Biology Reference
In-Depth Information
2.8 Sound Propagation at Forest Edges
The impact of the forest edge on acoustic propagation is not well known, it is
reasonable to expect a strong discontinuity of sound propagation from the abrupt
changes in microclimate, ground cover, and biomass present between interior
forest, edges, and open areas (rural or natural). Some recent measurements have
been carried out by, for example, Swearingen et al. (
2007
).
The hypothesis of a habitat-dependent noise condition has been evaluated by
Slabbekoorn (
2004
) in two tropical forest types (a primary and secondary rainforest
and a gallery forest) (forest ecotone). In the rainforest the spectral profile is
characterized by an increasing peak of frequency from category 1 to 7 (kHz), but
in the forest ecotone the spectrum was very monotonous with two peaks at 6 and
8 kHz, and the differences were significant (Fig.
2.4
). The rainforest is
characterized by distinctive frequency bands, but in the forest ecotone the peaks,
probably resulting from cicada activity, are present only at the higher frequencies.
The differences between the two types of forest probably are the result of wind-
generated noise and animal-borne sounds. The wind noise results are more impor-
tant in the forest ecotone.
The distribution of noise in specific frequencies probably affects the acoustic
selection of species, because animal signals have a great plasticity and can be
modified in a very short time according to environmental conditions. In conclusion,
it is reasonable to hypothesize that ambient noise may drive signal evolution. In
fact, ambient noise can mask the singing activity of males, concurring to reduce the
margin of affordability of female recognition of honest signals.
Fig. 2.4
Distribution of
relative amplitude level per
1.0-kHz interval from 0 to
8.0 kHz along eight sites of
rainforest and ecotone
forest of Cameroon (Central
Africa) (Reproduced with
permission from
Slabbekoorn
2004
)
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