After the last ice age began, the number of animal species in Australia changed rapidly. The relationship between the elapsed time,
t
tt, 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.