Question

The authors assessed changes in distribution “as the difference in the number of 10-km grid squares occupied between 1970-82 and 1995-99 (…), divided by the 1970-82 distribution size.” That’s confusing! What are these 10km grid squares, and why divide the difference by the 1970-82 number?

***Background: Distribution of a species was measured over many years using 10km girds. The authors of the research article took the difference between a 1970-1982 set and a 1995-1999 set and then divied it by the 1970-1982 set. Why would they divied the difference by the 1970-1982 set?

Answer 1

A 10-km square as the name suggests is a square 10 km by 10 km. A tetrad meaning four is a collection of four 1-km squares which are set into a 2 km by 2 km square. In each 10-km square there are 25 tetrads and these are marked A to Z, exclusive of O in order to avoid misunderstanding with zero), from bottom left (SW) to top right (NE). A tetrad is afterward given its full reference as the 10-km square code which is followed by the tetrad letter (e.g. TF73G).

They divide the difference by the 1970-1982 set in order to measure distribution change.