From genes to habitats : effects of urbanisation and urban areas on biodiversity

Urban areas are landscapes dominated by built-up structures for human
use. Nevertheless, nature can still be found within these areas. Urban
ecosystems can offer ecological niches, sometimes only found in cities. This
biodiversity in the form of genetic diversity, species diversity and habitat
diversity provided the structure of this thesis.
First, we studied the effects of urbanisation on genetic diversity. We
analysed the population structure of the wall lizard with highly variable
genetic markers. We sampled over 200 wall lizards from nine sites with
different degrees of isolation. High genetic differentiation was found
between all investigated sites. The two sites most isolated showed the
lowest allelic richness and the lowest observed heterozygosity. These
results were combined with a GIS model to identify relevant factors of the
connectivity between sites. The geographic model, based on cost-distances,
showed that the dispersal on railway tracks was best correlated with the
genetic data. We can therefore conclude that railway tracks provide
important corridor functions for the wall lizard in the region of Basel. The
allelic richness of the populations in Basel and Jura corresponded to the
allelic richness of northern European populations, whereas the Mediterranean
populations were allelic richer.
Second, we investigated species diversity in urban forests in the city
and the surroundings of Basel. We compared the arthropod diversity,
abundance and assemblages in forest patches of different sizes. Over a
period of six months, we run 45 pitfall traps on nine sites and analysed
three taxa: spiders, ground beetles and rove beetles. Three different
methods were tested to estimate the similarity of arthropod assemblages
considering under sampling. The species number of small urban forest
patches did not differ significantly from large urban forests. The species
assemblages however changed from smaller forest patches to larger
patches. The occurring spider and ground beetle species were
predominantly forest species regardless of the habitat size. In contrast, the
rove beetle assemblages were not dominated by forest species. This study
demonstrates that urban groves can contribute considerably to the species
diversity in an urban area. On two urban forest patches, Leistus fulvibarbis
was found. L. fulvibarbis is a ground beetle, with an Atlantic-European
distribution. This species has been absent from the Swiss Fauna for more
than 100 years. The distribution map based on a literature review suggests
that L. fulvibarbis is dispersing southwards since the early 90ties. The
dispersion follows most likely the Rhine and its confluents.
And third, we analysed the habitat diversity, which can develop on
disused railway sites. Ecologically, these sites are similar to large gravel
river banks and therefore offer important habitats for threatened pioneer
species. Yet, disused railway sites are of great economical interest because
their reclamation costs are low and they are often located near the city
centre. In an interdisciplinary study, we compared five urban development
projects on disused railway sites in Europe. We identified three strategies to
protect the natural sites in such railway brownfields: (1) protection of the
pioneer habitats in-situ, (2) reinstallation of similar habitats on roofs (exsitu)
and (3) safeguarding of the natural process of succession. The
comparison of the five projects illustrated that the current legislation varied
considerably and the images of open green space differed between the
stakeholders. As a consequence, the quantity and type of green space
allocated changed. For future brownfield redevelopment projects, we
encourage guidelines that consider the special kind of nature on such sites
and guarantee planning reliability for investors.
The multiscale approach to study the effects of urban areas and
urbanisation on biodiversity provided valuable results. The main effects of
urbanisation and urban areas investigated in this study were habitat
alteration, isolation and loss. Moderate habitat alteration had no effect on
the species diversity. Moreover, typical forest species were still present in
urban forests. To prevent habitat isolation, the habitat connectivity by
railway tracks was important to maintain genetic diversity. Lastly, habitat
loss was reduced with innovative conservation strategies and the
involvement of all stakeholders. Overall, biodiversity in the city can be
promoted at all levels from genes to habitats by a sustainable, scientifically
based management.