A. Invasive species often triumph as a result of good intentions gone wrong. Take Japanese knotweed (Fallopia japonica), introduced to Britain by enthusiastic Victorian gardeners who thought it an ornamental delight that doubled as cattle feed. But from just a scrap of root no bigger than a pea it could grow through tarmac, pavements, and brick walls. A century later, its spread is considered such a threat that planting or dumping knotweed is a crime, knotweed is so hated because it suffocates other plants, replacing them with an unproductive, leafy monotony. Then there is the Nile perch ( Lates niloticus), branded one of the world's worst invaders by conservationists. It is a freshwater fish that can grow to huge proportions. Again, with good intentions, it was introduced in 1954 to Lake Victoria, straddling Tanzania. Kenya, and Uganda. Since then, it has helped push over 200 well-established local fish species to extinction. Like the Nile perch, the cane toad (Bufo marinus) eats almost anything it gets its mouth around. Introduced for pest control, it turned out to be noisy, fast spreading, and a greater pest itself.

B. As it is in nature, so it is in the economy. Big superstores and chain retailers were allowed to spread by planners, town councils, and governments in awe of big business. But then it started to go wrong. The chains became the economic equivalent of invasive species: hungry, indiscriminate, often antisocial and destructive. When no one was paying much attention, the superstores and cloned shops grew to dominate and suffocate the economic ecosystem. They passed through planning regulations as easily as knotweed pushes through tarmac, devoured smaller and independent retailers with as much reflection as the Nile perch cleansing Lake Victoria of competition. They were often introduced to provide a specific service but outgrew their habitats until their cash till song could be heard on every street corner, forecourt, round about, and out of town shopping centre. Neither in balance, nor even a boom bust cycle with other similar, local species of shop; they began permanently to displace them.

C. Natural scientists use a whole new term to describe the current epoch of comprehensive. global human interference in ecosystems. Our time, they say, should be called the "Homogocene" to describe the way that distinctiveness and difference are being eroded. A combination of the creep of invasive species and habitats destroyed by development is driving a mass extinction. The World Conservation Union warns that such invasions are leading to the irretrievable loss of native biodiversity. Typical characteristics of an invasive species include the absence of predators, hardiness, and a generalist diet. Whatever the reason for their arrival and proliferation, invasive species tend to cause a disruption of the ecosystem that is catastrophic for native species.

D. The big, centralised logistical operations of the supermarkets are likewise driving the homogenisation of business, shopping, eating, farming, food, the landscape, the environment, and our daily lives. In the process, Britain is being sucked into a vortex of US-style, chain-store-led. clone retailing, both in towns and in soulless "big box" out-of-town shopping parks - what they call in the US, with its associated suburban sprawl, the "dead zone". They are spreading in the way "invasive species" spread in nature, lacking checks and balances, killing off diversity and "native" (in other words, local) species. Tesco is not the only guilty party (think of McDonald's. Starbucks, and Gap), but it is possibly the largest driving force. With around 2.000 stores in Britain, almost one third of the grocery market, and rapid international growth, city analysts believe the brand has the land and resources in place already to double its UK floor space. Can anything stop it?

E. Bear in mind those characteristics of an invasive species: the absence of predators (real commercial competition or effective regulators to hold them back); hardiness (the legions of corporate lawyers, financial leverage, and endless commercial cost cutting); and a generalist diet (supermarkets will sell virtually anything, and chain stores operate according to a low common denominator). If you want diversity in your world rather than one kind of plant in your garden, one kind of fish in your lake and only one type of venomous, croaking toad under your shed, then you have to manage for that outcome. When we garden, we hold back aggressive, opportunistic plants in order to keep space open for a celebration of variety and colour.

F. Like it or not (and it is something about which most policy makers and economists are in deep denial), weakly regulated markets give free rein to economic invasive .species and hence tend towards monopoly. This is the great modern economic irony. Advocates of free markets argue against checks and balances to counter the power of big business, but in doing so ultimately destroy the possibility of markets that could meaningfully be called free, or, rather, "open". They resist anti-monopoly regulation in the name of providing consumer choice, and in the process they ultimately destroy it. In some important ways, we are returning to an earlier phase of corporatism. Henry Ford told customers they could have any colour of car, as long as it was black. The scale and seriousness of Tesco's ambition mean that, before long, unless we recognise what is happening and have regulators up to the job, one day we will be able to shop anywhere we like, as long as it is Tesco.

A. In the 2002 football World Cup, France, the reigning world champions, suffered a humiliating defeat to unfancied Senegal. All 11 members of the victorious Senegalese team had played for European clubs. They were not alone. By 2000, the first and second divisions of Europe’s leagues had poached enough African players to field 70 teams. So, have greedy European clubs deprived Senegal of its best footballers, or has the prospect of a lucrative career in Europe encouraged more Senegalese to take up the beautiful game?

B. This question is posed by a new book, “Give Us Your Best and Brightest”, by Devesh Kapur and John McHale. The authors are development economists first, football fans second (if at all). They see the emigration of African players as a highly visible example of the “brain drain”. Less visible, but more worrying, is the departure of the poor world’s doctors, nurses, and teachers to more lucrative job markets in the rich world. Ghana, for example, has only 6.2 doctors per 100,000 people. Perhaps three quarters of its doctors leave within ten years of qualifying.

C. The answer to the Senegal conundrum is of course “both”: the best players leave, and the dream of emulating them motivates many others to take their place. The real question is whether the second effect outweighs the first, leaving the game in Senegal stronger or weaker than it otherwise would be. A few economists, including Andrew Mountford, of Royal Holloway (part of the University of London), and Oded Stark, of the University of Bonn, think the nett effect of the brain drain is similarly ambiguous. The prospect of securing a visa to America or Australia should tempt more people in poor countries to invest in education. Mr. Stark calls this a “brain gain”. If the temptation is strong enough, and the chances of landing a visa low enough, the poor country could even come out ahead: it might gain more qualified (if disappointed) doctors and engineers than it loses.

D. As with all debates about the brain drain, theory has run ahead of evidence. The numbers on international flows of people are much patchier than those on cross-border flows of goods or capital. In a recent paper, Mr. Stark and his co-authors investigate internal migration instead. The rural villages of Mexico lose many of their brightest sons and daughters to jobs in cities or border towns. Those Mexicans who leave their home villages tend to be better educated than those who stay. Despite this, the example the leavers set (and the job leads they provide) raises the average level of schooling of those left behind. Because they can aspire to a world beyond the village, even if they never reach it, young Mexicans have an added reason to stay in school beyond a ninth year, the authors show.

E. Even if the brain drain does leave a country with a better-educated populace, is this necessarily a good thing? Education is not free, and some of those who gambled on a diploma as a ticket overseas will regret their decision. Mr. Stark assumes that people in poor countries tend to demand too little education. A person’s productivity depends on the skills of those around him, as well as his own. Because of these spillovers, an individual’s education is worth more to the economy as a whole than it is to himself, and he will underinvest in it as a result. Mr. Stark sees limited emigration as one way to fix this market failure.

F. India’s software engineers are perhaps an example of this principle at work. Indian students had little reason to learn computer coding before there was a software industry to employ them. Such an industry could not take root without computer engineers to man it. The dream of a job in Silicon Valley, however, was enough to lure many of India’s bright young things into coding, and that was enough to hatch an indigenous software industry where none existed before.

G. India’s valley dwellers represent just one contingent in a much larger diaspora. According to the most exhaustive study of the brain drain, released last month by the World Bank, there were 1.04m Indian-born people, educated past secondary school, living in the 30 relatively rich countries of the OECD in 2000. (An unknown number of them acquired their education outside their country of birth, the report notes.) This largely successful diaspora is more than just something to envy and emulate. Its members can be a source of know-how and money, and provide valuable entrées into foreign markets and supply chains.

H. Messrs Kapur and McHale think India’s relatively happy experience with its educated émigrés is more likely to be the exception than the rule. Its million-strong brain drain represents just 4.3% of its vast graduate population, according to the Bank. By contrast, almost 47% of Ghana’s highly educated native sons live in the OECD; for Guyana, the figure is 89%. This is not a stimulative leeching of talent; it is a haemorrhage.

I. Emigration, as Mr. Stark suggests, might be a spur to greater accomplishment, and the poor world’s talent, like Senegal’s footballers, deserves a chance to compete on a global stage. It is not easy to run a managed “emigration” policy. The drain of educated minds from poor countries is mostly determined by host countries’ rules, not home countries’ interests. There will be tremendous pressure to loosen those rules in the future, not least because, as the baby-boom generation retires, it will seek to “backfill the taxpaying workforce behind it”, as Messrs Kapur and McHale put it. The rich world no longer welcomes the tired and the huddled; it looks set to compete ever more fiercely for the bright and the qualified.

Dark chocolate used to be an acquired taste. The trouble is, we have all acquired it. As cocoa prices soar, and a shortage looms, our changing tastes could be threatening the very source of our pleasure. A 25% jump in dark chocolate sales in the UK pretty much speaks for itself. As a nation we are turning our backs on the bland taste of milk chocolate, which is often saturated with vegetable oil. Dark chocolate now dominates counters in many supermarkets. Chocoholics should brace themselves. Fears over a global chocolate shortage have sparked a run on the dark stuff on world cocoa markets that has pushed the price up of cacao beans (used to make cocoa) to their highest level for more than two years. The price of cocoa futures, which is how the beans are traded on world commodity markets, has climbed steadily since last November, reviving fears of the cocoa crisis of 2002 that forced major manufacturers such as Nestle to hike the price of some of its chocolate bars.

Although speculators have been fingered for pushing the price of cocoa higher, British chocolate aficionados must share at least some of the blame for the run on the bean. Soaring demand for dark chocolate in the UK has put a rocket under the global price of cacao beans, compounding the impact of a severe drought in West Africa, which is home to much of the world’s cacao bean supplies. Consumers are doing the same thing in chocolate as in the rest of the food market. They are trading up and being more discerning about what they buy. All of the country’s top supermarket chains have added new premium dark chocolate varieties to their own brand ranges in response to this phenomenon. Supermarkets have also expanded the number of dark chocolate lines they stock, dwarfing their milk chocolaté variants in some cases.

Of course, it takes more beans to produce a bar of dark chocolate than are needed for a bar of milk - and that is at the root of the problem. Yesterday, the cost of cacao beans for delivery in July on the London cocoa exchange shot to ₤1,028 a tonne on exceptionally heavy trailing. This was up ₤22 on the day, and the highest price that the crop has fetched at any point since November 2004. The cost of buying cacao beans was pushed higher by canny investors who have placed record bets that prices will keep rising. Hedge funds, which make money by punting on whether commodity or share prices will rise or fall, have swallowed up a third of the cocoa contracts traded in New York, the other main cocoa trading centre. So far, industry buyers, such as Cadbury Schweppes, have refrained from joining, the frenzy, although traders believe that a jump in prices will prompt them to build up their stocks.

Prices have risen nearly thirty per cent since the beginning of December, largely on growing concern about what farmers in Ivory Coast, the biggest cacao bean producer in West Africa, have dubbed the worst drought in living memory. To make matters worse, demand is outstripping supply as the teeth of consumers in developing markets begin to sweeten. The International Cocoa Organisation predicts a global cocoa shortfall of around 100,000 tonnes this year alone, although private forecasts warn the deficit could be as much as 250,000 tonnes.

Getting the right beans is becoming crucial in a market that is fast following in the footsteps of wine and coffee in terms of getting consumers to appreciate its effect on the palate. People these days are willing to experiment more with food and drink. Rather like wine and coffee, people are realising there are varieties of chocolate available. Dark used to be a niche, but not any longer. With dark chocolate you are tasting the cocoa, whereas with milk you are tasting the recipe. There are 450 different flavour elements in a single cacao bean, which itself varies in taste depending on where it is grown. Chocolate from Colombia might seem peppery, while chocolate from Venezuela might smell like vanilla, for example. This is driving demand for an uber-premium version: the so-called singleestate chocolate bar, which sources its beans from a single location.

Scientists are falling over themselves to complete studies showing that moderate consumption of high-quality chocolate can be good for you. Only last week, a Harvard professor unveiled research suggesting that a nutrient in cocoa called epicatechin could lower the risks of cancer, strokes, heart disease, and diabetes. Dr Norman Hollenberg, of Harvard Medical School, based his findings on a study of the Kuna people of Panama, who are weaned on cocoa and can drink up to 40 cups of the stuff a week. Separate research at the Johns Hopkins University in Baltimore, Maryland, late last year found that snacking on dark chocolate decreased the development of potentially fatal blood clots. And other studies have found that dark chocolate has proportionately more antioxidants than other foods that are better known for their health-giving properties, such as red wine, green tea, and berries.

Health has probably played a role, but more as a justification for consumption than anything else. One of dark chocolate's big selling points is that it does not trap the chocoholic in the same downward sugar-craving spiral as milk chocolate versions. Parents should bear this in mind when they go shopping for snacks for their children. A new breed of fashionable diets has also helped to promote the cause of dark chocolate. Most dieticians agree that a little of what you fancy does you more good than outright denial. Some experts think more people are eating dark chocolate because the products have got better. The challenge with higher cocoa variants is to get the right trade-off between the percentage of bitter-tasting cocoa, which is where all the good micronutrients come from, and sugar. In the past, dark chocolate was bitter and reasonably gritty tasting. Some companies use a gentle-tasting cocoa bean which, if you are new to dark chocolate, does not scare you off. Adding a hint of vanilla can also help. Vanilla acts as a seasoning to solicit the bitter notes of the cocoa and to bring out the sweetness.