Detecting existing crossing structures with optimization potential for wildlife by calculating a StructurePermeabilityIndex

Existing crossing structures are becoming increasingly important to reduce the barrier effect of fenced highways as they can help to increase the permeability at little additional cost. Nevertheless, not every existing crossing structure is suitable for optimization because of inappropriate structure related attributes such as its dimensions, road-related attributes, surrounding habitat characteristics, and human disturbance levels. The detection of suitable structures along a highway section is labour-intensive as each structure has to be inspected on site. Furthermore, the assessment of the suitability relies on the personal opinion of the consultant, because no standard methodology is available to date.

We present a user-friendly method to calculate a Structure-Permeability-Index (SPI) for crossing structures of fenced highways for the identification of structures which are worthwhile for wildlife optimization.

The SPI considers structure type and its dimensions, the land-use in the surrounding of the structure (forest, agriculture, settlement, water surface), noise emissions by traffic in the surrounding and the usage of the structure by motorised traffic. Since the SPI has a value between 0 and 1, the permeability of crossing structures can be compared one with another and thus, the overall permeability of a highway section can be classified.

The quality of the SPI was tested using linear regression analyses. Therefore, we tested the relationship between the SPI and the number of measured animal crossings (camera survey) of 98 crossing structures (own data). Results show a strong statistical relationship between SPI and number of animal crossings (R2=0.45, p<0.001).

This method allows reducing the cost of the identification of crossing structures which are suitable for wildlife optimization.

existing crossing structure, joint-use passages, permeability-index, highway permeability, optimization of crossing structures, camera traps