The Ecological Effects of Roads on Wetlands

Wetlands, vitally important ecological systems, have a history of inadequate protection. The impacts of road construction and road operation on wetlands are numerous and broad in scope; negative impacts range from changes to the chemistry and biology of the local area to changes in hydrology that go well beyond the immediate area. Loss of wildlife habitat, loss of species and biodiversity, and introduction of alien species are among the consequences of such changes. These impacts result from a roadøs location, construction, maintenance, manner of use, and further effects that occur once the road is in place.

Wetlands are extremely important habitats with a variety of functions. One of the most important is the prevention and reduction of flooding due to their ability to hold large amounts of water. Because of their capacity to hold so much water, wetlands also control erosion. In addition, wetlands recharge groundwater, improve water quality and provide habitat for wildlife. Eighty percent of Americaøs breeding bird population and more than fifty percent of the eight hundred species of protected migratory birds rely on wetlands (Mitsch and Gosselink 1993).

Knowing these ecological functions, it is vital to understand what effects roads can have on them. Roads have six general effects on wetland ecosystems that those who are concerned about preserving them must consider (Trombulak and Frissell 2000; Findlay and Bourdages 2000).

Alteration of the Physical Environment

Wetlands are dependent on hydro-periods, where flooding occurs seasonally and for a length of time. Once the hydrological processes are disrupted, many changes occur in a dominoeffect. When a road is introduced into a wetland, it acts as a dam in the system, which can affect areas some distance away.

A road can cause the upland side of a wetland to flood and the downland side to drain, diverting the surface water flow in the process and causing the biological characteristics to change. A road can also critically impact the subsurface water flow in a wetland, depressing the water table and affecting the amount of groundwater available (Darnell 1976). This depression can affect many water-dependent fauna and plants.

Often during road construction, channels are excavated to divert water straight towards culvert installations. This channelization can substantially alter the rate and character of surface and subsurface flow (Darnell 1976). Channelization can destroy both upstream and downstream wetland areas by giving them either too much or too little water. This effect is reported to reduce the diversity of the habitat and cause shifts in species composition (Darnell 1976).

As soil erosion accelerates due to this alteration in water volume and levels, there is a reduction in bank stability, and therefore, an increase in sediment loading. These fine sediments can increase the turbidity of the water, clouding it and preventing sufficient light penetration, which adversely affects the health of the flora and fauna (Darnell 1976).

Alteration of the Chemical Environment

Roads facilitate the alteration of the chemical environment. Highways can introduce oil and heavy metals, such as lead, aluminum, and cadmium, which can contaminate a wetland (Adamus and Stockwell 1983). In aquatic environments especially, these contaminants can travel far and fast. Such contamination can have adverse impacts on wildlife health, especially to animals higher up the food chain.

Salt from deicing roads also alters the chemical environment by contributing ions to the soil, changing the pH, and altering the soiløs chemical composition (Darnell 1976).

Fragmentation of Habitat

Roads act as barriers that fragment wetlands habitat and have short and long term impacts on wildlife. îFragmenting landscapes into disjunct patches and restricting and isolating wildlife populations by amplifying the risks associated with movement have drastic consequences for the preservation of biological diversity" (Harris and Gallagher 1989). Over the short term, roads cause an obvious loss of habitat as well as increased wildlife mortality. Over the long term, the damage can be much more severe.

Roads can disrupt population and metapopulation dynamics that maintain local and regional wildlife populations (Jackson and Griffin 1998). Roads fragment the migration and interaction between populations, which can eventually cause a loss of genetic variability (Reh and Seitz 1990). Thus, roads that fragment habitat, particularly in critical breeding habitat or between separate and different habitats, can adversely affect species. For every population, there is a threshold of mortality. When mortality rates exceed this threshold, there is a risk of extinction (Means 1999).

Increased Wildlife Mortality Rates

Roads increase the chance of wildlife-vehicular collisions (Trombulak and Frissell 2000). Often wildlife is migrating toward decreasing habitat or between fragmented habitats and must cross a road, where mortality is more likely. Scavenger species, too, suffer higher mortality rates when they feed on roadkill and are struck, with a potential impact on predatorprey population dynamics (Bernardino and Dalrymple 1992).

Modification of Animal Behavior

Animal behavior is also modified through road avoidance and disturbance (Forman and Alexander 1998). Terrestrial animals, such as amphibians and turtles, as well as others, exhibit reluctance in crossing roads (Fahrig et al 1995). Also, roads create their own microclimates. As temperatures rise, a warm column of air causes an effective barrier to some animals such as birds and butterflies (Van der Zande 1980).

Bird species are particularly sensitive to traffic noise pollution and undergo population density depressions from the presence of a road at distances from 200m to 2000m (Van der Zande 1980). Many choose not to nest near roads. While some species avoid roads altogether, others, particularly reptiles, are drawn towards the heat of a road, which can have fatal consequences (Trombulak and Frissell 2000).

Introduction of Exotic Species

Roads act as a dispersal corridor, enabling exotic species to penetrate into previously inaccessible areas. Vehicles or vehicular effects may introduce exotic species. Plants mayspread along roads due to vehicle-caused air turbulence (Forman and Alexander 1998). In wetlands where roads have been constructed, the native plants are already stressed due to a disturbance in flood frequencies and therefore cannot fend off the colonizers (Mitsch and Gosselink 1993). This can have large impacts on the overall health and biodiversity of the wetland.

Conclusion

The ecological effects of roads on wetlands range from the alteration of the physical and chemical environment to the unfavorable impacts on wildlife populations due to habitat fragmentation, modification of animal behavior and collisions with vehicles. In view of such ecological impacts, it is important to look for ways in which to lessen the effects. Certainly avoidance of road construction through wetland systems is the best alternative available.

¤ Kinza Cusic is an Environmental Studies graduate student who interned with Wildlands CPR.

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