Ecological effects of roads: Theory, analysis, management, and planning considerations

Human population growth and land development over the last century has resulted in widespread habitat loss, fragmentation, and increased wildlife mortality on highways. Since 1936, growth in Florida has resulted in the loss of 54% of herbaceous wetlands and 22% of forest lands. Concurrent with increased land development, hard-surface roads are being constructed at the rate of 8.8 km per day. Interagency coordination in the 1990s toward establishing a statewide greenways and trails network prompted State transportation officials to look at greenway-highway interfaces where conflicts are likely to occur. First, a computer algorithm was developed to evaluate and prioritize the need for wildlife crossing structures or underpasses on state roads. A rule-based decision-support model was developed using geographic information systems (GIS). Of 15,644 road segments prioritized by the model, 81% were located in designated greenways. Chronic road-kill sites, focal species hotspots, greenway linkages, listed species presence, and strategic habitat conservation areas strongly influenced results. Ninety-five scheduled road construction projects coincided with high-priority road segments. Second, field inventories were conducted to determine mitigation needed at 1,232 highly ranked, ecological hotspots on highways. Features documented included parameters of existing bridges and culverts, characteristics of roadways, description of surrounding landscape features, and signs of present animal use. Mitigation measures recommended ranged from installation of underpasses or culverts to minor measures such as fencing, landscaping, signage, and speed restriction. Third, a 2-year monitoring study was conducted to determine the capacity of existing highway drainage-structure designs to function as wildlife passages. Fifty-five different organisms were identified from 47,955 records at 290 field sites. Twenty structural, environmental, and ecological factors affecting use or avoidance of highway-crossing structures by six faunal groups (birds, carnivores, herpetofauna, mesomammals, small mammals, and ungulates) were evaluated through logistic regression analysis. Significance of factors varied by faunal group. Lastly, landscape ecology principles applied to conservation and transportation planning are discussed. Gradients such as topographic relief can be used to design and situate underpasses that conform to on-site ecological flows and landscape patterns. Road-corridor design should include sufficient right-of-way clearance and reduced curve severity and slope steepness to increase driver visibility. Appropriate native vegetation at entry points to underpasses protects species either intolerant of open areas or subject to ambush predators. Fencing or other barriers should be used to direct wildlife toward the crossings and away from the road surface; and to restrict human access.