How many autonomous recording units (ARUs) do I need?

When you start thinking about your own project/protected area and the potential introduction of acoustic monitoring as a wildlife monitoring and human disturbance monitoring tool, you naturally will start wondering how many sensors you need to budget for.

There is no single answer as to how many ARUs your project will need. Unless your target area is quite small, you are unlikely to be able to cover the entire study area by acoustic sensors. You need therefore to design a survey grid that will be both maintainable given your budget/human resources and representative enough of your overall area. In order for your grid to be representative, you need (as with the deployment of other sensors – e.g. motion activated camera traps) to consider the spacing of the sensors and how to avoid inadvertently introducing spatial biases in your grid design. If you put all your sensors on/near major trails, you run the risk of not properly incorporating in your grid less accessible/steep/rugged/swampy areas, since trails – unlike transects – typically were developed along areas of least resistance.

How far to space the sensors?

This will depend on the primary sound that you are aiming to record and the terrain. If you are primarily deploying sensors to monitor wildlife presence, you may space the sensors closer than if you are monitoring illegal hunting 9gunshots) or logging (chainsaw) sounds. The ARUS we use do not have “superhuman-recording” abilities – they record more or less what a human ear can hear (but tirelessly, day or night, 24/7 year round…). So, a primate alarm call may be heard over 300-400 m away, where as our experience in Korup with shotgun shots is that they can be heard as far as 1-1.5 km away. Therefore, placing ARUs 3 km apart should limit overlap and double detection.

How much area am I surveying?

Again, this will depend on the sound you are recording. Assuming that terrain is more or less open and you are recording gunshots, one ARU could be recording gunshots over an area of ~4 km2. It is important that you plan to conduct a brief pilot study before rolling out your acoustic grid over a larger area, to help you understand the effect of the terrain/forest type on your ARUs detection range of the target sound (and create control gunshots/sounds if possible to test the detection range). If you wish to record relative abundance of a sound over time (trends), then knwoing teh effective detection range is less important. If you want to calculate the absolute abundance of your sound (detections/area), then a good understanding of your detection range is needed.