Question

A population of butterflies decreases exponentially at a rate of 18.3% per year.

Answer 1

The population of butterflies decreases exponentially at a rate of 18.3% per year which equal to 18.3%  per 12 month. The exponent function will look like:
an=a0(1+r)^t

If you want to convert year into the month, it is like convert t into 12 t. The calculation would be:
a0(1+r1)^t = a0(1+r2)^12t
(1-0.183)^1 = (1+r2)^12
(0.817) = (1+r2)^12
$\sqrt[12]{0.817}$ = (1+r2)^(12/12)
1+r2= 0.983298
r2= 0.98329-1= 0.01671= 1.67%