Is agriculture actually beneficial for avian biodiversity?

It is a common belief that human-modified areas, such as farmlands, support fewer bird species than natural habitats such as forests. However, new research by Zhao et al. (2024) challenges this idea. Their study of bird species diversity on islands off the coast of China has unexpected results that suggest that farmlands might play a more complex role in avian biodiversity than previously thought. How can agriculture, often seen as a driver of biodiversity loss, actually boost bird species richness? Let’s examine this unexpected twist.

Impact of agriculture

When excluding glaciers and barren lands such as deserts and salt flats, nearly half of the world’s land is used for agriculture (Ritchie & Roser, 2024). It would be predicted that the biodiversity in these parts would be lower than in surrounding, less urbanized areas. Agriculture is the largest driver of biodiversity loss (Dudley & Alexander, 2017). One of the reasons for this is that most farmlands are converted from natural ecosystems, and existing farming becoming more intensified; monoculture plantations replace small-scale farming. At the same time the use of fertilizers and insecticides is increasing, which are known to seriously damage many animals including birds (EASAC, 2015 in Dudley & Alexander). Human activities, including agriculture, have significantly altered habitats on 75% of the Earth surface, and islands form no exception (University of Texas at Arlington, 2024). 

Another factor impacting species richness on both natural and more urbanized islands is the Island Biogeography Theory. This theory predicts that larger islands would have a positive species-area relationship, and the more isolated islands a negative species-isolation relationship. In other words, larger islands would have higher species richness, while isolated islands would have lower species richness (Zhao et al., 2024).

New research

Recently, a group of researcher examined bird species in the Zhoushan Archipelago, an island chain off the Chinese coast. Over two years, they studied the birds on 34 different islands — some used for farmland, others more forested, and some more isolated than others. Their results were consistent with the predictions of the aforementioned Island Biogeography Theory; species richness increased with island area and decreased with isolation. Interestingly, they also found that the bird species richness increased with the proportions of human-modified habitats such as farmlands.

Zhao et al. (2024) proposed a few possible reasons for their unexpected findings. One reason is that resources in farmlands and other human settlements may be easier to access for some species, making these areas suitable place to live for those birds. Secondly, the species that they found in those farmland areas were known to have higher dispersal abilities than average, which would lead to higher observed richness. Another point they made was that perhaps the birds lived in the neighboring forests at night but went to the farmlands during the day. Since the research was conducted only during the daytime, the possible role of the forests at night wasn’t considered, potentially skewing the observed biodiversity. Lastly, the observed species on the islands with more farmlands were species known to thrive in agricultural habitats.

Biodiversity

There are different interpretations of biodiversity. It can constitute both taxonomic diversity (species richness) and phylogenetic diversity (evolutionary distance and relationship of species) and can be viewed locally or globally. Two of the mentioned reasons for the measured increase in species diversity on the farmland islands were that the species they found have higher dispersal abilities and that many were species commonly found in agricultural habitats. This could be seen as indicators of (urban) biotic homogenization— the range expansion of generalist species and simultaneous range contraction of regional/endemic species (Morelli et al., 2016). Consequently, this increases similarities across communities in space and time, lowering overall biodiversity.

Another study by Deng et al. (2024), confirmed that this is the case for avian communities in mainland China, but the research did not include the Zhoushan Archipelago. So even though there was an observed increase in bird species diversity on the islands with agricultural habitats, the question remains whether this is truly beneficial for global biodiversity.

In addition to these findings, it is crucial to consider the long-term impacts of agriculture on bird species. While some species might initially thrive in farmlands, the continuous use of pesticides and fertilizers can lead to habitat degradation and loss of food sources, ultimately harming bird populations. The conversion of natural habitats to agricultural lands also results in the loss of nesting sites and shelter, further threatening bird species in the long run. 

Conclusion

With these considerations in mind, it becomes clear that whether agriculture is beneficial for avian biodiversity is not straightforward. While the study in the Zhoushan Archipelago suggests that farmlands might be beneficial for bird species richness, it is essential to consider the broader implications for overall biodiversity.

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References

Deng, J., Zhu, Y., Luo, Y., Zhong, Y., Tu, J., Yu, J., & He, J. (2024). Urbanization drives biotic homogenization of the avian community in China. Integrative Zoology. https://doi.org/10.1111/1749-4877.12815

Dudley, N., & Alexander, S. (2017). Agriculture and biodiversity: a review. Biodiversity, 18(2–3), 45–49. https://doi.org/10.1080/14888386.2017.1351892

Morelli, F., Benedetti, Y., Ibáñez‐Álamo, J. D., Jokimäki, J., Mänd, R., Tryjanowski, P., & Møller, A. P. (2016). Evidence of evolutionary homogenization of bird communities in urban environments across Europe. Global Ecology and Biogeography, 25(11), 1284–1293. https://doi.org/10.1111/geb.12486

Ritchie, H., & Roser, M. (2024, February 21). Half of the world’s habitable land is used for agriculture. Our World in Data. https://ourworldindata.org/global-land-for-agriculture

University of Texas at Arlington. (2024, May 13). Island birds more adaptable than previously thought. ScienceDaily. Retrieved June 9, 2024 from www.sciencedaily.com/releases/2024/05/240513150449.htm

Zhao, Y., Mendenhall, C. D., Matthews, T. J., Wang, D., Li, W., Liu, X., Tang, S., Han, P., Wei, G., Kang, Y., Wu, C., Wang, R., Zeng, D., Frishkoff, L. O., & Si, X. (2024). Land-use change interacts with island biogeography to alter bird community assembly. Proceedings – Royal Society. Biological Sciences/Proceedings – Royal Society. Biological Sciences, 291(2018). https://doi.org/10.1098/rspb.2023.2245

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Featured image: © Christoph Moning, some rights reserved (CC-BY)