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.

Roads affect animals in three adverse ways. They act as barriers to movement (’fence effect’), enhance mortality due to collisions with traffic, and decrease habitat size. We study the relative importance of the first two effects using a spatially explicit individual-based model of population dynamics. We discuss our results with respect to the suitability of fences along roads as a measure to reduce road mortality. The results reveal a much stronger effect of road mortality than of the ’fence effect’; the influence of traffic mortality is always much more significant when the proportions of individuals avoiding the road and those that are killed on the road (in relation to the number of individuals encountering roads) in the two situations compared are the same. The results indicate that putting up fences along roads might be a useful interim mitigation measure until more suitable measures will be applied. However, fences must be used with caution because they could increase extinction risk for species that have large area requirements and small population sizes. In the second part of this paper, we outline a comparison of different configurations of road networks. We ask if different spatial arrangements of the same amount of roads (e.g., ’bundling’ of roads) have consequences for the strength of both the ’fence effect’ and road mortality. The model results indicate longer times to extinction in case of a ’bundling’ of roads but the proportion of populations going extinct within 500 time steps does not change significantly.

Concern is growing over the fragmentation of habitats by roads and other transportation infrastructure. A number of measures to avoid, minimize, mitigate, or compensate for the detrimental effects of such fragmentation have been suggested. These are geared to specific scales, from culverts at the scale of a single road to plans for re-connecting habitats across entire countries or continents. They include the removal of roads, building of overpasses and underpasses at roads and railways to increase permeability for animals, restoration or creation of wildlife corridors and networks of wildlife corridors across transportation infrastructure, and the design of less fragmenting road network patterns, e.g., the bundling of traffic lines. However, it is still unknown which measures are the most effective in terms of restoring ecological processes. The investigation of their effectiveness, therefore, is an important and most urgent task because the most effective measures should be applied predominantly in order to use resources most efficiently. How can the effectiveness of such measures be evaluated (criteria and methods)? For example, possible criteria for the effectiveness of crossing structures are the reduction of road-kill frequencies, increased passage frequencies, presence of species on both sides of the road, genetic exchange across the road, recovery of lowered reproductive rates and skewed sex ratios, re-colonization success, recovery of skewed foraging intensities among foraging areas on either side of the road, and recovery of skewed predation rates. More generally, the measures should enhance landscape connectivity and restore ecological processes among habitat patches and across landscapes. During the last three years, considerable progress on measuring the effectiveness of such measures has been made in both Europe and North America. This session brought together the “Father of Road Ecology” Richard Forman with researchers from Europe (Austria, The Netherlands, etc.) and North America working at different scales and in different locations. They presented current methods and results on the success of various mitigation measures to foster cross-scale comparison and synthesis on this topic. The presentation included empirical studies, synthetic overviews, modeling studies, and conceptual studies.

Roads act as barriers to animal movement, thereby reducing the accessibility of resources on the other side of the road. Roads also increase wildlife mortality due to collisions with vehicles, and reduce the amount and quality of habitat. The purpose of this study was (1) to determine whether or not the configuration of road networks has an influence on the degree to which roads detrimentally affect wildlife populations and (2) to identify characteristics of road network configurations that make road networks less detrimental to the persistence of animal populations. To explore these questions, we used a spatially explicit individual-based stochastic-simulation model of population dynamics. A measure assumed to reduce the effects of the road network is the bundling of roads and traffic in order to keep as large areas as possible free from disturbances due to traffic. However, the suitability of this measure may be questionable because a group of several roads bundled together, or an upgraded road with more traffic on it, creates a stronger overall barrier effect that may be more detrimental to population persistence than the even distribution of roads across the landscape. Our modelling results clearly supported the bundling concept. Population persistence was generally better (and never lower) when all traffic was put on one road than when it was distributed on several roads across the landscape. If traffic cannot be combined on one road, the model results suggested it is better to bundle the roads close together than to distribute them evenly across the landscape. We also were interested in the question of whether the effect of a road network was determined by the number and size of the pieces (“patches”) that it fragments a landscape into or by the total length of roads in the landscape. We expected that the effect of a road network would be the more detrimental the more patches it creates. The results were surprising: The expectation that fragmenting the landscape into more patches would be more harmful to population persistence (while total road length is kept constant) was contradicted by the model results in the case where the degree of road avoidance by the animals was low. This implies that for animals that do not very strongly avoid roads, it is more important to preserve core habitats at a sufficient distance from roads than to keep the number of patches low. Our results are an important step towards a network theory for road ecology and towards the design of lessdetrimental road networks. Empirical studies comparing landscapes with differing road network configurations should be conducted in the future to validate the predictions and to provide a basis for developing more practical models for use in planning and designing of highway networks. Keywords: barrier effect, bundling of roads, core habitat, landscape connectivity, landscape fragmentation, population viability analysis (PVA), road avoidance, road configuration, roads, spatially explicit population model (SEPM), traffic mortality.

Habitat fragmentation due to transport infrastructure and other human development poses a threat to many wildlife species. This threat may differ depending on the species and types of fragmenting elements. There is a need to quantify the level of habitat fragmentation and the impact of habitat fragmentation on different wildlife species for use in transportation planning. Such measures would be useful in assessing the cumulative impacts of multiple road projects on wildlife connectivity and habitat suitability, for long-range wildlife impact mitigation planning for transportation projects, and as an indicator for the environmental monitoring of habitat fragmentation due to roads.

Effective mesh size (meff) is a biologically relevant landscape metric that quantifies the degree of landscape fragmentation. The definition of the effective mesh size is based on the probability that two randomly chosen points in a region will be located in the same non-fragmented area of land. We calculated effective mesh size to assess the level of land¬scape fragmentation in the State of California, USA, based on four fragmentation geometries defined by a combination of highways, minor roads, urbanized areas, agricultural areas, and natural fragmenting features (e.g., rivers, lakes, and alpine areas). The effective mesh size for these four fragmenting geometries were calculated for the entire State of California using eight sets of planning units: 1) transportation planning districts, 2) municipal county boundaries, and 3) six levels of watersheds. To demonstrate the methodology, we examined how effective mesh size may impact two species important to transportation planning in California: mule deer (Odocoileus hemionus) and mountain lion (Puma concolor). The calculated effective mesh sizes were compared with the home range sizes and daily movement distances of the selected focal species to determine the potential impacts of habitat fragmentation and to identify areas where transportation projects will potentially impact these focal species.

Based on the results of this analysis, we suggest that integrating an effective mesh size-based tool into transportation planning frameworks would be valuable to improve identification of potential landscape level impacts early in the planning process. The calculation of effective mesh size will give transportation planners a way to analyze the cumulative impacts of roads in districts, counties, and watersheds and can be used as an environmental indicator for ecological assessment of transportation system impacts.