Digging Deep: How animal soundscapes reveal levels of Amazon forest degradation

A study published in PNAS this month finds how biodiversity is affected as a result of the degradation of the Brazilian Amazon rainforest. Of course, any survey of biodiversity on Earth brings with it its challenges and limitations. To further this problem, Rappaport et al. (2022) Observation of sounds coming from the forest in different pockets and checking its agreement with the specialized acoustic hypothesis.

The Acoustic Specialized Hypothesis (ANH) says that in a well-established ecosystem, different species take advantage of different bandwidths to avoid competition. Therefore, each genre sings in a unique tone/frequency at unique times. However, in poorly formed disturbed ecosystems, a certain bandwidth is exploited by more than one species while other bandwidths may be left completely unused.

Danielle Rappaport, lead author of the study, says in an email with indianexpress.com. The ANH argues that there is a positive linear relationship between the extent of habitat integrity (safety is measured by aboveground biomass, i.e. trees, etc.) and the occupancy of acoustic space by the animal community.

An increase in ASO means that more “sound channels” are being used by species symbiosis in habitats. In other words, species emit sounds in a greater range of hours and frequencies during the 24-hour cycle’, Rappaport added.

This is similar to the concept of resource division or specialized differentiation that is often observed in terms of food. Two or more species that devour a particular fruit eat different parts of the fruit, or forage for food at different times of the day.
The team of researchers from the University of Maryland measured how many bands were recorded after three different stages of degradation: once burned, frequently burned, and logged. Acoustic measurements were made for each of the three locations at 1 minute and 1 hour time—naturally, these audio clips are affected by degradation from logging and fire.

However, patterns of change do not consistently follow the predictions made by the acoustic niche hypothesis, “instead, acoustic analyzes revealed a paradoxical effect of fire and log on the vocal community structure,” the study reports. First of all, daily ASO increased with aboveground biomass after fire, but not after logging.

The distinction in ASO in the combustion environment was largely insect-driven, because it was during the insect-dominated periods of the day that the change in the percentage of ASO was most markedly observed. For the regions that were recorded, the only time periods that showed a percentage change in ASO ended up showing a negative correlation, that is, the greater the time elapsed since recording, the greater the decrease in ASO.

The time periods when insects dominate – mid-morning, noon and night – are rarely sampled in most surveys, because they focus so heavily on birds. In fact, this study specifically did not observe a significant difference in ASO during dawn and dusk, which are bird-dominated time periods.

In frequently burned forests, ASO was the lowest, occupying the smallest frequency space across all time periods. Compared to the recording, “With a recurring fire, we see some animal cues dominating the 24-hour acoustic landscape, rather than a more differentiated acoustic landscape representing a more diverse animal population,” says Rappaport.

Moreover, the paper states that in forests that were once burned and logged, one can still observe frequencies below 3.7 kHz and above 5.2 kHz. But frequencies outside the mid-range (3.7-5.2 kHz) are less prevalent. Aside from determining the total ASO, the team also evaluated and modeled the interactions of individual audio cues. Rappaport adds that, “In the case of frequently burned forests, the 24-hour communication networks with animals are more homogeneous from day to night (fewer distinct peaks and choruses as you might expect when you consider a tropical rainforest soundscape).

What could explain this discrepancy between Rappaport et al.’s study and the vocal stature hypothesis? Why, “contrary to the hypothesis predictions, did the less intact forests contain more acoustic lacunae (i.e. empty acoustic ports)”?

One of them can be that the vocal niche division is the product of a long-term development; And when there is a disturbance, such as a fire or logging, the specialized vocal differentiation loses its relevance. Another reason may be that the study actually looked at the time periods in which the insects were active, and they tended to have more precise responses to perturbations. For example, species of cricket are very good at splitting frequencies, while species of cicada tend to have frequent interferences. Therefore, cicadas have evolved to live and transmit sounds at different altitudes, making them more susceptible to temperature changes, such as those caused by fire. Thus, the acoustic landscape produced in the burning forests is the result of the complex interaction between the structure of the forest, the local climate and the habitats of the animals.

Overall, the study highlights that potential environmental and acoustic studies should monitor biodiversity change in the event of decline, because “the competition for acoustic space is the strongest.” For example, this study allowed researchers to perceive links between habitat conditions and communication with animals other than those of only birds. In the future, studies measuring acoustic differentiation could be combined with satellite measurements and ground assessments to shed light on biodiversity responses especially in highly diverse tropics (see, for example, Aide et al. 2017 or Planque et al. 2008), the team argues.

The author is a Research Fellow at the Indian Institute of Science (IISc), Bengaluru, and an independent science panellist. Tweet on @critvik

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