Chronic warming, not heatwaves, predicts tropical fish colonization in the Mediterranean

The opening of the Suez Canal abruptly reconnected the Red Sea and the Mediterranean Sea, collapsing a geological barrier that had separated their biotas for millions of years. This exchange offers a natural experiment to determine whether tropical species' spread at their cold-edge range is driven by acute thermal shocks (marine heatwaves) or by progressive easing of physiological constraints under sustained warming. We analyzed 1,059 first-arrival time series for 75 non-indigenous fish species across the Mediterranean, explicitly accounting for progressive salinification, oceanographic connectivity, and historical canal enlargement. Our models demonstrate that colonization dynamics are overwhelmingly dominated by ecological momentum-the time-dependent geographic spread of populations-and facilitated by chronic background warming. Marine heatwaves explain negligible variance and show no detectable effect on colonization success. Instead, warming operates as a stage-dependent continuous climatic facilitator, accelerating post-establishment expansion, rather than triggering introductions. Thus, this shift toward warm-adapted communities is governed by chronic warming, with the Suez Canal triggering the reorganization of Mediterranean biogeography.