‘Change is the only constant in life’ and in the Anthropocene the nature of change is being altered too. Though we often associate human activities with increasing rates, a new study of urbanization shows that in some places we are in effect slowing down time instead.
Above: Seasonal dissimilarity of bird community composition in urbanized and less urbanized areas across several continents.
The urban population is increasing, promoting an expansion of cities and urban densification. Therefore, to know how these processes affect biodiversity in urban areas is fundamental to designing more sustainable cities. But, what is biodiversity? The number of species that can be found in a site is the alpha diversity, and this type of diversity has been predominantly analyzed in urban ecology. Another type of biodiversity is the change of species composition between sites, or spatial beta diversity. Therefore, if two sites do not share species, between them there is high beta diversity. Spatial beta diversity has been analyzed in urban ecology, regarding the biotic homogenization phenomenon (McKinney 2006). Biotic homogenization is the increased similarity of two sites over time, associated with the colonization of widespread species in urban areas. A third type of biodiversity is the change in species composition over time in a site, or temporal beta diversity. Seasonal or interannual changes of species are examples of temporal beta diversity. This type of beta diversity has been much less studied in urban ecology.
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Leveau, L. M., Jokimäki, J., & Kaisanlahti-Jokimäki, M. L. (2021). Urbanization buffers seasonal change in composition of bird communities: A multi-continental meta-analysis. Journal of Biogeography 48: https://onlinelibrary.wiley.com/doi/10.1111/jbi.14236
Due to urban areas seeming to stabilize the temporal variation of resources and habitat structure for birds, the temporal beta diversity may be reduced in cities. When I started to write a review about the temporal stabilization of resources for birds in urban areas, I realized that the biotic homogenization may have a spatial and a temporal component (Leveau 2018). The similar conditions of urban areas located in different biomes may promote the colonization of similar widespread species, increasing the spatial homogenization of biotas (Figure 1a). On the other hand, the similar conditions over time in a city may promote a temporal persistence of species compared to non-urban areas, increasing the temporal homogenization of biotas (Figure 1b).
The temporal homogenization has different scales: day/night, week/weekend, seasonal and interannual. Because I realized that several studies regarding the seasonal scale have been published worldwide, I decided to analyze global patterns with my colleagues Jukka Jokimäki and Marja-Liisa Kaisanlahti-Jokimäki. The large-scale patterns of climate seasonality give us the opportunity to answer two questions. (1) Is the seasonal homogenization of urban avifaunas a global phenomenon? 2) Does the degree of seasonal homogenization vary according to latitude or other large-scale variables?.
Schematic diagram showing the processes of spatial and temporal homogenization of biotas. The spatial homogenization (a) occurs when two sites become more similar in species composition (circles of different colors) due to urbanization. The temporal homogenization (b) occurs when the temporal variation of composition in a site is reduced due to urbanization.
We found that the seasonal homogenization of avifaunas was higher in those cities near the poles and in cold regions. Avifaunas in natural areas near the poles are dominated by migratory species. Thus, their seasonality is higher than avifaunas in tropical regions. As a result, the contrast between the seasonal change in urban areas and natural or rural areas is the highest near the poles. Another interesting finding was that the seasonal change of bird composition is higher in urban than in natural areas near the Equator. A case study from Belém (Brazil, 1°27′S; Lees et al. 2017) reflects our results. The authors found that the proportion of migrant bird species in the city was over-represented in comparison with the regional pool of species.
We also explored the effect of hemisphere and study design. We found that the seasonal homogenization of urban avifaunas was more pronounced in the Northern Hemisphere. This pattern could be related to several factors, such as the higher proportion of migrant birds or a more stabilization of resources in urban areas in the Northern Hemisphere. On one hand, we found a significant dampening of urban bird composition seasonality in those studies that compared urban versus rural or natural areas. On the other hand, studies that compared green areas in different urban contexts did not show differences in bird composition seasonality. This result suggests that urban green areas are refuges for migrant birds and can sustain part of the temporal beta diversity of the region.
Finally, I would like to thank all the colleagues that generously shared their valuable data with us: Dan Chamberlain, Mohan Kukreti, Gabor Lövei, Maria Dolores Juri, Nélida Villaseñor, Bibhu Panda, Neeraj Sharma and Gerardo Linares Hernándeza.
Lucas Leveau, PhD, Departamento de Ecología, Genética y Evolución, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires – IEGEBA (CONICET – UBA), Ciudad Universitaria, Pab 2, Piso 4, Buenos Aires 1426, Argentina
Lees, A. C., & Moura, N. G. (2017). Taxonomic, phylogenetic and functional diversity of an urban Amazonian avifauna. Urban Ecosystems, 20(5), 1019-1025.
Leveau, L. M. (2018). Urbanization, environmental stabilization and temporal persistence of bird species: a view from Latin America. PeerJ, 6, e6056.
McKinney, M. L. (2006). Urbanization as a major cause of biotic homogenization. Biological conservation, 127(3), 247-260.