species是一个1，0矩阵，要每一行与最后一行的值进行逻辑判断
n<-ncol(species)
> m<-nrow(species)
> b<- 0
> for (i in 1:m-1)
+ {  
+   for (j in 1:n)
+   {
+     if(species[i,j]==1 & species[m,j]==0)
+     b<-b+1
+   }
+ }
Error in if (species[i, j] == 1 & species[m, j] == 0) b <- b + 1 :
  argument is of length zero
>