Individuals can be counted. There is space between calls. Index Value 2 . Calls of individuals can be distinguished but there is some overlapping of calls. Index Value 3 .
Calls are constant, continuous, and overlapping.
The bias in this situation is severe, even though I know of no validation studies that have directly explore the relationship between count and true population. Note though that in no case does the index act like Line 3 in Figure 2, where an increase in population size result in a decrease in the index. As tree coverage changes so does the mix of amphibian populations. Some species are favored and others not. These are the types of changes we would like a monitoring program to track.