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# Interpret change in exponential models: changing units

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## Problem

After the last ice age began, the number of animal species in Australia changed rapidly.
The relationship between the elapsed time, t, in years, since the ice age began, and the total number of animal species, S, start subscript, start text, y, e, a, r, end text, end subscript, left parenthesis, t, right parenthesis, is modeled by the following function:
$S_{\text{year}}(t)=25{,}000{,}000\cdot \left(0.78\right)\^{\Large t}$
Complete the following sentence about the rate of change in the number of species in decades.