Promoting wildflower biodiversity in dense and green cities: the important role of small vegetation patches

Abstract

The conservation and promotion of biodiversity in urban spaces has become a core concern in urban greening. As cities continue to densify, however, urban green spaces are becoming smaller and more isolated. Many hope that wildflowers, and with them also faunistic biodiversity, can be maintained through networks of small informal green spaces. Currently, there are little data available for city planners to answer the questions of how patch size and ecological connectivity affect the population persistence of wildflowers in small vegetation patches. To address these concerns, we surveyed all vegetation patches greater than 1 m2 in public spaces across seven 1 km2 quadrats distributed across the city of Zurich, Switzerland. Our survey focused on a list of 166 herbaceous vascular plant species. We analyzed vegetation patch topologies (frequency distributions of size and connectivity) and related it to alpha and beta species diversity. Dispersal traits of investigated species were used to interpret connectivity patterns. Over 75 % of patches were <20 m2 in size, however, these small patches made up only 3.4 % of total vegetation area. Species richness was significantly positively correlated with patch area. Nevertheless, taken together, small patches had similar species richness and higher beta diversity than larger patches. These small patches contribute far more than expected to overall wildflower species richness in the city – “punching above their weight”. This held true even for less common species. However, our research indicates that connecting these patches to one another and to larger green spaces is essential to unlocking their potential as habitats and potentially allowing for the natural colonization and reinforcement of existing populations. As cities densify, we must acknowledge that small vegetation patches such as tree discs or road margins are essential for the survival of wildflower populations and associated fauna, especially in highly urbanized areas.