Braiding Sweetgrass

They tell us that frogs and toads take only about fifteen seconds to
cross the road and mostly escape the cars. The spotted
salamanders average eighty-eight seconds. They may have evaded
countless predators, survived the summer drought, endured the
winter without freezing, but it all comes down to eighty-eight
seconds.
The students’ efforts on behalf of Ambystoma go beyond
roadside rescue. The highway department could install salamander
crossings, special culverts that allow the animals to avoid the road,
but they’re expensive and the authorities need to be convinced of
their importance. The class mission tonight is to take a census of
the amphibians crossing the road to estimate the total numbers of
animals who move from the hills to the pond, and the number who
die en route. If they can get adequate data to show that road
deaths endanger the viability of this population, then they may be
able to persuade the state to take action. There’s just one problem.
In order to obtain accurate estimates of salamander mortality, they
have to count both the animals who make it across the road and
those who don’t.
It turns out that tallying death is easy: they’ve developed a
system for identifying the species of animal by the size of the blotch
it leaves on the road, which is then scraped up to avoid counting it
again on the next traverse of the road. Sometimes death occurs
even without collision. Salamanders are so soft bodied that merely
the pressure wave generated by a passing vehicle can be fatal. The
missing number is the denominator of the death equation—the
number of animals who do make it safely across. How can they
take inventory of the successful crossings on a long stretch of road
in total darkness?
At widely spaced intervals along the road, drift fences have been