Assessment of macroinvertebrate biodiversity on green roofs

Abstract

In the current era of urbanization, the pressure on biodiversity is greater than ever. Urbanization causes the destruction and fragmentation of natural habitats, which in turn causes biodiversity to decline. Biodiversity plays a crucial role in driving ecosystem functioning and in the provision of ecosystem services. Green roofs, a unique form of novel urban green infrastructures, are a prime example of how cities can mitigate biodiversity loss while also offering a multitude of ecosystem services such as thermal regulation, storm water management and mitigating the heat island effect. In this study, I focus on extensive green roofs as they are applied widely within the urban landscape and they have many benefits. Extensive green roofs are characterized by a shallow substrate, lowmaintenance requirements, and a high cost-effectiveness and planted or sown with plants and mosses that have stress-tolerant characteristics. They can be installed on most existing roofs because of their limited weight, require minimal maintenance, and are low in cost. Despite the recognised benefits of green roofs, there is still a need for more research and more specific guidelines to fully realize their potential and support urban biodiversity. Two of the key questions that remain are (i) how green roof characteristics affect biodiversity and (ii) what species traits are needed to survive in these urban environments. The benefits of green roofs are often case-specific, and with the lack of comprehensive green roof studies in Flanders, further investigation into the potential of these urban environments in promoting biodiversity in general, and of macroinvertebrates specifically, is of paramount importance. The main objective of this study is to investigate macroinvertebrate biodiversity on green roofs in Flanders, with a focus on determining the relationships between roof characteristics and measures of biodiversity such as species richness and abundance. With very limited research available on belowground biodiversity in green roofs, the first part of this study specifically focuses on Collembola as the dominant soil-dwelling arthropod group in green roofs. In this part, the study investigated roof characteristics impact species richness, abundance, or diversity of Collembola. Additionally, I examined the preferred species traits in Collembola communities on green roofs, specifically concerning vertical distribution, body size, and moisture preference. I collected my data on 12 extensive green roofs in three different cities of Flanders; Antwerp, Ghent and Hasselt, and sampled 7006 individuals belonging to 10 different species. Apart from the age of the roof that showed a positive significant impact on the abundance of Collembola, our results suggested that the collembolan fauna showed no significant differences in abundance, species richness or diversity between roofs with different characteristics. In the second part of this study my focus shifts toward aboveground macroinvertebrates. To gather the data, I sampled 20 green roofs in the city of Antwerp. I identified 26,986 individuals to species level and found a total of 341 different species. Here as in the previous part, I investigate whether roof characteristics impact species richness, abundance, or diversity of macroinvertebrates. None of the physical roof characteristics appears to explain differences in macroinvertebrate species richness and abundance. The purpose of the third part of this study is to determine if there is a significant difference in abundance of macroinvertebrate taxa between green roofs and adjacent ground level habitats. This part was mainly conducted to assess whether green roofs have a comparable ability to ground-level habitats in terms of supporting macroinvertebrates’ abundance in urban environments. I examined eight green roofs and adjacent ground level habitats in the city of Antwerp, Belgium. All higher-level macroinverbrate taxa found were present in both types of habitats without significant differences in overall abundance between green roofs and ground level habitats. However, significant differences in abundance were discovered between the two types of habitats when specific taxa were compared. Some taxa, such as beetles, had a significantly higher abundance at ground level, while others, such as cicadas, showed the opposite trend. In the subsequent section of the thesis, I focus on pollinator species. The aim of this part is to investigate whether roof characteristics influence pollinator (wild bee and hoverfly) diversity, abundance and species composition of green roofs in an urban environment. The community weighted means were analysed for pollinators to determine the average community traits, such as body size, social behaviour, flower visiting, and nesting type of wild bee species on green roofs. In total I found 40 wild bee and eight hoverfly species on the green roofs. Roof wild bee communities were identified as social polylectic individuals, with a preference for ground nesting. In conclusion, this thesis provides insight into the potential of green roofs in the Flanders region in terms of biodiversity, species richness, and abundance of macroinvertebrates. This study indicates that the studied roof characteristics, such as the height of the green roof, have little to no influence on the biodiversity, species richness or abundance of the macroinvertebrate species present. Overall, the results of this study also indicate that desirable species traits for living on green roofs include having strong dispersal abilities, adaptability to dry and hot conditions, and a broad diet. The research findings can inform future biological landscape planning on green roofs in the region, aimed at maximizing macroinvertebrate biodiversity. Integrating the newly acquired knowledge from this research with the conclusions of the EcoCities project holds great potential for a synergistic effect that promotes sustainable development in urban areas. As a final part, recommendations for future directions in green roof research are provided.