Prologue

The idea of combining social species, information, perturbations, and nonlinear responses related to dispersal originated naively a long time ago, in the Gulf of Roses in the western Mediterranean. As a kid, I used to spend holidays in a tiny village nearby the ruins of Empuries, a magical place where an ancient Greek colony was founded in 575 BC, later occupied by the Romans. I remember going to the beach where I would place my towel sheltered from the wind behind a large section of the ancient Greek dock built on huge stones. More than 2100 years later, one can still enjoy the mosaics, the temple columns, and the large walls protecting the Roman city from the outside. Once, while visiting this place with my parents, I asked them why that magnificent settlement was abandoned, vanished, and was buried by dust, but I did not get a convincing answer (even now, I would not be able to answer this question if asked by my own kids). Archaeologists believe that the collapse of Empuries was caused by a combination of factors, namely the appearance of other flourishing communities (Barcino and Tarraco, or Barcelona and Tarragona as they are known today) and a perturbed environmental regime, caused by an accumulation of sediments resulting from a nearby river, which disabled the use of the harbour. These factors likely contributed to dispersal, which ended up in the abandonment of the city. In any case, my wonderings about Empuries remained dormant for the next 40 years. But these questions slowly awakened when one of my fieldwork studies monitoring Audouin’s gulls at the Ebro Delta was unexpectedly affected by a perturbation that began in the mid-1990s. This breeding patch, which came to hold almost 75% of the total world population of this once endangered species, has collapsed in recent times, but strikingly it remained apparently resilient for many years (Figure P1). The Ebro Delta shared with Empuries the characteristic of being an exceptionally suitable habitat allowing a population to flourish, prior to eventual collapse. Empuries and the Ebro Delta represent all of the issues I have come to be interested in as a researcher: a social group thriving in a favourable patch, perturbations generating dispersal, and a nonlinear response leading to patch extinction (as a form of a new stable state). Some years ago a reading of Marten Scheffer’s book about critical transitions was also very inspiring. Understanding why Empuries and the Ebro Delta collapsed has intrigued my curiosity over the past several years, and has led me to take the leap in writing this book....