diff --git a/models/ecolab_model.cc b/models/ecolab_model.cc
index 1f9144c..6179f58 100644
--- a/models/ecolab_model.cc
+++ b/models/ecolab_model.cc
@@ -93,6 +93,15 @@ void EcolabPoint::generate(unsigned niter, const ModelData& model)
   density = ROUND(n);
 }
 
+unsigned EcolabPoint::nsp() const
+{return sum(density!=0);}
+
+array<unsigned> SpatialModel::nsp() const
+{
+  array<unsigned> nsp;
+  for (auto& i: objects) nsp<<=i->nsp();
+  return nsp;
+}
 
 void EcolabPoint::condense(const array<bool>& mask, size_t mask_true)
 {
@@ -129,6 +138,17 @@ void PanmicticModel::condense()
   EcolabPoint::condense(mask, mask_true);
 }
 
+void SpatialModel::condense()
+{
+  array<int> total_density(species.size());
+  for (auto& i: *this) total_density+=i->as<EcoLabCell>()->density;
+  auto mask=total_density != 0;
+  size_t mask_true=sum(mask);
+  if (mask.size()==mask_true) return; /* no change ! */
+  ModelData::condense(mask,mask_true);
+  for (auto& i: *this) i->as<EcoLabCell>()->condense(mask, mask_true);
+}
+
 
 
 /* 
diff --git a/models/ecolab_model.h b/models/ecolab_model.h
index 0d23fd0..b39bd01 100644
--- a/models/ecolab_model.h
+++ b/models/ecolab_model.h
@@ -63,7 +63,8 @@ struct EcolabPoint
   array<int> density;
   void generate(unsigned niter, const ModelData&);
   void condense(const array<bool>& mask, size_t mask_true);
-  array<int> mutate(const array<double>&); 
+  array<int> mutate(const array<double>&);
+  unsigned nsp() const; ///< number of living species in this cell
 };
 
 struct PanmicticModel: public ModelData, public EcolabPoint
@@ -89,10 +90,11 @@ class SpatialModel: public ModelData, public graphcode::Graph<EcoLabCell>
   EcoLabCell& cell(size_t x, size_t y) {
     return *objects[makeId(x,y)];
   }
+  array<unsigned> nsp() const;
   void makeConsistent();
   void generate(unsigned niter);
   void generate() {generate(1);}
-//  void condense();
+  void condense();
   void mutate();
 //  void migrate();
 };
diff --git a/models/spatial_ecolab.py b/models/spatial_ecolab.py
index 4f4c3ac..6d5ccc7 100644
--- a/models/spatial_ecolab.py
+++ b/models/spatial_ecolab.py
@@ -39,10 +39,11 @@ def randomList(num, min, max):
 def step():
     ecolab.generate()
     ecolab.mutate()
-    #ecolab.condense()
+    ecolab.condense()
     nsp=len(ecolab.species)
     statusBar.configure(text=f't={ecolab.tstep()} nsp:{nsp}')
     plot('No. species',ecolab.tstep(),nsp)
+    plot('No. species by cell',ecolab.tstep(),ecolab.nsp()())
     for i in range(numX):
         for j in range(numY):
             plot(f'Density({i},{j})',ecolab.tstep(),ecolab.cell(i,j).density(), pens=ecolab.species())