Hokkaido University finds that the dietary habits of grasshoppers altered by automobile noise through fecal analysis — Does intense stress cause dietary intake diversity?

A research group composed of Associate Professor Masayuki Senzaki of the Graduate School of Environmental Science at Hokkaido University and Head Taku Kadoya and Senior Researcher Haruko Ando of the Biodiversity Division, National Institute for Environmental Studies announced their discovery that automobile noise diversifies the dietary habits of omnivorous grasshoppers, and the impact occurs within a range of approximately 300 m from noise sources, even with sufficient weakening of the noise level. Using outdoor noise experiments together with the DNA metabarcoding technique, they discovered that grasshoppers exposed to noise stopped focusing on specific foods and began consuming a wider variety of other items. This study revealed that anthropogenic noise extends over a broader area than previously assumed, and the results were published in the October 13 issue of the international academic journal Ecology Letters.

In recent years, the impact of anthropogenic noise from sources such as automobiles and aircraft on organisms and ecosystems has been a cause for concern. For example, vertebrates such as birds and mammals have been observed to experience heightened stress or difficultly in detecting the calls of other individuals when exposed to noise. These effects can lead to reduced breeding success, lower survival rates, and the abandonment of habitats.

Despite that, insects and other invertebrates play crucial roles in primary production and material cycling in ecosystems as major members of the community, the subjects of research on the impact of anthropogenic noise are often vertebrates, and the impact on invertebrates remained unclear.

The research group conducted field experiments to reproduce automobile noise in grasslands in the Iburi region of Hokkaido. They investigated how such noise impacts the feeding behavior of omnivorous grasshoppers as representative terrestrial invertebrate consumers.

In this experiment, they set up 6 sites (each measuring 80×80 m²) in which noise was emitted from speakers and another 6 sites where no noise was emitted. Within each site, both close- and far-range zones extending from the speaker positions were established. The noise emitted in the close-range zone was set at approximately 50−70 decibels, whereas that in the far-range zone was set at approximately 35 decibels, similar to background noise (non-artificial). The close- and far-range zones were approximately 150 m apart, and each site was selected in areas with similar vegetation and other conditions.

During two periods, before and during noise emission, in each zone, the research group collected approximately 600 fecal samples from eight identified grasshopper species inhabiting the same grassland site. These samples were analyzed using the DNA metabarcoding method to determine the plant and animal matter within the feces. Specifically, the researchers captured approximately five individuals of each of the eight grasshopper species from both the close- and far-range zones. They isolated each individual and collected the feces produced by it for analysis. At the same time, they investigated the noise effects on the abundance of grasshoppers and birds (which are predators of grasshoppers) and on the foraging pressure of birds.

Statistical analysis of the obtained data revealed that exposure to noise in both the close- and far-range areas had increased the dietary diversity of grasshoppers. This trend was observed in both plant- and animal-based diets. Exposure to noise led to a shift from the consumption of only a few specific foods to the intake of a wider variety of food items. Generally, different grasshopper species exhibit different preferences for specific types of vegetation, such as certain grass species, and tend to consume the same type in the absence of stress.

The noise had led to foraging pressure for birds and a decrease in their presence. This shift in feeding habits is believed to be directly caused by the effects of noise. The noise impact was shown to extend to the far-range zone where the noise level was sufficiently lowered. Despite the noise level in the far-range zone being comparable to background noise, it nonetheless contributed to stress in grasshoppers.

Senzaki stated, "In this study, it has become evident that noise affects a broader range of ecosystems than previously thought. Currently, the establishment of nature reserves for ecosystem conservation does not consider noise. I believe that noise mitigation could be achieved by establishing sufficient buffer zones around protected areas. In the future, I aim to proactively identify specific measures to determine how we can decrease its influence."