diff --git a/virtual_ecosystem/data_variables.toml b/virtual_ecosystem/data_variables.toml
index 199be53d5..33311f8a1 100644
--- a/virtual_ecosystem/data_variables.toml
+++ b/virtual_ecosystem/data_variables.toml
@@ -2,7 +2,7 @@
 axis = [] # ["community_id", "functional_group_id"]
 description = "Density of animal populations."
 name = "population_densities"
-unit = "???"
+unit = "individual m-2"
 variable_type = "float"
 
 [[variable]]