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Researching the history of ecosystems, it is not long before you make an arresting discovery. Great abundance of the kind that exists in the tropics – or existed until recently – was once almost universal.

With a very few exceptions, every major ecosystem had a megafauna; every major ecosystem witnessed vast migrations of mammals, birds or fish; every major ecosystem possessed an abundance of animal life orders of magnitude greater than current abundance in the temperate nations. In some cases the ecosystems these life forms created were a world apart from those we now know.

Take, for example, the North Sea. Olsen’s Piscatorial Atlas of the North Sea, English Channel, and St. George’s Channels, published in 1883, marks an area of the North Sea the size of Wales as oyster reef. (I am indebted to Prof Callum Roberts, whose magnificent book The Unnatural History of the Sea reproduces this map). This area is far from any coast: it would have been among the least exploited regions.

By then, trawling in the North Sea had been taking place for at least 500 years (the first written record in England dates from 1376). Given that there is no obvious difference in habitat between the region marked on the map and many other parts of the North Sea, the most likely explanation for the distribution mapped in 1883 is that the oyster beds had been fished out and broken up throughout the more accessible areas.