The impacts of urbanisation and forest size on antagonistic ecological interactions and forest ecosystem services

One of the main human-induced changes to the natural environment is the rapid expansion of urban areas throughout the world. Urbanisation can lead to a reduction of habitat sizes and increased spatial isolation of the remaining green areas. In European cities, forests are one of the most frequent type of urban green areas. These green areas are subjected to urbanisation-associated abiotic changes such as increased temperature. These environmental changes can affect the composition of plants and their nutritional quality, which in turn can influence other antagonistic and beneficial trophic levels dependent on living tissues (herbivores) and dead plant matter (decomposers). To acquire a better understanding of the effects of urbanisation and forest size on arthropods, I focused my thesis on a variety of arthropods that previously only received limited attention within an urban context.
The aim of the first study was to evaluate whether plant-galling infestation rates and leaf damage by mining and chewing arthropods on three common tree species (sycamore, beech, and ash) are influenced by the degree of urbanisation and forest size. For this purpose, I assessed the frequencies of gall infestation and leaf damage in 20 forests in the city of Basel and its surroundings. There were contrasting responses among the plants and herbivorous arthropods investigated. Total galling infestation rates on sycamore and infestation rates of beech gall midges were highest in forests situated in low urbanised areas, whereas only beech gall midges had a clear negative response to decreasing forest size. Regarding the herbivorous arthropods on sycamore, the total leaf area damage by mining and chewing arthropods was influenced by urbanisation and also increased with increasing forest size. Leaf miners on beech leaves tended to be affected by degree of urbanisation.
The leaf litter decomposition process is vital for nutrient recycling and uptake of nutrients for plants in forests. In the second study, the aim was to examine whether litter decomposition rates and the associated mesofauna are influenced by the degree of urbanisation and forest size. To assess the rate of leaf litter decomposition in urban and sub-urban areas of Basel, I used standardised litterbags with three different mesh sizes filled with a mixture of leaves of three common tree species (sycamore, beech and ash) that differ in their natural rate of decay. Litterbags were exposed for 3, 6 or 9 months in 17 forests. The leaf litter decomposition rate was negatively affected by the degree of urbanisation, while forest size had no significant influence. The mesofauna (consisting mainly of Acari and Collembola) contributes to the process of leaf litter decomposition. Therefore, I also examined how the mesofauna in the litterbags responded to urbanisation and forest size. The mesofauna showed contrasting responses to urbanisation. The oribatid mites (Acari) were most abundant in forests in moderately urbanised areas, while Collembola were more abundant in forests situated in highly urbanised areas. Furthermore, forest size influenced the mesofauna. Medium-sized forests had the lowest abundances of Acari and large forests the highest abundance of Collembola. There was, however, no shift in Collembola species composition in response to urbanisation. Nevertheless, urbanisation affected Collembola species with specific traits. Collembolans with a globular body shape were most abundant in forests of medium-size and situated in low urbanised areas, and the eudaphic (mainly soil-living) collembolans tended to be less abundant in small forests.
Deadwood is an integral component of forests in terms of nutrient recycling, forest regeneration, water retention, and also provides an essential habitat for nearly a quarter of the forest arthropod species. In the third study, the aim was to investigate factors that influence the deadwood-dependent (saproxylic) insect and fungal communities along a rural-urban gradient. I exposed freshly cut beech and oak branches as bait for saproxylic insects and fungi in 25 forests situated in Basel-city and its surroundings for 8 months to examine effects of landscape factors (degree of urbanisation), local factors (forest characteristics such as tree species composition at the forest site and volume of naturally occurring deadwood) and branch characteristics (e.g. moisture content of branch deadwood) on the saproxylic community. I also used subsamples of drilling chips from each branch to determine fungal species richness (operational taxonomic units OTUs) and abundance using a molecular technique. Altogether more than 193,500 insects emerged from the branches and were identified to the lowest possible taxonomical level. Urbanisation reduced the number of total saproxylic insects, bark beetles, longhorn beetles, flies, moths and ichneumonid wasps. Species richness of saproxylic insects was not affected by the degree of urbanisation or any forest characteristic. In contrast, the taxonomic composition of all insect groups was altered by wood moisture content (a branch characteristic) and the beetle species composition shifted with the degree of urbanisation and pH of branch wood. In saproxylic fungi, the total number of OTUs decreased with increasing degree of urbanisation. The total number of fungal OTUs was also positively influenced by the volume of naturally occurring fine woody debris at the forest site. The composition of fungal OTUs was affected by the degree of urbanisation and wood pH. These detected shifts in the composition of both saproxlic insects and fungi result in alterations in the associations between saproxylic insects and fungi along the rural-urban gradient.
In my thesis I was able to demonstrate that urbanisation has the potential to disrupt interactions between arthropod and living as well as dead plants (litter and wood). The difference in responses to urbanisation by the variety of arthropods investigated indicates that taxonomical differences can respond contrastingly to the same urban pressure. This is a result of differences in life-history and morphological traits, resource availability, and sensitivities to the altered micro-climatic conditions within urban areas. The changes in species composition in some arthropod groups (e.g. saproxylic insects and fungi) show that urbanisation can filter for certain species better adapted to the urban environment. This might not only lead to reductions in the abundance or even species richness of insects, but also reduce the efficiency in ecosystem services of urban forests such as litter and deadwood decomposition.