Human preferences may generate purely anthropogenic positive feedback loops reinforcing either declines of overexploited populations or expansions of prized introduced species, potentially leading to species substitutions. I illustrate this process based on a content analysis of 20th century Spanish newspapers that document a rapid, country-wide substitution of an originally exploited crayfish (Austropotamobius italicus) by two North American species (Procambarus clarkii and Pacifastacus leniusculus). The substitution was driven by anthropogenic positive feedbacks acting over a short period, coinciding with an exponential increase of crayfish market value, high numbers of crayfish-related articles and a rapid change in the thematic content of those articles. Similar substitutions involving economic or socially valued organisms occur in other regions and concerning other taxa. Human preferences and choices may constitute relevant drivers of several species declines and invasions, which can be very quick due to positive feedback loops. Conservation biology in general and invasion biology in particular have been traditionally focused on biological aspects, but, as shown here, would largely benefit from social perspectives dealing with human motivations and preferences.