diff --git a/src/simulator/crates/mc-compute/src/ecology.rs b/src/simulator/crates/mc-compute/src/ecology.rs
index a6c20a26..e4329f96 100644
--- a/src/simulator/crates/mc-compute/src/ecology.rs
+++ b/src/simulator/crates/mc-compute/src/ecology.rs
@@ -5,7 +5,7 @@
 //! full-grid input buffers on every device, partition-sized output buffers.
 
 #[cfg(feature = "gpu")]
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 #[cfg(feature = "gpu")]
 use mc_core::grid::GridState;
@@ -141,7 +141,7 @@ impl GpuEcologyEngine {
     pub fn upload<S>(
         &mut self,
         grid: &GridState,
-        tile_populations: &HashMap<(i32, i32), Vec<S>>,
+        tile_populations: &BTreeMap<(i32, i32), Vec<S>>,
         species_registry: &HashMap<u32, SpeciesLike>,
     ) where
         S: PopSlotLike,
@@ -179,7 +179,7 @@ impl GpuEcologyEngine {
     pub fn download_into(
         gpu_tiles: &[GpuTileEcology],
         grid: &GridState,
-        tile_populations: &mut HashMap<(i32, i32), Vec<PopSlotOutput>>,
+        tile_populations: &mut BTreeMap<(i32, i32), Vec<PopSlotOutput>>,
     ) {
         for (i, tile) in grid.tiles.iter().enumerate() {
             if i >= gpu_tiles.len() {
diff --git a/src/simulator/crates/mc-compute/src/gpu/ecology_buffers.rs b/src/simulator/crates/mc-compute/src/gpu/ecology_buffers.rs
index 602532fb..c66df656 100644
--- a/src/simulator/crates/mc-compute/src/gpu/ecology_buffers.rs
+++ b/src/simulator/crates/mc-compute/src/gpu/ecology_buffers.rs
@@ -7,7 +7,7 @@
 //! Input buffers are full-grid sized (same pattern as climate buffers). Output buffers
 //! are partition-sized for multi-GPU dispatch.
 
-use std::collections::HashMap;
+use std::collections::{BTreeMap, HashMap};
 
 use mc_core::grid::GridState;
 use wgpu::util::DeviceExt;
@@ -156,7 +156,7 @@ impl EcologyBuffers {
         &self,
         queue: &wgpu::Queue,
         grid: &GridState,
-        tile_populations: &HashMap<(i32, i32), Vec<S>>,
+        tile_populations: &BTreeMap<(i32, i32), Vec<S>>,
     ) where
         S: PopSlotLike,
     {