diff --git a/.project/objectives/p2-58a-tilestate-fauna-fields.md b/.project/objectives/p2-58a-tilestate-fauna-fields.md
new file mode 100644
index 00000000..ddc113a6
--- /dev/null
+++ b/.project/objectives/p2-58a-tilestate-fauna-fields.md
@@ -0,0 +1,43 @@
+---
+id: p2-58a
+title: "TileState fauna fields — fauna_density + fauna_index for AmbientTileCtx"
+priority: p2
+status: done
+scope: game1
+owner: game-systems
+updated_at: 2026-05-07
+blocked_by: []
+evidence:
+  - src/simulator/crates/mc-core/src/grid/mod.rs — fauna_density + fauna_index added to TileState (p2-58a section, lines ~277-290)
+  - "cargo test -p mc-core tilestate_fauna: 3/3 pass (tilestate_fauna_fields_default, tilestate_fauna_roundtrip, tilestate_fauna_density_serde_default_on_missing_field)"
+  - "cargo check --workspace: clean (pre-existing solo_dominion errors only)"
+---
+## Summary
+
+Adds `fauna_density: f32` and `fauna_index: Vec<SpeciesId>` to `TileState` in mc-core
+so `AmbientTileCtx` (mc-ecology) can be populated from the live GameState in the
+per-tile-moved encounter hook (p2-58b).
+
+`SpeciesId` is the existing string newtype from `mc-core::ids` (snake_case fauna species
+identifier, e.g. `"grey_wolf"`). No new type was created.
+
+## Acceptance
+
+- [x] ✓ `TileState.fauna_density: f32` field added with `#[serde(default)]`
+- [x] ✓ `TileState.fauna_index: Vec<SpeciesId>` field added with `#[serde(default)]`
+- [x] ✓ `cargo test -p mc-core tilestate_fauna` passes — 3/3 tests green, including
+      backward-compat deserialization (old save files without these fields default cleanly)
+- [x] ✓ `cargo check --workspace` clean of new errors (pre-existing solo_dominion
+      MapUnit/GameState field errors are pre-existing tech debt, not introduced here)
+
+## Population note
+
+`fauna_density` and `fauna_index` default to 0/empty. The ecology pipeline
+(`mc-ecology::fauna_select::pick_fauna_for_tile`) must be wired to write these fields
+back onto `TileState` after worldgen — tracked in p2-58b. Until then, encounter rolls
+will always see density=0 and skip (correct behaviour for uninitialized maps).
+
+## Out of scope
+
+- Wiring the ecology pipeline to populate these fields (p2-58b)
+- The actual `AmbientTileCtx` call site in `mc-turn::movement` (p2-58b)
diff --git a/.project/objectives/p2-58b-ambient-encounter-hook.md b/.project/objectives/p2-58b-ambient-encounter-hook.md
new file mode 100644
index 00000000..6a5d25c0
--- /dev/null
+++ b/.project/objectives/p2-58b-ambient-encounter-hook.md
@@ -0,0 +1,40 @@
+---
+id: p2-58b
+title: "Ambient encounter hook — mc-turn::movement calls roll_ambient_encounter per tile step"
+priority: p2
+status: stub
+scope: game1
+owner: unassigned
+updated_at: 2026-05-07
+blocked_by: [p2-58a]
+---
+## Summary
+
+With `TileState.fauna_density` and `TileState.fauna_index` now populated (p2-58a),
+the per-tile-moved hook in `mc-turn::movement` (or `processor.rs` movement phase)
+can build `AmbientTileCtx` from the live `GameState` and call
+`mc_ecology::encounter::roll_ambient_encounter(...)`.
+
+Also needed: the ecology pipeline must write `fauna_density` + `fauna_index` back
+onto `TileState` after worldgen (currently `pick_fauna_for_tile` in
+`mc-ecology::fauna_select` uses an ephemeral context; the result needs to persist
+on the tile for `mc-turn` to consume at runtime).
+
+## Acceptance
+
+- [ ] `mc-ecology` finalise pass (post-worldgen) writes `fauna_density` and
+  `fauna_index` onto each `TileState` from `pick_fauna_for_tile` results.
+- [ ] `mc-turn::movement` (or `processor.rs` movement phase) builds `AmbientTileCtx`
+  from `GameState::tiles[tile_idx].fauna_density` + `.fauna_index` per step.
+- [ ] Calls `mc_ecology::encounter::roll_ambient_encounter(ctx, unit_kind, &mut rng)`
+  and appends any `Some(EncounterSpec)` to the turn's event log.
+- [ ] `cargo test -p mc-turn test_ambient_encounter_fires` — seeded 50-step walk
+  through `fauna_density=0.8` tile yields ≥1 encounter.
+- [ ] GUT integration test: scout in wilderness, assert `EventBus.encounter_started`
+  fires within 20 moves at density=0.8.
+
+## Out of scope
+
+- Pioneer escort rules (p2-59).
+- Lair siege/assault/raid (p3-10).
+- Encounter narrative text / event cards — separate UI objective.
diff --git a/src/simulator/crates/mc-core/src/grid/mod.rs b/src/simulator/crates/mc-core/src/grid/mod.rs
index 8f79f740..c4cd7401 100644
--- a/src/simulator/crates/mc-core/src/grid/mod.rs
+++ b/src/simulator/crates/mc-core/src/grid/mod.rs
@@ -4,6 +4,7 @@ pub mod biome_registry;
 // works without drilling through the biome_registry module.
 pub use biome_registry::terrain_tier_cap;
 
+use crate::ids::SpeciesId;
 use serde::{Deserialize, Serialize};
 use std::collections::HashMap;
 
@@ -274,6 +275,16 @@ pub struct TileState {
     /// BFS distance to nearest river/lake hex. 0 = on water; u8::MAX = beyond MAX_RIPARIAN_DISTANCE.
     #[serde(default = "default_riparian_distance")]
     pub riparian_distance: u8,
+    // ── Fauna ambient fields (p2-58a) ─────────────────────────────────────────
+    /// Aggregate ambient fauna density (0.0 = none, 1.0 = saturated). Populated
+    /// by the ecology pipeline after fauna placement; used by AmbientTileCtx in
+    /// the per-tile-moved encounter roll (p2-58b).
+    #[serde(default)]
+    pub fauna_density: f32,
+    /// Candidate species present on this tile (filtered by trophic + domain gates).
+    /// Populated by mc-ecology::fauna_select after worldgen; consumed by encounter roll.
+    #[serde(default)]
+    pub fauna_index: Vec<SpeciesId>,
 }
 
 impl Default for TileState {
@@ -375,6 +386,8 @@ impl Default for TileState {
             stream_order: 1,
             lake_id: None,
             riparian_distance: u8::MAX,
+            fauna_density: 0.0,
+            fauna_index: Vec::new(),
         }
     }
 }
@@ -1115,5 +1128,47 @@ mod tests {
         assert!(f.road, "road feature was wiped by migration");
         assert!(!f.river, "road edge incorrectly marked as river");
     }
+
+    // ── p2-58a: TileState fauna fields ──────────────────────────────────────
+
+    #[test]
+    fn tilestate_fauna_fields_default() {
+        let t = TileState::default();
+        assert_eq!(t.fauna_density, 0.0);
+        assert!(t.fauna_index.is_empty());
+    }
+
+    #[test]
+    fn tilestate_fauna_roundtrip() {
+        let mut t = TileState::default();
+        t.fauna_density = 0.75;
+        t.fauna_index = vec![SpeciesId::new("grey_wolf"), SpeciesId::new("dire_bear")];
+        let json = serde_json::to_string(&t).unwrap();
+        let t2: TileState = serde_json::from_str(&json).unwrap();
+        assert_eq!(t2.fauna_density, 0.75);
+        assert_eq!(t2.fauna_index, vec![SpeciesId::new("grey_wolf"), SpeciesId::new("dire_bear")]);
+    }
+
+    #[test]
+    fn tilestate_fauna_density_serde_default_on_missing_field() {
+        // Tiles serialized before p2-58a won't have fauna_density/fauna_index.
+        // Round-trip a default tile (which has the correct enum variants), strip
+        // the two new keys, and verify they come back as zero/empty.
+        let mut t = TileState::default();
+        t.col = 3;
+        t.row = 5;
+        let full_json = serde_json::to_string(&t).unwrap();
+        // Remove fauna_density and fauna_index from the serialized form to simulate
+        // an old save file that predates p2-58a.
+        let stripped: serde_json::Value = serde_json::from_str(&full_json).unwrap();
+        let mut map = stripped.as_object().unwrap().clone();
+        map.remove("fauna_density");
+        map.remove("fauna_index");
+        let old_json = serde_json::to_string(&map).unwrap();
+        let t2: TileState = serde_json::from_str(&old_json).unwrap();
+        assert_eq!(t2.fauna_density, 0.0, "missing fauna_density should default to 0.0");
+        assert!(t2.fauna_index.is_empty(), "missing fauna_index should default to empty vec");
+        assert_eq!(t2.col, 3);
+    }
 }