Roads and fragmentation in northern Wisconsin, U.S.A., 1937-1999

Collaborators: Volker C. Radeloff, Roger B. Hammer, Murray K. Clayton, Charlotte E. Gonzalez-Abraham, and Susan I. Stewart

Roads directly remove habitat, alter adjacent habitat, and interrupt and redirect ecological flows. They fragment wildlife populations, foster invasive species spread, change the hydrologic network, and increase human use of adjacent areas. Understanding past changes in road networks and the resulting effect on landscape pattern is necessary for understanding past and anticipating future impacts of roads. We mapped roads from aerial photography at five dates between 1938 and 1999 in 17 townships in predominantly forested landscapes in northern Wisconsin, USA. Patch-level landscape metrics were calculated on terrestrial area outside of 15m road buffers. We found substantial increases in both road density and landscape fragmentation over time. Road density more than doubled and median, mean patch size and largest patch size were reduced by a factor of four, while patch shape became more regular. Increases in road density were different among ecological subsections, and were positively related to increases in housing density. Fragmentation was largely driven by increases in road density, but housing density influenced largest patch area and patch shape. However, the influence of road density on landscape pattern decreased over time, suggesting that fragmentation reaches a certain level and then decreases in rate. The full impact of road related changes on surrounding ecosystems may only be realized in coming decades because of lag effects. Furthermore, the patterns imposed by roads are not easily reversed as ecological effects may persist even after road closure or removal. Wisconsin legacy of road development has left a lasting mark on the landscape and will constrain landscape-scale management options for the future.

Acknowledgements:
This research was funded by a U.S. Department of Agriculture McIntire-Stennis grant and the North Central Research Station of the U.S. Department of Agriculture Forest Service.

Publications:
Hawbaker, T. J., V. C. Radeloff, R. B. Hammer, and M. K. Clayton. 2005. Road density and landscape pattern in relation to housing density, land ownership, land cover, and soils. Landscape Ecology, in press.

Hawbaker, T. J. and V. C. Radeloff. 2004. Road and landscape pattern in northern Wisconsin based on a comparison of four road data sources. Conservation Biology 18, no. 5:1233-1244. PDF (contains color, 0.6 Mb)