them as a kid." Hong is from Hainan Island, China and wants to study invasive plant problems when she has finished her doctoral degree. She jokes about how delicious Chinese water spinach is: "I want to introduce it here. It's so tasty!" Unfortunately, it has already has already been introduced in several parts of the state and the plant has proved difficult to eradicate.
Jed has conservation in his blood. His grandfather was a soil scientist in Oklahoma during the dust-bowl years of the 1930s. As a child, Jed heard how his grandparents couldn't eat for seven years without first blocking the gaps around the doors and windows of their house with wet towels to keep out the choking, powder-fine dust that billowed off the ruined farmland of the prairie: "I expect my lifetime to be a critical time. I expect to witness mass extinction. Most all my fellow grad students feel the same way. For me, my research is a continuation of what my grandfather did."
Most visitors to this part of the Everglades either make straight for the boardwalk just inside the entrance at Royal Palm, where there are spectacular views of the wildlife at close quarters, or they tear down the highway to the tip of the peninsula to launch their boats at Flamingo. We will do neither but are going instead to the "Hole in the Donut," a former agricultural area in the middle of the Everglades where there is a really serious Brazilian pepper problem. After a short drive, we pull up by the side of the road and get out. The day is beginning to get decently hot, but it is the dry season and there are mercifully few mosquitoes about. Looking down the road straight ahead, its two sides present an astonishing contrast.
On the left is a solid, impenetrable thicket of Brazilian pepper about five meters high, stretching away into the distance. Living stems of the bushes are interwoven with fallen, dead branches, forming a barrier that an English hedgelayer would be proud to have created, only this barrier is not the depth of a hedge—it is the depth of an entire field. At first, it looks as if there are no other trees in this horrendous thicket, but then John notices something. "Look! Its Ardisia. And here, that's guava! Guava forms thickets just like this in Hawaii." Guava and shoebutton ardisia are both shade-tolerant alien shrubs with fruit that many birds just love. Both are sprouting beneath the Brazilian pepper: round one in a three-cornered battle among demons.
On the other side of the road, not ten meters away, is an entirely different scene—a subtropical pine forest. The pines are widely spaced, tall, and graceful with no lower branches, so that their lofty canopies cast only a dappled shade on the plants below. They are southern Florida slash pines, belonging to the same species that we saw in the scrubs at Archbold, only here they are growing not in pure sand, but on solid rock. The two familiar palmettos from the scrubs are here too, but there the similarities end. Instead of the acid sand of the scrubs, the pine rockland, as this plant community is known, grows on solid limestone. There appears to be no recognizable soil, just plants springing direct from the rock, as if secured by bolts rather than not roots. I turn over a broken piece of rock cautiously, in case it conceals a scorpion or a rattlesnake, and find a tiny seedling sprouting in a crevice. Its roots are exposed and you can see how they thread their way through small holes in the rock that are like the pores of a sponge. Porous limestone can hold a lot of water, and this must be how the plants here survive. It is natural hydroponics.
John and Hong scour the ground for some of the wildflowers that occur here in abundance. "The flora here is very diverse," says John. "You can get ten different herb species in just a ten-by-ten-centimeter square, and about 15 percent of the flora is endemic." John points to a delicate mauve flower of the rockland ruellia and Hong finds a specimen of the ground cherry with nodding yellow flowers, each with a purple center. These will produce small fruits like a cherry tomato wrapped in a tiny, papery lantern. Jed is searching in vain for a small cycad called the "Florida arrowroot." The cycads are an ancient group of plants related to the conifers. They have very tough leaves and no doubt were once the food of herbivorous dinosaurs. Now Florida arrowroot is rare because so many of the plants have been dug up for their tubers, which are processed to produce a very fine starch that is sold as culinary arrowroot.
The diversity and density of species in the ground layer of the pine rockland is reminiscent of that found in chalk grassland (chapter 6), and the similarity is no coincidence. Both communities occur on thin, calcareous soils whose nutrient poverty limits the spread of demon plants that would otherwise crowd out the many small, slow-growing species.
But what about the demons next door? Why are they growing there? John explains that more than half a century ago, the native plants were torn down and a rock-crusher was used to chew up the surface, producing a "soil" of limestone gravel. Fertilizers were added and corn was grown in the middle of the Everglades. Now the fields have been abandoned, but the damage is done and demons have taken control.
There must be a huge rain of Brazilian pepper seeds into the remaining natural habitat from just across the road. Why haven't they invaded it? "We do sometimes find seedlings," Jed says, "but low nutrients and regular fire keep them out. This particular site is burned every two years." Once again, the importance of wildfire.
Florida's natural habitats are supremely elemental—they are bathed in water, nourished by rock, and protected by fire. Where fires have been prevented in the Everglades, or where nutrients have been added such as in the former fields in the Hole in the Donut, Brazilian pepper invades. Demons often need feeding, and this particular one is not well adapted to fire.
Understanding how Florida's natural ecosystems work is essential to protecting them from invasion. Unfortunately, there are alien plants that can invade even well-managed natural habitats. Florida's latest scourge is the Old World climbing fern from Southeast Asia, which can blanket forest trees. When burned, the fern carries fire into tree canopies that are normally protected from fire by their height from the ground. The trees have fire-resistant bark, but a crown fire kills them.
Unfettered demons such as Brazilian pepper, paperbark, and the rest of Florida's plagues seem to have enormous niches with limits that are impossibly wide. By completely trouncing the competition, these plants spread into habitats where they would not normally be able to grow at home. In Queensland, Australia, for example, paperbark is confined to seasonally wet places, but in Florida it also invades areas with standing water and well-drained uplands. By comparison, alien plants that have been successfully controlled by the introduction of natural enemies have shriveled niches that are much more specialized.
A fascinating example is the perforate St. John's wort (or Klamath weed), an alien from Europe that was once a pest in the grasslands of California and that is now controlled by an introduced beetle. St. John's wort hasn't disappeared altogether, but is now only to be found in shaded places where once it enjoyed the sun. The reason is that the beetle that lays its eggs on the plant and whose grubs eat it avoids shade, so plants growing there find a refuge from this herbivore. The combination of natural enemies that defoliate the plant and other plants that compete with it have turned St. John's wort into a species that skulks in dark corners. Looking at its distribution in California today, who would dream that St. John's wort was once a demon of the pastures, not the shy and retiring plant it now appears to be? These changes were wrought by predation and competition acting together. Just like in grasslands (chapter 6), herbivory weakens the demon and competition then confines it to a narrow niche. How many native plants are similarly restricted we do not know, perhaps many.
It would be very useful to be able to predict which among the thousand or so nonindigenous plants growing wild in Florida that are presently harmless will one day become demonic. If this could be done, these species might be removed before they prove problematic. Unfortunately, as we already know, there is a demon in every plant and research has not yet discovered a reliable method of predicting which plants will realize this potential. The best predictor of trouble is a plant's record of prior offenses. If a species is already a demon in one area of introduction, it is highly likely to cause problems elsewhere too. The difficulty is that not every demon has a previous record of invasiveness. The Exotic Plant Pest Council of Florida keeps a list of species that have known invasion potential. Despite the recognized dangers, many of these "usual suspects" are still commercially available in Florida.
Each alien species has its own history and an individual demonology, but there is a common feature found in many plant invasions that offers a warning and an opportunity. In the histories of many invasive plants in Florida, there is a delay of several decades between its introduction and its eventual spread. The warning in this knowledge is that there must already be new demons in the pipeline. The opportunity is that delays may provide time to spot demons and to eradicate them before they get out of hand. Brazilian pepper trees took fifty years to become pervasive and paperbark was present for a similar period before a problem was recognized. Japanese honeysuckle was grown in gardens for eighty years before it ran amok from the Gulf of Mexico to Massachusetts, reaching as far west as Kansas. Delays of the kind seen in the spread of alien plants in Florida have also been observed elsewhere and may be an inherent feature of the invasion process. Perhaps a species must reach a critical density of colonies before its spread begins to take off, or maybe it takes time for natural selection to bring about adaptation to a new habitat.
There is only one case in Florida where the cause of the delay is really understood—that of the laurel fig. A native of Asia, this fig was grown as a street tree in Florida for many decades; it never set fruit, however, because it requires pollination by a particular species of tiny fig wasp, which had not been imported with the tree. Then, sometime in the 1970s, fig wasps must have arrived with a consignment of new plants, because laurel figs started to mature their fruit and to spread by seed. Fig seeds are dispersed by birds and the seedlings can grow in rock crevices, on buildings, and in the leaf-bases of palm trees. Laurel fig has now invaded many natural areas in southern Florida. Knowing that invasive plants tend to have a refractory period before their spread takes hold suggests that there is a window of opportunity when a small effort at control could avoid big problems later: a stitch in time saves nine. Research is needed on what happens at the transition between the refractory period and the period of spread, so that invasions by new species can be nipped in the bud.
Florida's troubles with alien invasive plants illustrate the practical need for a better understanding of what turns plants into demons abroad. Joseph Hooker summed up the problem rather well in 1853 in the book he wrote on returning from his travels in the Southern Hemisphere (chapter 1): "We have the apparent double anomaly, that Australia is better suited to some English plants than England is, and that some English plants are better suited to Australia than those Australian plants which have given way before English intruders."
To these anomalies might be added those Australian plants such as paperbark that grow better in Florida than do the natives of the Everglades. One might have thought that such anomalies would have worried Darwin because they strongly suggest that native plants are not as well adapted to home conditions as are species from halfway around the world, and that plants can grow better in foreign parts than in the region in which they evolved. How can natural selection explain this? (And, lest any creationist take comfort from this question, note that it is just as much of a problem if you believe that God placed each species in its right place.) Strangely, this is a question Charles Darwin ignored in the two chapters on geographical distribution in The Origin of Species.
Eighty years after Joseph Hooker puzzled over the anomalous behavior of traveling plants, another Englishman raised the same question. In the words of the arch snob and elegant wit Noel Coward: "Why, oh why, do the wrong people travel, when the right people stay at home?"
Is the explanation of demon behavior that the wrong plants travel, or that travel itself creates the problem? Since we can compare how the same species behave in indigenous and in alien populations, we do actually have an answer to this question, but it comes in two parts. The first has to do with the likelihood of a plant reaching foreign shores. Species with larger natural geographic ranges are more likely to turn up as aliens elsewhere. In this respect it is the plant that "elects" to travel. The second part of the question has to do with whether, once arrived, an alien plant then becomes an invasive demon or just sojourns awhile. Here, the answer is that there is no reliable correlation between a plant's abundance at home and how it behaves abroad. A good example is found in a group of three herbs in the genus Impatiens that occur as aliens in the British Isles. The most demonic of the three is Himalayan balsam (Impatiens glandulifera), but this species has the smallest natural geographic range, being confined to a narrow altitudinal belt between two and two-and-a-half thousand meters in a part of the western Himalaya. Impatiens capensis, the least widespread alien of its genus in Britain, has the widest natural geographic range of the three.
Evidently, travel itself turns some plants into demons. The ultimate proof of this is where two regions have swapped plants. The Everglade's gift to the home of paperbark is pond apple. In an irony of ironies, pond apple is now becoming scarce in Florida, but it has invaded Australia's paperbark swamps where it is considered the greatest of threats to the tropical wetlands in northern Queensland. Paperbark itself is a declining species in Queensland. Paperbark and pond apple each seem to grow better in the other's habitat than in their own. This is a severe shock to anyone who believes in the perfection of nature. What is going on?
The likeliest explanation is that travel enables demons to escape from the natural enemies such as insects, fungi, and diseases that attack them. The successes of biological control, which reunites plants with their enemies, bear this idea out. When we travel abroad we tend to catch local diseases or succumb to bugs in the water or the food. For plants it is quite the reverse. When they leave their native range they carry no baggage (not even leaves if they travel as seeds) and they usually leave behind all their natural enemies.
In Europe, purple loosestrife is plagued by a whole specialized fauna of herbivorous insects, but in North America these are absent and not one of the native insects has developed a taste for this exotic dish. Instead, not surprisingly, North American insects eat North American plants. This puts native plants at a disadvantage when they come into competition with aliens. The native wetland flora is weakened by attacks from its natural enemies, but the alien is not. This may well tip the balance into demonic behavior for a plant like purple loosestrife that is already so well prepared for this role. In sports competitions it is the home team that usually has an advantage over any visiting team. In the natural world it is often the reverse and visitors dominate the play.
The altered conditions that aliens encounter outside their natural range must also explain why, on occasion, they actually do better than either their confreres back home or the natives that are adapted to local conditions. Evolution is not a magic wand, and even though it can provide a plant with armaments against its natural enemies, it cannot banish those enemies entirely because they too evolve in response. By contrast, plants that, with human help, leap ocean barriers do get a magic carpet ride to a land of Cockayne where no disease will touch them and every herbivore finds them distasteful.
If escape from natural enemies explains the demonic behavior of alien plants, there still remains another mystery that is less easily explained. Why is the New World so troubled by plants from the Old one, while Europeans have much fewer problems with New World weeds? Darwin even pulled the leg of his American friend Asa Gray on the subject, asking him in a letter "Does it not hurt your Yankee pride, that we thrash you so confoundedly? I am sure Mrs. Gray will stick up for your own weeds. Ask her whether they are not more honest, downright good sort of weeds." To which her riposte was that American weeds were "modest, woodland, retiring things; and no match for the intrusive, pretentious, self-asserting foreigners."
Joseph Hooker described the situation as a "total want of reciprocity in migration." Trade and travel brought aliens plants from Europe to North America, but although human colonization was one-way, the traffic of goods and ships was not. Raw materials transported from all over the globe brought the seeds of alien plants to Britain in the nineteenth and early twentieth centuries. Imports of wool were a particularly rich source of seeds, amounting to 900 million tons a year just before World War II. Wool from Australia, Africa, southern Russia, Asia, and South America arrived at the woolen mills where it was washed and the seed-laden water was passed back into local rivers. Galashiels, which stands on the River Tweed in Scotland, just over the border from England, became a mecca for botanists seeking chance encounters with exotic plants. The wool supplied the manufacture of tweed cloth that was once in great demand.
In 1919, Ida Hayward and Claridge Druce published a book cataloging their discoveries made on the banks of the Tweed. They identified no fewer than 348 species of "wool alien." Nearly half were from Europe and the Near East, but significant numbers also came from Australia, South Africa, and the Americas. Now these plants are as rare in Galashiels as a tweed jacket on a Paris catwalk. The only survivor from the 348 species recorded by Hayward and Druce is a New Zealand plant of the rose family, the pirri-pirri bur. This plant found its way onto the Holy Island of Lindisfarne, which lies at the mouth of the River Tweed, where it is now a local nuisance. The Holy Island is famous for the illuminated Lindisfarne Gospels produced on the island in the eighth century. A translation of the Gospels from Latin into Old English was added between the lines of the manuscript in the tenth century, making Lindisfarne the source of the earliest surviving copy of the Gospels in the English language. The woolen industry of Galashiels might have been designed for the specific purpose of introducing troublesome plants into Scotland, so efficiently did it gather, concentrate, and propagate aliens along the banks of the Tweed. What an irony that all that now remains of the wool aliens is a castaway demon on Holy Island!
The fate of the wool aliens of Galashiels could not underline more strongly the "total want of reciprocity in migration" that Joseph Hooker described a century and a half ago. Only about 5 percent of the plant species found wild in Europe come from elsewhere, while the comparable figures for other regions of the Northern Hemisphere, such as Ontario in Canada or Florida in the United States, are about 30 percent. The wool aliens demonstrate that this difference cannot be due to a failure of the seeds of alien plants to reach Europe, and certainly Europeans are as keen to introduce and grow plants from foreign places as gardeners anywhere are. Hundreds of thousands of varieties and species of plants are grown in European gardens and have only to leap a fence to run wild. Surprisingly, few are successful. Why is this?
Climate must be part of the answer. Plants that can tolerate the climate of northern Europe when tended by a gardener cannot do so when exposed to the wild, where they also have to contend with competition. As always, Darwin perceived the situation clearly. He remarked that there were a "prodigious number of plants in our gardens which can perfectly well endure our climate, but which never become naturalised, for they cannot compete with our native plants, nor resist destruction by our native animals."
The cooler the climate, the harder it is for aliens. There are many more alien plants in the south of England than in the chillier north or in Scotland, and the same pattern is repeated on a larger scale throughout Europe. However, an inhospitable climate cannot be the whole reason for the different success of aliens in the Old World and the New. Even in warmer parts of Europe, few aliens turn demonic. A dozen of the pasture weeds that infest the grasslands of northern North America, including St. John's wort, are native in British grasslands, but not one alien grassland species has managed to reciprocate. Climate cannot explain this imbalance, so what other explanation can there be?
The North American grasslands that have been invaded by aliens are not those few remnants of native prairie where once the bison roamed. The grasslands that alien weeds invade are as foreign to North America as the aliens themselves and they are grazed by livestock whose ancestors also came from Europe. In fact, these grasslands are whole transplanted ecosystems. The grassland plants from Europe that are palatable to livestock have acquired a Green Card and obtained legitimate employment feeding cattle and horses. Some have even acquired an American alias such as "Kentucky bluegrass," which back home in England goes by the name of "smooth-stalked meadow grass." The disowned, invasive weeds are simply their less palatable traveling companions.
These North American grassland communities were created, wittingly or otherwise, by European settlers raising European animals. It should therefore be no surprise that European plants found the conditions created by this kind of farming, which was unknown in North America before European settlement, so much to their liking. The introduction of European livestock into North America, Argentina, Australia, New Zealand, and South Africa quite literally prepared the ground for the invasion of alien plants from the same source. Realizing this, and looking back across the ocean, it suddenly seems less surprising that Mrs. Gray's "modest, woodland, retiring" American weeds have failed to invade Europe. Were North American woodland environments to be recreated in Europe, the story might be quite different.
Alien plant invasions are very rarely accidents of nature. Some such as kudzu or paperbark were deliberately aided by misguided human action. Others, like the aquatic plants released into Florida's waters to provide commercial wholesalers with a convenient source of aquarium plants, were accidents waiting to happen. Another group are pasture weeds on a spree in habitats that feel to them just like home, but without their accustomed pests and diseases. There are many flavors of folly, and now that we are familiar with some of them it would be wise to avoid past mistakes. This must mean looking with a wary eye at any possible source of new demonic plants.