function n = metapop(imi,ext,ini,alfa,dim,iffig, nostep)
% This function simulates the proportion of occupied islands 


% imi - immigration rate
% ext - extinction rate
% ini - proportion of habitats initially occupied (0 - one individual in the centre)
% dim - number of rows of cells (dim^2 is the number of habitats)
% iffig - whether to draw a spatial figure
% nostep - number of simulation steps


if ini > 0
   a = rand(dim,dim) < ini;
else
   a=zeros(dim,dim);
   a(dim/2,dim/2)=1;
end

n = [];

for t=1:nostep
   nact = sum(sum(a>0)) ./ dim^2;
   n = [n nact];
   %anew = anew .* (rand(dim,dim) > ext);
   a = a .* (rand(dim,dim) > ext/(1+alfa*ext*nact));
   anew = a + (rand(dim,dim) < (imi .* nact));
   a = anew > 0;
   
   %plotting
   colour=['w','g'];
   if iffig
      hold on;
      axis([1,dim+1 1 dim+1]);
      for (i=1:dim)
         for (j=1:dim)
            fill([i,i,i+1,i+1],[j,j+1,j+1,j],colour(a(i,j)+1))
         end
      end

      pause(0.1);
      clf
   end
end