Of course, there are qualities that predispose a species to be a successful invader. Weedy species the world over have been crafted by evolution into skilled opportunists. They often have resistant seeds or spores that can wait as long as it takes for the right moment to germinate. It helps if you grow fast, produce lots of offspring quickly, and don’t mind mating with relatives. It is also good not to be too picky about where you live and to enjoy novelty and variety in your diet. I wouldn’t have singled out lionfish as likely invaders of the Caribbean from my encounters with them in the Red Sea. They seemed too shy and delicate and were never particularly common. Surrounded by animals that know well to avoid their dangerous beauty, they have to work hard for a meal. There was no hint of the profligate glutton they would become in the all-you-can-eat buffet of the Caribbean.
Few places on this planet have not been reached by some marine invader or other. Not surprisingly, some of the worst affected places have the highest ship traffic. The U.S. West Coast, northern Europe, and the Mediterranean are all hot spots of invasion. But some places are more vulnerable than others. Hawaiian seas have been invaded time and time again.16 Like the islands above them, Hawaii’s seas are rich in species found nowhere else. On land, invasives have devastated plants and animals ill-equipped to deal with aggressive conquerors. In the sea around Oahu many areas are now dominated by invaders: mangroves from Florida line Pearl Harbor; one of the most common shore barnacles is newly arrived from the Caribbean; and blacktail snappers from Moorea and bluestripe snappers from the Marquesas are among the most abundant large fish in shallow waters. Over three hundred alien plants and animals are now known in Hawaiian seas, and these numbers do not take into consideration potentially hundreds more that may be alien but whose histories are less well-known. On the face of it, Asian seas have been less affected by aliens, with reported numbers less than a fifth the number of new arrivals into the U.S. West Coast and Hawaii. But the long-term presence of Southeast Asia at the epicenter of trade in consumer goods tells us to distrust these statistics. Asian waters are probably crawling with invasive species but surprisingly few people have cared to look so far. A Caribbean variety of zebra mussel, the black-striped mussel, has established at high densities all around Singapore and part of Malaysia, for example.17
The least invaded seas in the world are at the poles. At the time of this writing there was just one marine invader known in the Southern Ocean and nine in the Arctic. The poles have been spared for several reasons. Not many people live there so boat traffic is sparse. Polar conditions are extreme and few species from anywhere but the opposite pole could cope with them. It is hardly likely that they could find a way between the poles. Times are changing, though, and it is very probable that polar seas will soon find themselves home to many more alien species. Loss of sea ice in the Arctic Ocean due to climate change has led to the mixing of seawater from the North Pacific with the North Atlantic for the first time in eight hundred thousand years. We know this from the arrival in the Atlantic of a species of North Pacific planktonic diatom that has been absent that long.18 A far bigger Pacific traveler reached the Mediterranean in 2010, where astonished Israeli scientists spotted a gray whale.19 The last Atlantic gray whales were slaughtered by New England whalers in the seventeenth or eighteenth century. This one must have lost its way and inadvertently navigated the newly ice-free route to the Atlantic.
Global climate change is warming polar seas enough to make life there possible for species from lower latitudes. On a changing planet, these range expansions will come to make up a large fraction of new introductions. They might also gain a helping hand from us. The prospect of an ice-free Arctic in summer has once more excited great interest in a northwest passage for ships between the Atlantic and Pacific. Dozens of explorers lost their lives between the sixteenth and nineteenth centuries in search of this elusive route, their efforts doomed by savage ice. As the lost gray whale shows, it might soon be a reality, and without great care, ships could take many new species into the Arctic and provide a fast track between the Atlantic and Pacific for countless others. The increasing quantities of floating plastic and fishing debris to which fouling species can attach themselves may also carry aliens to polar seas, although there is little firm evidence of that yet.20 Ocean garbage has multiplied the means of hopping from place to place in the sea and can go where few ships travel.
Not all alien species cause problems. Some may blend into their new homes with little fuss. They add color and variety, like a new picture hung on the living room wall. But significant numbers, perhaps the majority, are harmful, as judged by their impacts on native wildlife or to human interests. An estimate made in the year 2000 suggests that invasive species cost us $1.4 trillion every year in trying to undo the damage caused, eradicate the invaders, or in economic losses sustained from degradation of ecosystem services, such as clean water provision or soil stabilization.21 Those troublesome Chinese mitten crabs have so filled Germany’s levees with burrows that they are crumbling. The vast majority of this damage can be attributed to terrestrial invaders. So far little attention has been given to alien species in the sea, but we can be certain that the costs of aggressive new colonizers, like Seattle’s gribbles, are quietly building beneath the waves.
While much of the attention around invasive species has concentrated on costs, some create new opportunities. When the French engineer Ferdinand de Lesseps achieved his life’s goal to join the Mediterranean and Red seas by digging a canal between them, he managed what countless others had dreamed of since pharaonic times. The Suez Canal runs at sea level. As soon as it opened in 1869, water began to flow between seas that had been parted for over eleven million years and has carried tide upon tide of emigrants. Since Mediterranean Sea levels are lower than those of the Red Sea by four feet, most of the flow has been northward, and invasions were much facilitated by the lower biodiversity of the Mediterranean. To begin with, the invaders’ paths to virgin territory was blocked by a series of briny lakes—the Bitter Lakes—in the middle of the canal. Over the course of a century, flushed by seawater, the lakes became less inhospitable. The rate of colonization of the Mediterranean increased from drip to trickle to steady stream.
Early colonists burst forth into a place with few predators or competitors to challenge them. The eastern Mediterranean had long been thought of as impoverished, with low productivity and few species compared to the richer west. The contrast from north to south through the canal was even greater. The Red Sea hosts an extravagant diversity of life that thrums around vibrant coral reefs. While few corals can tolerate the cool Mediterranean, many reef inhabitants were more cosmopolitan and soon settled in. Several of them now sustain valuable commercial fisheries in Israel, Egypt, and along the North African coast as far as Tunisia.22 They include rabbitfish, lizardfish, goatfish (only the lizardfish looks anything like its terrestrial counterpart; imagination seems often to have deserted the namers of fish), and a clutch of prawns, three of which were fat and juicy enough to be welcomed.
It is premature to say this without fear of contradiction, but places with few species seem to be easier to invade than richer communities.23 The idea is that resources are less fully used so would-be invaders can gain a foothold more readily. According to this logic, the eastern Mediterranean was ripe for the taking. The problem is, by extending this logic one has to conclude places where human or other impacts have picked off the natives will be highly vulnerable to alien invaders. Mass mortality of corals and sponges from disease and heat stress has left vast areas of free space, for instance. Given the ubiquity of human sources of stress and their multiplicity of effects, it is a near certainty that most ecosystems in the sea have lost at least some of their resistance to invaders. Their defenses are down just when they need them most. The onslaught has only just begun.
In some ways, the changes to nature that we have unleashed through travel and trade mirror the effects of globalization on human culture. Some cultural influences are l
ike aggressive invaders that, within the space of half a century, have subsumed and erased cultures that flourished for millennia in isolation. Explorers and filmmakers today struggle for days or weeks through sweltering jungle, across torrents and treacherous ravines, only to find on arrival people sporting ragged T-shirts emblazoned with Coca-Cola or Toshiba. So it is that in countryside and town, lake and ocean we find familiar faces in unusual places. What we often miss are the plants and animals they have displaced, that in the past made these environments exotic and strange.
Invasive species are one of the five horsemen of the apocalyptic loss of biodiversity that is sweeping the planet (the others are habitat loss, pollution, climate change, and overkill by hunting or fishing). Yet, strangely, invasives have been neglected relative to the others, particularly at sea. Although they have purged habitats and ecosystems of their rightful owners—just as with habitat loss, pollution, or climate change—they do not yet seem to have caused many outright extinctions in the sea.24 Native species often hang on in pockets of poor quality habitat, just as peoples quashed by conquering armies find themselves marginalized and browbeaten. Native mullet in the Mediterranean have been pushed deeper by invasive goatfish. But local population crashes can eventually lead to disappearance, and there are many scientists who believe that invasive species in the sea may yet drive natives to extinction given enough time. The first may be closer than we think.
The Derwent River estuary of Tasmania is home to a little fish with the bizarre habit of walking on its fins. Its pectorals—the fins on either side of the body behind the gills—look like arms, and each ends in a webbed “hand,” hence its name, the handfish. Spotted handfish, as this one is called, have flesh-colored bodies speckled red, fins edged in lemon, and dark eyes lined with gold. They display to one another in a semaphore of fin flashes and showy standoffs. Nobody knows why they are found here and nowhere else, but such a limited geographic range means local population loss and global extinction are one and the same. In the 1990s, their universe was upheaved by the arrival of the North Pacific starfish, an undiscriminating predator from Japan and East Asia whose numbers have since swelled to millions. Among their predilections are spotted handfish eggs, laid in nests on the seabed. There could be as few as a couple of hundred to a couple of thousand spotted handfish left in the wild, but it is hard to know, since they are difficult to count. While all species have a right to exist, the loss of one so charming seems especially heartbreaking.
Geologists recognize distinct epochs in Earth’s history according to the fossils and rocks present and give them names. The most recent, the Quaternary, covers the last 2.6 million years and is subdivided into the Pleistocene and Holocene. The Pleistocene spans the time of the ice ages, while the Holocene covers the twelve thousand years from the end of the last ice age to the present. Some scientists with a humorous bent have suggested that the present era should be called the Homogecene, because it is a time of unprecedented mixing of animals and plants from one part of the world to another. While efforts are being made to control the spread of invasive species, I wonder about their efficacy in the long term. This is an uncontrolled experiment with life. Who knows what will come of it?
When I was a student in the 1980s my professors made much of the problems of alien species and how they could lay waste to the native fauna and flora in their new homes. Rabbits ate their way through Australia, water hyacinths choked lakes from shore to horizon, killer bees swarmed into the southern United States, and Nile perch chomped their way through an irreplaceable evolutionary showcase of brightly colored African cichlid fish. The solution, in those days, usually involved introducing something else, following the logic of the old nursery rhyme about the woman who swallowed a fly, and then a spider to catch the fly, and a bird to catch the spider, and so on until she had eaten her way up the food chain. Like this hapless lady, who dies in the end as you may recall, the risk is that further introductions will do more harm than good.
There was a dramatic case of such an effect in French Polynesia. Giant African land snails were accidentally introduced and began to damage crops, so a voracious predator was brought from its homeland to control it. This snail, called the rosy wolf snail, developed a taste instead for the colorful tree snails that lived deep in moist valleys. Every valley had its own species of tree snail that had evolved in isolation, separated from the others by craggy mountain ridges they could not cross. The rosy wolf snail soon jumped these barriers to munch its way through an evolutionary cornucopia whose remnants now hang on only in zoos scattered across the world. As far as I know there haven’t yet been any tit-for-tat introductions in the sea, so dreadful mistakes like this haven’t been made, but the temptation is there.
Introduced species—biological pollution—is one of the hardest kinds to clean up. People have spent lavishly on efforts to rid tiny scraps of land from the scourge of rats so as to spare rare seabirds or unique species like lizards. The biggest islands on which they have been successful to date are little more than a square mile in area, and they are unlikely to get much bigger. If you can catch an invader early and hit it hard, success is possible.25 The black-striped mussel, a native of the Caribbean now established in Singapore and Fiji, was detected in Darwin Harbor in Australia in 1999. Within nine days the harbor was quarantined and pumped full of bleach and copper sulfate, which killed pretty much everything, including the mussel. In California, a parasitic worm that invades commercially valuable abalone was introduced with animals brought from South Africa. It established at Cayucos and was successfully eradicated by hand removal of infected abalone and reduction in overall abalone densities below the level needed to sustain the parasite. This was easy to do for an animal that lives in shallow water stuck to rocks and is the coveted target of skilled fishermen. Once an invader has spread beyond a few sites, however, it is pretty much hopeless to try to eliminate it. Local control is the only option, such as the enthusiastic bands of spearfishers attempting to rid Caribbean reefs of lionfish, and the chefs who encourage us to eat them.
Until recently it was almost heresy in conservation circles to admit that introduced species were here to stay and should be treated as valid members of their new ecosystems. They are persecuted and discriminated against and treated as the most reprehensible forms of life. In the UK, some conservationists still treat species with contempt that arrived five hundred years ago! When do you draw the line? I have loved beech trees since I was small, with their muscular gray trunks, smooth coppery leaves, and funny triangular nuts. It was only years later that I realized what a fool I had been to be taken in by its shallow appeal. This tree was an alien! An imposter that had only made the hop from mainland Europe in the Middle Ages. I am joking, of course; I still love beeches. People mourn loss of the familiar, but there are good reasons to resist the blending of the world’s fauna and flora as the planet shrinks beneath our globalizing influence. This mixing represents a real loss of diversity and variety, and sometimes it takes generations to realize how much intruders have altered their new environments. When I look at a picture of a coral reef I can tell from the species present where it was taken, give or take a couple of thousand miles. Each place has its own distinctive collection of species.
Even if only a small fraction of introduced species cause trouble, their effects on native ecosystems can be hugely disruptive and expensive. Like pollution by toxic chemicals or plastic, the best means of control is prevention. Since the two main vectors for the spread of invasives are ships and aquaculture, the onus is on these industries to deal with the problem. One simple solution for aquaculture would be to stop raising species outside their native ranges. With nearly four hundred species in culture right now, that shouldn’t restrict options much and would benefit consumers by giving them a wider range of ingredients to choose from. Ships could solve the problem of stowaways relatively easily by installing purification systems that use spare heat from the engines or UV light to kill unwanted passengers. Another simple
measure that can reduce harm is to exchange ballast water offshore rather than in coastal seas. The idea is to avoid exchanging coastal species since these are the ones that cause most harm. Jim Carlton, from Williams College in Connecticut, thinks he has seen fewer invasions of U.S. seas due to ballast water since open-ocean exchange became mandatory in 2004. Similarly, he has heard far fewer reports from Australia or New Zealand, where similar regulations were introduced a decade ago.
On the outside of boats, the usual approach to invasive prevention has been to slather toxic chemicals over the hull, but some of these, like tributyltin, proved terribly toxic to other species. An alternative approach involves coating the hull with a nontoxic layer of vinyl resin reinforced with glass,26 which reduces drag and limits the ability of fouling species to get a grip. It can be maintained by high-pressure tools that kill any species that do manage to establish on the ship. Like agreements on toxic chemicals, these measures can only succeed with international regulation.
CHAPTER 13
Pestilence and Plague
The Ocean of Life Page 21