diff --git a/src/simulator/crates/mc-ecology/src/engine.rs b/src/simulator/crates/mc-ecology/src/engine.rs
index 628541c6..cb2396c1 100644
--- a/src/simulator/crates/mc-ecology/src/engine.rs
+++ b/src/simulator/crates/mc-ecology/src/engine.rs
@@ -325,6 +325,24 @@ impl EcologyEngine {
         // 3. Habitat-gradient dispersal between tiles
         self.run_dispersal(grid);
 
+        // 3b. Carrying-capacity migration: species over a tile's carrying
+        //     capacity push surplus population into the best eligible neighbour.
+        //     Computed from the current populations, then applied in-place.
+        //     Deterministic — `compute_migrations` is a pure function of the
+        //     populations, registry, grid, and `seed`; `offset_neighbors`
+        //     walks neighbours in a fixed order.
+        {
+            let (w, h) = (grid.width, grid.height);
+            let migrations = crate::generation::compute_migrations(
+                &self.tile_populations,
+                &self.species_registry,
+                grid,
+                seed,
+                |col, row| mc_core::algorithms::hex::offset_neighbors(col, row, w, h),
+            );
+            crate::generation::apply_migrations(&mut self.tile_populations, &migrations);
+        }
+
         // 4. Tier advancement
         self.run_tier_advancement(grid);