Communication Theories - Soundscape Ecology: Principles, Patterns, Methods and Applications - page 97

reduction of half their size if the actual climatic trend in rising temperature is
confirmed. One risk of deforestation depends on fires, which have doubled in recent
years. Forests damaged by fires are easily attacked by species of opportunistic
insects. This concern recently has been extended also to temperate forests that
under drought stress are attacked by bark beetles.
It is evident that some damages are the result of just a unique species of insect; in
Alaska has been reported that the spruce bark beetle ( Dendroctonus rufipennis ) has
killed 4.4 million acres of forests. We do not enter into the details of the damages
caused also by other species in the American subcontinent or in Siberia, but the
situation is in continuous evolution and forests are facing an unprecedented risk of
radical modification of their assets with consequences on the carbon storage
balance.
Among the several hypotheses of a modified relationship between insects and
trees, the bioacoustic ecology hypothesis represents a novel field of investigation.
The changes in temperature and moisture create conditions for insect outbreaks,
changing the relationship between plants and insects. Insects are poikilothermic
organisms and a change in temperature, for instance in winter, can favor the active
presence of insects all year around.
Trees have no capacity to react at the same speed of climatic changes. At the
southern range plants under an increased temperature reduce the reproductive cycle
and die; on the northern border they migrate at a low speed, and the total surface
occupied by a species is progressively reduced.
Between the different aspects that favor the spreading of insect outbreaks in
many northern biomes, little is known about the role of bioacoustics in this process
because the insects responsible for the decline of forests use intensively acoustic
mechanisms that are not well understood at the present.
In many bark beetles, there is a pars strident organ; at least 14 types of
stridulating organs have been described in this group of insects for 30 different
families. In Ips confusus beetles this organ is located on the back of the head and is
stroked by a plectrum on the underside of the dorsal anterior edge of the prothorax.
In other species the pars strident has a different anatomical position. The sound
produced by these organs consists of a single-impulse click or a range of different
multi-impulse chirps. All these sounds are considered stridulations.
In some species, as in the genus Dendroctonus , only the male has a stridulation
organ but the female is the pioneer. The opposite is observable in the Ips genus
where the male is the pioneer and without pars stridens and it is the female that has
such an organ.
It is evident that bark beetles use either chemical and acoustic signals to regulate
aggression, attachment on host trees, courtship, mating behavior, and population
density regulation. The chemical signals seem to be used for communicating at a
distance and the acoustic signal for short-distance communication. In the complex
behavior of this species chemical and acoustic cues are intensively utilized,
although little information is yet available on the hearing organ mechanisms.
On the other hand plants emit, when under hydric stress, acoustic signals
produced by cavitation phenomena. Cavitation is the breaking of the water column