PROCEEDINGS of the
23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Effects of traffic noise, land use types and ecotones on the
distribution of resident birds in a natural reserve in the Ruhr area,
Germany
Philipp ANTONIOU1; Dr. –Ing. Bryce LAWRENCE2
1

Technical University of Dortmund, Germany

2

Technical University of Dortmund, Germany

ABSTRACT
While being one of the most densely populated and formerly heavily industrialized regions of Germany, theRuhr area still offers space for many diverse ecosystems whose preservation is regulated by law. Neverthelessauditory impairments, especially by traffic noise and due to spatial restrictions are likely. This study examines
acoustic effects of traffic noise on the spatial distribution of birds in a natural reserve being in close proximity
to two roads with heavy traffic. Furthermore the effects of the land use and transition zones between two land
uses (ecotones) on avian distribution were evaluated. To answer the posed research questions soundmeasurements were taken throughout the whole natural reserve and analyzed utilizing five acoustic indices.
The results of the indices were used in a statistical analysis to investigate correlations between the distanceto roads and the amount of biophony as well as differences between various land uses and ecotones.
Methodological insights regarding the use of acoustic indices, correlations between the distance to roads andthe amount of biophony, as well as significant differences between acoustic environments recorded indifferent land use types were found.
Keywords: Soundscape, traffic noise, biophony

1. INTRODUCTION
The aim of the present work is to investigate factors influencing the distribution of bird
vocalizations in natural and protected areas using the example of the nature reserve 'Im Siesack' using
acoustic indices. The factors considered to explain vocalizations are time, land cover type (including
the transition areas between land uses) and distance to roadway. Given the scope of the study, it was
not possible to analyze the effects of other factors (such as distance from nearby villages, or wildlife
population characteristics) and apply further analysis to understand explanatory power of many factors.
Rather, the focus of the study was limited to the effects of roadways and land cover on the acoustic
environment and content of biophonic sound in German nature reserves. The determination of theinfluence of traffic noise on nature conservation areas is of particular relevance in the densely
populated Ruhr area, as many nature conservation areas are located in the immediate vicinity of busy
traffic routes. According to Section 23, Paragraph 2 of the Federal Nature Conservation Act, "allactions that could lead to the destruction, damage or alteration of the nature reserve or its components
or to a lasting disturbance (...) are prohibited". Accordingly, it must be determined to what extent thenatural acoustic environment of the study area is altered by the noise of the adjacent roads. The
acoustic environment can in turn be used to draw conclusions about the distribution of birds and thespecies diversity. In addition, the influence of land use on the acoustic environment will be determinedin accordance with Pijanowski et al. (1) in order to identify connections between the natural
1
2

Philipp.antoniou@tu-dortmund.deBryce.lawrence@tu-dortmund.de171

environment and the composition of the acoustic environment. Of particular interest are the ecotones
as transition zones between two land uses.
Farina et al. define the term soundscape and its components geophony, biophony a nd anthrophonyas follows: "The soundscape can be defined as every sound produced by any abiotic and biologicalcomponent of an ecosystem (geophonies and biophonies, respectively) together with anthropogenic
sounds (anthrophonies). It is thus the result of the energy released by both natural processes andhuman technologies (2)." The geophony was represented in the study area mainly in the form of wind,the biophony was formed almost exclusively by bird vocalizations and the anthrophony is dominatedby the traffic noise of the two adjacent major traffic routes. Acoustic indices were used to analyze theacoustic environments. Acoustic indices are automatically calculated and represent, among otherthings, the relationship between biophony and anthrophony as well as characteristic properties of theacoustic environment such as its acoustic diversity (3).

2. STATE OF RESEARCH
2.1

Influence of noise on bird vocalization and habitat distribution

Numerous studies have already investigated the influence of noise on the distribution of birds.
Particularly noteworthy is Garniel et al. (4), whose systematic study of 132 bird species forms thebasis for the legal regulations on the consideration of bird habitats in road construction projects (5).
It was found that for some species, such as the Little Bittern, at a critical sound level (52 dB(A))
habitats are completely devalued, while other species such as many birds of prey are almost
completely insensitive to road noise (5). Experimental studies by McClure et al. show a decline in thenumber of species by a total of 25% along a noise gradient of 44 - 51 dB(A) at a linear noise source(6). In addition to the influence on the absolute number of species and species composition as
described above, it has also been demonstrated that noise changes the behaviour of birds and inparticular the timing, frequency, level and redundancy of their vocalization, which in turn can affectreproduction success (7).

2.2

Influence of land use and ecotones on the spatial distribution of bird

vocalizations
The influence of land use on the acoustic environment was investigated by Pijanowski et al. ( 1).
The study shows that the acoustic environment of forested areas shows characte ristics associated withbiophony to a greater extent than agricultural or urban areas. It should be noted, however, that thebiophony can be composed not only of bird vocalizations but also of insects, which is not the case in
the present study. With regard to the influence of margins or ecotones on species diversity, Walz
describes that they have a clear influence on species diversity and composition. Studies in agrarian
areas in Saxony-Anhalt show that the diversity of species in the margins is up to twic e as high as insurrounding areas (8).

3. METHODS
3.1

Study site

The study area was selected according to the following criteria: It had to be a natural area withseveral types of land use, protected as a nature reserve and in the immediate vicinity of major tr afficroutes. The nature reserve 'Im Siesack' fulfils these criteria. It covers almost 170 ha and has a very
heterogeneous landscape structure. The main components are arable land (17% of the total area), bushand grassland (18%), meadows and pastures (20%) and woodland (30%). The area was designated anature reserve due to its high biodiversity and structural richness ( 9). It is also in the immediate
vicinity of a motorway (called BAB 2) and a country road with heavy traffic (called L 609). According172

to the environmental noise mapping of the Ministry for the Environment, Agriculture, Nature
Conservation and Consumer Protection of the State of North Rhine-Westphalia, the traffic noise
exposure of the area lies between 60 and 70 dB(A) above a 24-hour level in the north and west of thenature reserve (10). In addition, an industrial site is located directly south of the nature reserve, but it
does not emit any significant noise emissions. Due to the limited accessibility of numerous open land
and forest areas used as pastures, the study area was narrowed down after a first measurement run.
Figure 1 shows the study area, the narrowed-down study area and the location of the traffic routes.

Figure 1 – Overview of the study area and the traffic routes (L 609, BAB 2) in the surrounding area. The
study area is marked in pink, the narrowed-down study area is marked in white (Source: the author's own
compilation based on the Web Map Service 'DOP NRW' by: Land NRW (2019) Datenlizenz
Deutschland - Namensnennung - Version 2.0 (www.govdata.de/dl-de/by-2-0))
3.2

Soundscape recordings

All measurements were carried out at the end of September in a total of three measurement cycles.
Due to new insights during the conduct of the study, the methodology of the measurements varie d.
First measurement cycle: A grid of 128 points with a constant distance of 115 m to eachother was created in ArcGIS. Due to the limited accessibility, however, measurements could
only be carried out at 102 points. The measurements took place between 12:00 and 16:00 inorder to ensure a constant traffic load during the entire measurement period. The du ration ofeach measurement was five minutes.
Second measurement cycle: Because of the low song activity of birds at the time of the firstmeasurement, the second measurement was performed between 06:50 (sunrise) and 9:30. In
order to make optimum use of this short period, the measurement time was reduced to threeminutes and the measurements were carried out on the numerous paths of the study area thatcross the study area evenly. The distances between the measuring points were 100 m. 31

173



3.3

measurements were taken. This was a test of the extent to which the measurements deviatedfrom the first run.
Third measurement cycle: Due to the positive results of the second measurement cycle, therecording times and lengths were retained during the third measurement cyc le and themeasurements were carried out in the narrowed-down study area. According to a modified
approach of the 'sampling by land use category' by Brown and Lam (11), the 72 measuring
points were mainly aligned with the edges of the land use patches, sup plemented bymeasurements in the geometric center of each land use patch.
Data analysis

The software 'Rstudio' was used to calculate acoustic indices for the 204 usable recordings. The
following indices were calculated:
Acoustic Complexity Index (ACI): Calculation of the variability of sound intensity. The
higher the ACI, the higher the proportion of biophony in the acoustic environment.

Acoustic Diversity Index (ADI) and Acoustic Evenness Index (AEI): Splits the acoustic
environment into frequency bands and applies the Shannon Index (ADI) or Gini Index (AEI)
to them. The higher the proportion of biophony, the greater the diversity and the lower theevenness.
Bioacoustic Index (BIO): The BIO is a function of the sound level as well as the frequency
ranges occurring and exclusively reflects the degree of biophony in the recording.
Normalized Difference Soundscape Index (NDSI): The ratio between biophony andanthrophony is calculated, whereby anthrophony is defined as components of the acoustic
environment with low frequencies and biophony as components of the acoustic environment
with high frequencies. (3)The distribution of the index values determined over the study area is graphically displayed using
a heat map in order to be able to recognize patterns.
In the following step, the determined index values were statistically evaluated using the software
SPSS. For this purpose, the distance of each point to the L 609 and the BAB 2 was calculated using
ArcGIS and each point was assigned a land use. The land use categories used were kept non -detailed
in order to ensure that the sample size was as large as possible for each category. The data was checked
for normal distribution using the Kolmogorov-Smirnov test. The Spearman's correlation test was used
to answer the question as to whether the values of the index changed as a function of the distance tothe traffic routes, based on Pieretti et al. (1). The Kruskal-Wallis test was used to answer the questionof the extent to which the index values differed significantly depending on land use (based on Liu andKang, 13).

4. RESULTS
4.1

Spatial distribution of index values

Figure two (left) shows the distribution of the NDSI as determined in the first measurement cycle.
It can be clearly seen that in the areas which are closer to the traffic routes, the values are significantly
lower and thus more strongly influenced by anthrophony than in areas further away from traffic routes.
The environmental noise mapping of the MUNLV NRW as shown on the right side of figure two showsa similar pattern but differs regarding the degree of traffic noise impact by the L 609 and BAB 3 . In
contrast to the NDSI, the graphical representation of the distribution of the ACI, ADI, AEI and BIO
show no clear dependence on the distance to the traffic routes or the land use. Only trends are
discernible: Figure three shows that the linear woods between the fields in the south of the study area
have higher (or in the case of the AEI lower) values for the ADI, AEI, BIO a nd NDSI than the fields
themselves and can therefore be assumed to have a more pronounced biophony.

174

Figure 2 – Distribution of the NDSI during the first measurement run in comparison to the environmental
noise mapping of the MUNLV NRW 2017 (Source: own research; Web Map Service‘Umgebungslärmkartierung NRW’ and ‘DOP NRW’ by: Land NRW (2019) Datenlizenz
Deutschland - Namensnennung - Version 2.0 (www.govdata.de/dl-de/by-2-0))

ADI

AEI

BIO

NDSI

Figure 3 – Distribution of the index values in an agricultural area in the south of the study area; third
measurement cycle (Source: own research)

175

4.2

Influence of traffic noise on the acoustic environment and the spatial distribution of

bird vocalizations
The following table shows the results of Spearman's correlation test. Only statistically significant
results (significance level > 0.05) are presented.

1st Measurement run

2nd and 3rd Measurement run

Distance

Distance

Distance

Distance

L 609

BAB 2

L 609

BAB 2

ACI

0,204

-

ACI

-

-

ADI

-

-

ADI

-

0,327

ADI2

-0,274

-

ADI2

-

0,346

AEI

0,392

-

AEI

0,260

-0,195BIO-0,265-BIO-0,340

-

BIO2

-

-

BIO2

-0,274

-

NDSI

0,608

-

NDSI

0,304

0,529

NDSI2

0,600

-

NDSI2

0,348

0,551

Table 1: Significant Spearman's correlation coefficients of the three measurement runs (Source: own
research)

The calculation of the indices was carried out with the following parameters:
 ADI, AEI: Division of acoustic environments into 1,000 kHz wide frequency ranges
 ADI2, AEI2: Division of acoustic environments into 100 kHz wide frequency ranges
 BIO: Definition of biophony as the frequency range between 2,000 - 8,000 kHz ranges
 BIO: Definition of biophony as the frequency range between 3,000 - 11,500 kHz ranges
 NDSI: Definition Anthrophony: 1,000 - 2,000 kHz; Biophony: 2,000 - 8,000 kHz ranges
 NDSI2: Definition Anthrophony: 1,000 - 3,000 kHz; Biophony: 3,000 - 8,000 kHz ranges
The graph of the NDSI distribution and the corresponding Spearman's correlation coefficient showthat the greater the distance to the roads, the more anthrophony decreases. Consequently, therelationship between anthrophony and biophony also changes across the study area. However, due tothe fact that the BIO does not correlate with the distance to the roads in the expected way, it can beconcluded that the intensity of the biophony is not significantly influenced by traffic noise. Traffic
noise therefore decreases with increasing distance from the traffic routes, but the biophony does notincrease. Concerning the degree of acoustic diversity, no such clear correlations as with NDSI could
be found, but the weak to moderate correlations of ADI and AEI with the distance to the BAB 2 in thesecond and third measurement cycle indicate that the distance to traffic routes influences the
distribution of birds. However, this is only one of many factors and the influence of traffic noise is aminor one.

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4.3

Influence of land use and ecotones on the acoustic environment and the spatial

distribution of bird vocalizations
To determine the influence of land use, all points were divided into four land use categories:

1: Field, meadow, pasture (n = 53)
2: Small-scale woodland (n = 41)
3: Ecotones (n = 35)
4: Forest (n = 53)

Points that could not be assigned to any of the four categories were not used. Figure four shows
the results of measurement cycles two and three for the various land uses. Clear differ ences between
the different land uses are visible: fields, meadows and pastures tend to have low values (or higher
values in the case of the AEI), forests have high values, small-scale woodland and ecotones occupy
an intermediate position. At measurement run one, less clear differences could be observed due to thelower biophony, possibly because of the time of day.

Figure 4 – Results of measurement cycles two and three, sorted by land use (Source: own research)
The Kruskal-Wallis test shows that there are significant differences between varying land use types
regarding the ACI, AEI, BIO, NDSI and NDSI2 for the first measurement cycle. This is the case for
the ADI, ADI2, NDSI and NDSI2 for the measurement cycles two and three. A correlation between
land cover types and indices was not considered in this study but is recommended in future similar
studies to determine if land cover type or roadway noise have more relationship to the acoustic
environment.

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5. CONCLUSIONS
The results of this study demonstrate that the distance to disturbance sources influence the valuesof (bio)acoustic indices and thus most likely the distribution of bird vocalizations in the nature reserve'Im Siesack'. They also show that the surrounding traffic routes and especially L 609 emit noise into
the nature reserve. Furthermore, the variation of the acoustic indices depending on the land use
indicate a relationship between the land use or even avian habitat structure and the acoustic
environment in the study area. Nevertheless, there is a large number of factors that influence the
distribution of bird vocalizations that has not been part of this study. Garniel et al. ( 4) provide a good
overview of these influencing factors. Methodological insights were also gained. For example,
changes in the parameters with regard to the frequency ranges used to calculate the indices may havea significant effect on the results and should be adapted to the specific conditions of the study area.
In addition, it was recognized that the results from Pijanowski et al. (1) could be partly reproducedwith a short recording time and a simplified approach adapted to the terrain when determining themeasuring points. In this respect, an initial overview of the biophony and noise in a protected area inthe sense of the 'Rapid Acoustic Survey for Biodiversity Appraisal' ( 14) could be obtained.

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