Urban trees can increase avian and insect diversity and abundance in tropical cities

Urbanization will continue to be a dominating trend in upcoming decades, threatening habitats, and biodiversity. It is projected that 50% of the population of the tropics will live in urban areas by 2050. Retaining or planting trees in urban areas may mitigate biodiversity losses and potentially provide other benefits such as carbon sequestration, local cooling and reduction of pollution. Yet, there is a large knowledge gap on the potential benefits of urban trees in tropical cities We therefore conducted a systematic literature review using “web of science” on 27 March 2021 to test the hypothesis that urban trees increase animal diversity and abundance in tropical cities. We initially captured 265 articles, then reviewed them by title and abstract, finally retaining 21 studies. We analysed them using a vote counting system by comparing numbers of significantly positive or negative effects and those where no differences were deemed significant. Studies were from the Americas and Asia, but none was from Africa. The areas assessed in these studies ranged greatly from 80 m2 to 7854 ha, with a median of 0.9 ha. The two most featured countries were Brazil and Mexico. More than three-quarters of the studies were on birds and for this taxon, the effect of trees was clearly positive. Remaining studies were on ants, bats, butterflies and, also the urban pest and disease vector, Aedes aegypti. Nine out of 21 studies found a significantly positive effect of urban trees on abundance, biodiversity, or both. We conclude that there is a research gap on the importance of urban trees for conserving biodiversity in tropical cities, particularly for Africa. Potentially urban trees can increase diversity and abundance of birds, but more studies are necessary on this and other taxa.