Effects of highways on bird distribution and soundscape diversity around Aldo Leopold’s shack in Baraboo, Wisconsin, USA

Highlights

• Avian diversity was assessed with point counts and passive acoustic monitoring.

• Highway noise and its edge effect negatively affected diversity in certain habitats.

• Effects differed between interior and edge specialist species.

• Avian acoustic activity increased with distance from highways.

Abstract

In the 1940s, Aldo Leopold took extensive notes on birds and their sounds near his iconic shack in Baraboo, Wisconsin, USA. His observations, along with his land management techniques, helped frame his seminal book, A Sand County Almanac. After his death, two interstate highways were built near his property and subjected this historically significant area to traffic noise. While highways currently represent vital transportation corridors, their observed and potential impacts on biodiversity and ecosystem services are cause for concern. As the area including Leopold’s shack is now an Important Bird Area, we sought to evaluate the impact of these highways on the bird community and its related acoustic diversity. In 2011, 150 avian point counts were conducted in the three main habitats composing the landscape—upland deciduous forest, floodplain forest, and herbaceous wetland. In 2012, soundscape recordings were collected in seven floodplain forest sites using automated passive acoustic recorders. We described the local bird communities and measured their acoustic diversity. Linear models accounting for additional factors including land cover and vegetation structure characteristics showed that as the distance from highways increased, bird community descriptors (overall abundance and species richness) and acoustic diversity increased (when relationships were significant). On the species level, forest interior specialists were negatively affected by the presence of the highways, contrary to edge specialists. In addition to the direct effects of the edges produced by the highway structure, this difference might be due to the masking effect of traffic noise on interior specialists’ low-frequency vocalizations and their reliance on acoustic, as opposed to visual, communication. We conclude that while habitat structure is a principle driver of bird diversity on a broader scale, highway-induced changes in both habitat structure and soundscapes may affect bird communities.

Introduction

In the study of environmental ethics and wilderness conservation, one cannot overstate the impact of Aldo Leopold’s (1887–1948) work, as his naturalistic observations became a cornerstone of the conservation biology movement and wildlife ecology (Burke, 2000, Callicott, 1990, Callicott, 1999, Flader, 1994). Much of the observation and writing that critically shaped his ideas took place on his property in south-central Wisconsin where he authored his seminal book, A Sand County Almanac. This groundbreaking work poetically describes the ethics, policies, and land management practices necessary to preserve ecological integrity while meeting human needs. Leopold observed that the soundscapes of his land were substantially influenced by bird populations (Bocast, 2013, Leopold, 1970), which are indicators of environmental health (Bocast, 2013, Gregory and van Strien, 2010). In the early 1960s, however, Interstate 90 (I-90) and Wisconsin State Trunk Highway 78 (now I-39) were routed near this historically significant area, consequently changing its landscape and the corresponding soundscapes.

Ecosystem functions depend on structural characteristics, including vertical vegetation profiles (MacArthur & MacArthur, 1961) and plant species composition (James & Wamer, 1982), and road construction changes these structural characteristics, often resulting in biodiversity loss or novel species assemblages (Pimm et al., 1995, Vitousek, 1994). For forest birds in particular, structural characteristics determine habitat suitability (MacArthur and MacArthur, 1961, MacArthur, 1964) by providing foraging and nesting opportunities as well as suitable locations from which vocalizations will propagate well (Farina and Belgrano, 2006, Pijanowski et al., 2011). More specifically, road networks affect bird populations (Benítez-López et al., 2010, Habib et al., 2007, Reijnen and Foppen, 2006) by physically fragmenting habitats and by generating traffic noise, which we define here as non-functional, unintentional, low-frequency sound (<2 kHz) caused by on-road vehicles. Traffic noise can drastically affect avian communication, as this frequency range overlaps with the frequency ranges in which some bird species produce sound (Halfwerk, Holleman, Lessells, & Slabbekoorn, 2011). Such continuous anthropogenic noise disturbs complex animal social structures (Cartwright, Taylor, Wilson, & Chow-Fraser, 2014), as acoustic communication is vital for birds to find mates, defend territories, hunt, and navigate landscapes (Catchpole and Slater, 2003, Farina and Belgrano, 2006).

Acoustic communication occurs in the context of a soundscape—the total collection of all biological, geophysical, and technological sounds (biophony, geophony, and technophony, respectively) occurring at a given place over a given time period (Mullet et al., 2016, Pijanowski et al., 2011, Pijanowski et al., 2011, Qi et al., 2008). Birds and other animals have evolved to communicate effectively in the natural physical structure and biophonic and geophonic conditions of their habitats (Lengagne and Slater, 2002, Luther, 2009, Morton, 1975). Technophony represents a potential impediment to such communication, however, and soundscape studies can be used to consider the propagation of anthropogenic noise and the interactions between biophony and technophony. Recently, soundscape analyses have also been used to quantify biodiversity and spatiotemporal ecological change (Dumyahn and Pijanowski, 2011b, Francis et al., 2011, Parris and Schneider, 2009, Pieretti and Farina, 2013, Shannon et al., 2016, Sueur and Farina, 2015, Summers et al., 2011). Some adverse impacts of roads on birds, including edge effects, population isolation, and road mortality have been well documented (Barber et al., 2011, Barber et al., 2010, Forman and Alexander, 1998, Forman, 2003), although there has been insufficient research on how roads impact avian soundscape contributions (Duarte et al., 2015, Pieretti and Farina, 2013). In order to promote avian conservation, it is important to understand the impact of roads on bird populations and the resulting soundscapes composed of traffic noise and altered bird sounds.

It is necessary to understand the sonic and non-sonic impacts of roads on bird communities in order to implement landscape-level conservation strategies to sustain bird communities (Francis et al., 2011, Smith and Pijanowski, 2014). In this study, our objective was to evaluate the impact of two major highways (I-90 and I-39, hereafter referred to as “highways”), on the bird community in the Leopold-Pine Island Important Bird Area (LPIIBA). This impact was quantified through avian point counts and passive acoustic monitoring. This study was also intended to demonstrate the utility of soundscape studies in evaluating disturbance impacts. Birds are highly dependent on acoustic communication and monitoring the acoustic diversity of bird communities is a valuable strategy to assess the consequences of sonic and non-sonic ecosystem disturbances. Furthermore, we sought to understand how other dominant landscape drivers affect bird distribution in a landscape of upland deciduous forest, floodplain forest, and herbaceous wetland at LPIIBA. To achieve these objectives, we 1) quantified the effect of distance from highways on the overall abundance, species richness, and composition of the bird community, 2) quantified the relative soundscape contributions of biophony, here dominated by bird sounds, and technophony, produced by highways in the study area, 3) investigated the effect of highways on avian acoustic diversity, and 4) examined the impact of other habitat structure variables on the bird community.