There is a space and time in the zone of the twilight when the last rays of the sun are slowly engulfed by the crawling darkness in the forest when the sound is most intense. It is as if you can feel the collective 'raging against the dying of the light'. It is the sound of the collective sounds of insects from cicada to crickets, bats and some birds species and maybe some nocturnal creatures welcoming the dark. This magical hour is familiar in the rainforest of Mount Bulusan. It is referred to as the 'dusk chorus' in acoustic ecology.
Acoustic ecology is a branch of study for this seemingly unknown aspect of the forest. It is an area that deals with sound as it relates to the lives of the wildlife living on it:
One specific study is this: insects.oeb.harvard.edu/farrell_lab/undegrads/bseah/lit_review.pdf
It underscores the function of sound as stated at the start of the introduction:
Animals communicate by producing and receiving signals of various kinds, one of them
being sound. A wide variety of animals produce sounds – birds, mammals, amphibians,
arthropods, and even fish. These calls serve various functions, which are mostly
intraspecific: (1) advertisement calls – for males to attract females over long distances,
aids in localization, also involved sometimes in male-male aggression and competition,
(2) aggressive signals – specifically for aggression between competing advertisers, (3)
release calls – especially among anurans, produced by unresponsive individuals when
they are clasped by a male for mating, (4) courtship calls – sometimes also produced by
females, act over a shorter range, produced just prior to mating, (5) distress calls – also
called handling calls, when grasped by predators. (cf. Gerhardt and Huber p.12)
Animals produce acoustic signals for various purposes. Their calls are presumably subject to natural selection. Acoustic ecology is the study of the selective pressures that act upon the calls that animals make.
Thus, to my mind, to alter the acoustic ecology of a pristine area like Mount Bulusan rainforest with the planned introduction of industrial activities of a Geothermal power plant (with noise levels from their industrial operations reaching higher than the decibel range of urban noises) is tantamount to environmental invasion worse than introducing an invasive species of flora and fauna. Its impact to the wildlife residents of the forest is disastrous considering that almost all of these creatures are dependent on sounds for their survival from mating calls to the caring of their youngs.
Note:
For actual sounds of a South East Asian forest in the above mentioned study, this link: insects.oeb.harvard.edu/farrell_lab/endergrads/bseah_aepf_sounds.html provides a sound gallery (in mp3 format) ranging from sounds of insects such as cicadas and mammals such as bats. The time of the sound recordings differs to get a feel of the actual sound settings. The dawn and dusk chorus for example when the intensity of the sound of the forest is most pronounced is noted by the study.
Photographs by Alma P. Gamil
Bulusan Volcano Natural Park (BVNP), Bulusan, Sorsogon, Philippines
