Nature conservation and biodiversity: bridges as stepping stone biotopes

Birds, bees, butterflies and other animals can find more food on all the new green spaces on the Frankfurt Bridges - and structural provisions are also being made for them : The bridges functioning as a model research area for structural species protection. In addition, there are natural wild meadows on the Frankfurt Bridges where endangered species can flourish undisturbed. The Frankfurt Bridges are thus not only a green lung through the city, but also serve as a stepping stone biotope for species previously separated by roads and the city, which can now come together again and mate.

Content: The network of bridges offers new habitats and a stepping stone biotope structure with its diverse green spaces

Already during the construction of the bridges, nesting, breeding and staging areas for animals and insects are created on the bridges; and the excavated material produced during the construction project is used directly to create nature conservation mounds.

Thus, in the course of a major construction project, new habitats are simultaneously created for Frankfurt's flora and fauna.

Moreover, stepping stone biotopes are created: birds, butterflies, smaller mammals, but also bats can cross gray roads with the help of the green bridges.

Around 1 million m2 of green space will be created on the bridges - of which 400,000 m2 will be special protection areas for flora and fauna

On the Frankfurt Bridges, the wild meadows are located behind the lanes of the autonomous vehicles - in other words, in an area to which humans have no access. They are deliberately set aside so that the flora and fauna can develop undisturbed.

Google Earth Stiftung Altes Neuland Frankfurt

Accordingly, the bridges function in large parts as an ecosystem in the middle of the city

There is the "normal" urban greenery and there are wild meadow areas: In the case of the normal urban greenery, attention is already paid to a balanced selection that is friendly to animals and insects, plants with double flowers are preferred for nutrition, plants that require bog soil are avoided, and more wild-looking plants that provide breeding and feeding opportunities are also found on the decorative beds.

In addition, the wild meadow areas specifically have an almost natural flora in which biodiversity can develop undisturbed, as the areas are completely inaccessible to public traffic.

Wild meadows on Frankfurt bridges behind the roadway - example Mainzer Landstraße

How do wild meadows work?

Wildflower meadows are cut on the bridges at certain times once in summer after seed ripening and once in autumn - preferably manually. Because true wildflowers and meadow grasses are self-seeding, the meadow, properly treated, becomes more beautiful from year to year.

This creates a cycle: The plants grow, blossom and bear fruit, and as soon as they dry out and decompose, they serve as humus for the next year's plants.

Additional fertilizers thus become completely unnecessary. The watering of wild meadows is similarly economical: Watering is really only done in extreme dry phases.

This is how wild meadows can develop in the first place: Because as soon as you irrigate and fertilize generously, plants with high water and nutrient requirements assert themselves and displace other (often endangered) plant species that are more modest.

For wildlife and especially for insects, (almost) natural wild meadows are being created as a valuable biotope.

Even if natural scrub or insect hotels in the wild meadows are sometimes a bit unsightly, this is a way to promote biodiversity

Experts on the Topic

Bundesamt für Naturschutz (BfN) Biologische Vielfalt — Bundesministerium für Umwelt, Naturschutz, nukleare Sicherheit und Verbraucherschutz (BMUV) Stadtnatur — Technische Universität München Zentrum Stadtnatur und Klimaanpassung (ZSK)

With the help of the bridges, new ecosystems are also being created in the Frankfurt area: the excavated earth produced by the construction of the bridge columns is recycled on site and backfilled as "bio-hills" nearby

In this way, long distances for the removal of excavated material are avoided, and high-quality biotopes are created by backfilling in Frankfurt's surrounding area - true to the environmental compatibility principle on which the Frankfurt bridges are based.

The columns of the bridge have a base area of one square meter and go down 20 meters. With approximately 15,000 columns, this results in 300,000 m3 of excavated earth.

The concept should be applied to all infrastructure projects because it is sustainable in two ways

Reducing transport distances leads to significant CO2 savings for such immense quantities of excavated earth. At the same time, valuable protected ecosystems are created close to the city.

The backfills create structurally rich habitats and nature is left to its own devices as far as possible

The mounds will be designed to provide a valuable habitat for birds, amphibians, grasshoppers, bats butterflies, beetles, all other insects, fungi and some mammals. A permanent still water body will be created on the top of the hill, fed by rainwater.

On the sides are smaller depressions and pools that fill up during rain events and controlled overflow of the stillwater. The water can thus seep away with a delay, erosion events are prevented and the groundwater is enriched. Appropriate monitoring documents the development of this measure and, if necessary, provides nature conservation instructions for action to ensure lasting success.

With clay layers can be distributed on the hill shallow sealed troughs collect rainwater

Excavation in Frankfurt usually also unearths larger quantities of Frankfurt clay. This is well suited to creating particularly valuable wetland biotopes on the backfilled hills.

Experts on the Topic

Botanische Vereinigung für Naturschutz in Hessen — Hochschule Geisenheim Institut für Landschaftsplanung und Naturschutz — Universität Kassel Institut für Landschaftsarchitektur und –planung Fachgebiet Landschafts – und Vegetationsökologie

Species protection measures are also integrated directly at the bridge body during construction

In this respect, too, the Frankfurt bridges should serve as a model for the infrastructure projects of the future: Structural promotion of biodiversity should accordingly be an important component of planning from the outset.

The bridges are structurally designed for endangered species as a protective refuge

With various nesting boxes or built-in bricks on and in the body of the bridge along the entire route, countless roosts can be created for swifts, bats, various swallow and other Frankfurt bird species. The green areas along the bridge also provide a species-appropriate food and nesting base.

Species diversity is promoted in a variety of ways on all green spaces at the bridges

Public green spaces on the bridges, as well as gardens of bridge residents, are designed from the outset to promote the biodiversity of Frankfurt's plant and animal life.

Even if it is not obvious to the layman at first glance, a large part of the one million square meters of green space on Frankfurt's bridges is designed to be ecologically valuable: Even in splendor beds, plants with unfilled instead of filled flowers are planted, structurally rich nutritive woody plants are preferred to solitary ornamental woody plants, and so on. But not only ecologically valuable plants but also species-promoting elements such as nesting boxes, woodpiles, water troughs, etc. are integrated into the bridge green spaces.

Thorny shrubs as habitat for birds and food base for all fauna

Building blocks in the facade as a roost for bats

Temporary water puddle - source of drinking water for animals and habitat for insects and amphibians

Densely vegetated areas as habitat for sensitive animal species

Sandy area for sand baths for birds or warm up for reptiles

Various types of nesting boxes to provide nesting sites for Frankfurt's birdlife

Structural roosts as a refuge, breeding and resting place for smaller mammals.

Water trough near the ground for birds, insects but also small mammals including open earth floor

Bird bath and feeding station

Woodpiles as habitat for ground-dwelling insects and shelter for small mammals

Dry stone wall for heat-loving species such as reptiles - hibernation site for smaller creatures

Permanent water surface with rocky environment provides habitat and food source for diverse animals

Thanks to the green bridge network, animals can now once again mate with conspecifics from which they were previously cut off by concrete and asphalt: Frankfurt's bridges thus also function as stepping stone biotopes

Animals and plants can expand their habitats with the help of the bridges

The bridges serve as so-called "stepping stone biotopes", i.e. they connect green spaces in the city with each other: populations can thus use the new green corridors through the city to reproduce more with their conspecifics.

The bridges thus become an integral part of Frankfurt's species and biotope protection concept (ABSK)

The bridge can connect natural habitats classified as valuable in the ABSK.

It is examined in detail, through which target areas of the ABSK the bridge runs - Then the target concept is considered both on and in the immediate vicinity of the bridge and implemented with appropriate measures.

Experts on the Topic

Hochschule Geisenheim Institut für angewandte Ökologie Biodiversität und Ökosystemfunktionen — Universität Kassel Institut für Landschaftsarchitektur und –planung Fachgebiet Landschafts- und Vegetationsökologie — Bund Naturschutz Stadt als Lebensraum — Startseite Hessische Gesellschaft für Ornithologie und Naturschutz e.V

Conclusion: Frankfurt bridges create an intact urban ecosystem

The Frankfurt bridges are a major structure that already integrates species protection measures (structure-compatible) during the planning phase. This makes the Frankfurt bridges a model for future infrastructure projects in Germany - but also worldwide.

In addition, protected areas are created where animals, insects and plants can develop and reproduce undisturbed by humans.

The diverse green spaces of the Frankfurt bridges create new habitats for plants and animals that also function as stepping stone biotopes. In addition, this significantly supports the City of Frankfurt's species protection goal.

Greenery on the bridges

Plantlife along the bridges

Unsealing of the city center

Vitalization of the urban greenery

The green metropolis of the future

The goal: to arm the city against drought and floods

Water distribution with the bridge-ring-line

Frankfurt: Current urban climate