Real-time invasion dynamics reveal the drivers of predator spread and prey extirpation on an island

Non-native predators profoundly restructure island biological communities worldwide, yet their management is hindered by limited empirical understanding of the dynamics of predator spread and prey extirpation. Here, we integrate two decades of field surveys, citizen-science records and standardized transects to reconstruct the invasion of the snake Hemorrhois hippocrepis on Ibiza and its impact on the endangered, endemic lizard Podarcis pityusensis. We found a marked acceleration in range expansion despite intensified culling, evidence for the transition from establishment to spread stages of this invasion. Spatially explicit invasion maps combined with citizen reports of lizard disappearance show a nonlinear acceleration in the interval from predator arrival to local prey extirpation, shortening from over 10 years in early-invaded areas to 3 years in recently invaded areas. The observed patterns are consistent with density-dependent predator pressure, which generates a wave-like, rapidly advancing invasion front evidenced by declining snake body condition and reduced efficiency of snake captures in long-invaded areas. Our study empirically confirms theoretical predictions of nonlinear spread and impact dynamics and highlights how citizen science can crucially help documenting early invasion stages. Effectively incorporating the nonlinear dynamics of invasions into conservation strategies is urgent to mitigate invasion-driven transformations of fragile island ecosystems worldwide.