Local and Landscape Effects on Arthropod Communities along an Arable-Urban Gradient
von Hannah Reininghaus
Datum der mündl. Prüfung:2017-07-11
Betreuer:Prof. Dr. Christoph Scherber
Gutachter:Prof. Dr. Teja Tscharntke
Gutachter:Prof. Dr. Holger Kreft
EnglischThe increase and expansion of urbanisation and agriculture intensification has led to a decrease of natural habitats worldwide. Natural habitats offer resources such as food or nesting sites for different arthropod groups. Decline of these habitats could cause a loss of arthropod biodiversity. Semi-natural habitats in farmland, but also green spaces (e.g. parks and gardens) in cities that have high plant richness and flower cover, can potentially maintain this biodiversity and the associated ecosystem services. Pollinators provide vital ecosystem services and are highly dependent on flowering resources in natural habitats. Bumblebees (Bombus) are a particularly important pollinator group in Europe. In study 1, I examined how floral resources at the local and landscape scale affect bumblebee foraging behaviour and colony performance. I conducted an experiment with 32 Bombus terrestris colonies along a farmland to urban gradient. I analysed local and long-range movement patterns of bumblebees to assess where pollinators forage in urban areas. I measured if B. terrestris colony growth depended on resource availability in the direct surroundings of the colonies or on landscape scale. The colony performance was the same along the gradient and workers visited plants providing floral resources in the direct surroundings and foraged at greater distances to their colonies. My results indicate that resources at the local and landscape scale affect bumblebee behaviour and performance. It is important to provide sufficient amounts of vegetated area for pollinators in farmland and urban areas as they benefit from plant rich environments. Additionally, in study 2 I collected pollen from 48 bumblebee colonies in May and July 2015 and analysed which plant families were collected most frequently from bumblebee workers and if these plant families occurred in the surroundings of the colonies. Bumblebee workers showed high preferences for specific plant families with high flower cover and a high amount of nectar and pollen resources. Bumblebees foraged in greater distance to their colonies when these plant families were not present in the direct surroundings of the colonies. My results suggest that in bumblebee conservation and green space management these plant families should be taken into account. These plant families could help with the maintenance and restoration of good quality habitat for pollinators.In study 3, I conducted a pollinator observation experiment along a farmland to urban gradient. I planted phytometer plants along grassy margins in farmland sites, in village gardens and city gardens and observed plant-pollinator interactions. Plant-pollinator networks were more robust with higher interaction strength in farmland sites than networks in village and city gardens. Pollinator community composition changed with increasing urbanisation. In farmland sites, syrphid flies visited the phytometer plants more often, whereas wild-bees were more often present in gardens. In village gardens intermediate amounts of syrphid flies and bees visited the phytometer plants. My results suggest that farmland and urban landscapes support different pollinator communities and that the interface between the two extremes is of particular importance for the maintenance and restoration of a complementary pollinator community. In study 4, I sampled three arthropod taxa (Coleoptera, Araneae and Hymenoptera) along an urbanisation gradient from small villages to cities. Sampling was conducted in gardens and public green spaces in the edge or centre of settlements to investigate if the direct surroundings or the amount of urban area in the landscape affected arthropod community composition. Coleoptera community structure and composition changed depending on the local surroundings (position in the settlement and green space type) and the amount of urban area (landscape effect). Hymenoptera and Araneae were influenced predominantly by the local surroundings. Our study deepens our understanding of how arthropod communities respond to urbanisation, as it is the first to investigate the influence of both urban area size and position in an urban area. In conclusion, the results of my thesis show that arthropod groups respond to local habitat type and resource availability such as flower cover and plant richness in urban green spaces. Arthropod community composition and structure are influenced by landscape factors, such as degree of urbanisation or the amount of a mass flowering crop. Vegetated areas and urban green spaces have biodiversity value and my results suggest that landscape and the type of local habitat are intricately linked and therefore they should be evaluated together when designing landscapes to maintain and conserve biodiversity.
Keywords: urbanisation; pollinator; city size; gradient; Bombus terrestris; arthropods