R - Creating a column of IDs based on each time the condition within another column resets to 1

I have a data frame containing species populations and their abundance in each population. I would like to add another column which contains an ID for each species. The population column increases upwards from 1 and re-sets each time a new species occurs. The species ID column therefore will count along the species population column, and increase in value by +1 each time the population column resets to '1'.

I understand the principle of what I want to achieve, but have so far failed to obtain the correct result. I include a dummy example below containing populations and counts for 2 species:

df <- data.frame(Count=c(10, 5, 4, 10, 6),
                 Population=c(1, 2, 3, 1, 2))

Count Population
    10          1
     5          2
     4          3
    10          1
     6          2

The desired outcome would be:

Count Population species
  10          1       1
   5          2       1
   4          3       1
  10          1       2
   6          2       2 

Where a new column of species IDs has been generated, based upon each time the value in df$Population resets to 1. In reality the data set contains many hundreds of species each with varying numbers of populations.

I have been able to generate the following output using the following code:

df$species[c(df$b[-1], 0) == 1] <- df$species[c(df$b[-1], 0) == 1] + 1

Count Population species
 10          1       1
  5          2       0
  4          3       0
 10          1       1
  6          2       0

However, I cannot get the species column to accumulate in value along the sequence.

Thanks in advance for any suggestions. Apologies in advance if the question or example are not clear.

Mike