Wildlife & Traffic

A European Handbook for Identifying Conflicts and Designing Solutions

3 Effects of Infrastructure on Nature
Original version (2003)
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3.3 Primary ecological effects

Loss of wildlife habitat

Figure 3.3 - Diagram showing the impact of infrastructure development on the interior core of habitat, important for its special flora and fauna. The area of core habitat lost is far greater than that taken by construction due to the increased edge effect along the route of the road.

The direct impact of road construction is the physical change in land cover along the route as natural habitats are replaced or altered by transport infrastructure. The impact of this net loss of natural habitat is made worse by disturbance and isolation effects that lead to an inevitable change in the distribution of species in the landscape. Roads and roadsides cover an area of about 0.3% of the land surface of Norway to more than 5% in the Netherlands. Thus, at regional or national scales, the direct land uptake by infrastructure may appear to be only a minor issue. Locally, however, the allocation of space for infrastructure will necessarily lead to conflicts with other landuses such as nature conservation, recreation, agriculture or human settlement.

Barrier effects

The barrier effect of roads and railways is probably their greatest negative ecological impact. The dispersal ability of individual organisms is one of the key factors in species survival. The ability to move around a landscape in search of food, shelter or to mate, are negatively impacted by barriers that cause habitat isolation. Impacts on individuals affect population dynamics and often threaten species survival. The only way to avoid the barrier effect is to make infrastructure more permeable to wildlife by means of fauna passages, adapting engineering works or by the management of traffic flows. Carefully selecting the route of the road through the landscape can minimise the barrier problem.

Figure 3.4 - The effect of ecological corridors and road networks on the movement of species across landscapes.

A. In open landscapes without ecological corridors, species may not be able to move between habitats.
B. Small fragments of suitable habitat may serve as stepping stones connecting distant habitat patches.
C. Ecological corridors in combination with roads may attract animals but direct them towards the road where they might be killed when attempting to cross.
D. Mitigation measures such as fauna passages can help to re-link ecological corridors.

Physical barrier

For most of the larger mammals, transport infrastructure becomes a complete barrier only if fenced or if traffic intensity is high. For smaller animals, especially invertebrates, the road surface itself and road verges impose a considerably stronger barrier, either because the substrate is inhospitable or disturbance is too great.

Behavioural barrier

Many larger wildlife species are known to avoid areas near roads and railways related to the degree of human disturbance (traffic density, secondary development). Wild reindeer in Norway, for example, under-utilise their grazing resources within 5 km of roads. Other animals, such as small mammals and some forest birds, exhibit behavioural avoidance patterns particularly associated with crossing large open spaces.

Figure 3.5 - Traffic creates important problems for the otter population in the Czech Republic. Migrating otters (mostly males) often do not use small fauna passages and attempt to cross on the roads. (Photo by V. Hlaváˇc)

The relationship between road traffic density and the barrier effect on mammals. Fences along infrastructure increase the barrier effect of infrastructure. However, fences near passages can be used to lead animals safely to fauna passages.

Table 3.1 - The relationship between road traffic density and the barrier effect on mammals. Fences along infrastructure increase the barrier effect of infrastructure. However, fences near passages can be used to lead animals safely to fauna passages.

The alignment of two or more forms of transport infrastructure along the same corridor (in close proximity) can be beneficial for some species as only one barrier is created. It is, therefore, often advantageous to place two or more parallel routes as close as possible especially in the case of multimodal transport corridors (roads and railways). The disadvantage of multimodal transport corridors is that they can strengthen the barrier effect to some species. There is documented evidence of combined solutions acting as a significant barrier, fragmenting mountain habitats and isolating reindeer populations. Where parallel infrastructures are not placed in a single corridor, the zones between them often suffer local/regional biodiversity declines.

Fauna casualties

Mortality is probably the best-known impact of traffic on wildlife. Millions of individuals of a wide range of wildlife species are killed on roads and railways each year, and many more
are seriously injured. Large numbers of fauna casualties may not necessarily imply a threat to populations, but indicate that the species involved are locally abundant and widespread. Traffic mortality is considered responsible for just a small proportion (1-4%) of the total mortality of common species (rodents, rabbits, foxes, sparrows, blackbirds, etc.). However, for more sensitive species, traffic can be a
major cause of mortality and a significant factor in local population survival. In Flanders, for instance, more than 40% of the badger population is killed on the roads each year. Such losses represent a very serious threat to the long-term survival of badgers at the regional level.

Numbers of bird casualties can also be significant. Major road schemes adjacent to or crossing wetlands can result in a high density and diversity of birds being forced to fly across roads thus increasing the risk of mortality due to traffic accidents. Large birds such as raptors and owls are attracted to the grassy road verges to prey on the small mammal and songbird populations that concentrate there. Large numbers of theseç birds become road casualties as they fly low over the road while hunting

Figure 3.6 - At low traffic intensity (<2500) the small proportion of fauna casualties and animals repelled causes limited impact on the proportion of animals successfully crossing a road barrier. At medium traffic intensity (2500-10000) casualties are high, the number of animals repelled by the infrastructure increases and the proportion of successful crossings decreases. At high traffic intensity (>10000) a large proportion of animals are repelled and despite a lower proportion of fauna casualties there is only a small proportion of successful crossings. (Graph by Andreas Seiler, unpublished)

Figure 3.7 - A collision with large mammals such as moose can be dramatic, but these accidents are mainly a traffic safety issue. For smaller mammals and some birds, e.g. barn owls, road mortality can have a serious impact on local populations. (Photo (moose) by H. Corneliussen, Fædrelandsvennen and (barn owl) by G. Veenbaas)

Species especially sensitive to road barriers and traffic mortality:

  • Rare species with small local populations and extensive individual home ranges, such as large carnivores.
  • Species that have daily or seasonal migratory movements between local habitats. Amphibians are especially sensitive to road mortality when their seasonal movement to and from breeding
    ponds crosses roads. Some deer species use different habitats at different times of the day and often cross roads or railways to meet this need.
  • Species that have long distance seasonal migrations from summer to winter feeding grounds such as moose and reindeer.

Most of the measures taken to reduce the numbers of road casualties are taken for reasons of traffic safety. This is especially true for cases where larger animals such as moose, deer and wild boar are involved. The measures usually focus on stopping the animals gaining access to the road or the railway, but the need to lead animals to safe crossing points to minimise the fragmentation effect is often neglected.

The intensity and concentration of road and rail casualties varies with factors such as temperature, precipitation, season and time of day, and tend to follow the daily rhythms of traffic and animal activity. Seasonal variations in fauna casualties are influenced by breeding, dispersal, seasonal migration patterns and seasonal disturbances such as hunting. The landscape context of roads and railways also influences levels of wildlife traffic mortality. Roads that run parallel to or intersect the edges between forest and grassland are especially hazardous to the animals that move regularly between forest shelter and open foraging habitats.

Disturbance and pollution

Road and railway development and operation alter the ecological characteristics of adjacent habitats, which may induce changes in the way they are used by wildlife. Many of these changes can affect habitat quality at a significant distance from the infrastructure development. The following are the main types of disturbance associated with transport infrastructure.

Hydrological changes

Cuttings and embankments change landscape topography, and often induce large-scale changes in hydrology. Cuttings may increase soil erosion and drain aquifers. Embankments may change the water regime producing either drier or wetter conditions. These changes will affect vegetation, e.g. in wetlands and riparian habitats.

Chemical pollution

A wide range of pollutants are derived from road traffic and the road surface. Motor exhausts give rise to, for example, carbon monoxide, nitrogen oxides, sulphur dioxide, hydrocarbons including polycyclic aromatic hydrocarbons (PAH), dioxins and particles. Vehicles are sources of heavy metals such as lead, zinc, copper and cadmium. Sodium and chloride pollution comes from de-icing salt. The chemicals pollute surface and groundwater, soil and vegetation along roads. Compounds containing nitrogen and sulphur contribute to acidification and eutrophication. Pollutants can cause damage or disturbance to biological functions at several organisational levels, from cells through individuals to populations

Figure 3.8 - Pollution by traffic includes secondary effects such as pollution from deicing salts, which seriously damage roadside vegetation, as illustrated in this photo of a Norwegian road verge. (Photo by M. Smeland)

Noise and vibration

The disturbance from noise is mainly influenced by the type of traffic, traffic intensity, road surface properties, topography, rail type and the structure and type of the adjacent vegetation. Geological and soil characteristics influence the magnitude and spread of vibrations. Some species avoid noise-disturbed areas. For example, in the Netherlands, bird densities were shown to decline where the traffic noise exceeded 50 dBA, whereas birds in woodland were sensitive to noise levels as low as 40 dBA. Some species breed in normal densities in disturbed areas but with lower breeding success.

Lighting and visual disturbances

Artificial lighting can affect growth regulation in plants, disturb breeding and foraging behaviour in birds or influence the behaviour of nocturnal amphibians. Lights can also attract insects (mercury lamps) and, in turn, increase the local densities of bats along roads resulting in increased bat mortality. The movement of road and rail traffic is thought to disturb several sensitive wildlife species such as wild reindeer (see also Behavioural Barrier, page 5).

Ecological functions of verges

The value of infrastructure verges is a much debated topic. They can be important habitats for some species of wildlife, but they can also lead animals to places where mortality is increased or aid the spread of alien species. Verges can provide links in an ecological network and function as corridors for movement, especially in agricultural landscapes. Their function depends on their geographical location, vegetation, adjacent habitat, management and type of infrastructure. Positive values are more common in northern Europe and problems more often associated with southern Europe.

Habitat function

Numerous inventories in highly urbanised countries indicate the potential of verges as habitat for a diverse plant and animal life. Through careful management, infrastructure verges may complement and enrich landscapes where much of the natural vegetation has been depleted. Nevertheless, verges are unable to fully replace natural habitat due to disturbance and pollution effects. As a result, the species composition in roadside communities is often biased towards a higher proportion of nonnative and ruderal species.

Figure 3.9 - The corridor function of road verges in different landscapes.

A. In open, agricultural habitats, vegetated roadsides can provide a valuable movement corridor and habitat for wildlife.
B. In natural landscapes, open and grassy road verges introduce new edges and can increase the barrier effect of roads to forest species, but increase the corridor effect or provide new habitat for others.
C. Verges may serve as sources for species spreading to new or re-colonising vacant habitats, but may also favour invasive alien species invading natural habitats or the spread of predators.

In more urbanised countries verges can be important wildlife habitat. In the Netherlands, 796 plant species (more than 50% of all plant species) are found in roadside verges. This includes not only widespread species, but also less widespread and fairly rare species.160 rare species (representing 10% of the national total) are mainly found in verges. Low to moderately fertilized grasslands are rare in the Netherlands due to intensive agricultural production and verges represent important refuges for species requiring this habitat. Verge management has changed from lawn management in the 1950s and 1960s, to mowing only 1 or 2 times a year. The result is more colourful verges which are habitat for 50% of Dutch butterfly species (i.e. 80 species). It is estimated that 22 of these species can survive in the network of road verges. Even some species under threat in the Netherlands (such as brown argus and small heath) can survive in the verges.

Verge management has a strong impact on the value of verges as wildlife habitat. Management operations that affect biodiversity include: tree and bush pruning, mowing grassy vegetation, ditch cleaning and management of culverts, tunnels, fences, fauna passages and other measures. The principles of ecological verge management include careful timing of mowing grassy verges to mimic hay meadows, planting of native bushes and trees, minimising disturbance in the breeding season and reducing the use of chemicals for weed and insect control. Ecological verge management can increase biodiversity locally, but without careful planning can increase traffic accidents or create ecological traps for some species. Hence, planning must be sensitive to local circumstances.

Figure 3.10 - Invasive weed species may spread along transport corridors. In Spain, narrowleaved ragwort, a weed toxic to cattle, has spread along roads. (Photo by E. Bassols)

Corridor function

road and railway verges may function as wildlife corridors, enhancing the movement of species along the route. There are positive and negative effects of the corridor function of verges. Positive: this effect has been observed mainly for small mammals and insects, but corridors also lead wildlife into urban areas, for instance, roe deer, foxes, badgers and reptiles. Broad verges of low vegetation cut from forest may reduce accidents between road and rail traffic and large mammals by increasing visibility for both animals and drivers. Road verges are seen as important components of ecological networks in northern Europe. Negative: alien species or weeds may spread along transport corridors through the wind disturbance caused by traffic, or by seeds and propagules transported by vehicles. The examples of rhododendron in the UK and narrow-leaved ragwort in Spain (a weed toxic to cattle) provide evidence of the way alien species can rapidly spread over large geographical areas with the help of transport infrastructure. Road verges can also be a major source of forest fires in the Mediterranean countries. In Spain, for example, more than 24% of forest fires in 2000 were attributed to fires started in road verges (often by cigarettes) and, to a lesser extent, railway verges.

Roads and railways can also function as wildlife corridors enhancing the movement of unwanted species through the landscape. The linking of offshore islands to the mainland by bridges can also initiate the spreading of predators such as mink, martens and fox to otherwise isolated bird colonies. The result is increased bird mortality through predation and disturbance effects.

Verges rarely have the same value as natural corridors, since habitat conditions in road and rail verges are rarely constant over longer distances, and may vary greatly in quality. Roads often intersect with other infrastructure and may lead animals towards these intersections where the risk of accidents is high. Broad roadside verges that contrast with the surrounding vegetation (for instance, grassy verges in a forested landscape) may add to the barrier effect of the road and increase the isolation of habitats.