Mosaic of the air: partitioning the acoustic space
The idea that different species have to share a limited resource is common in ecology — whether that resource is actively competed for, or the use of the resource differs for other reasons.
These two ideas are related yet subtly different. In the first, competition between species leads to specialisation to avoid direct competition, for example by each feeding on a separate group of arthropods, or by one species feeding on the ground while another feeds in the tree canopy.
The second is more loosely defined and seems to be what is the case with acoustic communication by animals; there are solid evolutionary reasons for use of the resource to be divided somehow, but there is plenty of resource to be utilised. Direct competition seems rare.
Then music, the mosaic of the air
Andrew Marvell (Music’s Empire)
The use of ‘acoustic space’ to define how sounds can be partitioned comes from the formal use of the term ‘space’ in mathematics. In a simple acoustic space defined only by frequency and amplitude each sound could be plotted as points on a graph with frequency and amplitude axes. While this becomes significantly harder to visualise beyond three dimensions, the mathematics does not. The use of ‘acoustic space’ is not (yet?) as routinely used as the concept of morphospace in studies of biological forms and structure, and the use of ‘space’ is often more metaphorical than formal.
How can space be partitioned?
An analogy I often use to demonstrate sharing of the acoustic space is that of a person alone in the mountains with little else but a radio, and the need to get a weather forecast for the days ahead. What challenges would this imaginary person have in getting the information they need?
The metaphor of radio broadcasts has previously been used for cricket song, including the title of John Himmelman’s book Cricket Radio.
The word communication will be used here in a very broad sense to include all of the procedures by which one mind may affect another.
Warren Weaver (The Mathematical Theory of Communication)
Frequency
One problem our intrepid survivor may face is knowing on which frequency he should listen out for the forecast. If they are not tuned in to the correct frequency then they could be surrounded by the communication they desire, but unable to detect it. There is often a nocturnal raucous in the ultrasound (bats, bush-crickets, dolphins, …) that we can only perceive by artificially enhancing our senses.
Incidentally, the subtitle of Himmelman’s book is “tuning in the night-singing insects”.
Amplitude
It is possible of course that our explorer started off with their radio set tuned to a station that provided a weather forecast every hour, but they have either travelled so far that they can no longer hear it clearly, or their route around the mountain has placed them in a ‘shadow’ where it is impossible to get reception. These physical limits — amplitude diminishing with distance, and the effect of waves interacting with the environment — confine the effective distance over which a signal can be used to communicate.
Time
Weather forecasts, like news reports, tend to happen at the same time each day. Many animals also have daily patterns in calling activity (as well as patterns on longer, generally annual, cycles).
Structure
Finally there are ways that can distinguish between signals making use of the same frequency bands. In Europe the Gomphocerine grasshoppers are known for their broadband songs (covering a wide frequency range) and relatively complex song structures. As the frequency bands of different species regularly overlap, the structure of the song provides a reliable method for the songs of individual species to be identified. To extend the radio analogy consider a radio station that produces two weather forecasts each in a different language; the frequency and amplitude are identical, the time of day is very similar, but the messages are intended for two different audiences.
There are of course complexities. The dawn chorus (where many bird species sing at the the same time) suggests that sometimes species not dividing their songs by time of day is beneficial; there are other factors — biological and physical — that might select against partitioning. The conceptual framework however I believe is important, knowing how the acoustic space can be partitioned may provide insights into why, sometimes, it is not.
Understanding acoustic space partitioning is important not just for understanding the ecology of a region or species. As we move into an era where widespread, autonomous, acoustic monitoring becomes more feasible we can combine our knowledge of how a species uses the acoustic space, with its distribution and phenology to refine and improve species-level acoustic identification and monitoring.
References
Walter, G. H. (1991). What is resource partitioning? Journal of Theoretical Biology, 150(2), 137–143. doi:10.1016/s0022–5193(05)80327–3
