And these disruptions occur sometimes more than a mile distant from the highway. Other species are at risk too. Excessive noise causes nausea in some animals. Kangaroo rats are substantially more vulnerable to predators for several days after being subjected to a loud vehicle, because of the disruption to the rats' hearing. Salmon disappear when the silt, dirt and chemicals brought by roads flow into once-pristine streams.
Some calves fail to thrive because road-induced stress inhibits their mother's milk production.
Streetlights confuse amphibians and turtles, limiting reproduction. A species' genetic viability may diminish when a road acts as a physical barrier dissecting one population into two, and in extreme cases, roads may cause genetic changes in species, by changing movement patterns. There are solutions to these problems. Forman believes that proper planning and implementation of mitigation measures can halve the 20 percent figure. Preliminary evidence from Europe and Australia suggests that he may be correct.