Dry and barren, the island had little appeal for passing ships except for collecting fresh meat, and was not claimed for the Portuguese Crown. Mariners could hunt for the numerous seabirds and the enormous female green turtles that laid their eggs on the sandy beaches. The Portuguese also introduced goats as a potential source of meat for future mariners.
And if it is a paradise today, blame Charles Darwin. Again from the BBC article:
when the British garrisoned Ascension to discourage French attempts at rescuing Napoleon - who had been exiled to St Helena, the nearest point of land, 700 miles southeast - they too could find virtually no fresh water.
"Near the coast, nothing grows," wrote Charles Darwin. "The island is entirely destitute of trees."
Darwin discussed how to make Ascension more habitable for humans with his friend Joseph Hooker, later director of the Royal Botanic Gardens at Kew, who visited in 1843. Hooker devised a plan. He would plant trees all over the 859m (2817ft) summit of Green Mountain, Ascension's highest point. Foliage would trap moisture from the warm southeasterly winds that sweep continuously over, letting it drip down to ground level to assure a water supply for the troops. He would introduce grasses to create pasture for livestock, and soil to plant vegetables.to make the ecology worse, in the 1980's, someone brought in another bush with a deep taproot that is now taking over the place:
Engineers arrived in the mid-60s to construct transmitters to beam World Service programming to Africa and South America. They built a new village, a few miles inland from the sleepy capital, Georgetown - and planted a type of mesquite, known as Mexican thorn, to bind the dry soil.
Today the dry, stiff-thorned mesquite runs riot, dominating whole swathes of Ascension's terrain. "A conservative estimate is that there's now about 38,000 of these bushes," says Weber. "It's very difficult to control physically - it puts down tap roots that can be 20-30m deep - so we're looking at biological methods of control, bringing in pest species that are specific to this shrub from its native range."well, that might make the ecology types unhappy, but the Mesquite bush does have value as a food plant etc. LINK link2
The Wiki article notes that the UK would like to claim sovereignty of the local octean (for it's mineral and petroleum rights).
finally, the island has an emergency landing strip for the US Space Shuttle, which is now, of course, no longer in service.