Effects of the Configuration of Road Networks on Landscape Connectivity
Wildlife biologists, traffic planners, and decision makers are increasingly concerned about the effects of landscape fragmentation caused by transportation infrastructure. Data on the degree of landscape fragmentation are urgently needed for monitoring environmental change, identification of trends, and as a basis for investigating the effects of fragmentation on larger scales. The method of effective mesh size is currently used in several countries for national environmental reporting, e.g., as one of 24 core indicators in Germany. The objectives of this paper are to develop a new method for the quantification of landscape connectivity that incorporates variable barrier strengths into the effective mesh size, and to apply it to the question of how the configuration of transportation networks affects landscape connectivity, using empirical data on ungulates and amphibians. The paper also addresses the question of how crossing structures can enhance landscape connectivity most efficiently depending on their placement and spatial arrangement.
The outcomes include the following principles: (1) The more crossing structures were implemented, the higher the resulting landscape connectivity. (2) The higher traffic volume, the larger the difference between the configuration with and without crossing structures, and the more pronounced the differences among the various configurations with crossing structures. (3) The more patches can be accessed from any patch by few road crossings (i.e., high number of nearest neighbours and next nearest neighbours), the higher the degree of landscape connectivity. (4) The closer to each other the roads are (i.e., the more bundled the roads are), the higher the degree landscape connectivity. (5) However, putting all traffic on one road can be better or worse for landscape connectivity, depending on how quickly crossing success decreases with increasing traffic volume. (6) The number and quality of crossing structures are highly relevant. Wildlife passages that are not satisfactorily functional provide little benefit to landscape connectivity. (7) Large patches should be connected first. Only once the large patches are well enough connected does the additional connection with smaller patches provide higher additional connectivity than an improvement of the connectivity between the large patches.
The results demonstrate that the topology-sensitive effective mesh size is a suitable tool to study the effects of road network configuration and wildlife passage location on landscape connectivity. Because traffic volume may vary over time, landscape connectivity can vary over a day, week, or year. This new method will probably be applied widely in the future as the current lack of quantitative empirical data on the barrier strength as a function of road type, traffic volume, and animal species is currently addressed more and more systematically by wildlife biologists.