Birds in a tree. Photo by A. DeSantis, Wikimedia Commons
(Click here to listen to an audio version of this blog!)
Monitoring sounds in an ecosystem (also called “acoustic monitoring”) is a common technique for determining the presence of organisms that are hard to visually detect. Bird songs, for example, are frequently used to determine the presence of shy or furtive species or to locate birds that inhabit particularly dense vegetative habitats. The detection of these birds by sound is an accepted technique for precise species identification in the field.
Bird songs also encode a great deal of information about a biotic community and can relay important survival clues to a wide range of members of that community. Back in July, 2015 I wrote a blog that explored the information content of bird alarm-songs that were triggered by the presence of predators. Different bird species “announced” the presence of particular predators and other species in the community (both birds and mammals) specifically listened to an often species-specific set of these bird alarms and reacted accordingly! The speed at which these alarm signals raced across a habitat were also calculated and shown to be many times faster than the top speeds at which the predators can move!
Photo by D. Sillman
A recent study in Ohio published in PlosOne (September 4, 2019) explored the effect of “bird chatter” (resting, peaceful bird songs and chirps) on eastern gray squirrels. The squirrels (neighborhood street and park dwellers in a small Ohio town), were exposed to a recording of a red-tail hawk (their principal avian predator). These recordings caused the squirrels to freeze in place with their heads and eyes turned upward searching for the incoming hawk. Immediately after the hawk recording, the researchers played either a recording of ambient noise or a recording of ambient noise plus bird chatter. The squirrels who got the recording of just noise remained motionless and vigilant significantly longer than those who were played the bird chatter tape. The squirrels, apparently, interpreted the relaxed bird sounds as an “all clear” signal for the passing hawk and relaxed their defensive postures and returned to other tasks like looking for food or tormenting neighborhood dogs!
The movement of sound through a vegetative ecosystem can be rapid, but its transmission velocity is affected by a variety of site factors. Very open habitats often are exposed to wind, and wind sounds interfere with sound transmission. Very dense, forest habitats, on the other hand, can have significant masses of vegetative materials that absorb sound or cause directional sound waves to echo about. These factors can both dampen sound energy and also decrease its fidelity and, therefore, information content.
Photo by D. Sillman
A factor we have discussed many times before that affects the density and complexity of forest vegetation here in the eastern United States is browsing by white-tailed deer. White-tailed deer population densities are currently at levels here in the East far greater than their pre-European settlement numbers, and their feeding activities have greatly reduced the ground vegetation cover, forest understory complexity and tree seedling densities! Not surprisingly, in these very open, heavily deer-browsed forests the flow of sound (including bird chatter and bird alarm calls) travels over greater distances and with much less distortion than is observed in a denser, non-deer browsed forest (this research was outlined in a second PlosOne article (February 13, 2019)). The consequences of this facilitated sound transmission on bird territory dimensions or community responsiveness have not yet been determined.
Compared to the cacophony of the above-ground portions of an ecosystem, the below-ground sections, i.e. the soil itself, should be very quiet, indeed. That is until you actually tune into it and listen!
Stag beetle grubs. Photo by L. Enking, Flickr
A soil ecologist at Hochschule Geisenheim University in Germany was interested in studying the amount of greenhouse gas (carbon dioxide, to be exact) beetle grubs generate when they feed on plant roots in an agricultural field. The problem was, in order for her to know where the beetle grubs were (and how many there were and what species they were, etc.) she was forced to dig up the field soil and thus destroy the grubs’ habitat. She wondered if she could locate grubs in the intact soil by sound possibly zeroing in on their chewing noises and then do her carbon dioxide measurements on the intact soil.
So, though contacts and previously published research she located an acoustics researcher at the University of York in England who designed some sturdy microphones that could be driven into the soil. Working in the York scientist’s lab, the two scientists used these microphones on soil containing grubs like the ones living in the fields in Germany and found that the beetle grubs, while burrowing underground, made “chirping” noises! These chirps (called “stridulations”) are made when the grub rubs its middle and hind legs together!
They further determined that the two main species of beetle grubs that infest the study area fields in Germany made recognizably different patterns of chirps! Both the location of the grubs and their species identification, then, could be determined by acoustic analysis alone! The ability to know which beetle grub species was dominant in a particular field could be most beneficial for the crop manager or farmer and lead to the application of the most effective method of pest control!
The German soil ecologist and the English acoustics researcher published a paper describing their findings in Scientific Reports this past summer (12 July, 2019). They are not sure why the grubs chirp underground. They are hopeful, though, that after they gather sufficient data they should be able to come up with models that relate the stridulation patterns and intensities to the density of the grubs in a given soil volume. They also hope to develop techniques to identify and measure a wide range of crop damaging pests using these remarkably non-invasive, acoustic monitoring technologies!
The soil ecologist is also hopeful that she will be able to connect these auditory data to the metabolic rates of the grub community and then be able to make estimates of their production of carbon dioxide after all!
Now there are a three very important lessons or axioms about Science that are wrapped up in this narrative!
- Scientists need to communicate and cooperate with other scientists and share ideas and tools!
- A scientist always keeps their eyes open when dealing with natural phenomenon (SOMETHING unexpected (and interesting) is almost always going to happen!).
- A scientist should not blindly stick to their research plan! They must never hesitate to change and follow some new, beckoning path! (and maybe that path, just like the T.S.Elliot poem, will eventually curve back to the place that you began!)