Salted roads and sodium limitation in a northern forest ant community


Michael Kaspari, Department of Zoology, University of Oklahoma, Norman, OK 73019, U.S.A. E-mail:


1. Road salt is a common, anthropogenic source of NaCl in many temperate ecosystems. Sodium is also an essential and potentially limiting element for inland animal populations. This suggests that Na limitation in inland ecosystems, and hence attraction to Na sources, should increase with distance from salted roads.

2. In a North Temperate forest, we tested the prediction that Na recruitment would increase, as soil [Na] decreased, with distance from a salted two-lane highway. We presented ants with three concentrations of NaCl and sucrose solution along four pairs of transects ca 1, 10, 100, and 1000 m from the road.

3. Consistent with the Na-limitation hypothesis, the ratio of NaCl to sucrose use increased with distance from the road from 1:13 vials at 1 m to 1:5 vials at 1 km. Genera characterised by high Na use did not accumulate farther from the road. For the common and widespread Tapinoma sessile (Say), a 10-fold increase in distance from the road resulted in ants doubling their use of NaCl relative to sucrose.

4. Road salt is a well-known pollutant, especially of freshwater ecosystems. However, by suppressing plants and potentially promoting consumers, road salt may have more complex effects on terrestrial ecosystems, especially those far inland from oceanic aerosols.