Extensive green roofs as protected habitat for endangered species by the example of the blue-winged grasshopper (Oedipoda caerulescens)
During the years 2009-2012 extensive green roofs in the urban area of Basel were examined on the presence of the blue-winged grasshopper. It turned out that green roofs can be of use as habitat or stepping stone for this red-list species and are an important ecological element, especially in urban areas. The results point out that presence of O. caerulescens is to favour by increasing roof area, decreasing roof height and rising roof age.
O. caerulescens shares the fate of many grasshopper species (Acridoidea) which in recent years were partly shocked by drastic population declines in Switzerland and Europe. Extensive special habitats, ones that are preferred and used by O. caerulescens, such as burned areas, floodplains and other dynamic pioneer sites in early successional stages, are increasingly disappearing.
Besides the often described habitat requirements of O. caerulescens (temperature, expansion, coverage, substrate) and the animal’s absence and presence in dependence of the same, little is known about spatial distribution of subpopulations, migration and mobility. The distancing of habitats and the consequently poor gene flow and thus, the insulation increase the risk of a collapse of an existing metapopulation. The disappearance of the blue-winged grasshopper seems, in addition to the aforementioned phenomena, as the result of a lack of exchange among subpopulations, or reasoned, of missing stepping stones between them.
The blue-winged grasshopper likes it dry. It will be found primarily on south-expanded locations, such as rocky lawns and pastures, ruderal areas, quarries and gravel pits, in dry pine forest and pioneer areas of various kinds, such as floodplains. Specifically, railway areas should be mentioned, as they with sandy and fine-grained substrate, the almost total solar expansion and the sparse vegetation are valuable habitats, refuges and stepping stone for dissemination.
Larvae first appear in May, adult animals from mid-July, with numerically highest strength in August till their beginning decease in mid-October. During this time, the adults mate and the females lay eggs in earthy or sandy soil. O. caerulescens shows no distinct diet preferences and feeds opportunistically on a variety of herbs and grasses. Accordingly, the judgment of the food can not be concluded that they offer habitat preference. Determining factors are the spatial structure and the degree of coverage, and the aforementioned dry and moisture.
On the background of this demanding activity the sensitivity of this creature to disturbing influences gets significant and can sometimes explain its rare occurrence. In Switzerland the species is on the Red List and deemed to be potentially endangered.
Investigations and observations were made during June-October. Each potential area was recorded twice. Was after two visits now individual of O. caerulescens to be found, the roof was classified as vacant. The study sites were paced in wavy lines. Animals were flushed and caught with a net of 40 cm diameter.
Results show that for Oedipoda caerulescens green roofs can serve as habitat. The occurrence is dependent on, or favoured by several factors and criteria. A deep vertical distance (height), a large roof area, a high roof age, an average-vegetation cover (25-70%) and one occurrence in the immediate vicinity of the roof are to abet the presence of the blue-winged grasshopper on the roof. Besides that, all roofs the animal was found on, were highly structured and offered thereby a rich palette of different microhabitats.
It seems remarkable that O. caerulescens was found on greenroofs without beeing present in potential habitats in the vicinity of the roof. This highlights the importance of extensive green roofs in urban areas in terms of conservation.