After the last ice age began, the number of animal species in Australia changed rapidly. The relationship between the elapsed time, ttt, in years, since the ice age began, and the total number of animal species, S(t)S(t)S, left parenthesis, t, right parenthesis, is modeled by the following function: S(t)=4,200,000⋅(0.72)t Complete the following sentence about the yearly percent change of the number of animal species.