The biome runs across the interior of the Northeast behind the coastal fringe of Atlantic Forest, covering parts of nine states: Piauí, Ceará, Rio Grande do Norte, Paraíba, Pernambuco, Alagoas, Sergipe, Bahia and a slice of Minas Gerais. That comes to 912,529 square kilometres, close to a tenth of Brazil. Of Brazil's six major biomes, this is the only one that is entirely Brazilian - no part of the Caatinga extends across any national border, which means a large share of its biological heritage exists nowhere else on the planet. More than two thousand species of plants, fish, reptiles, amphibians, birds and mammals have been recorded. Among the birds are Lear's macaw and Spix's macaw, both among the rarest parrots in the world, along with the moustached woodcreeper and the Caatinga parakeet.

Rainfall averages somewhere between 20 and 80 centimetres a year, and it is intermittent and violent rather than steady - irregular winds arrive from every direction and the water comes down hard when it comes at all. Every plant here is an answer to that arithmetic. Cacti and other succulents store what they can. Non-succulent species carry small, hard leaves and branch intensely near the ground, giving the shrubby, tangled structure that makes walking through untouched caatinga so difficult. Palms cluster where the water table allows: carnaúba and babaçu mostly, sometimes tucumã or macaúba. There is no single Caatinga vegetation but a mosaic, usually split into the dry sertão and the moister agreste, though some botanists count as many as eight distinct regimes. Scattered through it are enclaves of humid tropical forest, tall-treed islands that persist where a plateau wrings extra rain out of the air.

This is not empty country. Twenty-six million people live in the Caatinga, farming ground that gives grudgingly and only sometimes. Much of the agriculture is slash-and-burn, and much of it works with native species rather than against them. One of the most productive is a bee: Melipona subnitida, the stingless jandaíra, kept across the sertão and yielding up to six litres of honey a year per colony - real income in households that have few other reliable sources of it. Drinkable water is scarce. Harvesting is difficult, not because the soil is poor but because the rain cannot be counted on, and a crop planted on the strength of one good shower may find no second shower behind it. Living here has always meant planning around an event that may not arrive.

The São Francisco changes the equation wherever irrigation reaches it. The soil is genuinely fertile, and irrigated valleys along the river now export grapes, papayas and melons; the promise held out is that the Northeast becomes a breadbasket. The catch is chemistry. Irrigating in a climate where evaporation outruns rainfall concentrates salt in the topsoil, and salinisation can take good land out of production permanently. Away from the irrigated blocks the pressures are older and simpler: overgrazing and the cutting of trees for fuelwood have stripped vegetation across large areas, and parts of the biome have begun to desertify in the manner of the Sahel.

Estimates put somewhere between a quarter and a half of the Caatinga's original vegetation as already transformed by economic development. Against that, only about one percent of the biome sits inside strict-protection conservation areas, with a further six percent under sustainable-use designations. Europeans have been describing this landscape for a long time - Marcgrave described it in 1638, Spix and Martius between 1817 and 1820 - and the fossil record suggests it was once part of a much larger dry belt across South America. What it has never had is the attention given to the Amazon or the Atlantic Forest. A biome that belongs to one country alone, and to no one else's conscience, turns out to be easy to overlook.