Habitat heterogeneity determines plant species richness in urban stormwater ponds

Urbanisation is seen as one of the most important drivers for species extinction. The aim of our study was to compare vascular plant species richness of 35 artificial urban stormwater ponds (STOPON) designed to control water flow, with 35 control ponds (CONTROL) in the surrounding landscape. For both plot types, we gathered a variety of environmental parameters to determine the drivers of plant species richness. Based on the results, we discuss the relevance of STOPON for biodiversity conservation in urban areas and give recommendations for biodiversity-friendly construction and pond management. Our study showed that the environmental conditions clearly varied between STOPON and CONTROL. STOPON were especially larger than CONTROL; as a consequence, the aquatic, semi-aquatic and terrestrial zones had a greater extent at STOPON. Accordingly, the overall and mean number of aquatic, salt-tolerating aquatic and threatened plant species was higher at STOPON than at CONTROL. However, species density of all three plant indicator groups did not differ among the two plot types. The greater plant species richness of STOPON was driven by a higher habitat heterogeneity which was especially explained by (i) the genuine effect of habitat area on habitat heterogeneity, (ii) the existence of three similarly large habitat zones (terrestrial, semi-aquatic, aquatic) and (iii) the regular management, which represses shading plants, creates open soil and activates the soil seed bank. Due to the high species richness of threatened plant species STOPON play an important role in biodiversity conservation in urban areas.