'Urban filter' explains decline in bird diversity in Campinas

IB research highlights the importance of investing in green areas as refuges for species

Walking through a neighborhood, sitting in a square, or standing near a large green space, can you hear birdsong? You may not know it, but these sounds—in their vast variety—are part of a complex ecological system, capable of conveying valuable information for planning green cities with greater fauna diversity. Based on this observation, biologist Pâmela Rodrigues Braga conducted her master's dissertation at the Institute of Biology (IB) at Unicamp, investigating how urbanization affects bird diversity in Campinas.

"Our goal was to understand the influence of the urban landscape and other factors related to urbanization on the bird community in green areas," explains Braga. The research, led by IB professor Wesley Rodrigues Silva, a specialist in bird community ecology, was divided into two complementary parts.

In the first, the biologist compared intra-urban areas (located within the city's urban core) with extra-urban areas (outside this core), analyzing how urbanization intensity—measured by the area's distance from the urban edge—and tree cover influence species richness and composition. In the second, Braga sought to understand the impact of specific landscape variables, such as impervious cover, the density of woody vegetation fragments, and noise levels, on the region's avifauna. The work was co-supervised by Professor Milton Ribeiro of the Institute of Biosciences at São Paulo State University (Unesp).

The choice of Campinas as the research location was no accident. With approximately 1,2 million inhabitants, the city is located in one of the most threatened phytophysiognomies of the Atlantic Forest: the seasonal semideciduous forest, typical of the interior of São Paulo state. Braga sees the municipality as a natural laboratory for urban ecology. "It's a large city, recognized for its afforestation efforts. This creates a complex and very interesting scenario for studying urban biodiversity."

The results confirmed that there are fewer species in intra-urban green areas than in extra-urban ones. Furthermore, a tendency toward greater similarity in the composition of birds in the city center was observed. This phenomenon led the biologist to suggest the existence of a "semi-permeable urban filter": fewer species are able to cross it and persist in the environment.

"Some species will be able to colonize intra-urban areas and remain there. Others, more dependent on forest habitats or specific resources, simply disappear," he states. Silva adds: "The main species filters are food and shelter. If the environment doesn't offer these resources, many birds lose their place."

The research also showed that increased urban cover and decreased density of woody vegetation patches negatively influenced species richness. This means that a high number of small green areas and sparse trees may offer less support for birdlife compared to large patches of vegetation.

"This result suggests that larger parks are more beneficial for biodiversity than many small squares scattered throughout the city," explains Braga. The data has direct relevance for urban planning, according to Silva. "This encourages the creation of larger green areas, which, in addition to housing more species, promote connectivity between populations."

Listen to tell

Over the course of three months, Braga traveled to 30 locations throughout the city, making four visits to each site for fieldwork. The method adopted was the listening point: for 15 minutes, the biologist recorded every species seen or heard. "If I hear a bird on one side and then the same species sings on the other, I know there are at least two individuals," she explains.

This work required Braga's practice and auditory sensitivity. "It was fun, and I learned a lot. Being in the field made me better understand the multidimensionality of sounds," says the biologist, who completed part of her master's degree at the University of Helsinki (Finland). "This experience contributed primarily to the method for delimiting the urban environment's edge, which was used to classify green areas."

Implications and future

Research confirms that urbanization leads to the simplification of bird communities, favoring generalist species and reducing overall diversity. Even so, urban green spaces serve as important refuges.

Braga argues that Campinas should take this fact into account as it grows. "We have extensive green areas in the urban area, and how this growth progresses is a key factor." As a solution, the researcher suggests creating new green areas in strategic locations, especially "near this region where the urban area blends with the urban area, increasing the connection between the two."

Silva goes further: "Connectivity is important not only for physical space, but also for genetic diversity. Without it, populations become isolated, lose variability, and become more vulnerable to environmental changes."

The advisor emphasizes that the traditional view of urban ecology pits city against nature. "There's a perception that these are opposing elements, making it challenging to understand that both can coexist harmoniously." Research demonstrates that this integration is not only feasible but also offers mutual benefits.

In its central message, Braga's work emphasizes that biodiversity permeates the urban fabric and deserves recognition and appreciation. "Our cities are home to an astonishing biological wealth," Silva emphasizes. "You don't need to travel to the Amazon to encounter nature. It manifests itself daily in the squares, parks, and gardens of our neighborhoods." According to the professor, this recognition strengthens both environmental education and the demand for more sustainable public policies.