metapop = function (imi,ext,ini,alfa,dim,iffig, animation,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=matrix(as.numeric(runif(dim^2)<ini),nrow=dim)
else {
   a = matrix(rep(0,dim^2),nrow=dim)
   a[dim/2+1,dim/2+1]=1;
}

n = NULL;

for (t1 in 1:nostep) {
   nact = sum(a>0) / dim^2;
   n = c(n, nact)
   
   a = a * (as.numeric(runif(dim^2)> ext/(1+alfa*ext*nact)))
   anew = a + (as.numeric(runif(dim^2) < (imi * nact)))
   a = anew > 0;
   
   #plotting
   if (iffig) {
      image(a,col=c("white","green"),main=paste(t1))
     if (animation) 
       Sys.sleep(0.5)
   }
   
}
n
}