Page 19 - Special Issue

bioacoustic Monitoring Contributes
to an Understanding of Climate Change
features of echolocation calls differed between rainy and dry seasons. In rainy seasons, when precipitation and humidity were high, absorption was higher (0.58 dB/m greater than in the dry season) and echolocation sounds were both lon- ger in duration and lower in frequency. An increase of this magnitude would translate into about 6 dB greater absorp- tion, and thus a lower sound intensity, for a bat detecting a prey 5 m away. Such changes may impact the bat’s foraging success by altering the distance over which its echolocation operates.
The calling behavior of tawny owls (Strix aluco) also is af- fected by precipitation. During dry days with lower sound absorption, these birds showed better signal-to-noise ratios for detection of their territorial calls and thus a larger com- munication range (Lengagne and Slater, 2002). Tawny owls did not call during nights of heavy rain, even within the breeding season. The authors estimated that to maintain the same communication range during rainy and dry nights, the owls would need to increase the intensities of their calls by 20 dB, increases that are not physiologically realistic (Len- gagne and Slater, 2002). Thus, if overall precipitation levels increase and rainy seasons become longer, then owl acoustic behaviors will become less efficient.
Precipitation also affects communication by its impact on vegetation. Moller (2011) compared the location of calling sites (‘song posts’) in 34 species of Danish birds in the period 1986-89 and in 2010. Over this time span, local ambient temperatures increased on average by 2° C (20%), precipita- tion increased by 75 mm (30%), and vegetation was overall higher. Over all species sampled, male birds sang on aver- age from higher perches in 2010 than in 1986, an 18% (1.2 m) increase in “song post height.” Moller (2011) suggested that the increased precipitation and temperature produced increased vegetation which in turn triggered changes in song post height. The extent to which an individual species of bird moved to higher perches was related to their local microhabitat: Birds singing in forested areas showed larger increases in song post height than birds singing in grassland. We do not know how these changes in song post height af- fect the propagation or degradation of song. Details of the mating interactions within a species as well as their suscep- tibility to predation also influenced the magnitude of the changes in song post height. Because not all species reacted in the same way to similar climatic changes, this suggests that local ecosystems might become unbalanced by favoring some species over others.
Temperature
Like fishes, ectotherms such as insects, reptiles, and amphib- ians are physiologically vulnerable to external temperature fluctuations (Deutsch et al., 2008; Paaijmans et al., 2013). Ectotherms living in temperate zones have been shown to have a broader tolerance range for changing temperatures than those living in the tropics (Deutsch et al., 2008). Be- cause of this broader tolerance range, these species might suffer fewer physiological, and thus behavioral, consequenc- es of increased temperatures than tropical species (Navas, 1996).
Laboratory studies in both insects and anuran amphibians have shown that temperature affects call production and the operation of the ear and auditory nerve (Stiebler and Narins, 1990; Gerhardt and Huber, 2002; Meenderink and van Dijk, 2006). For example, wing-stroke rate in stridulating, sing- ing insects is linearly related to temperature over a fairly broad range. And in some anurans, the rate and duration of the male’s advertisement call vary with temperature, al- though these effects are not always linear but show consider- able individual and species variability (Gerhardt and Huber, 2002). Currently, we have little information as to how these effects would manifest in the natural environment under conditions of global warming.
How a changing thermal environment affects calling behav- ior in temperate zone anurans has been examined by Llusia et al. (2013). These investigators monitored atmospheric temperatures and calling temperatures (atmospheric tem- peratures during the times the animals were actively vocal- izing) of 10 species of Iberian anurans for a continuous three year period. Males of all of these species vocalized across a range of environmental temperatures, exhibiting a wide “thermal breadth” of activity. These data suggest that an- urans can adapt to changing environmental temperatures. Species living in habitats with hotter temperatures had lon- ger calling seasons and longer periods of calling than those living in habitats with cooler temperatures. For both hot- ter and cooler habitats, when ambient temperatures rose, so did calling temperatures. Llusia et al. (2013) suggested that temperate zone anurans can acclimate to increased external temperatures by shifting their preferred calling tempera- tures, but again, different species do not all react in the same way.
The onset of the breeding season in temperate zone frogs is typically indexed acoustically, by the beginnings of chorus formation and advertisement calling (Blair, 1961). Gibbs and Breisch (2002) showed that over the time periods 1900-
Spring 2020, Special Issue | Acoustics Today | 19 Reprinted from volume 10, issue 3
12 | Acoustics Today | Summer 2014