The impacts of urbanisation and habitat size on local biodiversity and ecosystem functioning

Urbanisation is increasing worldwide and regarded a main driver of environmental change. Urban development and associated factors like increased spatial isolation, reduced habitat size and various types of disturbances can alter the dynamics of plant and animal populations in the remaining green areas. Furthermore, changes in abiotic site conditions (e.g. temperature, moisture content) can influence habitat quality and, consequently, the species richness, species composition and functional diversity of plants and animals, which in turn can affect the functioning of ecosystems. Nonetheless, urban areas can harbour a remarkably high species richness and can be of high conservation value by serving as a refugia for many rare and threatened species. It is, therefore, of central importance in conservation biology to disentangle the various drivers of biodiversity in urban landscapes. Within the scope of this thesis, three studies were conducted to examine the consequences of urbanisation on the biodiversity and ecosystem functioning of green areas in the urban region of Basel, Switzerland. The aim of the first study was to assess the impacts of habitat size and landscape composition of the closer surroundings on the species diversity of three taxonomic groups differing in trophic rank (vascular plants, Orthoptera and Lepidoptera) in meadows and ruderal sites. I also related the response of three traits (body size, dispersal ability and food specialisation) to habitat size in Orthoptera and Lepidoptera. For this purpose, I analysed data of species from the natural heritage inventory of Basel. I found that the response of different groups of species considerably varied depending on the habitat type, taxonomic group and species trait examined. The species richness of Orthoptera and Lepidoptera was positively related to meadow size but not to the size of ruderal sites, while the opposite was true for plants. For Lepidoptera in ruderal sites, the percentage of ruderal sites in the closer surroundings was a better predictor of species richness than habitat size per se. Forests belong to the most frequent green areas in urban landscapes and provide a wide range of ecosystem functions and thus play a major role for human well-being in cities. The aim of the second study was to examine the potential effects of degree of urbanisation, forest size and the corresponding interaction on the species diversity and functional diversity of vascular plants, ants and spiders. The two arthropod groups do not show species-specific mutualistic or exploitative relationships with plants in contrast to those in the first study. I conducted vegetation surveys and pitfall trapping to sample soil surface-active ants and spiders. In plants, species richness decreased with the degree of urbanisation. Ants and spiders at higher trophic rank showed more pronounced shifts in species composition with increasing degree of urbanisation, while the percentage of forest specialists in both arthropod groups was positively related to forest size. Local site characteristics were also important determinants for species diversity and functional diversity. In forests, the decomposition of leaf litter is an important component of the process of nutrient cycling and the formation of soil. In this way, litter decomposition contributes to the maintenance of several other ecosystem functions and services. The third study aimed to investigate the effects of urbanisation on leaf litter decomposition process in forests. Standardised litter of Fagus sylvatica leaves was used to assess the impact of urbanisation-related factors on the early stage of decomposition and seasonal microbial activity. I found combined effects of degree of urbanisation and forest size on the decomposition rate of leaf litter (klitter) indicating that forests of similar size differed in abiotic and biotic forest characteristics depending on the degree of urbanisation in the closer surroundings. Furthermore, moisture content of litter was the best predictor of microbial activity, followed by forest size. The findings of this thesis highlight the necessity to consider different taxonomic groups and functional groups in urban planning to maximise conservation value of urban green areas. In addition to degree of urbanisation, also habitat size was important for the diversity of some groups and leaf litter decomposition process in forests. It was also encouraging to find that even small green sites have the potential to make a significant contribution to biodiversity conservation and essential ecosystem functions in urban landscapes. I recommend that urban planners develop more flexible management strategies to satisfy the different requirements of various groups of species in the corresponding habitat type. Locally adapted management practices may provide a way forward to enhance habitat quality in a way to maximise species diversity and thus ensure the functioning of ecosystems; albeit large-scale factors also remain important

Ecosystem functioning in cities: Combined effects of urbanisation and forest size on early-stage leaf litter decomposition of European beech (Fagus sylvatica L.)

Environmental changes associated with urbanisation can affect the functioning of ecosystem processes. In cities, forests are among the most frequent types of green areas and provide a wide range of ecosystem services including air cleaning, decomposition of leaf litter and recreation. The European beech (Fagus sylvatica) is a frequent and widespread deciduous tree in temperate forests in Central Europe. In this study, we examined the effects of urbanisation on decomposition processes of F. sylvatica leaves in different-sized forests in the urban region of Basel, Switzerland. We used standardised litterbags (mesh size: 2 mm) with F. sylvatica leaves to assess the impact of degree of urbanisation (indicated by the percentage cover of sealed area in the surroundings) and forest size on the early stage of leaf litter decomposition and seasonal microbial activity. We found combined effects of degree of urbanisation and forest size on the decomposition rate of leaf litter (klitter). Large forests showed the highest klitter in areas with sparse settlements and the lowest klitter in densely settled areas, whereas the opposite pattern was recorded for small and medium-sized forests. This indicates that abiotic and biotic forest characteristics of forests of similar size differently influenced klitter depending on the degree of urbanisation. Moisture content of litter was the best predictor of microbial activity, followed by forest size. We assume that factors acting at the landscape scale such as the degree of urbanisation might be too coarse to detect any differences in microbial activity. Our results revealed that even small urban forests contribute to this important ecosystem function. As decomposers are at the bottom of the food chain, management actions that support the biological activity in soil might be also beneficial for species at higher trophic ranks

Habitat- and matrix-related differences in species diversity and trait richness of vascular plants, Orthoptera and Lepidoptera in an urban landscape

Urban growth is considered to be a major driver of environmental change. Urbanisation can affect urban biodiversity in different ways. So far, most studies focused on the impact of urbanisation on single taxa in one habitat type. In this study, we used data of species inventories and GIS-based landscape elements to examine the effects of habitat size and landscape composition on the species diversity of three taxonomic groups (vascular plants, Orthoptera and Lepidoptera) in meadows and ruderal sites in the urban region of Basel, Switzerland. We also related the responses of three species traits (body size, dispersal ability and food specialisation) to habitat size in Orthoptera and Lepidoptera. We found that species of the different taxonomic groups differed in their response to habitat size and landscape composition both in meadows and ruderal sites depending on the traits examined. The species richness of Orthoptera and Lepidoptera was positively related to meadow size but not to the size of ruderal sites, while the opposite was true for plants. For Lepidoptera in ruderal sites, the percentage cover of ruderal area in the closer surroundings was a better predictor of species richness than habitat size per se. To sustain high levels of urban biodiversity, we recommend that urban planners develop adequate management strategies to satisfy the different requirements of various taxonomic groups and to increase the quality of green sites surrounding the target habitat

Diverse effects of degree of urbanisation and forest size on species richness and functional diversity of plants, and ground surface-active ants and spiders

<div><p>Urbanisation is increasing worldwide and is regarded a major driver of environmental change altering local species assemblages in urban green areas. Forests are one of the most frequent habitat types in urban landscapes harbouring many native species and providing important ecosystem services. By using a multi-taxa approach covering a range of trophic ranks, we examined the influence of degree of urbanisation and forest size on the species richness and functional diversity of plants, and ground surface-active ants and spiders. We conducted field surveys in twenty-six forests in the urban region of Basel, Switzerland. We found that a species’ response to urbanisation varied depending on trophic rank, habitat specificity and the diversity indices used. In plants, species richness decreased with degree of urbanisation, whereas that of both arthropod groups was not affected. However, ants and spiders at higher trophic rank showed greater shifts in species composition with increasing degree of urbanisation, and the percentage of forest specialists in both arthropod groups increased with forest size. Local abiotic site characteristics were also crucial for plant species diversity and species composition, while the structural diversity of both leaf litter and vegetation was important for the diversity of ants and spiders. Our results highlight that even small urban forests can harbour a considerable biodiversity including habitat specialists. Nonetheless, urbanisation directly and indirectly caused major shifts in species composition. Therefore, special consideration needs to be given to vulnerable species, including those with special habitat requirements. Locally adapted management practices could be a step forward to enhance habitat quality in a way to maximize diversity of forest species and thus ensure forest ecosystem functioning; albeit large-scale factors also remain important.</p></div

Diverse effects of degree of urbanisation and forest size on species richness and functional diversity of plants, and ground surface-active ants and spiders - Fig 3

<p><b>NMDS of (a) plant, (b) ant and (c) spider species composition.</b> Forests sites are grouped according to their degree of urbanisation (low, medium, high).</p

Diverse effects of degree of urbanisation and forest size on species richness and functional diversity of plants, and ground surface-active ants and spiders - Fig 2

<p>Plant species richness (a; mean ± SE), percentage of forest specialists (b), Shannon diversity (c) and evenness (d) in forests, which were located in areas with different degrees of urbanisation.</p

Summary of GLM analyses examining the effects of degree of urbanisation, forest size and shape, forest management (time since last thinning), disturbance (indicated by path density), canopy closure, soil characteristics (moisture, pH, soil orgN and orgP) and cover of ground vegetation on the species richness, percentage of forest specialists, Shannon diversity and evenness of vascular plants.

<p>Summary of GLM analyses examining the effects of degree of urbanisation, forest size and shape, forest management (time since last thinning), disturbance (indicated by path density), canopy closure, soil characteristics (moisture, pH, soil orgN and orgP) and cover of ground vegetation on the species richness, percentage of forest specialists, Shannon diversity and evenness of vascular plants.</p

Summary of GLM analyses examining the effects of degree of urbanisation, forest size and shape, forest management (time since last thinning), disturbance (indicated by path density), canopy closure, soil organic matter content, litter characteristics (moisture, pH) and structural diversity measures (litter biomass, vegetation structure and amount of dead wood) on the species richness, percentages of forest specialists and generalists, Shannon diversity and evenness of ants.

<p>Summary of GLM analyses examining the effects of degree of urbanisation, forest size and shape, forest management (time since last thinning), disturbance (indicated by path density), canopy closure, soil organic matter content, litter characteristics (moisture, pH) and structural diversity measures (litter biomass, vegetation structure and amount of dead wood) on the species richness, percentages of forest specialists and generalists, Shannon diversity and evenness of ants.</p

Summary of GLM analyses examining the effects of degree of urbanisation, forest size and shape, forest management (time since last thinning), disturbance (indicated by path density), canopy closure, soil organic matter content, litter characteristics (moisture, pH) and structural diversity measures (litter biomass, vegetation structure and amount of dead wood) on the species richness, percentages of forest specialists and generalists, Shannon diversity and evenness of spiders.

<p>Summary of GLM analyses examining the effects of degree of urbanisation, forest size and shape, forest management (time since last thinning), disturbance (indicated by path density), canopy closure, soil organic matter content, litter characteristics (moisture, pH) and structural diversity measures (litter biomass, vegetation structure and amount of dead wood) on the species richness, percentages of forest specialists and generalists, Shannon diversity and evenness of spiders.</p

Location of the study area in Northwestern Switzerland and the distribution of the forests examined in the area of Basel-Stadt.

<p>The investigation area is surrounded by dense settlements in Germany (north), France (northwest) and Switzerland (south-west).</p