By Maria Fernanda Ziegler | Agência FAPESP – Urbanization and the resulting shrinkage of green spaces in cities can be considered a boon for mosquitoes that transmit diseases, such as Aedes aegypti (dengue) and Culex quinquefasciatus (lymphatic filariasis).
These insects have adapted to urban areas and are benefiting from the decline in populations of other mosquito species. Brazil’s biggest city, São Paulo is a perfect example of this relationship, according to a study conducted by researchers at the University of São Paulo’s Public Health School (FSP-USP) under the aegis of the FAPESP Research Program on Biodiversity Characterization, Conservation, Restoration and Sustainable Use (BIOTA-FAPESP).
With the collaboration of the City of São Paulo’s Animal Health Control Center and its Department of Parks & Green Areas, the researchers collected 37,972 specimens of the family Culicidae. Laboratory analysis showed that these mosquitoes belonged to 73 species and 14 genera.
Although the specimens collected in nine municipal parks monitored by the researchers highlight the city’s rich species diversity, the study points to a problem regarding the distribution and composition of these species in the city’s green spaces.
Among the results published in an article in Scientific Reports are the findings that the number of mosquito species is decreasing and that species that act as vectors of pathogens that cause diseases in humans end up benefiting adaptively.
Five species accounted for 68% of the insects collected: Culex nigripalpus, Aedes albopictus, C. quinquefasciatus, A. fluviatilis and A. scapularis. Other vector species – C. declarator, A. aegypti, C. chidesteri, Limatus durhami and C. lygrus – were also found more frequently in the urban parks.
“There’s a correlation between the size of green spaces and species diversity. Smaller green areas tend to have a subset of the species found in larger green areas, and the range of species in smaller areas tends to comprise mainly vectors,” said one of the authors of the study, Antônio Ralph Medeiros-Sousa, a PhD student at FSP-USP with a scholarship from FAPESP.
According to Medeiros-Sousa, fragmentation and reduction of green areas benefit vector mosquitoes owing to extinction of wild species.
“The vector species have adapted to urban environments,” he said. “As green areas are reduced, wild species gradually disappear, and urban-adapted species dominate the territory. They are precisely the species that transmit the most pathogens,” he said.
The study also shows significant differences between the parks in terms of species richness. Sixteen species were collected from Ibirapuera Park, which is 1.58 km² in area, compared with 47 species from Anhanguera Park (9.5 km²). As expected, smaller green fragments are more susceptible to environmental disturbances, which mainly drive out the less abundant insect species.
“Collecting almost 50 mosquito species in an urban park is impressive,” Medeiros-Souza said. “We didn’t expect such a large number. It was a surprise, even though we knew some areas, including Anhanguera Park, must have higher diversity because of their size.”
Despite finding higher concentrations of vector mosquitoes – seven of the eight most common species transmit pathogens to humans – the researchers stress that this does not necessarily entail a higher risk of transmission, but only a greater likelihood of contact between vector mosquitoes and humans.
“It doesn’t mean there will be more disease. The other factor that determines the incidence of disease is the presence of pathogens, such as dengue, Zika or yellow fever virus. The study shows that there’s an imbalance, with less species diversity in smaller and less conserved areas,” said another author of the study, Mauro Marrelli, associate professor at FSP-USP and Medeiros-Sousa’s PhD supervisor.
According to the authors, the data analyzed in the study highlight the need for further research to better understand how habitat loss and fragmentation due to urbanization affect vector insects and influence the risk of pathogen transmission.
The correlation between area and diversity is explained by the equilibrium theory of island biogeography, which US ecologists Robert MacArthur and Edward O. Wilson formulated in the 1960s. According to this theory, species richness on islands represents a dynamic equilibrium between immigration and extinction rates, which are affected by the island’s size and degree of isolation. This theory can be applied to urban green spaces, since these form island-like habitats isolated by built-up areas.
In the case of mosquitoes, which do not live long or travel far (leaving aside mechanical dispersal, when they travel inside vehicles, for example), extinction has an even greater impact on species equilibrium.
“We show that the theory of island biogeography applies to the city of São Paulo,” Marrelli said. “We also note that smaller green areas tend to have greater similarity of species, since species that have adapted best to the urban environment tend to be selected. Almost 70% of the mosquitoes collected in our study belonged to only five species. That really is a problem.”
Mosquitoes are a highly diverse group, with more than 3,500 known species. Studies of mosquito diversity in urban green spaces are therefore useful both to elucidate processes that lead to patterns of diversity in urban ecosystems and to understand the role of biodiversity in reducing or increasing the risk of pathogen transmission.
For this study, the team of researchers performed monthly collections between 2011 and 2013 in nine of São Paulo’s parks: Alfredo Volpi, Anhanguera, Burle Marx, Chico Mendes, Ibirapuera, Piqueri, Previdência, Santo Dias and Shangrilá.
The Scientific Reports article “Mosquitoes in urban green spaces: using an island biogeographic approach to identify drivers of species richness and composition” (doi:10.1038/s41598-017-18208-x) by Antônio Ralph Medeiros-Sousa, Aristides Fernandes, Walter Ceretti-Junior, André Barreto Bruno Wilke and Mauro Toledo Marrelli can be read at nature.com/articles/s41598-017-18208-x.
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