This post was contributed by Rachel Darling, a master’s degree student at the University of Michigan and a 2021 WOS Research Grant recipient.
We’ve lost 3 billion birds since 1970. Why? A menagerie of reasons, chief among them habitat loss. As the human population continues to grow and the footprint of our urban and suburban areas expands, anthropogenic (human-caused) alterations to environments are likely to increase and shrink the amount of suitable habitat available to birds. While certain anthropogenic effects on birds are well known to the general population (you may be familiar with the effects of DDT on bald eagle reproductive success, the threat domestic cats pose to bird survival, or the dangers of night light and glass for migratory birds), others, such as the effects of anthropogenic noise, are less well known, especially during birds’ breeding season.
I became interested in studying the effects of anthropogenic night light and noise (ANLN) on breeding birds during my master’s degree in conservation ecology at the University of Michigan School for Environment and Sustainability. I had studied the social sciences as an undergraduate, but after receiving my first pair of binoculars, I increasingly became fascinated with birds, their behaviors, and humans’ interactions with and effects on them, and so I decided to go back to school for a master’s degree that would allow me to conduct practical bird-related research. My first semester of graduate school acquainted me with the concept of ecosystem services, the benefits that people can gain from different parts of the natural world (such as wetlands providing flood protection, scavengers preventing the spread of disease, etc.). Of course, as an inquisitive bird-lover, I wanted to know how birds fit into this framework.
I found an excellent jumping off point in Şekercioğlu, Whelan and Wenny’s exploration of the subject, and after reviewing the literature, I discovered only a small number of studies drawing links between ANLN and ecosystem services provided by birds. Investigating changes in avian ecosystem services associated with ANLN seemed to me a good way to demonstrate the importance of humans’ effects on birds to people who might not otherwise be interested in wildlife. Via a connection to Clinton D. Francis through my thesis advisor’s lab, the Conservation and Coexistence Group, I had access to data on both the location of breeding birds through the years and ANLN across the United States. So, with an aim to provide useful information to practitioners and habitat managers, I decided to pursue this topic for my master’s thesis research.
An important part of my project was categorizing bird species according to the ecosystem services they provide. For simplicity’s sake, I chose to examine the ways in which humans benefit most directly from birds: seed dispersal, invertebrate pest control, pollination, nutrient cycling, and carrion disposal. Other ecosystem services such as ecosystem engineering (making changes to the physical structure of ecosystems) do benefit humans, but those benefits involve several intermediate stages or processes in addition to the direct actions of birds and are challenging to measure. An example of this is woodpeckers creating nesting holes, which speeds the decay of the tree, returning nutrients to the soil and helping other plants establish and grow, which may be plants of interest to humans…etc. I also only included bird species for which I could find specific measurements of the services they provided, and species that were included in the breeding bird and ANLN dataset. This left me with sixty bird species to work with.
Once the species were sorted into their ecosystem service categories, I analyzed whether each species’ presence or absence across the contiguous United States was influenced by anthropogenic light and/or anthropogenic noise, taking into account other variables associated with the built environment, such as human population and the amount of impervious surface. Does a species tend to avoid increasing levels of light or noise, is it attracted those pollutants, or does it not seem to care? And is there a net effect of light or noise on each of the ecosystem services? I wanted to answer these questions, and the Paul A. Stewart grant from the WOS allowed me to do so, alleviating some financial stress from the pandemic and allowing me to focus my energies on my research rather than unrelated part-time work.
Given the many studies demonstrating the negative effects of night light on birds during migration, I was surprised to find that during breeding season, only 8% of species responded negatively to light and 10% to noise. Perhaps even more surprising, 12% of species responded positively to noise. Though many species were not drastically affected by low ANLN levels, over certain light or noise level thresholds, some bird species entirely disappeared from an area. These maximum light and maximum noise thresholds vary greatly based on which species you are examining. For example, as illustrated in the graph below, among species providing invertebrate pest control, the Black-capped Chickadee and Western Meadowlark have the highest maximum noise thresholds (they aren’t restricted by noise levels throughout their range), while the Magnolia Warbler shows the most sensitivity to noise, only occurring in areas up to about 11 decibels louder than natural background noise (i.e. wind/weather, noise from rivers, other animals, seismic activity, etc.).
To estimate the effects of ANLN on ecosystem services across the contiguous United States, I had a lot of fun with ArcGIS Pro (read: spent many, many hours Googling error messages and searching help forums!), a spatial data analysis software program. For each of the five ecosystem services I was interested in, I compared the number of species that would be able to provide a service in an area in the absence of any ANLN effects (imagine layering bird species range maps on top of each other, see top map for example with seed dispersal species) to the number of species that actually provide a service in an area based on the ANLN statistical models mentioned above (see bottom map).
Based on my analysis, seed dispersal is the ecosystem service most at risk from anthropogenic night light and noise, and noise may cause greater declines in seed dispersal by birds than night light. This occurs across the contiguous U.S., with highest losses due
s to noise occurring in the Southwest, the coastal range of California, and areas of central and southern Texas. Invertebrate pest control sees small to moderate losses due to noise, again concentrated in the Southwest. Pollination and carrion disposal are relatively unaffected, with losses only occurring in and around metropolitan areas. Finally, nutrient cycling was not well represented across the contiguous U.S., with one widespread species (the Red-winged Blackbird) disproportionately impacting the data. Because of this, I won’t make any claims about the potential impact of ANLN on this ecosystem service.
Why should you care about all these models and maps? Let me start with the positives: more than 75% of the 60 species I studied do not react strongly to night light or noise. If this pattern holds for the rest of the species that breed in the US (there are no guarantees whether it does or does not—more research is needed!), this means that though birds may vacate many of our most urbanized, light- and noise-filled areas, they are not pushed into small corners of undisturbed habitat. Instead, they are living with the light and noise pollution we are creating, up to a point. Night light and noise can increase stress hormones, affect mating and reproductive success, alter birdsong frequency, and potentially cause phenological mismatches for birds, but they continue to persist on the landscape. This means they still provide ecosystem services beneficial to humans (and plants, and other animals too). However, as the human footprint continues to expand, areas of high noise and night light will presumably also increase, and species more sensitive to these pollutants will leave. Birds lose habitat and we lose ecosystem services, including the joy that comes from birds’ presence. This brings me to another reason to care about this research: I believe that the study of ecosystem services is a way for avian researchers to bridge the gap in understanding between those who love the natural world and those who view it strictly as a resource. Some birds do need relatively undisturbed habitat, while others do not. As I have designed these maps and dataset to be accessible and easily updated, I believe they could help guide future development projects and provide a roadmap to decrease night light and noise pollution in sensitive areas, thus protecting habitat for wildlife and enabling birds and other species to coexist with humans.