CPhill has covered it well but I will stress something here.
I specifically want to look at $$f(x)=5^{-x}$$
If x is negative this becomes
$$5^{--number}=5^{+number}=positive\:\:number$$
If x=0 then 50=1 = positive number
If x is postitive then
$$5^{-number} = \dfrac{1}{5^{+\:number}}=\dfrac{1}{+\:\: number}=positive\:\: number$$
When the + number on the bottom gets very big this will approach 0 but it won't ever actually get there.
So f(x)=0 is an ASYMTOTE f(x)>0
so the range is $$(0,\infty)\quad \mbox{ or put differently }\quad 0
I have written some posts on negative indices it may be useful for you to revise.
http://web2.0calc.com/questions/indices-especially-negative-indices