我有一个 Dataframe df1，其中我有属于不同地区（df 1 $Regions）的不同站点（df 1 $Site），其中我有关于食草证据及其类型的数据（df 1 $Herbivory_type）。当没有食草时，df 1 $Herbivory_type是NA。下面我展示了我的 Dataframe 的一个例子：

df1 <- data.frame(Region=c("ALI1","ALI1","ALI1","ALI1","ALI2","ALI2","ALI2","ALI3","ALI3","ALI3","ALI3","ALI5","ALI5"),
                  Site=c("ALI1_A","ALI1_B","ALI1_C","ALI1_D","ALI2_A","ALI2_B","ALI2_C","ALI3_A","ALI3_B","ALI3_C","ALI3_D","ALI5_A","ALI5_B"),
                  Herbivory_type=c(NA,"S",NA,NA,NA,NA,NA,NA,"S","S",NA,NA,"S"))

df1$Herbivory_type <- as.factor(df1$Herbivory_type)

df1

   Region   Site Herbivory_type
1    ALI1 ALI1_A           <NA>
2    ALI1 ALI1_B              S
3    ALI1 ALI1_C           <NA>
4    ALI1 ALI1_D           <NA>
5    ALI2 ALI2_A           <NA>
6    ALI2 ALI2_B           <NA>
7    ALI2 ALI2_C           <NA>
8    ALI3 ALI3_A           <NA>
9    ALI3 ALI3_B              S
10   ALI3 ALI3_C              S
11   ALI3 ALI3_D           <NA>
12   ALI5 ALI5_A           <NA>
13   ALI5 ALI5_B              S

我需要知道在df1$Site的计数中考虑到NA的地区食草性事件的数量。我希望得到以下结果：

df2

   Region N_Hervivory_S
1   ALI1             1
2   ALI2             0   # All sites have `NA`, thus, herbivorims is 0 in this region.
3   ALI3             2
4   ALI5             1

我试过这个：

as.data.frame(df1 %>% group_by(Region,Herbivory_type) %>% summarise(N = n()))

但产量不是我所期望的

Region Herbivory_type N
1   ALI1              S 1
2   ALI1           <NA> 3
3   ALI2           <NA> 3
4   ALI3              S 2
5   ALI3           <NA> 2
6   ALI5              S 1
7   ALI5           <NA> 1

有人知道怎么做吗？
先谢了