diff --git a/src/simulator/crates/mc-ecology/src/engine.rs b/src/simulator/crates/mc-ecology/src/engine.rs
index ff6565ed..68e6cecc 100644
--- a/src/simulator/crates/mc-ecology/src/engine.rs
+++ b/src/simulator/crates/mc-ecology/src/engine.rs
@@ -1,4 +1,4 @@
-use std::collections::{BTreeMap, HashMap, HashSet};
+use std::collections::{BTreeMap, BTreeSet, HashMap};
 
 use mc_core::algorithms::hex;
 use mc_core::grid::GridState;
@@ -418,7 +418,10 @@ impl EcologyEngine {
 
         // Pre-pass: count T10 slots per diet across all tiles.
         // Diets exceeding the cap get their terrain_tier_cap reduced to 9 this tick.
-        let mut t10_count_by_diet: HashMap<Diet, usize> = HashMap::new();
+        // BTreeMap/BTreeSet used instead of HashMap/HashSet so iteration order is
+        // deterministic across processes — RandomState hashing would otherwise
+        // vary turn-to-turn and perturb float accumulation in downstream systems.
+        let mut t10_count_by_diet: BTreeMap<Diet, usize> = BTreeMap::new();
         for slots in self.tile_populations.values() {
             for slot in slots {
                 if slot.tier >= 10 {
@@ -428,7 +431,7 @@ impl EcologyEngine {
                 }
             }
         }
-        let saturated_diets: HashSet<Diet> = t10_count_by_diet
+        let saturated_diets: BTreeSet<Diet> = t10_count_by_diet
             .iter()
             .filter(|(_, &count)| count >= Self::GLOBAL_T10_PER_DIET_CAP)
             .map(|(&diet, _)| diet)
diff --git a/src/simulator/crates/mc-ecology/src/traits.rs b/src/simulator/crates/mc-ecology/src/traits.rs
index 3d22c224..6fcbe9e9 100644
--- a/src/simulator/crates/mc-ecology/src/traits.rs
+++ b/src/simulator/crates/mc-ecology/src/traits.rs
@@ -10,7 +10,7 @@ pub enum Size {
     Huge,
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash, Serialize, Deserialize)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Serialize, Deserialize)]
 #[serde(rename_all = "snake_case")]
 pub enum Diet {
     Producer,