Introduction
Most people are aware that the human population is growing at an exponential rate; however, many people do not realize the full extent of how our rising population is negatively affecting the environment around of us and Earth as a whole. The current world population is currently at 7.8 billion people and is estimated to reach 10 billion people by 2050 [1]. Especially in the past fifty years, our world has been transformed due to the ever-expanding global trade that has been cultivated by the increased human population; however, with this comes trade-offs that are being ignored. Increased consumption of products has led to many natural resources being overused or simply put destroyed. For example, agriculture is responsible for eighty percent of global deforestation [2]. This deforestation not only is detrimental in terms of carbon emissions and mammalian and plant biodiversity, but it also has the potential to negatively affect many species of birds.
As human population continues to grow, the amount of human-bird interactions will also increase. These interactions can be categorized into four human-caused variables that are known to influence bird populations; road density, amount of development, percent forest cover, distance to the edge of nearest different land cover class. One study used these four variables to create a model to predict the population of five species of birds between the years of 2000-2050. This model illustrated that all five species decreased in occupancy probabilities by nineteen to thirty-eight percent. They found the highest decrease in occupancy occurred outside of city centers where human population was higher and thus higher amounts of housing density. [3] These results are highly worrying as it is projected that urban land is projected to increase at least 5% by 2050 which is a larger area than the state of Montana. Congruently, forestland is expected to decrease due to urbanization by 118,300km2, which is about the size of Pennsylvania. [4]
This projected decline in bird populations would have detrimental effects on many ecological processes that are necessary for the success of the human population. One ecological service that birds provide is pest control, and there have been a multitude of studies that have demonstrated the importance of pest control for a variety of species, including some human agricultural practices. One study done in the Dutch apple orchards found that placing nesting boxes for great tits (Parus major) reduced caterpillars and fruit damage, resulting in a sixty-six percent increase in fruit yield. Another ecological service that birds provide is the through the process of pollination as over five percent of cultivated plant species are pollinated by birds. The importance of birds for pollination was illustrated after a hurricane in the Bahamas essentially eliminated two bird species that pollinated a specific type of shrub, resulting in a seventy-four percent decline in fruit set. The final ecological service that birds provide is through seed dispersal as birds shape the plant community’s composition in a variety of environments. Seed dispersal is critical to the success of plants through gene flow, escape from areas of high mortality, colonization of new sites, and dispersal towards favorable sites. Overall, birds contribute to the overall ecosystem of the earth through many avenues of plant production and protection, which in turn benefits humans. However, humans are still contributing greatly to the habitat loss of birds and neglecting to acknowledge the important role birds play in the livelihood of humans. [5]
Based on the information from the past research and the differing human population densities in three Bloomington, IN neighborhoods, a study was able to be conducted to determine if/how increased human population density affects the biodiversity of birds. This study was conducted in the neighborhoods of Bryan Park (5124 people/miA2), Elm Heights (9276 people/miA2), and Garden Hill (12861/miA2) through naturalistic observation and surveys to find quantitative data on species richness of birds[6]. They hypothesis for this study was that the species richness of birds will be lower in Bloomington neighborhoods with higher human population density due to increased
human-bird interactions.
Methods
The first part of the study was conducting in the neighborhood of Bryan Park, in which one hour every other week was spent walking around and observing which species of birds were present and the number of birds per species. The species of bird was determined through identifying unique characteristics such as feather colors and size, and then comparing with photos and habitat locations from online bird journals. This data was then recorded in an excel Spreadsheet. During this part of the study, evidence of human activity (i.e. presence of vehicles, litter, birdfeeders, and pedestrians) was also observed and recorded in excel. After the data had been collected for five weeks, a diversity index were calculated using the Simpson’s Diversity Index formula.
The second part of the study involved a sending a survey to residents living in the two Bloomington neighborhoods of Garden Hill and Elm Heights. The survey began with a map of Bloomington with the two neighborhoods outlined to ensure those participating in the survey resided in said neighborhoods. Many of the survey questions were in regard to the types of birds that the participants see in their respective neighborhoods. The species of birds included in these questions were chosen based on birds that are common in Indiana, and a picture of each species was included to limit the possibility of misidentification. The other questions in the survey were in regard to how often the participant saw the presence of human activity in these neighborhoods.
The third part of the study consisted of comparing the data from the collection in Bryan Park to the data collected in the surveys from residents in Garden Hill and Elm Heights
Results
Bryan Park
In the data collection of Bryan Park, six different species and a total of 106 birds were observed. (Figure 2) Using this information, the diversity index for this neighborhood was calculated to be 0.39. In regard to human activity in Bryan Park, there was an average of fifteen vehicles driving, ten pieces of litter, and eight humans per week in which data was collected.
| Week 1 (9/13-9/10) | Week 2 (9/27-10/3) | Week 3 (10/11-10/17) | Week 4 (10/25-10/31) | Week 5 (11/8-11/14) | Total # of individuals | |
|
Northern Cardinal
|
1 | 2 | 0 | 2 | 0 | 5 |
|
House finch
|
8 | 4 | 9 | 7 | 5 | 33 |
|
Red Bellied Woodpecker
|
2 | 0 | 1 | 1 | 2 | 4 |
|
House Sparrow
|
10 | 12 | 9 | 14 | 12 | 57 |
|
Blue Jay
|
2 | 0 | 0 | 2 | 1 | 5 |
| Hawk | 1 | 0 | 0 | 1 | 0 | 2 |
Figure 2: The number of individual birds in each species seen per week in Bryan Park
Garden Hill
Of the 10 people who participated in the survey that currently reside in the Garden Hill neighborhood, residents reported that they had observed eight of the twelve birds included in the survey. Residents also reported seeing the Chipping Sparrow, which was not recorded in any other data set of this study.
Figure 3: Summary of species of birds observed in Garden Hill with the percentage of residents that reported an observation of said species
In regard to the presence of human activity, most respondents in Garden Hill recognized the human impact on the environment within their neighborhood with most resides responding that they see high amounts of litter and vehicles each week. Although, 0 out of the 10 respondents reported seeing bird feeders in their neighborhood.
Elm Heights
In the ten survey responses from the neighborhood of Garden Hill, residents reported the observations of eight different birds, with the addition of the American Tree Sparrow which was not identified in any other data set.
Figure 6: Summary of the species of birds observed in Elm Heights with the percentage of residents that reported an observation of each species.
In the surveys from the neighborhood of Elm Heights, most residents reported that they observed at least four pieces of litter each week, with only twenty percent reporting one to three pieces of litter each week. In regard to, vehicles seen per week, most respondents indicated seeing at least five vehicles per week and fifty percent of respondents reported seeing more than ten vehicles per week. Three out of the ten participants from Elm Heights acknowledged the presence of bird feeders in their neighborhood.
Discussion
Based on the research done in this study, a few assumptions can be drawn from the data collected, but overall the original hypothesis of this study was not supported. There was not a significant difference between the amount of bird species or evidence of human activity between the three neighborhoods to suggest human population density directly affects biodiversity of birds. Despite the hypothesis not being supported, there still are many aspects of the relationship between human and birds that can be further explored through the data collected.
One of the most consequential limitations of this study was the close proximity between Bryan Park, Elm Heights, and Garden Hill because it partially negated the importance of the human population density differences within this study. This impacted the data due to the possibility that birds being observed in one neighborhood could travel to the other neighborhoods quite easily and frequently. This limitation was also illustrated in the data on the presence of human activity as all three neighborhoods had very similar results. Although, this data on human activity did not depict the differences in human population density as predicted, the considerable amount of litter and cars reported in each neighborhood is significant because it has been established that avian species richness is negatively related with these human disturbance variables [7].
An important data contradiction to address is that although Bryan Park had the lowest human population density compared to the other neighborhoods, it had the lowest species richness in the data set. It is also important to note that the diversity index of Bryan Park was 0.39, which is quite low considering that the Simpson’s Diversity Index is measured on a scale of 0-1, with 0 being the least diverse and 1 being the most diverse. This could be deduced to some of the limitations of this study like the possibility of misidentifying species and the close proximity of the three neighborhoods, or this could be a byproduct of other factors such as the increased presence of bird feeders in Bryan Park. These anthropogenic food subsidies represent the ways that humans are trying to help reverse the effects of urbanization, but unfortunately these bird feeders have been found to negatively impact birds. One negative impact of bird feeders is the increased competition that has been shown to create between-species dominance hierarchies. As competitive interactions increase individuals in low ranking species often lose a greater number of aggressive interactions, suffer more injuries, and endure higher energetic costs than dominants.[8] This competition could also be increased in Bryan Park due to the large numbers of House Sparrows because House Sparrows are an aggressive invasive species that have been related to a significant decrease of bird species richness in other studies. [9]. Another problem that arises from bird feeders is they have been shown to increase infectious disease in birds. One study determined that 8.3 percent of birds found at the feeders included in the study exhibited symptoms of conjunctivitis, pox, dermal disease or cloacal disease. [10] Both of these effects of bird feeders could explain the decrease in species richness for Bryan Park compared to the other two neighborhoods because disease and increased competition could cause a decrease in the number of individuals per species and affect which birds were observed during the data collection. Overall, these results parallel with many studies that found urbanization did not necessarily reduce bird species richness, but actually increased the number of birds in a few dominant bird species [11].
Although the species richness in Bryan Park was surprisingly low, the species richness in Garden Hill and Elm Heights was higher than what was predicted at the beginning of the study. One thing to consider is that this could be an outcome of the limitations of the survey; such as having a relatively low amount of completed surveys, the possibility of misidentification of birds, and the realistic chance that people don’t pay that close of attention to the species of birds in their surroundings. However, with the limitations in mind the data is somewhat congruent with other studies revolving around the impact of human urbanization on birds. One aspect of birds that could explain the increased species richness in Elm heights and Garden Hill is the plasticity of bird behavior as birds are more able to adapt to changing environments compared to other vertebrates because they are highly mobile and have high mass-specific metabolic rates [12]. This adaptability could explain the higher species richness as the birds in these neighborhoods could have made small adjustments in their behavior in order to be able to survive and avoid some of the competition that has arisen in Bryan Park. Although this data suggests birds are able to handle the rate of human urbanization, other studies have established that urban environments act more as a filter to bird populations because urban areas tend to host a relatively limited number of bird species as not all species can adapt quickly enough or at all [13].
This study did not find enough evidence to support that there was a negative correlation between species richness of birds and Bloomington neighborhoods with differing human population density; however, it did highlight certain anthropogenic actions that are have the potential to be negatively impacting birds in Bloomington. Further exploration of these anthropogenic actions is critical to understanding human-bird interactions. One example of a future study could be exploring the relationship between bird species richness and areas of Bloomington with different anthropogenic noise levels as other studies indicated this as a potential problem of urbanization for bird biodiversity [14]. Another potential study that would be beneficial is exploring the implications of birdfeeders on bird biodiversity, with an emphasis on the possibility of increased competition. The last future study that could arise from this study is an investigation into the impact of an invasive species on biodiversity because biological invasions are often driver of biodiversity loss [15]. Overall, this evidence of human activity is extremely significant because most anthropogenic-caused issues do not become easily recognizable within an ecosystem until the damage has been magnified to the point of impacting the everyday lives of humans. Birds provide various essential services to many different ecosystems, and these services are imperative for humans’ health and overall quality of life. As the human population continues to grow and urbanization becomes more rapid, society needs to prioritize the protection of biodiversity of all species, but especially birds. During this time of high rates of urbanization and deforestation, the recovery and protection of avian biodiversity will be in the hands of humans to restore, protect, and maintain a multitude of diverse habitats and limit human-bird interactions.
Citations
[1] Lidicker, W. Z. (2020). A Scientist’s Warning to humanity on human population growth. Global Ecology and Conservation, 24. doi:10.1016/j.gecco.2020.e01232
[2] WWF (2020) Living Planet Report 2020 – Bending the curve of biodiversity loss. Almond, R.E.A., Grooten M. and Petersen, T. (Eds). WWF, Gland, Switzerland.
[3] Brown, M. L., Donovan, T. M., Schwenk, W. S., & Theobald, D. M. (2014). Predicting impacts of future human population growth and development on occupancy rates of forest-dependent birds. Biological Conservation, 170, 311-320. doi:10.1016/j.biocon.2013.07.039
[4] Nowak, David J., Walton, Jeffrey T. (2005). Projected Urban Growth (2000-2050) and Its Estimated Impact on the US Forest Resource. Journal of Forestry. December: 383-389.
[5] Whelan, C. J., Wenny, D. G., & Marquis, R. J. (2008). Ecosystem Services Provided by Birds. Annals of the New York Academy of Sciences, 1134(1), 25-60. doi:10.1196/annals.1439.003
[6] Https://statisticalatlas.com/place/Indiana/Bloomington/Population. (n.d.).
[7]Verma, S. K., & Murmu, T. D. (2015). Impact of Environmental and Disturbance Variables on Avian Community Structure along a Gradient of Urbanization in Jamshedpur, India. Plos One, 10(7). doi:10.1371/journal.pone.0133383
[8] Francis, M. L., Plummer, K. E., Lythgoe, B. A., Macallan, C., Currie, T. E., & Blount, J. D. (2018). Effects of supplementary feeding on interspecific dominance hierarchies in garden birds. Plos One, 13(9). doi:10.1371/journal.pone.0202152
[9] García-Arroyo, M., Santiago-Alarcon, D., Quesada, J., & Macgregor-Fors, I. (2020). Are invasive House Sparrows a nuisance for native avifauna when scarce? Urban Ecosystems, 23(4), 793-802. doi:10.1007/s11252-020-00963-x
[10] Wilcoxen, T. E., Horn, D. J., Hogan, B. M., Hubble, C. N., Huber, S. J., Flamm, J.,. . Wrobel, E. R. (2015). Effects of bird-feeding activities on the health of wild birds. Conservation Physiology, 3(1). doi:10.1093/conphys/cov058
[11] Tu, H., Fan, M., & Ko, J. C. (2020). Different Habitat Types Affect Bird Richness and Evenness. Scientific Reports, 10(1). doi:10.1038/s41598-020-58202-4
[12] Barrett, K., Romagosa, C. M., & Williams, M. I. (2009). Long-Term Bird Assemblage Trends in Areas of High and Low Human Population Density. Research Letters in Ecology, 2008, 1-4. doi:10.1155/2008/202606
[13]Santini, L., González‐Suárez, M., Russo, D., Gonzalez‐Voyer, A., Hardenberg, A., & Ancillotto, L. (2018). One strategy does not fit all: Determinants of urban adaptation in mammals. Ecology Letters, 22(2), 365-376. doi:10.1111/ele.13199
[14] Carvajal-Castro, J. D., Ospina-L, A. M., Toro-López, Y., Pulido-G, A., Cabrera-Casas, L. X., Guerrero-Peláez, S., . . . Vargas-Salinas, F. (2019). Birds vs bricks: Patterns of species diversity in response to urbanization in a Neotropical Andean city. Plos One, 14(6). doi:10.1371/journal.pone.0218775
[15] Enedino, T. R., Loures-Ribeiro, A., & Santos, B. A. (2018). Protecting biodiversity in urbanizing regions: The role of urban reserves for the conservation of Brazilian Atlantic Forest birds. Perspectives in Ecology and Conservation, 16(1), 17-23. doi:10.1016/j.pecon.2017.11.001
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