The implementation of urban greenspaces and corridors as a solution to biodiversity decline is well supported by the literature, and similar methods have demonstrably improved biodiversity metrics in a wide range of urban areas and geographies.
Seen in Figure 1, an analysis of cities across five continents concluded that the availability of vegetated habitat area influences urban biodiversity more than any other factors.[1]
Park space in particular has a powerful effect when compared to other green spaces. In an analysis of several northern European cities, converting urban land into park space was one of the single most effective ways to increase urban species richness and species diversity across species of birds, invertebrates, mammals, amphibians, and reptiles.[3] The study reviewed previous data, finding that in 64.3% of the analyzed cases, urban parks had higher species richness than other green spaces such as riverside banks and street niches.[4]
While large area greenspaces such as natural parks promote biodiversity, connectivity between separated habitats is also crucial for promoting biodiversity in urban areas. One of the mechanisms of connectivity is increased gene flow. Isolated populations tend to have lower genetic diversity than well connected ones, ultimately lowering the population’s resilience to threats such as disease and environmental changes, thereby weakening conservation efforts.[6] Depicted in Figure 2, a simulation study which modeled four species groups concluded that higher connectivity results in higher genetic diversity.[7]
Connectivity also increases species richness across many taxa. While buildings and other urban structures pose an obstacle to larger mammals, they also impede the movement of highly mobile animals. For example, the biodiversity of arthropods and carabids, which includes insects like spiders and beetles, is impacted by connectivity. In a study comparing gardens connected by green corridors to isolated gardens, both species abundance and richness were greater in the connected garden.[8] Despite their high mobility, even birds benefit from connectivity, as indicated in a case study of bird biodiversity in Madrid. Modifications as simple as tree-lined streets between urban parks were associated with increased native bird populations as opposed to parks alone.[9] This impact was attributed by the authors of the study to the availability of appealing resting places, food sources, and habitats outside of the large parks.[10]
The mobility of animals through large scale wildlife corridors has been confirmed in easily monitored, isolated areas. As seen in Figure 3, a study near the Savannah River in South Carolina created artificially separate multi-hectare habitats, in uninhabited land and introduced connectivity through 150 meter by 25 meter corridors between some of these habitats and tracked populations for 18 years.[12] Throughout this experiment, extinction rates were lower by 2% and colonization rates were higher by 5% in the connected habitats, demonstrating the positive role of connectivity.[13]
Further investigation into specific species movement in this experiment revealed the effect of connectivity across different taxa.[14] While some taxa move through corridors at a higher rate, movement between connected habitats was 68% greater in comparison to movement between unconnected habitats for all tracked taxa (small mammals, butterflies, plants, and pollinators).[15]
These solutions perform well not only in controlled environments but also in real urban areas. Several extant urban environments with large green space areas and high connectivity from corridors have achieved impressive biodiversity metrics. A causal relationship between connected greenspaces and biodiversity can be observed in Indonesia, where rapid biodiversity loss due to monocropping and urbanization led to a requirement in law no. 26/2007 which obligates cities in the river stream area to be composed of 30% forest area.[16],[17] This political trend resulted in extended greenification efforts in Sentul City.[18] Fifteen years after projects that implemented corridors of native trees, a study quantifying native and exotic species diversity and numbers reported improved biodiversity metrics.[19]
Similar effects can be observed in urban areas with different ecosystems. For example, a case study investigates the unusually high levels of plant diversity within and between the Italian urban areas, Palermo, Cagliana, Ancona, Rome, and Milan.[21]The authors attribute the high species richness and the similarity between the species outside and within city limits to the high connectivity built into city design, exemplified in Figure 4, and relatively large amount of urban green area as opposed to less biodiverse Italian cities.[22]
Finally, one of the most internationally recognized greenified urban areas, the city-state Singapore, consistently achieves its regularly updated biodiversity goals. Its two most recent national reports for the Convention on Biological Diversity assert that overall urban biodiversity has increased in response to ongoing greenification efforts.[24],[25] A smaller report provides further details of these projects and their impacts. For example, the creation of a landscaped area at Alexandra Hospital led to the return of 102 butterfly species to the area occupied by the hospital.[26] Another project, Singapore’s Park Connector, mapped in Figure 5, is a green pedestrian corridor that doubles as a bridge between smaller green areas in the city. Since its creation, the corridor has attracted 550 species of birds and butterflies to the area and allowed gene flow between parks.[27]
Together, these analyses, controlled experiments, and case studies indicate that introducing connected greenspaces can successfully mitigate urban fragmentation and the urban biodiversity crisis.