Demons in Eden considers these questions using the latest scientific discoveries from the plant world. Readers join Silvertown as he explores the astonishing diversity of plant life in regions as spectacular as the verdant climes of Japan, the lush grounds of the Royal Botanical Gardens at Kew, the shallow wetlands and teeming freshwaters of Florida, the tropical rainforests of southeast Mexico, and the Canary Islands archipelago, whose evolutionary novelties—and exotic plant life—have earned it the sobriquet "the Galapagos of botany." Along the way, Silvertown looks closely at the evolution of plant diversity in these locales and explains why such variety persists in light of ecological patterns and evolutionary processes. In novel and useful ways, he also investigates the current state of plant diversity on the planet to show the ever-challenging threats posed by invasive species and humans.
Bringing the secret life of plants into more colorful and vivid focus than ever before, Demons in Eden is an empathic and impassioned exploration of modern plant ecology that unlocks evolutionary mysteries of the natural world.
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Demons in EdenThe Paradox of Plant Diversity
By Jonathan Silvertown
University of Chicago PressCopyright © 2005 Jonathan Silvertown
All right reserved.
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 knownbecause 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.
Excerpted from Demons in Eden by Jonathan Silvertown Copyright © 2005 by Jonathan Silvertown. Excerpted by permission.
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Table of ContentsPreface
1. An Evolving Eden
2. The Tree of Trees
3. Succulent Isles
4. Demon Mountain
5. The Panama Paradox
6. Nix Nitch
7. Liebig's Revenge
9. New Demons?
10. The End of Eden?
Scientific Names of Plants Mentioned in the Text
Sources and Further Reading