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B7) Soundscapes

Ecoacoustic monitoring of vocalising organisms along forest structural gradients

Michael Scherer-Lorenzen & Sandra Müller
Doctoral researcher: Taylor Shaw (since 2020)

University of Freiburg, Faculty of Biology, Institute of Biology II, Department of Geobotany


Background

Soundscape ecology, or “ecoacoustics”, seeks to understand how biophonic, geophonic and anthrophonic sound sources interact at various spatial and temporal scales. Ecologically relevant characteristics of digital audio recordings can be quantified by converting audio files into spectrograms and deriving acoustic indices of diversity, complexity, and spectral occupancy (to name a few). As soundscapes vary across ecological and human-disturbance gradients, acoustic indices provide metrics with which to test hypotheses about observed acoustic patterns and to better understand how vocalizing organisms respond to changes in their environment. Much of terrestrial acoustic monitoring focuses on avian taxa, because they are highly vocal, exhibit wide geographic distribution, play diverse roles in ecosystem functioning, and are often used as indicators of environmental change. Birds are important bioindicators for forest habitats: species richness of both specialist and generalist forest birds can increase with forest age and size, and bird assemblages of forest ecosystems provide valuable information about habitat quality and biodiversity indicators on a European-scale.

The response of birds to structural complexity is a long-studied subject, and theory predicts that the number of species positively correlates with resource range, which increases with forest structural complexity. Vertical structure in particular directly affects birds through its influence on perching, nesting, and foraging sites, and areas with greater vertical structure thus provide more niches. Forest management plays a crucial role in shaping structural features which can either promote or suppress avian diversity. Managers closely monitor structural features and collect metrics which can be used for monitoring both forestry and biodiversity outcomes. Previous studies on ecoacoustics, avian diversity, and forest structure separately provide evidence that stand structural features and acoustic indices can be identified and used to predict avian diversity, however research synthesizing these factors is lacking. Thus this project aims to better understand the relationship between management-driven structural features and acoustic diversity, mediated by relevant ecological factors such as avian community composition, resource availability, landscape context and meteorological variation. This research also aims to explore which spatial and temporal scales are most effective for identifying acoustic differences across these gradients.

 

Research questions and hypotheses

The overarching aims of this study are:

  1. Develop a standardized baseline soundscape dataset for the southern Black Forest via a suite of acoustic indices that quantify its acoustic diversity
  2. Using these indices, describe shifts and patterns in the distribution of sound at different temporal scales (seasonal shifts, within-season shifts, and daily shifts in birdsong phenology)
  3. Correlate acoustic diversity and forest structural data with avian point count data (from ConFoBi project B6) to assess the predictive power of acoustic and structural indices combined on avian diversity in a temperate managed forest context
  4. Determine which forest structural elements reflect aspects of avian diversity that are not well-captured by acoustic indices, and vice versa
  5. Outline a combined set of structural and acoustic indices which together provide an efficient, broad avian diversity monitoring method useful for forest managers

 

Specific research questions include:

  1.  Acoustic diversity has been shown to generally positively correlate with avian diversity in a managed temperate forest ecosystem; do these findings replicate in the southern Black Forest, and if so, which acoustic indices have the strongest positive correlation with bird diversity? Which indices have the strongest positive correlation with specific life-history guilds?
  2. Based on literature of avian response to forest structure, we hypothesize that acoustic diversity will respond positively to measures of vertical and horizontal structure and negatively to certain aspects of stand composition (e.g. percentage of conifers). Does acoustic diversity data support this hypothesis, and if so, which structural explanatory factors have the strongest effect?
  3. How important a factor is dead wood (a key management-driven structural element and currently a popular retention structure in managed forests), in influencing the diversity of avian vocalizations? Do dead wood metrics affect avian diversity and acoustic diversity differently?
  4. Landscape context is likely to be an important driver of acoustic diversity, given that birds can fly and are not restricted to habitats at the plot-level. We hypothesize that acoustic diversity will increase with forest patch size, and even more strongly with landscape matrix quality.

 

Approach, methods and linkages

Automated acoustic recorders are deployed at ConFoBi plot centerss to capture acoustic events during to the bird breeding season. Each device is programmed to record from 18:00 to 9:00 the following day, totaling 90 one-minute recordings per plot per day, throughout spring. This schedule will capture both the dusk and dawn choruses and their respective shifts throughout the season, which is important because the bird community changes throughout spring: short-distance migrants and resident birds vocalize as early as April, but long-distance migrants typically do not begin vocalizing upon their arrival in May. The dusk chorus will also be recorded, as some research has shown that differences between certain forest plots are more detectable in dusk rather than dawn choruses (S. Müller, pers. comm., 30/3/2019). The devices will record overnight, between the dusk and dawn chorus, to additionally capture any nocturnal birds whose presence is difficult to detect through traditional daytime survey methods. This is a useful addition to the sampling design, as we may find that different plots are preferentially selected by an assemblage of nocturnal species - including the Nightjar (Caprimulgus europaeus) and certain owl species of high conservation interest - differences a morning point count survey alone may not be able to parse. If so, this would provide presence/absence data that more traditional sampling methods do not capture, which is relevant for all ConFoBi projects also using bird diversity datasets. It would also provide a subset of acoustic data for which structural variables may have greater explanatory power.

Over one million raw data files are then processed by a combination of manually cleaning weather events and automatically filtering background noise. Next, a suite of acoustic indices is computed and related to datasets from other projects. Temperature, atmospheric pressure, humidity, and light regime are also recorded at ten-minute intervals at each plot center by the acoustic devices, and correspond directly with the acoustic recording schedule. Avian point count surveys occur twice from April-June in all ConFoBi plots within ConFoBi Project B6. Thus, the resulting bird species richness and abundance datasets will have been generated from the same sampling period. This project also plans to use light data from Project B2 and structural inventories from Projects A1 and A2.

 

Findings

Although this project was recently added to ConFoBi (Jan 2020), results from an earlier pilot study demonstrated significant differences in acoustic complexity values along gradients of standing dead wood and connectivity in surrounding 25 square km., which preliminarily confirms our primary hypothesis. It provides initial evidence that structural differences in ConFoBi plots produce detectable variations in avian acoustic activity, findings which needed to be established prior to any further research. Additionally, acoustic experiments have been conducted prior to the start of the project, because there is no standard recording methodology in the literature for surveying on sloped terrain, on which many ConFoBi plots are situated. These results will identify which device orientation captures the largest range of frequencies at the widest possible radius, prescribing optimal installation practice for omnidirectional microphones in locations with steep inclines. These findings will be implemented in future acoustic research within ConFoBi.