How many bats are there?
By Tim Lucas
Knowing whether bat populations are growing or shrinking tells us about the health of the bat population (see Dr Kate Barlow’s blog). It also gives us information about the health of the ecosystem in general, as discussed by Dr Robin Freeman. However, these are relative measures; are there more bats this year than last year? Sometimes we want to know an actual number. Are there a thousand bats? Ten thousand? A million?
For example, if we are trying to conserve a rare bat species we need to estimate the actual number of bats. At very low population sizes inbreeding starts to become a significant problem for conservation. Ten percent fewer bats than last year could mean a decrease from 100,000 bats to 90,000 bats or it could mean a decrease from 1,000 to 900 bats. While both situations are worrying, only in the later case do we need to start considering genetic inbreeding.
So how do we estimate the size of a bat population from our acoustic surveys? This is not a simple problem. If I go out with my bat detector and count 200 bats, what does this mean? Maybe I actually detected every single bat making the population size only 200. Maybe I detected one percent of all bats making the population size 20,000.
In essence the problem is to work out how many bats you expect to detect per hour of surveying. This depends on a number of factors. The flight speed of a species is important. You are more likely to come into contact with bats that fly quickly. If you sit by a motorway you will see more cars than if you sat by a country lane even if the total number of cars is the same on each road.
Secondly the actual area of land that you are surveying is clearly important and this depends on the distance within which you can detect a bat. Louder bats can be detected from further away, just as larger animals can be seen from further away. This means you are surveying a larger area if you are looking for loud species than if you were looking for quiet species.
The frequency of the bat call also matters as high frequency sounds travel less far than low frequency sounds.
As an example, the spectrogram shown has a loud ‘hockey stick’ call at about 25kHz but also a quieter harmonic above it, at about 60 kHz. Even without considering that the harmonic is quieter (it is less bright on the spectrogram) it will travel about half the distance of the lower frequency call. The maths to work out these distances is long and boring so I hope you don’t mind me not showing my working!
So, if we can make a good guess of how large an area we are actually surveying we can work out what proportion of the bat population we saw. Did we survey 1% of land in Europe, or 0.001% and so did we see 1% of the bat population, or 0.001%. Using this information and information about how fast bats fly we can try to work out how many bats there are in total and use this information to conserve them as best we can.