Living on the Edge: Ecological and Evolutionary Dynamics Along Invasion Fronts.

Invasion fronts are the advancing edges of non-native species' ranges and represent dynamic, non-equilibrium boundaries where colonization, biotic interactions and rapid evolutionary processes interact. Although biological invasions are increasingly well studied, the ecological and evolutionary mechanisms operating at invasion fronts remain conceptually fragmented despite their disproportionate influence on spread dynamics, exerted impact and management. Here, we synthesize how invasion-front geometries arise from interactions among propagule pressure, landscape permeability, long-distance dispersal and environmental heterogeneity, producing continuous (including satellite colonies), patchy/fragmented or coalescing fronts that shift with invasion stage and spatiotemporal scales. We integrate ecological and evolutionary evidence from macro-organisms to show how gradients from core to front can result in declining population density, increased trait divergence, spatial sorting, serial founder effects, expansion load, and behavioural and physiological differentiation. We further draw parallels with climate-driven range expansions, while emphasizing the stronger disequilibrium and novel biotic contexts characteristic of non-native invasion fronts. Finally, we map these dynamics onto impact trajectories, demonstrating how trait-mediated interactions, resource reallocation and system-level reconfiguration can emerge sequentially along the invasion gradient. By unifying ecological and evolutionary processes across appropriate spatial and temporal scales, we establish invasion fronts as powerful natural laboratories and critical leverage points for predicting, monitoring and managing biological invasions.