Determining the permeability of a South Texas state highway for wildlife using expected crossing frequencies

T. Miles Hopkins, University of Texas Rio Grande Valley
Thomas Yamashita, University of Texas Rio Grande Valley
Anna Rivera Roy, University of Texas Rio Grande Valley
Kevin Ryer, University of Texas Rio Grande Valley
John Young Jr., University of Texas Rio Grande Valley
Richard Kline, University of Texas Rio Grande Valley
Topic Area
Terrestrial Wildlife and Ecosystem Interactions with Transportation

The endangered ocelot (Leopardus pardalis) is estimated to have fewer than 80 individuals remaining in the United States, with the entire population constrained to South Texas. Within this population, 40% of known ocelot deaths are due to vehicle collisions. To prevent additional road mortalities on state highway 100, Cameron County, Texas, Texas Department of Transportation (TxDOT) constructed 11.5 km of wildlife exclusion fencing and eighteen wildlife guards. To maintain landscape connectivity, five wildlife crossing structures were modified or constructed. Camera trap arrays at each road mitigation structure monitor actual wildlife use through wildlife crossing structures and at wildlife guards. Expected crossing frequencies were determined using control sites within close proximity to the wildlife crossing structures. Sites were placed within 150 meters from the roadway and within 500 meters of the comparable structure. The objective of this study was to determine the permeability of state highway 100 for ocelots and other target species. The expected crossing frequency was compared to the actual crossing rate for each wildlife crossing structure to determine overall mitigation structure performance. To estimate infrastructure permeability for target species, we used ArcGIS to measure the length of the entrance to each wildlife crossing structure as well as the length of the mitigation fencing. This comparison provides a proportional performance for each species using the activity rate in the surrounding habitat compared to the activity rate at the structures, calculating the permeability of the road for each target species. The final permeability estimate approximates the percent of state highway 100 that must be permeable to allow the same movement rates across highway as through the surrounding area.

Abstract Keywords
Road Mitigation Structures Linear Infrastructure Permeability Expected Crossing Frequencies