The lizard is Podarcis lilfordi lilfordi, first described in 1874, but on Menorca it goes by a plainer name: the sargantana negra, the black lizard. Black is the fashion here, though not the rule, for the animals can shift their skin colour to melt into whatever they are sitting on. Isolated on their island with few threats, they have grown bold and abundant, unbothered by the visitors who step ashore. Endemic subspecies like this one are evolution's local dialects, shaped by a single place over countless generations. Lift these lizards off their island and there would be no more of them anywhere; this scrap of rock is the whole of their world.

The plum trick is real, and it says something about how tightly life is wound on a small island. With little to fear and a limited menu, the lizards respond to any windfall of food with startling speed and numbers, gathering in a dark, quick-moving cluster around a single piece of fruit. It is the kind of behaviour that only develops where predators are scarce and opportunity is scarcer still. Watching them converge, you are watching an ecosystem with the lid off, its normal caution stripped away by generations of relative safety. The islet has become a small living laboratory, and not only in the figurative sense.

The land itself is modest: a circumference of about 3.3 kilometres and, until people intervened, a highest point of just fifteen metres above the sea. That changed with the building of the Illa de l'Aire lighthouse, which lifted a beam above the old summit to warn ships off the southeastern approaches. Most of the island, its accessible western side and the lighthouse included, is open to those who make the short crossing. From the mainland, the low silhouette forms a scenic backdrop to the resort of Punta Prima and the wider municipality of Sant Lluis, a familiar shape on the horizon for everyone sunning themselves on the beach opposite.

For all its scientific interest, Illa de l'Aire is not off-limits. Plenty of people make the short crossing from Punta Prima, drawn by the clear water and the promise of the lizards, and they come ashore on the accessible western side, which takes in most of the island and the lighthouse both. It is a curious kind of visit. You share the rock with an animal that lives nowhere else, watching the dark shapes flicker over the warm stone as you swim off the low shore. From the mainland the islet is simply scenery, a familiar silhouette closing the view from the beaches of Punta Prima and the municipality of Sant Lluis. Up close it becomes something stranger: a self-contained world small enough to walk, yet complete enough to have evolved its own inhabitants.

The lizards are not the island's only introduced curiosity. Illa de l'Aire also carries a population of around three hundred rabbits, brought by human hands, and in the year 2000 those rabbits made the islet the setting for a striking piece of science. It hosted the first field trial of a self-spreading, or transmissible, vaccine, engineered so that immunity to two rabbit diseases, myxomatosis and rabbit hemorrhagic disease, could pass from animal to animal on its own. The idea was to protect a population without having to catch and inoculate every individual. On this isolated stage, walled off by the sea, researchers watched a vaccine try to spread like the very contagions it was built to stop.